Improvements in and relating to data centre construction

Abstract

A chassis unit for forming a preferably pre-fabricated structural section of a floor of a data centre is suitable for constructing multi-storey building, preferable where the unit comprises both a low

Description

Field of the Invention The present invention concerns data centres and methods of constructing and operating data centres. More particularly, but not exclusively, this invention concerns data centre buildings, for example provided in sectional form. The invention also concerns a kit of parts for constructing a data centre. Background of the Invention A data centre is understood to be a large group of networked computer servers, typically provided in a dedicated space within a building. For example, a modern data centre may include high density, high capacity IT systems in a specialist dedicated space having a carefully controlled environment. A state of the art data centre may include multiple data halls each containing hundreds or even thousands of computer servers, typically contained in racks arranged in rows. In general, each server is provided with electrical power and network connection. During operation, components of servers typically generate heat, which should be dissipated to avoid overheating and damage to equipment. Organisations, and also individuals, treat data centres as mission-critical facilities, meaning that even short periods of down time due to equipment malfunction is not tolerated. With the increase in demand for 'cloud' computing services, there is a need not only for additional data centre facilities, but also higher capacity facilities. Alongside increases in computing power of individual servers, the number of servers in each rack, the number of racks in each data hall increases with each new generation of data centres. Various measures have been used to compare and contrast modern data centres, including the number of servers, building area, and power consumption. According to Data Centre Magazine, a hyperscale data centre should exceed 5,000 servers and 10,000 square feet, and the three largest data centre facilities in the world have total areas exceeding 7 million square feet. Hyperscale data centres often include multiple data halls, each being a single continuous space for accommodating servers. An individual data hall may include IT servers having a total power consumption of 3 MW or more. Racks, or cabinets, housing servers are typically arranged in rows separated by personnel aisles. In order to make efficient use of space, racks have become larger, and aisles longer. For example, a common modern rack design is capable of holding 42 servers stacked one on top the other (a '42U' rack). The dimensions of server racks have become highly standardised, particularly in terms of width. Racks are typically 600mm wide, and a 42U rack may have a height of about 2300 mm, a depth of 1050mm. A single row of racks in the data hall of a Hyperscale data centre may be made up of 22 or more racks, with the data hall accommodating 14 or more rows. Such a high concentration of servers in a relatively small area can present challenges in data centre cooling, especially as increased server power can result in increased heat generation during operation. Servers used for machine learning / artificial intelligence, for example, often generate more heat and thus require greater cooling. Cooling management is considered especially important in modern data centres, where there is a continuing drive for lower PUE (power usage effectiveness) values. PUE is the ratio of total data centre power consumption to server power consumption, and so minimising energy use for equipment cooling is often a high priority. It has been found that liquid cooling (i.e. using a liquid coolant to transport heat away from servers) can offer increased cooling capacity and / or improved efficiency as compared to air cooling (i.e. using a gaseous coolant to transport heat away from servers), at least in part due to the higher heat capacity of liquids as compared to gases. Liquid cooling systems may utilise servers configured to receive a flow of cooling liquid that passes through conduits embedded in or close to server components requiring cooling. For example, some servers may include chips having integrated microfluidic channels allowing heat to be extracted from targeted areas by fluid flowing through the channels, while others may feature cold-plate heat exchangers immediately adjacent chips (the heat exchangers receiving cooling fluid). Such systems may be referred to as 'direct-to-chip' liquid cooling systems, for example utilising 'direct-to-chip' servers. Other systems may utilise racks having air-to-liquid heat exchangers, thus allowing cooling liquid to be used to cool air in close proximity to severs, which are themselves cooled by the cooling air. Other arrangements may also be envisaged. A feature common to all such arrangements is that a cooled liquid coolant is supplied to, and warmed liquid coolant removed from, the servers or their immediate surroundings. In certain arrangements, liquid coolant may flow in a closed loop extending between a cooling unit (which is configured to receive warmed fluid, cool it down to a suitable temperature, and then provide the cooled fluid for re-use) and a heat exchange unit (which is configured to receive cooled fluid, use the fluid to provide cooling, and then return warmed fluid). It will be appreciated that the heat exchange unit may, for example, be configured for liquid-to-air exchange, or liquid-to-chip heat exchange. Data centres are sophisticated installations usually requiring high levels of accuracy in construction. Such requirements can make deployment of new facilities costly and time consuming. Traditionally, data centre facilities have been built using conventional construction methods, where a bespoke building shell is constructed first, and then fitted out with the necessary services and equipment on-site. In some instances, sophisticated 'volumetric' modular systems have been developed where modules of a data centre are factory-finished off-site, then transported to site where all that is required is to connect modules together. An example of such a system is described in WO2010139921 (Bripco BVBA). WO2010139921 also describes an especially energy efficient data centre layout, in which the data centre is subdivided into segregated hot and cold zones, including alternating hot and cold aisles separated by rows of server cabinets. Each cold aisle is supplied with cooling air from a cold air corridor, that also functions as a personnel access corridor, that leads from an air handling unit. Another type of data centre construction is disclosed in WO2013021182 (Bripco BVBA). A construction method is disclosed comprising the steps of: providing at least one ceiling portion, providing a plurality of supporting members, mounting services on the at least one ceiling portion, arranging the at least one ceiling portion and the plurality of supporting members into a first volume, transporting the ceiling portion and supporting members so arranged, and assembling the ceiling portion and supporting members to form a section of a data centre, the section so formed having a second volume. The first volume is smaller than the second volume. A further type of system is the 'plug and play' kit of parts disclosed in WO2017129448 (Bripco BVBA). WO2017129448 discloses a kit of parts that can be used to rapidly and efficiently convert an existing building into a data centre, by componentising the parts needed to equip a data hall. There remains a need for a data centre construction system and method adapted for liquid cooling of at least a portion of servers, as well as a system and method that allows for future installation of liquid cooling equipment without necessitating excessive reconfiguration of the data centre building. There is also a need to manage or eliminate the risk of damage to computing devices in the event of liquid coolant leaks. Summary of the Invention The present invention provides, according to a first aspect, a chassis unit for forming a structural section of a floor and / or ceiling of a data centre building. Optionally, the chassis unit has a length and a width and comprises a cavity extending through at least a portion of the length of the chassis unit. Optionally, the cavity is located at least partially, for example entirely, within the structure of the chassis unit, for example within a volume bounded by side and end beams of the chassis units. In other words, the cavity is optionally in the form of a recess in the structure of the chassis unit, and not, for example, located in a casing or structure positioned above or below structural beams of the chassis unit. Optionally, the cavity extends continuously through at least 75%, such as at least 90%, for example at least 95%, of the length of the chassis unit. Optionally, the chassis unit comprises at least one, optionally at least two, access covers movable between a first position closing an opening to the cavity and a second open position allowing hand access to the cavity through the opening. It will be appreciated that a cover provides hand access when the cover and opening are sized and configured so that a typical adult worker is at least able to reach through the opening so that they can position their hand inside and access the relevant parts of the cavity. It will be further appreciated that larger openings and covers (for example sized to allow access to more of a worker's body) necessarily also provide hand access. Optionally, the access covers and the opening may be large enough to allow hand access to the cavity (e.g. to equipment in the cavity) through the opening. It has been found that such arrangements confer a number of advantages. For example, pipes and other equipment, such as electronics, may require repair or malfunction. This arrangement allows access to said pipes and equipment in order to carry out repairs. Alternatively, the pipes or other equipment within the chassis may need updating to comply with new systems, this arrangement allows for easier installation and removal. The present inventors have recognised that providing access to the cavity in the chassis unit allows for easier access to the cavity within the chassis. Optionally, the cavity extends continuously through at least 75%, such as at least 90%, for example at least 95% of the width of the chassis unit. This creates space within the majority of the chassis for the storage of pipes or other equipment. It will be understood that at least one, optionally at least 2, access cover(s) may provide hand access to substantially all of the length of the cavity when moved to the second open position. This may be in the form of one large access cover that spans the entirety of the length of the cavity. Alternatively, this may be in the form of a plurality of access covers positioned along the cavity. Optionally, this may also be the case when the chassis unit comprises a plurality of said access covers, optionally the plurality of access covers together provide hand access to substantially all of the length of the cavity when moved to the second open position. Optionally the plurality of access covers may comprise three or more access covers. Optionally the plurality of access covers may extend across substantially the entire length of the cavity, such as across at least 95% of the length of the cavity. Optionally, at least a portion of the cavity is arranged to be positioned under a row of server racks, and another portion is arranged to be positioned under a personnel area. Optionally, the one or more access covers are sized and configured to provide hand access to substantially all (for example all) of the portion of the cavity arranged to be positioned under a personnel area. Optionally, the chassis unit may have upper face and a lower face. The at least one access cover may be positioned on the upper face, thereby providing hand access to the cavity from above when the chassis unit forms said structural section of the building. Such an arrangement may provide convenient access for repairs or other work on components situated within the chassis, such as replacement. Optionally, the structural section of the building may be a section of a floor of the building, and optionally a ceiling of the building. Optionally the at least one access panel may be a floor access panel forming a section of a floor of a personnel area when moved to the first closed position. Optionally the at least one access panel may be a ceiling access panel. Alternatively access to the cavity via the one or more access panels may be possible via both the ceiling and the floor access panels. Such an arrangement may provide improved access to components within the cavity. Optionally the chassis unit has a substantially planar upper surface that may be suitable for providing flooring of a personnel area in the data centre building. Optionally, the access cover may comprise at least one transparent section and / or a section with a plurality of openings allowing visual inspection of the cavity when the access cover is in the first closed position. Such an arrangement may enable the contents of the cavity to be inspected without requiring the access cover to be removed, thereby allowing a user to identify faults that may have occurred with the components located within the cavity. It will be understood that the section having a plurality of openings, which may for example be referred to as a perforated section, may have openings of any kind. For example, the section may be formed of and / or comprise a sheet material with openings cut into it, a mesh material or a grilllike structure (such as metal grillage); suitable flooring materials include composite and metal materials, for example. Optionally, the access cover is a flooring panel having a load rating of at least 10 kN / m2, for example at least 15 kN / m2. Additionally or alternatively, the chassis unit provides a floor having a load rating of at least 10 kN / m2, for example at least 15 kN / m2. Optionally, the cavity is a liquid containment cavity, for example wherein the chassis unit comprises a liquid containment barrier for holding liquid within the cavity. Advantageously, any liquid that may be released in the cavity (for example released from components housed in the cavity) may thus be contained within the cavity and not allowed to spread any further within the building structure. Optionally the cavity is bounded by a bottom surface and a plurality of side surfaces, wherein said bottom and side surfaces may be sealed to prevent escape of liquid from the cavity. Optionally the cavity has a volume, and the chassis unit is configured to contain in the cavity a volume of liquid at least 50% of the volume of the cavity, for example wherein the liquid containment barrier is sized and configured to contain in the cavity a volume of liquid at least 50% of the volume of the cavity. Optionally the liquid containment barrier is sized and configured to contain in the cavity a volume of liquid at least 60%, such as at least 80%, of the volume of the cavity. Such an arrangement may allow the liquid containment barrier to store a large amount of liquid relative to the size of the cavity. It has been found that such an arrangement may be beneficial in the event of a components stored in the cavity releasing liquid; in which case a large amount of liquid could be released into the cavity. Optionally the chassis unit comprises a liquid detection system for identifying the presence of liquid (e.g. liquid coolant) in the cavity. Such an arrangement may allow for automated and / or rapid leak detection and / or other malfunction within the data centre building. Optionally the chassis unit is a fire-rated chassis unit, for example configured to provide at least 1 hour of fire resistance, e.g. when exposed to fire from below. Optionally, the chassis unit comprises a heat insulation layer, for example positioned within the cavity below the liquid containment barrier. Optionally, the insulation layer comprises thermally insulating material, for example in the form of a board or sheet. It will be appreciated that any suitable material may be used, such as fiberglass, cellulose, and mineral wool. It has been found that such an arrangement provides the chassis unit with a particularly effective fire rating. Fire safety is an important aspect of building design, and national building regulations typically set out strict requirements for fire safety. For example as set out in the UK in 'Approved Document B - Volume 2 - Buildings Other Than Dwelling Houses' of 'The Building Regulations 2010'. In one aspect, fire safety regulations indicate to what extent and for how long 1) a building's structural integrity should be maintained in the event of a fire, and 2) spread of fire through a building should be prevented. It is often the case that in large buildings (such as data centres) some degree of compartmentalisation is incorporated in the structure of the building in order to inhibit or at least slow down the spread of a conflagration. Where a building is separated into compartments, 'Approved Document B' states that every compartment wall and compartment floor should form a complete barrier to fire between the compartments they separate. That document further indicates the minimum fire resistance that should be provided by a compartment wall or compartment floor. Typically, fire resistance is defined in terms of the time (in minutes) that a barrier is able to prevent fire penetration and / or transfer of excessive heat. In the UK, fire rating tests are specified in British Standard BS:476, with specifications for walls in buildings contained in British Standard BS:476-22. It will be appreciated that the fire resistance of an internal wall or floor may be upgraded by providing it with a lining of material having a higher fire resistance, for example by providing a layer of fire resistant insulation material in the structure of a chassis unit. Optionally, the chassis unit is configured to provide a floor that functions as a fire barrier, e.g. having a '1 hour' rating, in terms of integrity and / or insulation. Integrity refers to the ability of the fire barrier to remain standing for the specified time. For example, a 1-hour integrity fire rating test may involve exposing one side of a free-standing sample of the barrier to flames for an hour, throughout which the barrier must prevent passage of flames from one side to the other. In some tests, the barrier is subsequently sprayed with water at the end of the 1-hour period to ensure that the integrity of the barrier is maintained under fire-fighting conditions. Additionally or alternatively, a 1-hour insulation fire rating test may involve exposing one side of a sample of the barrier to flames for an hour, throughout which the temperature on the other side of the barrier should not exceed a pre-defined set point. In the UK, fire ratings are often assessed using British Standard BS:476-22. Optionally, the chassis unit is configured to provide a fire barrier, such as a 1-hour fire barrier, such as a 1-hour integrity / insulation fire barrier according to BS:476-22. Optionally, the cavity is bounded by a top surface having one or more openings allowing liquid to enter the cavity, for example when the at least one access panel is in the second closed position. Optionally, the at least one access panel, if present, has one or more openings allowing liquid to enter the cavity when said access panel is in the second closed position. The one or more openings in the top surface may enable liquid to flow into the cavity even when the panel is in the closed position. Such an arrangement may be particularly beneficial if the building comprises a liquid-based fire suppression system, for example thereby allowing liquid released by the fire suppression system to be caught and retained in the cavity. It has been found that uncontrolled release of fire suppressant liquid may be prohibited in certain locations for environmental regions, and provision of a single, centralised tank for collection and storage of fire suppressant fluid prior to controlled release may be prohibitively costly and / or take up valuable space in or adjacent to a data centre building. Optionally, the chassis unit may comprise a drain conduit for removal of liquid from the cavity, optionally wherein the drain conduit is in fluid communication with a pump for pumping liquid out from the cavity. It will be understood that such an arrangement may provide a particularly convenient method of removing liquid from the cavity. Optionally, the cavity comprises a gulley for collecting liquid, and an opening positioned in the gulley and in fluid communication with the drain conduit. Optionally the drain conduit is comprised in a drainage system further comprising one or more downpipes in fluid communication with a water storage reservoir outside and / or underneath the data centre building. Optionally the cavity is sized to accommodate at least one liquid coolant conduit for transporting liquid coolant to and / or from liquid cooling equipment in the data centre. It has been found that siting such conduits within the chassis unit (i.e. in the cavity) provides a highly space-efficient data centre, for example reducing the space taken up by the liquid coolant conduit that may otherwise be positioned in corridors and personnel areas. Furthermore, it has been found that positioning such conduits in a cavity in a chassis unit allows for particularly convenient manufacture and transport of chassis units. In particular, conduits may be installed in chassis units in a central manufacturing location, and subsequently transported to a data centre construction site for assembly. Locating conduits in a cavity within the structure of a chassis unit may also allow for chassis units to be stacked and / or transported and handled in a flat-pack manner. Optionally, the chassis unit comprises a plurality of removable floor joists positioned within the cavity, for example supporting flooring and / or one or more access panels at least partially covering the cavity. Optionally, the chassis unit comprises a conduit cassette, for example mounted or positioned substantially within (e.g. entirely within) the cavity, optionally attached directly or indirectly beams of the chassis unit. Optionally, the conduit cassette is releasably mounted, for example being removable from the cavity. Optionally, the chassis unit comprises a plurality of such conduit cassettes, for example wherein the conduit cassettes are separate from each other and / or releasably fastened together. Optionally the chassis unit comprises one or more conduit cassettes each configured to support a plurality of portions of liquid coolant conduits. Optionally, each conduit cassette comprises a framework to which said portions of liquid coolant conduits may be fastened. It will be understood that the chassis unit may, for example, comprise one or more such conduit cassettes without liquid coolant conduits attached. For example, the conduit cassette may be suitable for supporting components of liquid coolant conduits adding during later fit-out or retrofitting of a data centre. Optionally at least a portion of each conduit cassette is housed in the cavity of the chassis unit, for example wherein substantially all, or all, of the conduit cassette is housed within the cavity. It will be understood that substantially all of a conduit cassette is housed within the cavity when the cassette does not protrude from the sides or bottom of the cavity, and the cassette protrudes from the top of the cavity by a distance no more than 10% of the internal height of the cavity. It will be further understood that a cassette is housed within the cavity when no part of the cassette protrudes beyond the sides, bottom or top of the cavity. When a cavity is open-topped, the top is understood to be defined by the level of the parts of the chassis unit defining the sides of the cavity. Optionally, one or more conduit cassettes, and / or at least a portion of the conduit cassette is removable from the cavity. Optionally, the conduit cassette is configured to support one or more panels extending across and covering at least a portion of an opening of the cavity, for example an opening in a top of the cavity. Optionally, the conduit cassette is configured to support at least part of a floor provided by the chassis unit, for example the conduit cassette is configured to support one or more flooring sections or panels providing flooring for a personnel area and / or rack storage area situated above the cavity of the chassis unit. Optionally, the conduit cassette is configured to be supported by, and preferably attached to, structural elements (e.g. beams) of the chassis unit, and to support, and preferably be attached to, one or more sections of flooring, thereby providing load-bearing support for a floor surface above the cavity. Optionally, the chassis unit comprises one or more flooring sections attached to and supported by one or more conduit cassettes accommodated in the cavity, for example wherein at least one flooring section is releasably attached to the conduit cassette or otherwise movable from a closed position covering at least a portion of the cavity and an open position allowing hand access to the cavity. Optionally, the conduit cassette is configured to support (i) at least a portion of a liquid coolant conduit positionable within the cavity, and (ii) a portion of the at least one access cover when moved to the second closed position. Additionally or alternatively, the conduit cassette is configured to support a plurality of portions of liquid coolant conduits, for example wherein the conduit cassette comprises a framework to which may be fastened: (i) a plurality of portions of liquid coolant conduits, and (ii) one or more panels extending across and covering at least a portion of an opening of the cavity. Optionally, said panels are flooring panels, for example openable flooring panels configured to provide and / or control hand access to the cavity. It has been found that such an arrangement provides particularly convenient and effective support of flooring above the cavity, particularly through the conduit cassette providing the dual role of supporting liquid coolant conduit components and flooring components. According to a second aspect, the present invention provides a chassis unit assembly, comprising a chassis unit and at least a portion of a liquid coolant conduit. The chassis unit is for forming a structural section of a floor and / or ceiling of a data centre building, has a length and a width and comprises a cavity extending through at least a portion of the length of the chassis unit. Optionally, said at least a portion of the liquid coolant conduit is positioned in the cavity, for example wherein said at least a portion of the liquid coolant conduit extends within the cavity from a first position along the length of the chassis unit to a second position along the length of the chassis unit. Additionally or alternatively, at least a portion of a liquid coolant conduit extends within the cavity from a first portion across the width of the chassis unit to a second position across the width of the chassis unit. Optionally the chassis unit may comprise at least a portion of each of a plurality of liquid coolant conduits positioned in the cavity of the chassis unit. For example at least a portion of a liquid coolant supply conduit for transporting liquid coolant to liquid cooling equipment in the data centre, and at least a portion of a liquid coolant return conduit for transporting liquid coolant from said liquid cooling equipment in the data centre. Such an arrangement may allow liquid coolant to be efficiently distributed throughout the data centre. Optionally at least a portion of a liquid coolant supply conduit comprises a plurality of branch conduits for distribution of liquid coolant to separate items of liquid cooling equipment in the data centre, and / or wherein said portion of liquid coolant return conduit comprises a plurality of branch conduits for receiving liquid coolant from separate items of liquid cooling equipment in the data centre. The branch conduits may be of smaller diameter than the liquid coolant conduits. Optionally, the chassis unit may comprise at least one manifold connecting said at least a portion of the liquid coolant conduit to a plurality of liquid coolant branch conduits, for example wherein the manifold is operable to regulate flow between said at least a portion of the liquid coolant conduit and said plurality of liquid coolant branch conduits. Optionally, the manifold is operable to increase or decrease flow rate of the coolant between the liquid coolant conduit and the plurality of liquid coolant branch conduits, for example in dependence on IT equipment cooling requirements. Optionally, at least one manifold may be located in the cavity. Optionally, at least a portion of the liquid coolant conduit is fluidly connected to at least one additional portion of the liquid coolant conduit. Optionally, the additional portion of the liquid coolant conduit is located outside the cavity. Optionally, the liquid coolant supply conduit passes through an upper or lower surface of the chassis unit, for example an upper surface configured to provide a floor surface in the data centre. Optionally, liquid coolant conduits of the chassis unit assembly extend into, and / or out of, the cavity through a top and / or bottom (preferably top) surface of the chassis unit to provide fluid communication with inaisle liquid cooling equipment (such as racks of liquid-cooled servers and / or liquid coolant distribution units located in or adjacent to racks. Additionally or alternatively, one or more liquid coolant conduits pass through openings provided in outer side and / or end beams of the chassis unit (preferably only through end beams and / or not through side beams). It has been found that such arrangements allow for efficient fluid communication between chassis units. Optionally, at least a portion of the liquid coolant conduit is mounted on a conduit cassette, for example a conduit cassette inserted into the cavity and / or attached to the chassis unit. Optionally, the conduit cassette is releasably attached to the chassis unit, for example being separable from the chassis unit and / or removable from the cavity. Optionally, the chassis unit comprises a plurality of conduit cassettes, for example wherein the conduit cassettes are separate from each other and / or releasably fastened together. Such an arrangement may enable a user to easily remove the conduit cassette(s) from the cavity, if required. Optionally the chassis unit comprises one or more conduit cassettes each configured to support a plurality of said portions of liquid coolant conduits. Optionally, each conduit cassette comprises a framework to which said portions of liquid coolant conduits may be fastened. Optionally at least a portion of each conduit cassette is housed in the cavity of the chassis unit, for example wherein substantially all, or all, of the conduit cassette is housed within the cavity. Optionally, one or more conduit cassettes, and / or at least a portion of the conduit cassette is removable from the cavity. Optionally, the conduit cassette is configured to support one or more panels extending across and covering at least a portion of an opening of the cavity, for example an opening in a top of the cavity. Optionally, the conduit cassette is configured to support at least part of a floor provided by the chassis unit, for example the conduit cassette is configured to support one or more flooring sections or panels providing flooring for a personnel area and / or rack storage area situated above the cavity of the chassis unit. Optionally, the conduit cassette is configured to be supported by, and preferably attached to, the chassis unit, and to support, and preferably be attached to, one or more sections of flooring, thereby providing loadbearing support for a floor surface above the cavity. Optionally, the chassis unit comprises one or more flooring sections attached to and supported by one or more conduit cassettes accommodated in the cavity, for example wherein at least one flooring section is releasably attached to the conduit cassette or otherwise movable from a closed position covering at least a portion of the cavity and an open position allowing hand access to the cavity. Optionally, the conduit cassette supports a plurality of portions of liquid coolant conduits, for example wherein the conduit cassette comprises a framework to which is fastened: (i) one or more portions of liquid coolant conduits, and (ii) one or more panels extending across and covering at least a portion of an opening of the cavity. Optionally, said panels are flooring panels, for example openable flooring panels configured to provide and / or control hand access to the cavity. Optionally, the chassis unit assembly comprises at least a portion of a liquid coolant distribution conduit, for example a distribution conduit in fluid communication with one or more liquid coolant conduits located at least partially in the cavity of the chassis unit. Optionally, the distribution conduit is configured for positioning above the floor of the chassis unit, for example positioned above the floor. Optionally, the distribution conduit is mounted in a frame. Optionally, the distribution conduit is configured for fluid communication with a liquid cooling unit. According to a third aspect, the present invention provides a data centre building comprising a plurality of chassis units assembled together to define a unitary structure. Optionally the chassis units will be of elongate shape, with a length and a width, opposed ends parallel to the width, opposed sides parallel to the length, and side beams extending along the opposed sides. Optionally, the side beams of neighbouring chassis units may abut (and are optionally joined together) at side joins. Optionally the plurality of chassis units may form a structural section of a floor and / or a ceiling of the data centre building. Optionally, each chassis unit is supported above the floor by a plurality of support posts. Optionally, the plurality of chassis units may comprise a chassis unit according to any preceding aspect of the invention, and or a chassis unit comprised in a chassis unit assembly according to the invention, for example comprising a plurality of such chassis units. Such chassis units may be referred to as 'cavity chassis units', for example to differentiate from similarly sized and / or structured chassis units not including such a cavity. Optionally, the cavity chassis unit forms a structural section of the floor of the building, and optionally a ceiling of the building. Optionally at least one access panel, when present, is a floor access panel and may form a section of a floor of a personnel area of the data centre building when moved to the first closed position. Optionally the chassis unit has a substantially planar upper surface suitable for providing flooring of a personnel area of the data centre building. Optionally, the data centre comprises a liquid fire-retardant fire protection system. This may prevent the spread of fire throughout the data centre building. Optionally, the data centre building comprises a plurality of cavity chassis units. Optionally, each cavity chassis unit comprises at least one liquid coolant supply conduit for supplying liquid coolant to electronic equipment in the data centre and at least one liquid coolant return conduit for receiving liquid coolant from said electronic equipment, wherein at least a portion of each of the supply and return conduits is positioned within the cavity. Optionally, the data centre building comprises at least one liquid coolant supply distribution conduit, and at least one liquid coolant return distribution conduit, wherein the supply and return distribution conduits are each in fluid communication with a liquid cooling unit operable to cool liquid coolant, wherein the supply distribution conduit is in fluid communication with a plurality of said liquid coolant supply conduits, and wherein the return distribution conduit is in fluid communication with a plurality of said liquid coolant return conduits. Optionally, the distribution supply conduit is in fluid communication with coolant supply conduits of a plurality of cavity chassis units, and / or the distribution return conduit is in fluid communication with coolant return conduits of a plurality of cavity chassis units. Optionally, the data centre comprises a plurality of said cavity chassis units. Optionally, the data centre comprises two or more cavity chassis units. Optionally, the side beams of the cavity chassis units abut and may be joined together at side joins. Optionally, the plurality of chassis units forms a structural section of a floor of the data centre. It will be understood that a structural section of a data centre building is a weight-bearing / load-bearing part of the structure of the building, for example intrinsic to structural integrity of the building. By way of example, services framework and / or ducting suspended from a structural framework or ceiling, or supported on a floor, are not considered to be structural sections of a data centre building. By way of further example, it will be understood that a chassis unit, forming a structural section of a floor of a data centre, is configured to bear the weight of personnel and equipment accommodated on the floor. Optionally, the data centre comprises at least ten rack storage areas positioned on the floor, each rack storage being arranged to accommodate a plurality of server racks and being positioned adjacent at least one personnel access area. Each rack storage area may have a width and length and may be arranged to accommodate a row of at least 10 server racks. Optionally, the cavity of said at least one cavity-containing chassis units is positioned under said at least one personnel access area. The personnel access area may give access to the server racks. Optionally wherein said at least one cavitycontaining chassis units forms at least a portion of the floor of said at least one personnel access area. Optionally, the chassis units are configured for arrangement side by side and / or end to end (preferably side by side) to form a continuous section of the floor of the data centre. Additionally or alternatively, the chassis units are optionally configured for attachment to a plurality of support post assemblies, for example so that each chassis unit spans one or more gaps between adjacent support post assemblies, and / or so that the chassis units and support post assemblies together provide structural support for a floor of the data centre. Additionally or alternatively, the plurality of chassis units optionally forms at least 75%, for example at least 90%, of the surface area of a floor of the data centre. Optionally, at least one rack storage area is configured for accommodating liquid cooled electronic equipment. Optionally said cavity of at least one cavitycontaining chassis unit may be positioned under a personnel access area adjacent said rack storage area, for example configured for accommodating liquid cooling equipment. This arrangement may improve space efficiency in the data centre. Optionally, the data centre comprises a plurality of personnel access areas, each adjacent to one or more rack storage areas. Optionally, the personnel access areas comprise alternating hot and cold aisles separating rack storage areas. Optionally, the data centre comprises a plurality of air handling units for providing cooling air to cold aisles. It will be appreciated that provision of cooling air may be desirable, for example if the data centre comprises a mixture of air-cooled and liquid-cooled equipment, and / or for cooling of electronic components not configured for liquid cooling. Optionally the data centre comprises a cold air supply corridor having a length extending perpendicular to and being in fluid communication with the cold aisles to receive from the cold air supply corridor cooling air supplied to the cold air supply corridor by a plurality of air handling units. Optionally the air handling units are direct air handling units. It will be understood that a direct air handling unit provides cooling air that comprises or consists of a portion of ambient air from outside the data centre building. Optionally where said ambient air is treated to adjust its temperature and / or humidity. Direct air handling units, and suitable control methodologies are disclosed in WO2011 / 148175A1 (Bripco BVBA), the contents of which are incorporated herein by reference. Alternatively, the air handling units are indirect air handling units. Suitable indirect air handling units are disclosed in WO2016 / 207323A1 (Bripco BVBA), the contents of which are incorporated herein by reference. It will be understood that an indirect air handling unit provides air that consists substantially of (for example consists entirely of) air from inside the data centre building, optionally having been treated to adjust its temperature and / or humidity. Optionally, an indirect air handling unit comprises a heat exchanger for transferring heat from air inside the building to air from outside the building, for example wherein internal air travels through the air handling unit along an internal airflow path segregated from an external airflow path along which air from outside the building travels. Preferably, the air handling units comprise adiabatic cooling units, and optionally are free from mechanical cooling units (such as direct expansion cooling units). It will be understood that a direct expansion mechanical cooling unit provides cooling by contacting air with coils containing a refrigerant that had been compressed then allowed to expand. Optionally, the data centre comprises at least one data level, such as a plurality of data levels. It will be understood that a data level may be a floor (or storey) of a building configured to accommodate multiple racks of servers. Optionally, each data level accommodates at least ten rack storage areas positioned on a floor, each rack storage area having a width and a length and being arranged to accommodate a row of at least ten server racks, the rack storage areas separating alternating hot aisles and cold aisles. Each cold aisle and each hot aisle has a width and a length. Preferably, each data level comprises a floor structure formed from a plurality of cavity chassis units, each chassis unit having an elongate shape with a length and a width, opposed ends parallel to the width, opposed sides parallel to the length, and side beams extending along the opposed sides. Optionally, each chassis unit comprises a cavity in a space bounded by the side beams. Optionally, each chassis unit is supported by a plurality of support posts. Optionally, each plurality of chassis units includes a plurality of cavity chassis units. Preferably, a cavity chassis unit forms a portion of the floor structure of a data level under at least part of an IT area. An IT area of a data centre is an area in which IT equipment, particularly server racks, are accommodated. As used herein, an IT area comprises hot aisles, cold aisles and rack storage areas. It will be appreciated that a cavity chassis unit may extend partially over further parts of a data level, for example extending across at least part of an ancillary area and / or a personnel access corridor (such as a cold corridor). Optionally, cavity chassis units comprise cold aisle chassis units and hot aisle chassis units, for example arranged with cold aisle chassis units interleaved with a plurality of hot aisle chassis units. It will be understood that a cold aisle chassis unit typically aligns with an overlying cold aisle, thereby forming a portion of the floor structure that extends under and along at least part of the length of a cold aisle. Similarly, it will be understood that a hot aisle chassis unit typically aligns with an overlying hot aisle, thereby forming a portion of the floor structure that extends under and along at least part of the length of a hot aisle. Preferably, the side beams of neighbouring chassis units abut (and are optionally joined together) at side joins. Optionally, there is an alternating side by side arrangement of cold chassis units and hot chassis units. Optionally, a plurality of data levels includes a first data level, and a second data level positioned above the first data level. Optionally, the floor structure of the second data level forms the ceiling of the first data level. Preferably, each side join between chassis units (such as each side join between cold aisle chassis units and hot aisle chassis units) of the floor structure of the first data level is positioned under and extends along at least a portion of the length of a rack storage area of the first data level so that said rack storage area of the first data level spans a join between neighbouring chassis units. Preferably, said side join runs along a line located in the middle third, for example approximately in the centre, of the width of said rack storage area. Optionally, each side join is a continuous side join extending from one end of each chassis unit to the other end of each chassis unit. It has been found that such a data centre building structure is especially effective and allows for particularly efficient manufacture and construction. In particular, forming the ceiling structure from chassis units allows the structure to be built up from a plurality of components that can be factory finished prior to arrival on site. Centralised factory manufacture provides a controlled environment and access to an experienced and highly skilled workforce, as well as allowing components to be constructed and tested while other works (such as ground preparation) take place on site. Dividing the ceiling structure into 'cold aisle' and 'hot aisle' chassis units allows the width of those units to be kept to a size that allows convenient transport, for example by road, and in particular allows the width to be kept within the limits of 'regular' load sizes, obviating the need for specialised equipment or additional safety precautions (such as road convoy escorts) during transport. A further advantage of dividing the ceiling structure into such chassis units, and arranging the units so that side joins underlie rack storage areas of the data level above is that the ceiling structure is strengthened at the points of highest load in a particularly efficient manner. Optionally, each data level has a height of at least 4.5 m, such as at least 5.5 m. it will be understood that the height of a data level is the distance from the floor of one data level to the floor of the data level immediately above. The height thus includes the thickness of a chassis unit as well as the separation between chassis units of adjacent levels. It will be understood that a rack storage area is a space sized and configured to accommodate the row of server racks, for example a row of at least 20 server racks. Optionally, the server racks that may be accommodated are 42U or larger server racks, meaning racking capable of holding 42 or more servers. In an air-cooled area of a data centre, IT equipment (which becomes hot during use) is cooled by contact with cooling air. Such cooling air may be supplied by one or more air handling units. As used herein: a 'cold aisle' of an operational data centre is a space adjacent a row of racks (for example between opposed rows of racks) from which rack-mounted IT equipment is able to draw cooling air; a 'hot aisle' is a space adjacent a row of racks (for example between opposed rows of racks) into which rack-mounted IT equipment is able to expel warm air. Typically, operational efficiency is improved by segregating hot aisles and cold aisles, thereby avoiding cooling air in a cold aisle mixing with warm air from a hot aisle before being used to cool IT equipment. It will be appreciated that the floor structure is a structural component of the building, for example being a load-bearing structure of the building. Data centre sections are also disclosed in WO2013021182 (Bripco BVBA), the contents of which are incorporated by reference. Optionally, side joins are formed by bolting or otherwise fastening adjacent chassis units together, for example via a bracket. For example, the abutting side beams of neighbouring chassis may be bolted together, e.g. via a bracket. Optionally, when chassis are bolted together via brackets, said brackets may for example be bolted to each chassis unit (such as a side beam of each chassis unit), or each welded to a respective one chassis unit (such as a side beam of a chassis unit). Optionally, the brackets are multipurpose brackets, such as multipurpose brackets for attaching neighbouring chassis units together and for attachment of data centre services suspended below each chassis unit. For example, each bracket is positioned and configured to fasten neighbouring chassis units together and to provide an attachment point for a drop rod and / or a beam of a slot channel frame system. Such brackets are further described in PCT / GB2023 / 053066 (Pripco Limited), the contents of which are fully incorporated herein by reference. Optionally, the side beams are load-bearing structural beams that provide the main structural support for the chassis unit along its length. Optionally, the side beams are fabricated from metal, such as steel. For example, the beams are rolled steel joists. Optionally, the side beams are T beams, so-called due to their crosssection having the shape of a capital letter i. Such beams are also known as universal beams, and / or 'H' beams. Optionally, each beam comprises top and bottom (e.g. planar) flanges joined by a (e.g. planar) web perpendicular to the flanges. It will be appreciated that such beams allow for neighbouring beams to abut along straight edges, forming a tight join between neighbouring chassis units. Optionally, each chassis unit has end beams extending along the ends of the chassis unit. For example, each chassis unit may have a 'ring beam' structure, where beams extending along the sides and ends are joined together to form an outer framework. Optionally, each chassis unit has a planar shape, for example taking the form of a panel. Optionally, each cavity chassis unit has a portion free from crossbeams extending between opposed sides or ends (preferably sides), thereby providing space for the cavity. It may be that such a ring beam structure provides a particularly rigid and strong construction. It will be appreciated that chassis unit beams may be formed from any suitable material, such as metal (e.g. steel). Optionally, the plurality of data levels comprises a third data level positioned above the second data level. Optionally, the floor structure of the third data level forms the ceiling of the second data level, and each side join between chassis units (such as between cold aisle chassis units and hot aisle chassis units) of the floor structure of the third data level is positioned under and extends along at least a portion of the length of a rack storage area of the third data level so that said rack storage area of the third data level spans a join between neighbouring chassis units. It will be appreciated that there may be further data levels, such as a fourth, fifth, sixth etc. data level, with corresponding arrangements of the floor structure. It will be understood that each support post may be formed form a single piece of material, such as a metal (e.g. steel) beam, or multiple pieces secured together (such as multiple metal, e.g. steel, beams welded, bolted or otherwise fastened together. Optionally, each support post supports a portion of at least one chassis unit of the floor structure of the first data level and a portion of at least one chassis unit of the floor structure of the second data level. For example, each support post may support a portion of two or more chassis units of the floor structure of the first data level and two or more chassis units of the floor structure of the second data level. Optionally, each chassis unit of the floor structure of the first data level is supported by four or more support posts that also support a chassis unit of the floor structure of the second data level. It will be understood that each support post may support at least a portion of two or more, such as four, chassis units of each floor structure, for example wherein the support post is positioned at the intersecting corners of two or more, e.g. four, chassis units. Optionally, when the data centre building comprises more than two data levels, the support posts each support at least a portion of at least one chassis unit of the floor structure of each data level. For example, the support posts may extend through two or more, such as all, data levels. Optionally, the chassis units are bolted or otherwise fastened to the support posts, optionally via one or more brackets. For example, the side and / or end beams of the chassis units are secured to the support posts. It will be appreciated that when brackets are used, such brackets may for example be bolted to the chassis and the post, or welded to one of the chassis or the post and bolted to the other (preferably welded to the post and bolted to the chassis, such as to a beam of the chassis). Optionally, each chassis unit is arranged end to end with another chassis unit. Optionally, each cold aisle chassis unit is arranged end to end with an adjacent cold aisle chassis unit. Optionally, each hot aisle chassis unit is arranged end to end with an adjacent hot aisle chassis unit. In such an arrangement, it may be that chassis units are arranged in complementary pairs, for example so that the cold / hot aisles associated with each chassis unit together form a single continuous cold / hot aisle that extends along at least part of the length of each chassis unit. Optionally, such end to end arranged chassis units are spaced apart, for example having an insert bridging the gap between their ends. Optionally, the facing ends of such end to end arranged chassis units abut and are secured to a plurality of support posts (such as two support posts), for example so that said support posts support said ends of the chassis units. Optionally, such end to end adjacent chassis units are secured to and positioned on opposite sides of at least one, preferably two or more, support posts. For example, said support posts are sandwiched between the facing ends of the chassis units, e.g. positioned in a gap between said facing ends. It has been found that such positioning of the support posts allows neighbouring side by side chassis units to have a continuous side join from one end of each chassis unit to the other. Optionally, when cavity chassis units abut end to end, one of more liquid coolant conduits pass upwards or downwards (preferably upwards) out of the cavity of one of said cavity chassis units, across any gap between the chassis units, and back downwards or upwards (preferably downwards) into the cavity of the other end-to-end arranged cavity chassis unit. For example, at least a portion of one or more such conduits extends along and above the floor of a hot or cold (preferably cold) aisle. It will be understood that each cold aisle chassis unit and each hot aisle chassis unit is associated with at least a portion of the length of an associated cold or hot aisle. For example, a cold or hot aisle chassis unit of the floor structure of an upper data level extends along and below a portion of a corresponding cold or hot aisle of that upper data level, and optionally extends along and above a portion of an aisle of a lower data level immediately below the upper data level. Optionally, the cold aisles of higher data levels overlie the cold aisles of lower data levels. Alternatively, it may be that the cold aisles of a higher data level overlie the hot aisles of a lower data level. Optionally, the width of a cold aisle chassis unit plus the width of a hot aisle chassis unit together correspond to: the width of a cold aisle, plus the width of a hot aisle, plus the width of two rack storage areas. For example, the width of a cold aisle chassis unit plus the width of a hot aisle chassis unit of the floor structure of the a data level correspond to the sum of the widths of the cold aisle, hot aisle and two rack storage areas of the data level overlying the chassis units, and preferably also to the sum of the widths of the cold aisle, hot aisle and two rack storages areas of another data level underneath the chassis units. It will be appreciated that since the side join between chassis units runs underneath a rack storage area, the width of 'two rack storage areas' referred to above is in practice made up of one complete rack storage area (spanning the join between the two chassis units), and portions of two separate rack storage areas that together add up to the width of a whole rack storage area (i.e. those that are on the outer sides of the two chassis units). Additionally or alternatively, the combined width of two neighbouring chassis units of the plurality of chassis units corresponds to: the width of a cold aisle, plus the width of a hot aisle, plus the width of two rack storage areas. Optionally, each cold aisle has a width of from lm to 2m, such as 1.4m to 1.6m. Optionally, each hot aisle has a width of from lm to 2m, such as lm to 1.4m. Optionally, each rack storage area has a width of from lm to 1.5m, such as about 1.2m. Optionally, the width of a cold aisle chassis unit plus a hot aisle chassis unit is from about 5m to about 7m, preferably about 5m to about 6m. Optionally, each cold aisle chassis unit and each hot aisle chassis unit (such as all chassis units) has a width of from about 2m to about 4m, such as about 2.5m to about 3.5m. It may be that such a width allows for particularly convenient transport. Optionally, each chassis unit has a width equal to or less than the maximum permitted width for conventional road transport (e.g. in the UK, Europe, the US, Canada or Australia), for example without requiring special measures for 'oversize' loads. In the UK in 2022, a width of up to 2.9m is permitted for normal loads. Optionally, the cold aisle chassis units and hot aisle chassis units have equal width. Optionally, all chassis units have an equal width. It may be that such an arrangement allows for particularly efficient manufacture and building layout flexibility. Optionally, chassis units have a length of from about 8m to about 20m, for example from about 10m to about 16m. A suitable cold corridor arrangement is described in WO2010139921 (Bripco BVBA), the contents of which are incorporated by reference. Specifically, that document describes a data centre layout which utilises personnel space for transport of large volumes of cooling air at low velocity, improving operating efficiency and making better use of space. Preferably, an air supply corridor is a personnel corridor, for example having a height of at least 3 m, such as at least 3.5 m, and a width of at least 2 m, such as at least 2.5 m, and optionally a length of at least 30 m, such as at least 40 m. Optionally, air handling units are positioned alongside and distributed along the length of an cold air supply corridor. Optionally, during operation of the data centre, cooling air from the air handing units enters the corridor through openings in the side of the corridor along its length. Optionally, the cold air supply corridor extends across a plurality of side joins between adjacent chassis units, such as adjacent cavity chassis units (e.g. cold aisle chassis units and hot aisle chassis units). It will be appreciated that in such an arrangement, at least a portion of the cold corridor occupies a space within an area bounded by cavity chassis units (e.g. said cold aisle and hot aisle chassis units). Optionally, said cold corridor(s) span the ends of a plurality of cavity chassis units (e.g. cold aisle chassis units and hot aisle chassis units). Optionally, the data centre comprises an uppermost data level, the ceiling structure of the uppermost data level forming at least a portion of a roof of the data centre building. Additionally or alternatively the plurality of data levels comprises a lowermost data level, for example wherein the floor of the lowermost data level is the ground floor of the multi-level data centre. Optionally, the ceiling structure comprises one or more service cassettes suspended from chassis units, such as cavity chassis units. Service cassette systems are also disclosed in WO2017129448 (Bripco BVBA), the contents of which are incorporated by reference. Specifically, that document discloses cassettes suspendable from a building structure and equipped with air entrainment panels and data centre services. Preferably, each service cassette comprises components of a plurality of data centre services, such as at least three services. For example, components may include a conduit for fire suppressant fluid, a cable tray, electrical power components (such as cabling and / or a bus-bar), networking cabling, lighting equipment and / or sensor equipment (such as motion, temperature and / or humidity sensor equipment). Additionally or alternatively, one or more such services may for example be mounted directly on chassis units, independently of such service cassettes. Optionally, each service cassette comprises aisle air entrainment panels configured to cooperate with racks of IT equipment to segregate the hot and cold aisles. Optionally, at least a portion of such aisle entrainment panels comprise one or more service pass-through openings for accommodating service components extending between hot and cold aisles, optionally wherein said pass-through openings comprise one or more sealing openings for sealing around said service components to maintain air segregation between the hot and cold aisles. Optionally, each service cassette comprises ceiling air entrainment panels configured to segregate cold aisles from a hot air plenum disposed between the ceiling air entrainment panels and said chassis unit, said hot air plenum being in fluid communication with one or more hot aisles. Optionally, such service cassettes are suspended from hot aisle chassis units. It has been found that with such an arrangement, aisle entrainment panels on such a service cassette may conveniently define a warm air passageway providing fluid communication between a hot aisle and the hot air plenum. Additionally or alternatively, the ceiling structure comprises a plurality of ceiling air entrainment panels suspended from chassis units, for example by drop rods secured to chassis unit brackets and / or beams of a framework secured to chassis units. Optionally, such panels are suspended from cold aisle chassis units. Optionally, each data centre comprises a hot air plenum for receiving hot air from hot aisles and transporting hot air to air handling units and / or exhaust vents (e.g. on the exterior of the building). Optionally, said hot air plenum extends above the cold aisles, for example being segregated from said cold aisles by ceiling air entrainment panels. Optionally, the plenum has an internal height of at least 0.5 m, such as at least 0.6 m, for example above each cold aisle. It will be understood that the internal height of the plenum is the distance across the gap between air entrainment panels segregating the plenum from a cold area, such as a cold aisle, and the ceiling structure. Optionally, the data centre building comprises a plurality of racks of IT equipment accommodated in the rack storage areas of each data level. Preferably, each rack is capable of holding at least 20, such as at least 30, optionally at least 40, items of rack-mountable IT equipment. Optionally, the racks together with air entrainment panels segregate the hot and cold aisles. Optionally, the data centre building is a hyperscale data centre building, for example accommodating at least 5,000 servers (rack-mountable items of IT equipment), and / or having an IT floorspace of at least 10,000 square feet. It will be understood that IT floor-space is the sum total of space occupied by rack storage areas, cold aisles and hot aisles, in the data centre building. Optionally, each data level comprises at least 15 rack storage areas each able to accommodate at least 20 racks, for example wherein each data level is configured to accommodate at least 300 racks, for example 300 racks capable of housing at least 40 severs. In such an arrangement, each data level is optionally capable of accommodating at least 12,000 servers. Optionally, each data level has a single IT area comprising rack storage areas, cold aisles and hot aisles. It will be appreciated that each data level may for example accommodate two or more separate IT areas. Optionally, each IT area is configured to accommodate at least 12,000 servers. Optionally, each IT area occupies a floor space of at least 1,000 square metres. A hyperscale data centre building may be one that is configured, when fully operational, to be capable of operating at total power levels of at least 40MW. While many benefits of the present invention may be particularly suited to embodiments in which the data centre building is a hyperscale data centre building, there may be comparable benefits to be had in large-scale, not necessarily, hyperscale data centre facilities. A large scale facility may for example accommodate at least 2,000 servers (rack-mountable items of IT equipment), and / or have an IT floor-space of at least 500 square meters and / or be configured, when fully operational, to be capable of operating at total power levels of at least 10MW. According to a fourth aspect, the present invention provides a method of constructing a data centre building. Optionally, the method comprises providing a plurality of chassis units of the first aspect of the invention, and / or a plurality of chassis unit assemblies according to the second aspect of the invention. Optionally, the method comprises transporting chassis units and / or chassis unit assemblies to an installation location separate from the manufacturing location, and optionally, assembling together the chassis units and / or chassis unit assemblies to form structural sections of the data centre building. It has been found that such a manufacturing method offers a particularly cost- and time-efficient method of construction. Optionally, the method comprises installing and / or testing one or more components of a data centre liquid cooling system in the cavity of at least one chassis unit prior to the transporting step. Optionally, the method comprises installing and / or testing a liquid containment barrier in the cavity of at least one chassis unit prior to the transporting step. Optionally, the method comprises providing a plurality of support posts for supporting the chassis units and / or chassis unit assemblies at the installation location. Optionally, the method comprises arranging the chassis units and / or chassis unit assemblies, and optionally a plurality of support posts, into a transportation configuration prior to the transporting step, wherein the transportation configuration has a sum volume smaller than a sum volume occupied by the chassis units and / or chassis unit assemblies, and optionally the plurality of support posts, once assembled together at the installation location. Optionally, the data centre building is a data centre building according to the third aspect of the invention. Optionally, the method comprises providing a kit of parts according to the fifth aspect of the invention. Optionally, the method comprises transporting the kit of parts in a first transport configuration, and subsequently assembling the kit of parts to form at least a section of the data centre building. Preferably, the kit of parts occupies a first sum volume in the transport configuration, and a second sum volume once assembled. Preferably, the first sum volume is smaller than the second sum volume, for example wherein the first sum volume is less than half, for example less than a third, of the second sum volume. Optionally, service cassettes are fastened to chassis units during the transporting step. Optionally, the service cassettes are arranged in a transport configuration during the transporting step. Optionally, the method comprises moving the service cassettes from the transport configuration to the deployed configuration during or after the assembling step. Optionally, the method comprises bringing together at an installation and / or construction site multiple prefabricated chassis units, for example each having an elongate shape with a length and a width, opposed ends parallel to the width, opposed sides parallel to the length, and side beams extending along the opposed sides. The method further comprises, at the construction site, supporting the chassis units so that they form a floor structure of a level of the building and so that the side beams of neighbouring chassis units abut each other at side joins. The method further comprises forming, above the chassis units, a plurality of cold aisles in the data centre interleaved with a plurality of hot aisles in the data centre such that a rack storage area is defined between each pair of adjacent hot aisle and cold aisle. Optionally, each such rack storage area is arranged to span the side join between neighbouring chassis units. Optionally, a floor structure of one level forms the ceiling of another level of the building below. Optionally, the method comprises utilising the kit of parts of the fifth aspect of the invention. Optionally the method comprises arranging chassis units and / or support posts as described in relation to the third aspect of the invention. It will be appreciated that the method may for example be a method of constructing a data centre building according to the third aspect of the invention. According to a fifth aspect, the present invention provides a kit of parts for forming one or more structural sections of a data centre building. Optionally, the kit of parts comprises a plurality of chassis units according to the first aspect of the invention, and / or a plurality of chassis unit assemblies according to the second aspect of the invention. Optionally, the data centre building is a data centre building according to the third aspect of the invention. Optionally, the kit of parts comprises a plurality of support posts for supporting the chassis units and / or chassis unit assemblies, optionally where each support post is configured for supporting at least a portion of each of two or more chassis units of a first data level and at least a portion of each of two or more chassis units of a second data level. It will be appreciated that the support posts may optionally be as described in relation to the first aspect of the invention herein above. Optionally, all chassis units of the kit of parts have equal width. Optionally, the kit of parts comprises a plurality of service cassettes configured for suspension from a chassis unit (e.g. a cavity chassis unit). Optionally, the kit of parts includes two or more service cassettes per hot aisle chassis unit. Optionally, each cold aisle service cassette comprises components of a plurality of data centre services, for example components as described in relation to the cold aisle service cassettes of the first aspect of the invention. Additionally or alternatively, each cold aisle cassette comprises aisle air entrainment panels configured to cooperate with racks of IT equipment to segregate the hot and cold aisles. Additionally or alternatively, each cold aisle cassette comprises ceiling air entrainment panels configured to segregate cold aisles from a hot air plenum disposed between the ceiling air entrainment panels and said chassis unit. Additionally or alternatively, the kit or parts optionally comprises a plurality of ceiling air entrainment panels suspendable from chassis units, and optionally a plurality of drop rods securable to chassis units. Optionally, such panels are suspended from cold aisle chassis units. Optionally, the kit of parts includes multiple ceiling air entrainment panels per cold aisle chassis unit. Optionally, said hot air plenum is in fluid communication with one or more hot aisles. It will be appreciated that the service cassettes may include any feature as described in relation to the service cassettes of the first aspect of the invention. Optionally, the services cassettes each comprise a frame upon which service components and / or entrainment panels are mounted. Optionally the kit of parts includes a plurality of cooling units, for example including liquid and / or air cooling units, optionally wherein said units are transportable separately to the chassis units. According to a sixth aspect, the present invention provides a method of liquid cooling electronic equipment in a data centre building, for example a data centre building according to the third aspect of the invention. Optionally, the method comprises transporting cooling liquid to said electronic equipment via one of more conduits located at least partially in a cavity of a chassis unit. Optionally, the chassis unit is a chassis unit according to the first or second aspect of the invention. According to a seventh aspect, the present invention provides a method of installing liquid cooling equipment in a data centre building, for example a data centre building according to the third aspect of the invention. Optionally, the method comprises inserting at least a portion of a liquid cooling conduit into a cavity of a chassis unit. Optionally, the chassis unit is a chassis unit according to the first or second aspect of the invention. Optionally, the method comprises providing liquid cooling in a data centre building previously configured only for air cooling of electronic equipment. In other words, the method is optionally a method for retrofitting a data centre building from air cooling to at least partially liquid cooling of electronic equipment. According to an eighth aspect, the present invention provides a method of controlling release of liquid from a data centre building, for example a data centre building according to the third aspect of the invention. Optionally, the method comprises capturing liquid in a cavity of a chassis unit, such as a chassis unit according to the first or second aspect of the invention. Optionally, the method comprises removing liquid from said cavity, and optionally transferring removed liquid to another location for treatment, disposal and / or re-use. Optionally, the liquid is one or more of liquid coolant and / or fire suppressant liquid. It will of course be appreciated that features described in relation to one aspect of the present invention may be incorporated into other aspects of the present invention. For example, the method of the invention may incorporate any of the features described with reference to the apparatus of the invention and vice versa. Description of the Drawings Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings of which: Figure 1 shows a side cross-sectional view of a data centre building according to the invention; Figures 2 to 4 show enlarged portions of the drawing of Figure 1; Figure 5 shows a side cross-sectional view of another data centre building according to the invention; Figure 6 shows an end cross-sectional view through a portion of the data centre of Figure 1; Figures 7 and 8 show enlarged views of portions of the drawing of Figure 6; Figure 9 shows a plan view of a data level of the data centre of Figure 1; Figure 10 shows an enlarged portion of the drawing of Figure 9; Figure 11 shows the plan view of Figure 9, overlaid with dashed lines to show the edges of chassis units; Figure 12 shows the plan view of Figure 9, annotated to indicate the positions of the views of Figures 1 and 6; Figure 13 shows the steps of a method of constructing a data centre building according to the invention; and, Figure 14 shows an end cross-sectional view through a portion of another data centre building. Detailed Description Figure 1 shows a side cross-sectional view of a data centre building 101 according to the invention. Figures 2 to 4 show enlarged views of portions of the view of Figure 1. The building 101 is shown as a multi-level building, but it will be appreciated that any number of levels is possible, including a single level. The building 101 comprises an optional mechanical and electrical services level 190 on the ground floor, and three data levels 102a, 102b, 102c on the floors above. Each level of the building 101 comprises a ceiling structure formed from a plurality of chassis units 103, chassis units 103a-l to 103a-3 forming the floor structure of level 102a and the ceiling structure of level 190, chassis units 103b-l to 103b-3 forming the floor structure of level 102b and the ceiling structure of level 102a, chassis units 103c-l to 103c-3 forming the floor structure of level 102c and the ceiling structure of level 102b, and chassis units 103d-l to 103d-3 forming the ceiling structure of level 102c and also supporting the roof of the data centre building 101. It will be understood that chassis units are in general referred to with the reference number 103, with specific chassis units and types of chassis units being given a suffix, and that a side-view of the chassis units 103 is shown in Figure 1. The ends of the chassis units are supported by and fastened to support posts, with the chassis units shown in Figure 1 being supported by eight support posts (only four support posts 104a-d are shown in Figure 1). The support posts 104a-d are made up of multiple sections secured end to end. The mechanical and electrical services level 190 houses ancillary equipment, including 'uninterrupted power supply' (UPS) equipment, and power and data distribution equipment, not shown in Figure 1. Each data level 102a-c comprises sixteen rack storage areas arranged between alternating hot and cold aisles, with each rack storage area accommodating a row of 32 server racks 105. The server racks 105 each hold up to 42 servers. The view shown in Figure 1 cuts along a cold aisle, and thus illustrates the fronts of server racks 105. Chassis units 103a-2 and 103a-3 form part of the ceiling structure of the services level 190, and the floor of the first data level 102a. Chassis units 103b-2 and 103b-3 form part of the ceiling structure of the first data level 102a, and the floor of the second data level 102b. Chassis units 103c-2 and 103c-3 form part of the ceiling structure of the second data level 102b, and the floor of the third data level 102c. Chassis units 103d-2 and 103d-3 form part of the ceiling structure of the third data level 102c, and the roof of the building 101. Chassis units 103a-2 / 3,103b-2 / 3 and 103c-2 / 3 are cold aisle chassis units configured to support a section of a cold aisle and at least a portion of rack storage areas extending alongside the cold aisle. The cold aisle chassis units each comprise cavities 118a / b / c extending along the length of the chassis unit. The cavities 118a / b / c are sealed to retain liquid. Each cavity 118a / b / c includes a liquid leak detection sensor (not shown in the figures) and a liquid outlet (a drainage outlet) connected to a liquid discharge system (also no shown in the figures). In the embodiment of Figure 1, the cavities are each lined with a liquid retention barrier (for the sake of clarity, only the liquid retention barrier 119c of cavity 118c is labelled in Figure 1), for example comprising a resin membrane material, but it will be appreciated that other approaches to liquid retention may be adopted. It will be appreciated that any suitable liquid detection system may be employed, such as a system comprising a liquid detector suitable for detecting liquid of the type expected to be discharged into the cavity in the event of a leak. Optionally, the system is configured for communication with a building management system, thereby allowing the data centre operator to be alerted in the event of a leak. It will be further appreciated that the liquid discharge system may comprise one or more pumps connected to chassis drainage outlets by one or more conduits, allowing liquid to be pumped out of the cavities, e.g. for disposal. Each data level 102a-c comprises a plurality of cooling units for providing liquid coolant to IT equipment in the cold aisles. Figure 1 shows cooling units 121a, 121b and 120c of data levels 102a-c, respectively. Illustrated in Figure 1 are the side covers and access doors of the cooling units 121,120. Cooling unit chassis units 103a-l, 103b-l and 103c-l form part of the floor structure of data levels 102a-c, respectively, below and supporting cooling units 121,120, respectively. Chassis unit 103d-l forms part of the roof of the building 101, the roof comprising roof covering sections 140-1,140-2 and 140-3. Cooling unit chassis unit 103a-l forms part of the ceiling structure of the services level 190. Cooling units 121a and 121b are liquid cooling units, supplying cooled liquid coolant, while cooling unit 120c is an air handling unit supplying cooling air. When the data centre 101 is operational, cooling air is supplied by cooling unit 120c to IT equipment via cold corridor 150c. Cooling air is also supplied to IT equipment from further air handling units (not shown in Figure 1) via cold corridors 150a, 150b. The cold corridors 150a-c run perpendicular to cold aisles. Cold corridor 150c receives cooling air from cooling unit 120c and transports said cooling air to multiple cold aisles on data level 102c. From the cold aisles of data level 102c, cooling air passes into server racks 105, thereby cooling the servers and becoming warm air and exiting into the hot aisles on data level 102c. Warm air passes upwards from the hot aisles to overhead warm air return plenum 153c, and then into a warm air return space 154c above air handler unit 120c. From that space, warm air may be ejected from the building through warm air exhaust vents (not shown in Figure 1) and / or passed into mixing chamber 155c upstream of the air handler unit 120c for mixing with ambient air from outside the building 101. Ambient air enters the mixing chamber 155c through air intake vents (not shown in Figure 1). IT equipment in racks 105 on data levels 102a and 102b are also supplied with cooling air from air handling units (not shown in Figure 1), providing cooling for ancillary electronic components not cooled by liquid cooling. Similarly to the arrangement described above in relation to data level 102c, cooling air is transported to cold aisles via cold corridors 150a / b, passes via racks 105 into hot aisles and up into warm air return plenums 153a / b and back to the air handling units. Cooling units 121a / b are liquid cooling units configured to supply cooled cooling liquid for cooling IT equipment in racks 105. Cooling liquid is cooled by cooling units 121a / b then supplied into cold corridor supply conduits 171a / b via cooling unit supply conduits 170a / b. Chassis supply conduits 172a / b receive cooling liquid from cold corridor supply conduits 171a / b. it will be understood that chassis supply conduits may also be referred to as branch conduits. In the embodiment shown in Figure 1, cold corridor supply conduits 170a / b are positioned above the floor in the cold corridors 150a / b, although it will be appreciated that other arrangements are possible. Chassis supply conduits 172a / b extend within a cavity 118a / b in the chassis units 103a-2 / 3 and 103b-2 / 3. On data level 102a (see Figure 2), chassis supply conduit 172a supplies cooled liquid coolant to manifold 173a positioned in the chassis cavity 118a under the cold aisle. From the manifold 173a, coolant is supplied to in-aisle coolant distributors 174a-2 and 174a-3 via cold aisle coolant supply conduits 175a that extend through the chassis cavity 118a and up into the in-aisle coolant distributors 174a-2 / 3. Coolant distributors 174a-2 / 3 distribute liquid coolant to heat exchangers (not shown in Figure 1) positioned in racks 105. Coolant used for in-rack cooling through the heat exchangers is warmed and returned to the in-aisle coolant distributors 174a-2 / 3, then returned to the manifold 173a via cold aisle coolant return conduits 176a. From the manifold 173a, warmed coolant is returned to a cold corridor return conduit 178a via chassis return conduit 177a, then back to the cooling unit 121a via cooling unit return conduit 179a. In order to pass from cavity 118a of chassis unit 102a-2 to cavity 118a of chassis unit 102a-3, cold aisle coolant supply and return conduits 175a / 176a extend through end beams of the chassis units 102a-2,102a-3. By that arrangement, coolant supply and return conduits 175a / 176a need not pass through structural side beams of the chassis units 102a-3 / 3. On data level 102b (see Figure 3), chassis supply conduit 172b supplies cooled liquid coolant directly to IT equipment in racks 105 via supply spurs 179b. Warmed coolant is returned to chassis return conduit 177b via return spurs 180b, then to cold corridor return conduit 178b, then back to the cooling unit 121b via cooling unit return conduit 179b. it will be understood that spurs may also be referred to as branch conduits. In order to pass from cavity 118b of chassis unit 102b-2 to cavity 118b of chassis unit 102b-3, chassis supply and return conduits 172b / 177b extend up out of the cavity 118b of chassis unit 102b-2, across the floor and back down into cavity 118b of chassis unit 102b-3. On data level 102c (See Figure 4), IT equipment in racks 105 are air cooled, and so no liquid cooling equipment is provided on the level. As noted above, cavities 118c of chassis units 102c-2 and 102c-3 are lined with a liquid retention barrier 119c. Examples of suitable cooling units are disclosed in International (PCT) application no. PCT / GB2023 / 053065 and UK patent application no. 2409261.1 (Pripco Limited), and examples of suitable arrangements for managing cooling airflow are disclosed in International (PCT) application no. PCT / GB2023 / 053070 (Pripco Limited). Systems for fastening together chassis units, and suspending data centre components from ceilings, are disclosed in International (PCT) application no. PCT / GB2023 / 053066 (Pripco Limited). The contents of those documents are incorporated herein by reference. Figure 5 shows a side cross-sectional view of another data centre building 201 according to the invention. Features of the data centre building 201 that are the same as those of the data centre building 101 shown in Figure 1 are labelled with the same reference numerals. The data centre building 201 includes a single data level 102, positioned above a services level (not shown in Figure 5). Cooled liquid coolant is supplied to an an-aisle liquid cooling control unit 181 via chassis supply conduit 172, which conduit 172 passes down into the cavity 118 from a connection with the cold corridor supply conduit 171, along the cavity and back up through the floor into the base of the control unit 181. In the embodiment shown in Figure 5, cooling control unit 181 passes cooled liquid coolant from cold aisle supply conduit 172 through a heat exchanger, to provide cooled liquid coolant for supply to IT equipment in racks 105. In alternative arrangements, cooling control unit 181 may instead act as a manifold, distributing the cooled liquid coolant to the racks 105, for example. Warmed liquid coolant is returned to the cold aisle return conduit 178 via chassis return conduit 177, which conduit 177 follows a similar path to that of chassis supply conduit 172. A cold aisle supply conduit 175 extends out from cooling control unit 181 in a loop, with supply spurs 179 providing fluid connection to liquid-cooled servers in racks 105, and a corresponding cold aisle return conduit 176 extends in another look with return spurs 180 also providing fluid communication to liquid-cooled servers in racks 105. In the embodiment shown in Figure 5, racks 105 contain direct-to-chip liquid cooled servers. However, it will be appreciated that other arrangements are possible, for example including racks 105 fitted with liquidair heat exchangers to provide air-cooling of rack equipment. Figure 6 shows an end cross-sectional view through a portion of the data centre 101 of Figure 1. Shown in Figure 6 is the full height of data level 102a, and the chassis units 103b of data level 102b. Equipment above the floor on data level 102b is omitted for clarity. A view cutting across the widths of chassis units 103 is shown in Figure 6. In the figure, ten chassis units 1003a-j are shown, with chassis units 1003a-e forming the ceiling and chassis units 1003f-j forming the floor of data level 102b (chassis units 1003a-e also form the floor of data level 102c). Chassis units 1003a-j are arranged side by side. The chassis units 1003a-j are fastened together by bolts that secure together brackets 1004 arranged on adjacent chassis units. As shown in Figure 6, joins between adjacent side by side chassis units 1003a-j are positioned under server racks 1060 and under the in-aisle coolant distributors 1074, so that the racks 1060 span the join and are supported by the side beams of two chassis units. Drop members 1006 and slot channel frame system beams 1040 are also bolted to the brackets 1004. Hot aisle service cassettes 1050 are suspended from brackets 1004 by drop members 1006, the service cassettes 1050 supporting fire suppression equipment 1051, air entrainment panels 1052 dividing hot aisles 1070 from cold aisles 1080, electrical cable trays 1053, and network cable trays 1054. Further air entrainment panels 1040 are suspended from the beams 1005 by drop rods 1041. As shown in Figure 6, chassis units 1003b, 1003d having brackets 1004 supporting service cassettes 1050 are positioned above the hot aisles 1070, while chassis units 1003a, 1003c, 1003e having brackets 1004 supporting the further air entrainment panels 1040 are positioned above the cold aisles 1080. The hot aisles 1070 and cold aisles 1080 are shown in an alternating arrangement. Note that Figure 6 shows only a portion of cold aisles 1080 and the associated chassis system components. Note also that the cassettes 1050 are made up of an open framework extending down from their associated chassis units 1003 and across the width of the hot aisles 1070. The air entrainment panels 1052 are mounted on the framework of the cassettes 1050, and extend along the lengths of the rows of server racks 1060 along each aisle, spanning and closing the gap between the tops of the racks 1060 and the warm air return plenum 1090. The hot aisles 1070 are in fluid communication with the overhead warm air return plenum 1090 along their length. The plenum 1090 also extends above the cold aisles 1080. Together, the hot aisles 1070 and the warm air return plenum 1090 for a hot zone, which is separated from the cold zone (comprising the cold aisles 1080 and cold corridor, not shown in Figure 10) by the air entrainment panels 1040,1052. Chassis units 1003a, c, e, f, h and j are hot aisle chassis units, positioned under the data centre hot aisles 1080 on each data level. In the embodiment shown in Figure 6, the hot aisle chassis units do not feature cavities, but such cavities (e.g. lined with a liquid retention barrier) could be provided in the hot aisle chassis units. Chassis units 1003b, d, g and i are cold aisle chassis units. Each cold aisle chassis unit features a cavity 1018 running the length of the chassis unit, and lined with a liquid retention barrier 1019. The cavities are covered with removable mesh flooring panels 1020, which provide a permeable upper surface allowing liquid to enter the cavities 1018 from the personnel space above. It will be appreciated that such flooring panels are arranged to be openable with the IT racks 1060 in situ. Cooled liquid coolant is supplied from chassis supply conduit 176 into receiver module 1075 of in-aisle coolant distributor 1074, and warmed liquid coolant is returned to chassis return conduit 175 from the module. Chassis coolant supply and return conduits 176,175 are supported on brackets 1021 located within chassis unit cavities 1018 above liquid retention barriers 1019. It will be appreciated that conduits may additionally or alternatively be mounted in or on a cassette, for example in the form of a framework, optionally insertable into and / or removable from the cavity. Figures 7 and 8 show enlarged views of portions of the drawing of Figure 6, with features labelled with the same reference numerals. As shown in Figures 7 and 8, the cold aisle chassis units are each made up of a pair of T beams 1202,1203 that extend on each side along the length of the chassis unit, each T beam having top and bottom flanges and a web extending from the top flange to the bottom flange. The T beams 1202,1203 are joined together by a cross-beam at each end (not shown in Figures 7 and 8), providing a frame around the cavity 1018. Figure 9 shows a plan view of data level 102a of the data centre 101 of Figure 1, labelled with the same reference numerals as used in Figures 1-8. In the floorplan shown in Figure 9, the IT area of the data level includes cold aisles 151, hot aisles 152, rack storage areas accommodating racks 105, and connecting corridor 114 (which links cold aisles 151 together). As shown in Figure 9, riser areas 115 accommodating various service conduits that extend between levels are located alongside the IT area, with the riser chassis units arranged site by side with IT chassis. In use, cooling air is provided by air handler units 120, which deliver cooling air into one side cold corridor 150, which transports and distributes cold air to cold aisles 151 through vented access doors 116, which provide controlled fluid communication and personnel access between the cold corridor 150 and cold aisles 151. Examples of vented doors are disclosed in WO2010139921 (Bripco BVBA), the contents of which are incorporated herein by reference. Personnel doors 117 provide access to hot aisles 152, while also maintaining air segregation between the cold corridor 150 and the hot aisles 152. Cooled liquid coolant is provided by liquid cooling unit 121. An enlarged view of a portion of Figure 9 is shown in Figure 10. Figure 10 shows that liquid cooling unit 121 supplies cooling liquid into cold corridor supply conduit 171a via cooling unit supply conduit 170a. The cold corridor supply conduit 171a is in fluid communication with chassis supply conduits 172a, which transfer cooled liquid coolant to manifolds 173, and thus into cold aisle supply conduits 175a. The arrangement of conduits shown in Figure 10 is simplified for the sake of clarity. Once used for cooling, warmed liquid coolant returns to the liquid cooling unit 121 via cold aisle return conduits 176a, manifolds 173a, chassis return conduits 177a, cold corridor return conduit 178a and cooling unit return conduit 179a. Note that chassis supply / return conduits are provided for each row of racks bounding the cold aisle. Figure 11 shows the plan view of Figure 9, overlaid with dashed lines to show the edges of chassis units forming the floor structure of data level 102a (those chassis units also forming the ceiling of the level below). Figure 12 shows the plan view of Figure 9, annotated to indicate the positions of the views of Figures 1 and 6. Line A-A indicates the position of the view of Figure 1. Line B-B indicates the position of the view of Figure 6. Figure 13 shows the steps of a method 1300 of constructing a data centre building according to the invention, comprising a step 1301 of providing a plurality of chassis units according to the first aspect of the invention, for example at a manufacturing location, and a step 1304 of transporting said chassis units to an installation location. It will be appreciated that the manufacturing and installation locations may be separated by any distance, and that any suitable form of transport may be used. For the transporting step, a plurality of chassis units and associated support posts are loaded onto a plurality of road vehicles, with the chassis units stacked to provide an efficiently small sum volume for transport. It will be understood that the chassis units and support posts may be loaded in any suitable configuration. Once at the installation location, the chassis units are unloaded and then assembled together to form structural sections of the data centre building in step 1305. Prior to step 1304 of transporting the chassis units, the cavity of each chassis unit is configured for retention of liquid by a step 1302 of installing and testing a liquid retention barrier in the form of a resin membrane. Following the step 1302 of installing a resin membrane to configure the cavity for liquid containment, a step 1303 of installing liquid coolant conduits in the cavity is performed, thereby providing liquid cooling infrastructure in the chassis unit ready for connection to liquid cooling equipment when the data centre is commissioned. It will be appreciated that, for example, step 1303 of installing liquid cooling conduits in the cavity could be performed at any stage, such as after transport and / or after initial commissioning of the data centre (e.g. in a future retro-fit to convert to or increase liquid cooling capacity in the data centre building). Figure 14 shows an end cross-sectional view through a portion of another data centre. Shown in Figure 14 is an end cross-sectional view of a single cold aisle chassis unit 1401, made up of a pair of T beams 1402,1403 that extend on each side along the length of the chassis unit 1401, each T beam having top and bottom flanges and a web extending from the top flange to the bottom flange. The T beams 1402,1403 are joined together by a cross-beam at each end (not shown in Figure 14), providing a frame around the cavity 1418. The chassis unit 1401 forms the ceiling of a lower data level and the floor of an upper data level, with server racks and in-aisle coolant distributors 1474 positioned on the floor. The cavity 1418 runs the length of the chassis unit 1401, and is lined with a liquid retention barrier 1419. The cavities are covered with removable mesh flooring panels 1420, which provide a permeable upper surface allowing liquid to enter the cavity 1418 from the personnel space above. It will be appreciated that such flooring panels are arranged to be openable with the IT racks in situ. Cooled liquid coolant is supplied from chassis supply conduit 1476 into receiver module 1475 of in-aisle coolant distributor 1474, and warmed liquid coolant is returned to chassis return conduit 1477 from the module. Chassis coolant supply and return conduits 1476,1477 are fastened to and supported by conduit cassette located within the chassis unit cavity 1418. In Figure 14, the liquid retention barrier 1419 is shown spaced apart from inner surfaces of the cavity 1418 for the sake of clarity - it will be appreciated that the barrier 1418 may, for example, be in the form of a membrane or coating applied to such surfaces, for example wherein the conduit cassette is secured over and / or through the barrier. The conduit cassette is made up of lateral supports (e.g. in the form of beams) 1481 for supporting the coolant supply and return conduits 1476,1477, and upstanding supports (e.g. in the form of legs) 1481 for supporting the flooring panels 1420. The lateral supports 1481 are fastened to the side beams 1402,1403 of the chassis unit. In the embodiment shown in Figure 14, the coolant conduits 1476,1477 and flooring panels 1420 are fastened to the conduit cassette 1480. A layer of fire-proof insulation material 1490 is positioned in the cavity below the liquid retention barrier 1419. Insulation is also provided between side beams of neighbouring chassis units (not shown in Figure 14).

Claims

1. A chassis unit for forming a structural section of a floor of a data centre building;wherein the chassis unit has a length and a width and comprises a cavity extending continuously through at least 90% of the length of the chassis unit;wherein the chassis unit comprises at least one access cover movable between a first position closing an opening to the cavity and a second open position allowing hand access to the cavity through the opening;and wherein the chassis unit has an upper face and a lower face, wherein the at least one access cover is positioned on the upper face, thereby providing hand access to the cavity from above when the chassis unit forms said structural section of the building.

2. The chassis unit of claim 1, wherein the cavity extends continuously through at least 90% of the width of the chassis unit.

3. The chassis unit of claim 1 or claim 2, wherein the at least one access cover provides hand access to substantially all of the cavity when moved to the second open position;optionally wherein the chassis unit comprises a plurality of said access covers, optionally wherein the plurality of access covers together provide hand access to substantially all of the length of the cavity when moved to the second open position.

4. The chassis unit of any preceding claim, wherein the chassis unit comprises a conduit cassette positioned at least partially within the cavity, wherein the conduit cassette is configured to provide support for: (i) at least a portion of a liquid coolant conduit positionable within the cavity, and (ii) a portion of the at least one access cover when moved to the second closed position.

5. The chassis unit of any preceding claim, where the structural section of the building is a section of a floor of the building, and a ceiling of the building,optionally wherein the at least one access panel is a floor access panel forming a section of a floor of a personnel area when moved to the first closed position; and / oroptionally wherein the chassis unit has a substantially planar upper surface suitable for providing flooring of a personnel area in the data centre building.

6. The chassis unit of any preceding claim, wherein the access cover comprises at least one transparent section and / or at least one section with a plurality of openings allowing visual inspection of the cavity when the access cover is in the first closed position.

7. A chassis unit for forming a structural section of a floor and / or ceiling of adata centre building;wherein the chassis unit has a length and a width and comprises a cavity extending through at least a portion of the length of the chassis unit;wherein the cavity is a liquid containment cavity for holding liquid within the cavity;and wherein the chassis unit comprises at least one access cover movable between a first position closing an opening to the cavity and a second open position allowing hand access to the cavity through the opening;optionally wherein the cavity is bounded by a bottom surface and a plurality of side surfaces, said bottom and side surfaces being sealed to prevent escape of liquid from the cavity.

8. The chassis unit of claim 7, wherein the cavity has a volume and the chassis unit is configured to contain in the cavity a volume of liquid at least 50% of the volume of the cavity, for example wherein the chassis into comprises a liquid containment barrier sized and configured to contain in the cavity a volume of liquid at least 50% of the volume of the cavity.

9. The chassis unit of claim 7 or claim 8, wherein the chassis unit comprises a liquid leak detection system for identifying the presence of liquid in the cavity.

10. The chassis unit of any one of claims 7 to 9, wherein:the cavity is bounded by a top surface having one or more openings allowing liquid to enter the cavity when the at least one access panel is in the second closed position; and / orthe at least one access panel has one or more openings allowing liquid to enter the cavity when said access panel is in the second closed position.

11. The chassis unit of any one of claims 7 to 10, comprising a drain conduit for removing liquid from the cavity, optionally wherein the drain conduit is in fluid communication with a pump for pumping liquid out from the cavity.

12. The chassis unit of any one of claims 7 to 11, wherein the chassis unit is a chassis unit according to any one of claims 1 to 6.

13. The chassis unit of any preceding claim, wherein the cavity is sized to accommodate at least one liquid coolant conduit for transporting liquid coolant to and / or from liquid cooling equipment in the data centre.

14. A chassis unit assembly comprising a chassis unit and at least a portion of a liquid coolant conduit;wherein the chassis unit is for forming a structural section of a floor and / or ceiling of a data centre building;wherein the chassis unit has a length and a width and comprises a cavity extending through at least a portion of the length of the chassis unit;wherein said at least a portion of the liquid coolant conduit is positioned in the cavity, for example wherein at least said at least a portion of the liquid coolant conduit extends within the cavity from a first position along the length of the chassis unit to a second position along the length of the chassis unit.

15. The chassis unit assembly of claim 14, comprising at least a portion of each of a plurality of liquid coolant conduits positioned in the cavity of the chassis unit, for example including (i) at least a portion of a liquid coolant supply conduit for transporting liquid coolant to liquid cooling equipment in the data centre, and (ii) at least a portion of a liquid coolant return conduit for transporting liquid coolant from said liquid cooling equipment in the data centre.

16. The chassis unit assembly of claim 15, wherein said at least a portion of a liquid coolant supply conduit comprises a plurality of branch conduits for distribution of liquid coolant to separate items of liquid cooling equipment in the data centre, and / or wherein said portion of liquid coolant return conduit comprises a plurality of branch conduits for receiving liquid coolant from separate items of liquid cooling equipment in the data centre.

17. The chassis unit assembly of any one of claims 14 to 16, comprising at least one manifold connecting said at least a portion of the liquid coolant conduit to a plurality of liquid coolant branch conduits, for example wherein the manifold is operable to regulate flow between said at least aportion of the liquid coolant conduit and said plurality of liquid coolant branch conduits.

18. The chassis unit assembly of any one of claims 14 to 17, wherein said at least a portion of the liquid coolant conduit is fluidly connected to at least one additional portion of the liquid coolant conduit, wherein the additional portion of the liquid coolant conduit is located outside the cavity;optionally wherein the liquid coolant supply conduit passes through an upper or lower surface of the chassis unit, for example an upper surface configured to provide a floor surface in the data centre.

19. The chassis unit assembly of any one of claims 14 to 18, wherein said at least a portion of the liquid coolant conduit is mounted on a conduit cassette, wherein the conduit cassette provides support for one or more panels extending across and covering at least a portion of an opening of the cavity.

20. The chassis unit assembly of any one of claims 14 to 19, comprising a conduit cassette supporting a plurality of said portions of liquid coolant conduits, wherein the conduit cassette comprises a framework to which is fastened: (i) said portions of liquid coolant conduits, and (ii) one or more panels extending across and covering at least a portion of an opening of the cavity;optionally wherein the framework is located substantially within the cavity.

21. The chassis unit assembly of any one of claims 14 to 20, wherein the chassis unit is a chassis unit according to any one of claims 1 to 13.

22. A data centre building comprising a plurality of chassis units assembled together to define a unitary structure, wherein the plurality of chassisunits forms a structural section of a floor and / or a ceiling of the data centre building;wherein the plurality of chassis units comprises at least one chassis unit or at least one chassis unit assembly according to any one of claims 1 to 21.

23. The data centre building of claim 22, comprising a liquid fire-retardant fire protection system.

24. The data centre building of claim 22 or claim 23, wherein the plurality of chassis units forms a structural section of a floor of the data centre, wherein the data centre comprises at least ten rack storage areas positioned on the floor, each rack storage being arranged to accommodate a plurality of server racks and being positioned adjacent at least one personnel access area;wherein the cavity of said at least one chassis unit or at least one chassis unit assembly is positioned under said at least one personnel access area;optionally wherein said at least chassis unit or at least one chassis unit assembly forms at least a portion of the floor of said at least one personnel access area.

25. The data centre building of claim 24, wherein at least one rack storage area is configured for accommodating liquid cooling equipment, and wherein the cavity of said at least one chassis unit is positioned under a personnel access area adjacent said rack storage area and is configured for accommodating liquid cooling equipment.

26. The data centre building of claim 24 or claim 25, comprising a plurality of personnel access each adjacent to one or more rack storage areas, wherein the personnel access areas comprise alternating hot and cold aisles separating rack storage areas;wherein the data centre comprises a plurality of air handling units for providing cooling air to cold aisles;optionally wherein the data centre comprises a cold air supply corridor having a length extending perpendicular to and being in fluid communication with the cold aisles to receive from the cold air supply corridor cooling air supplied to the cold air supply corridor by a plurality of air handling units.

27. The data centre building of claim 26, wherein at least a portion of the cavity of said at least one chassis unit is positioned under at least a portion of a cold aisle and / or under at least a portion of a hot aisle, optionally wherein the data centre comprises multiple said chassis units each having a cavity at least partially positioned under at least a portion of a cold aisle and / or under at least a portion of a hot aisle.

28. A method of constructing a data centre building, wherein the method comprises:providing a plurality of chassis units according to any one of claims 1 to 13, and / or a plurality chassis unit assemblies according to any one of claims 14 to 21, at a manufacturing location;transporting the chassis units and / or chassis unit assemblies to an installation location separate from the manufacturing location; assembling together the chassis units and / or chassis unit assemblies to form structural sections of the data centre building.

29. The method of claim 28, comprising one or more of the following steps: installing and / or testing one or more components of a data centre liquid cooling system in the cavity of at least one chassis unit prior to the transporting step;installing and / or testing a liquid containment barrier in the cavity of at least one chassis unit prior to the transporting step;30.31.32.33.providing a plurality of support posts for supporting the chassis units and / or chassis unit assemblies at the installation location;arranging the chassis units and / or chassis unit assemblies, and optionally a plurality of support posts, into a transportation configuration prior to the transporting step, wherein the transportation configuration has a sum volume smaller than a sum volume occupied by the chassis units and / or chassis unit assemblies, and optionally the plurality of support posts, once assembled together at the installation location.A method according to claim 28 or claim 29, wherein the data centre building is a data centre building according to any one of claims 22 to 27.A kit of parts for forming one or more structural sections of a data centre building, wherein the kit of parts comprises a plurality of chassis units according to any one of claims 1 to 13, and / or a plurality of chassis unit assemblies according to any one of claims 14 to 21, and optionally a plurality of support posts for supporting the chassis units and / or chassis unit assemblies;optionally wherein the data centre building is a data centre building according to any one of claims 22 to 27.A method of liquid cooling electronic equipment in a data centre building according to any one of claims 22 to 27, wherein the method comprises transporting cooling liquid to said electronic equipment via one of more conduits located at least partially in a cavity of a chassis unit according to any one of claims 1 to 21.A method of installing liquid cooling equipment in a data centre according to any one of claims 22 to 27, wherein the method comprises inserting at least a portion of a liquid cooling conduit into a cavity of a chassis unit according to any one of claims 1 to 21, optionally wherein the methodcomprises providing liquid cooling in a data centre building previously configured only for air cooling of electronic equipment.

34. A method of controlling release of liquid from a data centre building according to any one of claims 22 to 27, wherein the method comprises capturing liquid in a cavity of a chassis unit according to any one of claims 1 to 21, removing liquid from said cavity, and optionally transferring removed liquid to another location for treatment, disposal and / or re-use; optionally wherein the liquid is one or more of liquid coolant and / or fire suppressant liquid.s

Images