A prefabricated prestressed Y-shaped hollow bridge pier

Abstract

The present invention discloses an assembled prestressed Y-shaped hollow bridge pier, which belongs to the field of bridge engineering. The pile foundation and the pedestal are the foundation of the bridge. The bridge pier is arranged on the pedestal. The bridge pier includes an adjustment section, a plurality of standard sections and a pier cap section. The adjustment section, the standard section and the pier cap section are prefabricated reinforced concrete components. The adjustment section, the standard section and the pier cap section are all provided with channels, through which steel strands are passed. The steel strands for anchoring are arranged on the pedestal. The adjustment section, the standard section and the pier cap section are prestressed and anchored by steel strands to form an integrated assembled prestressed bridge pier. The bridge pier is prefabricated in a factory and assembled on site, which reduces the processes of tying steel bars, supporting formwork, pouring concrete, curing, and removing formwork for cast-in-place concrete, and shortens the construction period of on-site construction. The reinforced concrete components are prestressed and tensioned by steel strands to firmly fix the components. The prefabricated components of the bridge pier adopt a hollow structure, which reduces the weight of the bridge pier and reduces the bearing capacity requirements of the foundation.

Description

Technical Field

[0001] The invention relates to a bridge pier composed of reinforced concrete prefabricated components in the field of bridge engineering. Background Art

[0002] A bridge is generally composed of a superstructure, a substructure, a support and ancillary structures. The superstructure, also known as the span structure, is the main structure for crossing obstacles. The substructure includes abutments, piers and foundations. The piers are structures that support the span structure and transmit dead loads and vehicle live loads to the foundation. Bridge piers are divided into gravity piers and light piers. Gravity piers are generally solid structures made of concrete or stone. A pier cap is set on the pier body and connected to the foundation. Its characteristics are that it makes full use of the compressive properties of masonry materials, and uses its own large cross-sectional size and weight to withstand external forces in the vertical and horizontal directions. It has the advantages of being strong and durable, simple to construct, easy to obtain materials, and saving steel. The disadvantages are that the masonry work is large, the appearance is thick and heavy, and the foundation load is increased. It is particularly disadvantageous when the pier is high and the foundation bearing capacity is low. Because this type of pier is simple to bear and easy to construct, it is widely used in railways, highways and urban bridges. In addition to transmitting the upper load, the bridge pier also needs to resist the external force in the horizontal direction. Therefore, the bridge pier needs a larger cross-sectional size, and the vertical bearing cross-sectional area of ​​the bridge pier is relatively abundant. Lightweight bridge piers are designed to address the shortcomings of gravity bridge piers. They have a light and beautiful appearance, less masonry, can reduce the foundation load, and save the cost of foundation engineering. There are many main ways to achieve light bridge piers, and hollow bridge piers are one of them. The appearance of hollow bridge piers is similar to that of gravity bridge piers, but they are hollow thin-walled piers, which ensures a larger cross-sectional size and reduces the cross-sectional area. The weight of the pier body can be reduced. Reinforced concrete can be cast in place or prestressed concrete assembly structure can be used, which is more suitable for high bridge piers. The No. 3 pier of the Hanshui Bridge between Xiangfan and Chongqing Ziyang on the Xiangyu Line in China is 70.5 meters high and 60 centimeters thick. It is currently the tallest railway bridge pier in China. Most of the existing gravity bridge piers are cast-in-place. After the pile foundation and the pedestal are constructed, the steel bars are tied on the pedestal, the formwork is supported, and the concrete is poured. The formwork can only be removed after the concrete strength reaches a certain level, which results in a long construction period. If hollow piers are used for cast-in-place gravity bridge piers, the inner membrane needs to be supported. After the concrete is poured, the inner membrane is difficult to remove. The small amount of concrete has no obvious advantage over the construction difficulty. Therefore, the existing cast-in-place gravity bridge piers generally use solid piers. Summary of the invention

[0003] The purpose of the present invention is to provide an assembled prestressed Y-shaped hollow bridge pier, which overcomes the shortcomings of existing light hollow piers, such as the need to support the inner membrane, the difficulty in removing the formwork, and the long construction period.

[0004] A prefabricated prestressed Y-shaped hollow pier, the pile foundation and the cap are the foundation of the bridge, the pier is arranged on the cap, and the characteristics are: the pier includes an adjustment section, a plurality of standard sections and a pier cap section, the height of the standard section is a certain value, the standard section height of the piers of the same bridge is the same, the adjustment section is located at the bottom of the pier, and the height of the adjustment section is 0.5 to 1.5 times the standard section height, with 0.1 to 0.2 times the standard section height as the range, and each range increment as a specification. There are several prefabricated adjustment sections of different heights. During assembly, an adjustment section of appropriate height is installed on the pedestal, and the elevation of the pier base is adjusted in combination with the elevation of the pedestal top surface. The adjustment section, standard section, and pier cap section are prefabricated reinforced concrete components. The size, steel reinforcement, and concrete strength grade of the components are determined according to the bridge load design. The adjustment section is installed on the pedestal, and several standard sections are installed on the upper part of the adjustment section in sequence. The pier cap section is installed on the upper part of the top standard section. The adjustment section, standard section, and pier cap section are all provided with channels, and the steel strands are inserted into the channels of each section. Anchor points are set on the pedestal to anchor the steel strands. The adjustment section, standard section, and pier cap section are prestressed and anchored with steel strands to form an integral assembled prestressed pier.

[0005] An assembled prestressed Y-shaped hollow pier, characterized in that: an anchor groove is provided at the corresponding channel on the top surface of the pier cap section, the anchor is arranged in the anchor groove, a through-hole jack is installed on each steel strand, the steel strand is tensioned and prestressed, the tensioning speed should be basically synchronized, after reaching the designed tensioning stress, the jack slowly releases the pressure, the wedge-shaped clip of the anchor locks the steel strand on the anchor, the adjustment section, the standard section and the pier cap section are tightly fixed together, the bottom cross-section of the pier cap section is consistent with the cross-section of the standard section, the outer wall of the pier cap section gradually expands upward, the top surfaces on both sides of the pier cap section are two bearing platforms, the bearing platforms are bearing pedestals, the bearings are placed on the bearing pedestals, the middle of the top surface of the pier cap section is a groove, which is a maintenance platform for replacing the bearings, the two raised bearing platforms and the recessed maintenance platform of the pier cap section constitute the "Y"-shaped top of the pier.

[0006] An assembled prestressed Y-shaped hollow pier, characterized in that: after the tensioning of the steel strands is completed, cement slurry is injected into the channel, and the empty groove is densely filled with cement slurry during grouting, thereby increasing the contact surface of the upper and lower pier bodies and improving the bearing capacity of the pier. After the channel is filled with cement slurry, the anchor groove is filled with cement slurry, and the anchor is enclosed in the anchor groove. After the grouting is completed, the bearing cushion stone construction is carried out, and the elevation of the bearing platform on the top surface of the pier cap section is measured. According to the top surface elevation of the designed bearing, the thickness of the bearing is subtracted to calculate the height of the bearing cushion stone. The top surface elevation of the bearing can be accurately adjusted by the height of the bearing cushion stone. Formwork is supported on the bearing platform, and cast-in-place concrete bearing cushion stones are used.

[0007] A prefabricated prestressed Y-shaped hollow pier, characterized in that: the cross-section of the adjustment section is the same as that of the standard section, the shape of the outer wall is a rectangle with four rounded corners, the inner wall is a rounded rectangle with the outer wall offset inward by the same width, a pier body of equal thickness is formed between the outer wall and the inner wall, the interior of the pier body is a cavity, the interior of the pier after assembly is hollow, and a channel is arranged along the circumference of the pier body, the connection between the standard section and the adjustment section, between the standard sections, and between the standard section and the pier cap section is a socket connection, the lower bottom surface of the adjustment section, the standard section, and the pier cap section, the edges at the outer wall and the inner wall are sockets, the upper top surface of the adjustment section and the standard section, the outer wall and the inner wall are sockets, the sockets are offset toward the center of the pier body by the width of a socket, so that the sockets fit exactly when assembled, and the raised sockets and the sockets form an empty groove at the joint of the upper and lower pier bodies, and the empty groove is connected to the channel.

[0008] The advantages of the present invention are as follows: the bridge piers of the present invention are prefabricated with reinforced concrete and assembled on site, which reduces the processes of tying steel bars, supporting formwork, pouring concrete, curing, and removing formwork for cast-in-place concrete, shortens the construction period on site, and the reinforced concrete components are prestressed with steel strands to firmly fix the components. The prefabricated components of the bridge piers adopt hollow structures, which reduces the weight of the bridge piers and reduces the bearing capacity requirements of the foundation. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1 This is a schematic diagram of the bridge structure of the present invention;

[0010] FIG2 is a schematic elevation view of the pier assembly structure of the present invention;

[0011] Figure 3 is a cross-sectional view of the standard section;

[0012] Figure 4 is a top view of the pier cap section;

[0013] FIG5 is a cross-sectional view taken along line AA of FIG3 ;

[0014] Figure 6 It is a cross-sectional view of the joint structure;

[0015] In the figure: 1-pile foundation, 2-capping platform, 3-pier, 4-adjustment section, 5-standard section, 6-pier cap section, 7-channel, 8-steel strand, 9-anchor point, 10-anchor groove, 11-anchor, 12-outer wall, 13-inner wall, 14-pier body, 15-cavity, 16-support platform, 17-support cushion stone, 18-support, 19-maintenance platform, 20-socket, 21-plug, 22-empty slot. DETAILED DESCRIPTION

[0016] The implementation manner of the structure of the present invention is now described in detail with reference to the accompanying drawings:

[0017] The bridge piers of the present invention are prefabricated reinforced concrete components in the factory, and prestressed assembly is applied on site. The schematic diagram of the bridge structure of the present invention is shown in Figure 1. The pile foundation 1 and the pedestal 2 are the foundation of the bridge. The pier 3 is set on the pedestal 2. The upper part of the pier 3 is a box beam or other types of bridge span structures. The vertical schematic diagram of the assembly structure of the bridge pier of the present invention is shown in Figure 2. An assembled prestressed Y-shaped hollow pier, the pier 3 includes an adjustment section 4, several standard sections 5 and a pier cap section 6. The adjustment section 4, the standard section 5, and the pier cap section 6 are prefabricated reinforced concrete components. The size of the components, the reinforcement of the steel bars, and the concrete strength grade are determined according to the bridge load design. The adjustment section 4, the standard section 5, and the pier cap section 6 are all provided with a channel 7, and the steel strand 8 is passed through. Anchor points 9 are set at the positions of the holes 7 of the cap 2 corresponding to the adjustment sections 4, and the steel strands 8 are anchored in the cap 2. The adjustment sections 4 are installed on the cap 2, and several standard sections 5 are installed in sequence on the upper part of the adjustment sections 4. The pier cap section 6 is installed on the upper part of the top standard section 5. The steel strands 8 are inserted into the holes 7 of each section, and anchor grooves 10 are set at the holes 7 corresponding to the top surface of the pier cap section 6. Anchors 11 are set in the anchor grooves 10, and the steel strands 8 are tensioned to apply prestress. The anchors 11 lock the steel strands 8. Under the action of the prestress of the steel strands 8, the adjustment sections 4, the standard sections 5, and the pier cap section 6 are firmly fixed into an integral bridge pier 3, forming an assembled prestressed bridge pier. The cross section of the adjustment section 4 is the same as that of the standard section 5. The shape of the outer wall 12 is a rectangle with rounded corners at four corners. The inner wall 13 is a rounded rectangle with the same width of the outer wall 12 offset inward. The outer wall 12 and the inner wall 13 form a pier body 14 of equal thickness. The interior of the pier body 14 is a cavity 15. The interior of the assembled pier 3 is hollow. The channel 7 is set along the circumference of the pier body 14. The cross-sectional view of the standard section 5 is shown in Figure 3. The height of the standard section 5 is h, and the value range of h is 1 to 2 meters. h can be determined according to the total height of the pier 3, and the lifting capacity of the lifting equipment should also be taken into account. If the total height of the pier 3 does not exceed 20 meters, the height h of the standard section 5 is preferably 1 meter. If the total height of the pier 3 is between 20 and 40 meters, h is preferably 1.5 meters. If it exceeds 40 meters, h is preferably 2 meters. The height k of adjustment section 4 is 0.5 to 1.5 times h, with a range of 0.1 to 0.2 times h as the extreme value ⊿h. k increases from 0.5h to 1.5h with each extreme value ⊿h as one specification. There are several prefabricated components with different heights k.For example: the height of standard section 5 is h = 1m, with a range ⊿h of 0.1m, then the height k of adjustment section 4 has eleven components of heights: 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, and 1.5m, which is convenient for adjusting the base elevation of pier 3. The pier cap section 6 is the uppermost component of the pier 3 that supports the bridge span structure. The bottom cross section of the pier cap section 6 is consistent with the cross section of the standard section 5. The outer wall 12 of the pier cap section 6 gradually expands upward, as shown in Figure 2. The height j of the pier cap section 6 is 1.5~2.5h. There are two bearing platforms 16 on the top surfaces of the two sides of the pier cap section 6. On the bearing platforms 16 are bearing cushion stones 17. The height of the bearing cushion stone 17 is f. A bearing 18 is placed on the bearing cushion stone 17. There is a groove in the middle of the top surface of the pier cap section 6, which is a maintenance platform 19 for replacing the bearing. The two raised bearing platforms 16 and the recessed maintenance platform 19 of the pier cap section 6 constitute the "Y"-shaped top of the pier. The top view of the pier cap section is shown in FIG4 , which shows that the upper span structure is a middle pier of a continuous structure, and there is one support pad stone 17 on each side support platform 16, which is arranged in the middle. If the span structure is a simply supported structure or an end pier of a continuous structure, there are two support pad stones 17 on each side support platform 16, and there are also two supports 18. The form of the two support pad stones 17 is not shown in FIG4 . The connection between the standard section 5 and the adjustment section 4, between the standard section 5, and between the standard section 5 and the pier cap section 6 is a socket connection. The lower bottom surface of the adjustment section 4, the standard section 5, and the pier cap section 6, the edges of the outer wall 12 and the inner wall 13 are the socket 20, and the upper top surface of the adjustment section 4 and the standard section 5, the outer wall 12 and the inner wall 13 are the socket 21. The socket 21 is offset toward the center of the pier body 14 by the width of the socket 20, so that the socket 20 and the socket 21 are exactly matched when assembled, which is convenient for assembly and rapid positioning. The vertical section view of the standard section 5 is shown in Figure 5, which is the AA section of Figure 3. The socket of the adjustment section 4 and the socket of the pier cap section 6 are shown in Figure 5. The raised socket 20 and the socket 21 form a hollow groove 22 at the joint of the upper and lower pier bodies 14. The hollow groove 22 communicates with the channel 7. The cross-sectional view of the joint structure is shown in Figure 6. When assembling and constructing the bridge pier, select the appropriate height k of the adjustment section 4 according to the designed bridge pier height; select the height h and number n of the standard section 5. The height h of the standard section 5 of the piers of the same bridge should be the same. The number n of the standard section 5 used in a bridge pier 3 is determined according to the height of the adjustment section 4 and the pier cap section 6 combined with the standard section 5 and the height of the pier 3; select the height j of the pier cap section 6. The height j of the pier cap section 6 of the piers 3 of the same bridge should be the same.The same height of the standard section 5 and the pier cap section 6 of the same bridge is convenient for construction and more beautiful. According to the height of the pier, as well as the selected height k of the adjustment section 4, the height h and number n of the standard section 5, and the height j of the pier cap section 6, the top surface elevation of the cap 2 is calculated. The top surface of the cap 2 is generally buried 0.5 to 1.0 m underground. If the bridge piers are set on the river channel, the buried depth of the cap 2 should meet the requirements of preventing hydraulic scour.

[0018] The schematic diagram of the pier assembly structure elevation is shown in Figure 2. The adjustment section 4 is located at the bottom of the pier 3 and serves as the base of the pier 3. When the cap 2 is constructed, the steel strand 8 is anchored at the anchor point 9 corresponding to the position of the hole 7 of the adjustment section 4. The steel strand 8 can be a 1×7 (seven-strand) steel strand with a nominal diameter of φ12.7mm. During assembly, a section of the adjustment section 4 component suitable for the height k is used, and the steel strand 8 is passed through the hole 7 of the adjustment section 4. The adjustment section 4 is installed on the cap 2 and combined with the cap 2 to adjust the base elevation of the pier 3. When installing the adjustment section 4, it should be ensured that it is level with the connection surface of the standard section 5 to ensure that the assembled pier 3 is vertical. Wedge-shaped pads can be used between the adjustment section 4 and the cap 2 for adjustment, and cement mortar is used to seal the gap between the outer wall 12 and the inner wall 13 of the adjustment section 4 and the cap 2. The standard section 5 and the pier cap section 6 are installed on the adjustment section 4 in sequence, and the steel strand 8 passes through the corresponding channel 7. After each section is installed, the outer edge of the pier 3 should be inspected for verticality. After the pier cap section 6 is installed, the anchor 11 is put on the steel strand 8 and embedded in the anchor groove 10. A through-hole jack is installed on each steel strand 8 to tension the steel strand 8. The tensioning speed should be basically synchronized. After reaching the designed tensioning stress, the jack slowly releases the pressure, and the wedge-shaped clip of the anchor locks the steel strand 8 on the anchor. The adjustment section 4, the standard section 5, and the pier cap section 6 are tightly fixed together. After the steel strand 8 is tensioned, cement mortar is injected into the channel 7. The strength grade of the cement mortar should not be lower than the strength grade of the component concrete. During the grouting, the empty slot 22 between the two sections is filled with cement mortar. No matter how flat the connection surface of the prefabricated components is, it is impossible for the contact surface to completely match. The contact surface is supported by many points. An empty groove 22 is set on the contact surface, and cement mortar is used for grouting to increase the contact surface of the upper and lower piers 14, thereby improving the bearing capacity of the pier 3. After the channel 7 is filled with cement slurry, the anchor groove 10 is filled with cement slurry, and the anchor 11 is enclosed in the anchor groove 10. After the grouting is completed, the support pad stone 17 is constructed. The elevation of the support platform 16 on the top surface of the pier cap section 6 is measured, and the height f of the support pad stone 17 is calculated according to the top surface elevation of the designed support 18, minus the thickness of the support 18. The top surface elevation of the support 18 can be accurately adjusted by the height f of the support pad stone 17. Formwork is supported on the support platform 16, and the concrete cast-in-place support pad stone 17 is used.

[0019] In Example 1, the height h of the prefabricated standard section 5 is 1.0m, the height j of the pier cap section 6 is 1.5m, and the height k of the adjustment section 4 has eleven heights of 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, and 1.5m. The top elevation of a pier support 18 is 41.273, the ground elevation at the pier is 30.131, and the height of the pier exposed above the ground is 11.142m. The thickness of support 18 is 50mm, and the design thickness of support pad stone 17 is 100mm. The top elevation of support platform 16 is 41.123, and the height from the top of support platform 16 to the ground is 10.992m. The height of one pier cap section 6 plus nine standard sections 5 is 10.5m. A combination of 1.2m high adjustment sections 4 is selected. When cap 2 is constructed, the height of cap 2 must be constructed according to the design, and the position of cap 2 above and below pile foundation 1 is adjusted to control the top elevation of cap 2 to 29.423. The buried depth of adjustment section 4 is 0.708m, and 0.492m is exposed above the ground.

[0020] The above content is a further detailed description of the present invention in combination with specific preferred implementation methods, and it cannot be determined that the specific implementation of the present invention is limited to these descriptions. The structure described in the application documents and drawings of the present invention is only an embodiment of an assembled bridge pier. The bridge piers assembled by combining adjustment sections, standard sections, and pier cap sections are all within the protection scope of this application. For ordinary technicians in the technical field to which the present invention belongs, if several equivalent substitutions or obvious modifications are made without departing from the concept of the present invention, and the performance or use is the same, it should be deemed to belong to the protection scope determined by the claims submitted by the present invention.

Claims

1. An assembled prestressed Y-shaped hollow bridge pier, wherein a pile foundation (1) and a cap (2) are the foundation of a bridge, and a bridge pier (3) is arranged on the cap (2), characterized in that: The bridge pier (3) comprises an adjustment section (4), a plurality of standard sections (5) and a pier cap section (6); the adjustment section (4), the standard section (5) and the pier cap section (6) are prefabricated reinforced concrete components; the size, steel reinforcement and concrete strength grade of the components are determined according to the bridge load design; the adjustment section (4) is installed on the pedestal (2); a plurality of standard sections (5) are installed in sequence on the upper part of the adjustment section (4); the pier cap section (6) is installed on the upper part of the uppermost standard section (5); the height of the standard section (5) is a certain value; the height of the standard sections (5) of the piers (3) of the same bridge is the same; the adjustment section (4) is located at the lowest part of the pier (3); the height of the adjustment section (4) is 0.5 to 1.5 The height of the pier (3) is adjusted to a height of times the standard section (5), with a range of 0.1 to 0.2 times the height of the standard section (5), and each range of increase is a specification. There are a number of prefabricated components of adjustment sections (4) with different heights. During assembly, an adjustment section (4) with a suitable height is installed on the pedestal (2), and the elevation of the base of the pier (3) is adjusted in combination with the elevation of the top surface of the pedestal (2). The adjustment section (4), the standard section (5), and the pier cap section (6) are all provided with a hole (7). The steel strand (8) is inserted into the hole (7) of each section. Anchor points (9) are provided on the pedestal (2) to anchor the steel strand (8). The adjustment section (4), the standard section (5), and the pier cap section (6) are prestressed and anchored by the steel strand (8), so as to form an integral prestressed assembled pier.

2. The assembled prestressed Y-shaped hollow pier according to claim 1 is characterized in that: An anchor groove (10) is provided at the corresponding hole (7) on the top surface of the pier cap section (6), and the anchor (11) is arranged in the anchor groove (10). A through-hole jack is installed on each steel strand (8) to tension the steel strand (8) to apply prestress. The tensioning speed should be basically synchronized. After reaching the designed tensioning stress, the jack slowly releases the pressure, and the wedge-shaped clip of the anchor locks the steel strand (8) on the anchor (11). The adjustment section (4), the standard section (5), and the pier cap section (6) are tightly fixed together. The bottom cross section of the pier cap section (6) is consistent with the cross section of the standard section (5). The pier cap section (6) ) is gradually expanded upwards, and the top surfaces of both sides of the pier cap section (6) are two support platforms (16), on which are support cushion stones (17), and on which are placed support (18). The middle of the top surface of the pier cap section (6) is a groove, which is a maintenance platform (19) for replacing the support. The two raised support platforms (16) and the recessed maintenance platform (19) of the pier cap section (6) constitute the top of the "Y" shape of the bridge pier.

3. The assembled prestressed Y-shaped hollow pier according to claim 2 is characterized in that: After the steel strand (8) is tensioned, cement slurry is injected into the hole (7), and at the same time, the empty groove (22) is filled with cement slurry to increase the contact surface of the upper and lower pier bodies (14) and improve the bearing capacity of the pier (3). After the hole (7) is filled with cement slurry, the anchor groove (10) is filled with cement slurry, and the anchor (11) is sealed in the anchor groove (10). After the grouting is completed, the support pad stone (17) is constructed, and the elevation of the support platform (16) on the top surface of the pier cap section (6) is measured. According to the top surface elevation of the designed support, the thickness of the support (18) is subtracted to calculate the height of the support pad stone (17), and the top surface elevation of the support (18) is accurately adjusted according to the height of the support pad stone (17). Formwork is supported on the support platform (16), and concrete cast-in-place support pad stone (17) is used.

4. The assembled prestressed Y-shaped hollow pier according to any one of claims 1 to 3, characterized in that: The cross section of the adjustment section (4) is the same as that of the standard section (5); the shape of the outer wall (12) is a rectangular shape with four rounded corners; the inner wall (13) is a rounded rectangle with the same width as the outer wall (12) offset inward; a pier body (14) of equal thickness is formed between the outer wall (12) and the inner wall (13); the interior of the pier body (14) is a cavity (15); the interior of the assembled bridge pier (3) is hollow; the channel (7) is arranged along the circumference of the pier body (14); the connection between the standard section (5) and the adjustment section (4), between the standard section (5), and between the standard section (5) and the pier cap section (6) is a socket connection The lower bottom surface of the adjustment section (4), the standard section (5) and the pier cap section (6) has a socket (20) at the edge of the outer wall (12) and the inner wall (13). The upper top surface of the adjustment section (4) and the standard section (5) has a socket (21) at the outer wall (12) and the inner wall (13). The socket (21) is offset toward the center of the pier body (14) by the width of the socket (20) so that the socket (20) and the socket (21) fit exactly when assembled. The raised socket (20) and the socket (21) form a hollow groove (22) at the joint of the upper and lower pier bodies (14), and the hollow groove (22) communicates with the channel (7).

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