Pilaster structure of underground excavation station and construction method of pilaster structure

Abstract

The invention discloses a pilaster structure of an underground excavation station and a construction method of the pilaster structure. A top longitudinal beam is arranged below a top plate of the underground excavation station, and a bottom longitudinal beam is arranged on a bottom plate; during construction, an upper pilot tunnel and a lower pilot tunnel are formed; pilasters are arranged between the top longitudinal beam and the bottom longitudinal beam and at the positions, where the end walls are to be arranged, of the end parts; the pilasters are arranged in the vertical drill holes between the lower pilot tunnel and the upper pilot tunnel, and steel stand columns are arranged in the pilasters. The steel stand columns are manufactured in a segmented prefabrication mode and comprise a plurality of segments. The subsection is connected with the other subsection in a bolt splicing manner; the steel stand columns are all of frame structures made of steel, and steel bar reserved holes are formed in the positions, corresponding to the corresponding wall columns, of the steel bar cages and used for penetrating of subsequent corresponding steel bars of the steel bar cages and flowing of poured concrete. When the end wall is constructed, a reinforcement cage of the pilaster is bound on the steel stand column, and concrete is poured to form the pilaster. According to the method, the limitation of construction transverse channel arrangement and the contradiction between the pilaster and the waterproof quality and the engineering quality are solved.

Description

Technical Field

[0001] The invention relates to the technical field of construction of a dark-cut station, and in particular to a pilaster structure of a dark-cut station and a construction method thereof. Background Art

[0002] In the field of urban rail transit, there are more and more cases of using the dark excavation method to implement underground stations.

[0003] In the underground excavation station with continuous arch (such as the middle tunnel method, pile-hole method (PBA) etc.), Figure 1 As shown, pilasters must be set on the end wall of the station. Their function is to serve as the end support of the longitudinal beam of the continuous arch station to bear the vertical load of the longitudinal beam; at the same time, they are also important load-bearing components of the end wall to bear the horizontal load of the end wall.

[0004] Current status of related technologies in this field:

[0005] Conventional dark excavation station end wall columns are usually constructed simultaneously with the longitudinal beams, that is, the end cross channel is constructed first, then the guide hole is excavated, and the wall columns are constructed simultaneously when the longitudinal beams are constructed, so as to form a complete longitudinal frame system. The wall columns are made of reinforced concrete structure and cast together with part of the end wall, which is permanent and temporary.

[0006] The shortcomings of the conventional approach are:

[0007] 1) At the location of the pilasters at the end of the station, a construction transverse passage needs to be set up to have space for the pilasters. That is, the pilasters are in the transverse passage, and the upper and lower parts are excavated spaces for workers to tie steel bars, support formwork, pour concrete and maintain. However, due to limited site or the influence of surrounding buildings, it is often impossible to set up a construction transverse passage at the end, so the conventional pilaster setting form and construction method cannot be applied. Therefore, the conventional pilaster form has strict requirements for the layout of the construction passage and is not universal.

[0008] 2) Conventional pilasters can only be implemented synchronously with the longitudinal beams, but not with the end walls, because they pass through several layers of partitions in the transverse passage. This will form a vertical construction joint on both sides of each pilaster. In underground projects, especially in dark excavation stations, the location of the construction joints, the joints of waterproof materials, and the cold joints formed by the successive construction of concrete. Therefore, the existence of construction joints means an increase in water seepage channels or water seepage points, which is particularly unfavorable for the use of underground stations. Therefore, conventional pilasters are not good for stations from a waterproofing perspective.

[0009] 3) Similarly, conventional pilasters cannot be implemented synchronously with the end walls, so the reinforcement of the end walls must be provided with joints on both sides of the pilasters. This position is where the negative bending moment is large. Therefore, from the perspective of stress, it is unreasonable for conventional pilasters to provide reinforcement joints at locations where the bending moment is large.

[0010] Therefore, how to make the end wall columns of the underground excavation station safer, more economical and more efficient when the construction site is limited, the underground project waterproofing requirements are high, and the project quality requirements are strict has become a technical problem that technical personnel in this field urgently need to solve. Summary of the invention

[0011] In view of the above-mentioned defects of the prior art, the present invention provides a wall column structure of a dark-cut station and a construction method thereof, the purpose of which is to enable the end wall columns of the dark-cut station to be constructed more safely, economically and efficiently when the construction site is limited, the underground project has high waterproofing requirements and the project quality requirements are strict.

[0012] To achieve the above-mentioned purpose, the present invention discloses a wall column structure of a dark excavation station, which is arranged between the top plate and the bottom plate of the dark excavation station, wherein two or more top longitudinal beams parallel to each other are arranged below the top plate; and bottom longitudinal beams parallel to the corresponding top longitudinal beams are arranged on the top plate just below each top longitudinal beam;

[0013] When constructing the main body of the underground excavation station, an upper guide hole covering all the top longitudinal beams and a lower guide hole covering each of the bottom longitudinal beams are opened;

[0014] Between each of the top longitudinal beams and the corresponding bottom longitudinal beam, a pilaster is provided at the end where the end wall is to be provided;

[0015] Each of the wall columns is arranged in a vertical borehole between the corresponding lower guide hole and the upper guide hole, and is provided with a steel column inside;

[0016] Each of the steel columns is made by prefabrication in sections, including a number of sections;

[0017] The length of each segment is sufficient to be hoisted in the upper guide hole or the lower guide hole, and is connected to another segment by bolt splicing;

[0018] Each of the steel columns is a frame structure made of steel, and the corresponding steel cages of the wall columns are provided with steel bars holes for the subsequent binding of the corresponding steel cages and the flow of poured concrete;

[0019] After the main construction of the underground excavation station is completed and the end wall is implemented for force conversion, the steel bars of the steel cage of each pilaster are passed through the corresponding steel column for binding, and concrete is poured to form the corresponding pilaster.

[0020] Preferably, anchor bars are provided at both the upper and lower ends of all the steel columns and are cast into the corresponding top longitudinal beams and the corresponding bottom longitudinal beams, forming a longitudinally framed structure that shares the load with the corresponding top longitudinal beams and the corresponding bottom longitudinal beams.

[0021] Preferably, at the position of the bottom of the upper pilot tunnel and the position of the top of the corresponding lower pilot tunnel for each steel column, steel bars are provided and welded to the steel bars of the upper pilot tunnel and the corresponding lower pilot tunnel.

[0022] Preferably, each segment includes four angle steels located at the corners, an H-shaped steel provided at the vertical center line position, and a number of segmented connecting plates arranged along the length direction on each side. The four angle steels and the H-shaped steel are connected by the number of segmented connecting plates to form a tubular structure with a "day" character cross-section and rectangular through holes on the side;

[0023] For each H-shaped steel, a plurality of kidney-shaped holes for passing steel bars or for flowing concrete are provided on the web according to the steel reinforcement cage of the wall column and the flow of concrete for casting the wall column.

[0024] More preferably, each two adjacent segments are connected by setting bolts on the end plates with bolt holes;

[0025] A number of stiffening rib plates are provided between each end plate and the corresponding angle steel or the corresponding segmented connecting plate.

[0026] Preferably, the upper and lower ends of each steel column are cast into the corresponding top longitudinal beam and the corresponding bottom longitudinal beam through anchor plates with anchor bars;

[0027] A number of stiffening rib plates are provided between each anchor plate and the corresponding angle steel or the corresponding segmented connecting plate.

[0028] Preferably, at the bottom position of the upper pilot tunnel and the top position of the lower pilot tunnel for each steel column, steel bars are provided and welded to the primary support steel bars of the upper pilot tunnel and the lower pilot tunnel.

[0029] The present invention also provides a construction method for the wall column structure of an underground excavation station, including the following steps:

[0030] Step 1, excavate all the lower pilot tunnels and the upper pilot tunnels;

[0031] Step 2, in the upper pilot tunnel, at the position where the wall column is to be provided, drill vertically by mechanical hole forming or manual excavation, and excavate and penetrate the vertical drill holes from the upper pilot tunnel to each lower pilot tunnel;

[0032] Step 3, after completing the construction of all top longitudinal beams and bottom longitudinal beams, hoist all segments of all steel columns to the corresponding vertical drilled holes in place, connect the steel columns, and connect all steel columns with the corresponding top longitudinal beams and the corresponding bottom longitudinal beams to form an integral force-bearing frame;

[0033] Step 4: When constructing the end wall and performing force conversion, first pre-pave the waterproof layer on the outside of the end wall, then tie the steel bars of the end wall and the steel cage of the pilaster, and insert the steel bars of each steel cage into the corresponding steel bar holes or waist-shaped holes of the steel column to form composite stirrups that meet the requirements of the pilaster;

[0034] Step 5: synchronously pouring concrete of the pilasters and the end walls.

[0035] Preferably, the main reinforcement of the reinforcement cage of the wall column is fixed to the reinforcement in the corresponding top longitudinal beam and the corresponding bottom longitudinal beam by means of a reserved connector.

[0036] Beneficial effects of the present invention:

[0037] The invention solves the limitation of the arrangement of the construction transverse passage, the contradiction between the wall column and the waterproof quality and the engineering quality, and can complete the arrangement of the wall columns at the end of the dark excavation station more economically, reasonably, safely and efficiently.

[0038] The concept, specific structure and technical effects of the present invention will be further described below in conjunction with the accompanying drawings to fully understand the purpose, characteristics and effects of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 The figure shows a schematic diagram of the plan structure of an underground station in the prior art.

[0040] Figure 2 A schematic diagram of the plan structure of an underground station in one embodiment of the present invention is shown.

[0041] Figure 3 A schematic cross-sectional structure diagram of an underground station in one embodiment of the present invention is shown.

[0042] Figure 4 A schematic diagram of the structure of an upper guide hole and a lower guide hole in one embodiment of the present invention is shown.

[0043] Figure 5 A schematic structural diagram of a wall column in one embodiment of the present invention is shown.

[0044] Figure 6 A cross-sectional view of a steel column in one embodiment of the present invention is shown.

[0045] Figure 7A cross-sectional view of a steel column without a segmented connecting plate in one embodiment of the present invention is shown.

[0046] Figure 8 A schematic diagram of the partial structure of the outer side of a steel column in one embodiment of the present invention is shown.

[0047] Fig. 9 A schematic diagram showing that the web of an H-shaped steel is provided with waist-shaped holes in one embodiment of the present invention.

[0048] Fig.10 A schematic structural diagram showing a steel column segment provided with an end plate in one embodiment of the present invention.

[0049] Fig.11 A schematic diagram of the partial structure of the side surface of two adjacent steel column segments connected by an end plate in one embodiment of the present invention is shown.

[0050] Fig.12 A schematic structural diagram showing a steel column provided with an anchor plate in one embodiment of the present invention is shown.

[0051] Fig.13 A schematic diagram of the partial structure of the side surface of a steel column connected to a top longitudinal beam or a bottom longitudinal beam through an anchor plate in one embodiment of the present invention is shown.

[0052] Fig.14 A schematic structural diagram of the inner reinforcement structure of the pilaster and the end wall in one embodiment of the present invention is shown. DETAILED DESCRIPTION

[0053] Example

[0054] A subway underground excavation station was constructed by partial excavation. The station is an underground two-story island platform with an effective platform width of 15.5m, a net length of 255m inside the station, and a net width of 22.8m. It was constructed by underground excavation and has a double-column, three-span arch and straight wall section. Due to site reasons, the station construction cross passage is set in the middle of the station. There is no construction cross passage at the two end walls of the station, that is, there is no large space connecting the upper and lower floors, so it is impossible to set up conventional Figure 1 The station pilasters shown.

[0055] like Figures 2 to 5 ,and Fig.14 As shown, the pilaster structure of the underground excavation station is arranged between the top plate 1 and the bottom plate 2 of the underground excavation station, and two or more top longitudinal beams 11 parallel to each other are arranged below the top plate 1; and bottom longitudinal beams 21 parallel to the corresponding top longitudinal beams 11 are arranged on the top of the bottom plate 2 just below each top longitudinal beam 11;

[0056] When constructing the main body of the underground excavation station, an upper pilot hole 4 covering all the top longitudinal beams 11 and a lower pilot hole 5 covering each bottom longitudinal beam 21 are opened;

[0057] Between each longitudinal beam 11 and the corresponding bottom longitudinal beam 21, a pilaster 3 is provided at the position where the end wall 8 is to be provided at the end;

[0058] Each pilaster 3 is arranged in a vertical drill hole 6 between the corresponding lower pilot tunnel 5 and the upper pilot tunnel 4, and a steel column 31 is provided inside;

[0059] Each steel column 31 is made by a segmented precast method and includes several segments;

[0060] The length of each segment can meet the hoisting in the upper pilot tunnel 4 or the lower pilot tunnel 5, and is connected to another segment by means of bolt splicing;

[0061] Each steel column 31 is a frame structure made of steel, and corresponding reinforcing bar holes are provided for the reinforcing cage 33 of the corresponding pilaster 3, which are used for the binding of the subsequent corresponding reinforcing cage 33 and the flow of concrete pouring;

[0062] When the main body construction of the mined-out station is completed and the force conversion is implemented for the end wall 8, the reinforcing bars of the reinforcing cage 33 of each pilaster 3 are passed through the corresponding steel column 31 for binding, and concrete is poured to form the corresponding pilaster 3.

[0063] In some embodiments, anchor bars are arranged at the upper and lower ends of all steel columns 31 and poured into the corresponding top longitudinal beam 11 and the corresponding bottom longitudinal beam 21, forming a longitudinally framed structure that jointly bears force with the corresponding top longitudinal beam 11 and the corresponding bottom longitudinal beam 21.

[0064] In some embodiments, at the position of the bottom of the upper pilot tunnel 4 and the position of the top of the corresponding lower pilot tunnel 5 of each steel column 31, steel bars are arranged and welded to the steel bars of the upper pilot tunnel 4 and the corresponding lower pilot tunnel 5.

[0065] In practical applications, arranging steel bars on the steel column 31 and welding them to the steel bars of the upper pilot tunnel 4 and the corresponding lower pilot tunnel 5 can enhance the stability of the steel column 31.

[0066] As Figures 6 to 9 shown, in some embodiments, each segment includes four angle steels 311 located at the corners, an H-shaped steel 312 arranged at the vertical center line position, and several segmented connecting plates 313 arranged along the length direction on each side. The four angle steels 311 and the H-shaped steel 312 are connected by several segmented connecting plates 313 to form a tubular structure with a cross-section in the shape of a "day" character and a rectangular through hole on the side;

[0067] The web of each H-shaped steel 312 is provided with a plurality of kidney-shaped holes 314 for passing through reinforcing bars or for the flow of concrete according to the reinforcing cage 33 of the pilaster 3 and the flow of concrete pouring for the pilaster 3.

[0068] In actual application, the steel column 31 is composed of a full-length angle steel 311, an H-shaped steel 312 and a block connecting plate 313 to form a hollow rectangular cross-section. Each side of the rectangle is welded and fixed with a connecting plate, a full-length angle steel 311 and an H-shaped steel 312. The size of each component is determined by calculation based on the load transmitted from the upper top longitudinal beam 11.

[0069] The steel column 31 adopts a combined structure, which can greatly reduce the use of steel while ensuring vertical bearing and structural stability. The rectangular through holes between the angle steels 311 and between the angle steels 311 and the I-beams reserve space for the steel bar binding of the pilaster 3. On the web of the I-beam, waist-shaped holes 314 are reserved according to the steel bar cage 33 of the pilaster 3 to reserve space for the steel bars, which is convenient for the binding of the steel bars and the flow of concrete during pouring.

[0070] like Fig.10 and Fig.11 As shown, in some embodiments, every two adjacent segments are connected by providing bolts 318 on an end plate 315 with bolt holes 316;

[0071] A plurality of reinforcing ribs 317 are provided between each end plate 315 and the corresponding angle steel 311 or the corresponding block connecting plate 313 .

[0072] like Fig.12 and Fig.13 As shown, in some embodiments, the upper end and the lower ends of each steel column 31 are cast in the corresponding top longitudinal beam 11 and the corresponding bottom longitudinal beam 21 through an anchor plate 32 with anchor bars 321;

[0073] A plurality of reinforcing ribs 317 are provided between each anchor plate 32 and the corresponding angle steel 311 or the corresponding block connecting plate 313 .

[0074] In practical applications, the above technical means can enable the steel columns 31, the top longitudinal beams 11 and the corresponding bottom longitudinal beams 21 to form a longitudinal frame system that bears force together.

[0075] In some embodiments, each steel column 31 is provided with steel bars at the bottom of the upper guide tunnel 4 and at the top of the lower guide tunnel 5 and is welded together with the initial steel bars of the upper guide tunnel 4 and the lower guide tunnel 5 .

[0076] In practical applications, the steel column 31 can enhance the stability of the column by welding the steel bars to the initial supporting steel bars of the upper guide hole 4 and the lower guide hole 5 .

[0077] The present invention also provides a construction method for a pilaster structure of a dark excavation station, comprising the following steps:

[0078] Step 1, opening all lower layer guide holes 5 and upper layer guide holes 4;

[0079] Step 2: Drill holes vertically in the upper pilot hole 4 at the location where the pilaster 3 is to be set by mechanical drilling or manual excavation, and excavate and connect the vertical boreholes 6 from the upper pilot hole 4 to each lower pilot hole 5;

[0080] Step 3, after completing the construction of all top longitudinal beams 11 and bottom longitudinal beams 21, hoist all segments of all steel columns 31 to the corresponding vertical boreholes 6 in place, connect the steel columns 31, and connect all steel columns 31 with the corresponding top longitudinal beams 11 and the corresponding bottom longitudinal beams 21 to form an integral force-bearing frame;

[0081] Step 4: When the end wall 8 is constructed for force conversion, a waterproof layer is first pre-paved on the outside of the end wall 8, and then the steel bars of the end wall 8 and the steel cage 33 of the pilaster 3 are tied, and the steel bars of each steel cage 33 are inserted into the steel bar holes or waist-shaped holes of the corresponding steel columns 31 to form composite stirrups that meet the requirements of the pilaster 3;

[0082] Step 5: Concrete the pilasters 3 and the end walls 8 are poured simultaneously.

[0083] The concrete of the pilaster 3 and the end wall 8 are poured simultaneously, which avoids unnecessary construction joints on the end wall. The steel bars of the end wall 8 are not affected by the pilaster 3 and can directly pass through the steel column 31, and the joints can be set at reasonable positions.

[0084] In some embodiments, the main reinforcement of the reinforcement cage 33 of the wall column 3 is fixed to the reinforcement in the corresponding top longitudinal beam 11 and the corresponding bottom longitudinal beam 21 by means of a reserved connector.

[0085] In actual application, the reinforcement of the pilasters is directly connected through the reserved connectors during binding. During the implementation phase of the main structure of the underground station, the steel columns 31 do not need to be dismantled. After the permanent pilasters 3 are implemented, the force conversion is directly completed without the risk of force conversion and the structure is safe and reliable.

[0086] The preferred specific embodiments of the present invention are described in detail above. It should be understood that a person skilled in the art can make many modifications and changes based on the concept of the present invention without creative work. Therefore, any technical solution that can be obtained by a person skilled in the art through logical analysis, reasoning or limited experiments based on the concept of the present invention on the basis of the prior art should be within the scope of protection determined by the claims.

Claims

1. A wall column structure of a dark excavation station, arranged between a top plate (1) and a bottom plate (2) of the dark excavation station; characterized in that: Below the said top plate (1), there are two or more parallel top longitudinal beams (11); on the upper surface of the said bottom plate (2), corresponding to directly below each of the said top longitudinal beams (11), there are bottom longitudinal beams (21) parallel to the corresponding said top longitudinal beams (11). When constructing the main body of the said mined - out station, an upper pilot tunnel (4) covering all the said top longitudinal beams (11) and a lower pilot tunnel (5) covering each of the said bottom longitudinal beams (21) are opened. Between each of the said top longitudinal beams (11) and the corresponding said bottom longitudinal beam (21), at the position where the end wall (8) is to be set at the end, there is a wall pier (3). Each of the said wall piers (3) is arranged in a vertical drilling hole (6) between the corresponding said lower pilot tunnel (5) and the said upper pilot tunnel (4), and a steel column (31) is provided inside each. Each of the said steel columns (31) is made by a segmented pre - casting method and includes several segments. The length of each of the said segments can meet the requirement of being hoisted in the said upper pilot tunnel (4) or the said lower pilot tunnel (5), and is connected to another said segment by means of bolt splicing. Each of the said steel columns (31) is a frame structure made of steel, and corresponding to the reinforcement cage (33) of the corresponding said wall pier (3), there are steel - bar leaving holes for the subsequent binding of the corresponding said reinforcement cage (33) and the flow of concrete pouring. After completing the construction of the main body of the said mined - out station and when implementing the force transfer of the said end wall (8), the steel bars of the reinforcement cage (33) of each of the said wall piers (3) are passed through the corresponding said steel column (31) for binding, and concrete is poured to form the corresponding said wall pier (3).

2. The wall column structure of the underground excavation station according to claim 1 is characterized in that: Anchor bars are arranged at the upper and lower ends of all the said steel columns (31) and are poured into the corresponding said top longitudinal beams (11) and the corresponding said bottom longitudinal beams (21), forming a longitudinally - framed structure that jointly bears force with the corresponding said top longitudinal beams (11) and the corresponding said bottom longitudinal beams (21).

3. The wall column structure of the underground excavation station according to claim 1 is characterized in that: At the position of the bottom of the said upper pilot tunnel (4) and the position of the top of the corresponding said lower pilot tunnel (5) for each of the said steel columns (31), steel bars are arranged and welded to the steel bars of the said upper pilot tunnel (4) and the corresponding said lower pilot tunnel (5).

4. The wall column structure of the underground excavation station according to claim 1 is characterized in that: Each of the said segments includes four angle steels (311) located at the corners, an H - shaped steel (312) arranged at the vertical center - line position, and a number of segmented connecting plates (313) arranged along the length direction on each side. The four said angle steels (311) and the said H - shaped steel (312) are connected by a number of the said segmented connecting plates (313) to form a tubular structure with a cross - section in the shape of "day" and rectangular through - holes on the side. The web of each of the said H - shaped steels (312) is provided with a number of kidney - shaped holes (314) for passing steel bars or for the flow of concrete pouring according to the reinforcement cage (33) of the said wall pier (3) and the flow of concrete pouring for the said wall pier (3).

5. The wall column structure of the underground excavation station according to claim 4 is characterized in that: Between every two adjacent said segments, they are connected by setting bolts (318) on the end - head plates (315) with bolt holes (316). A plurality of reinforcing ribs (317) are provided between each of the end plates (315) and the corresponding angle steel (311) or the corresponding block connecting plate (313).

6. The wall column structure of the underground excavation station according to claim 1, characterized in that: The upper end and the lower ends of each of the steel columns (31) are cast in the corresponding top longitudinal beam (11) and the corresponding bottom longitudinal beam (21) through an anchor plate (32) with anchor bars (321); A plurality of reinforcing ribs (317) are provided between each anchor plate (32) and the corresponding angle steel (311) or the corresponding segmented connecting plate (313).

7. The wall column structure of the underground excavation station according to claim 1 is characterized in that: Each of the steel columns (31) is provided with steel bars at the bottom of the upper guide tunnel (4) and at the top of the lower guide tunnel (5) and is welded together with the primary steel bars of the upper guide tunnel (4) and the lower guide tunnel (5).

8. The construction method of the pilaster structure of the dark excavation station is characterized in that: The steps include: Step 1, opening all lower layer guide holes (5) and upper layer guide holes (4); Step 2, drilling holes vertically in the upper guide hole (4) at the position where the wall column (3) is to be set by mechanical drilling or manual excavation, and excavating and connecting the vertical boreholes (6) from the upper guide hole (4) to each of the lower guide holes (5); Step 3, after completing the construction of all the top longitudinal beams (11) and the bottom longitudinal beams (21), hoist all the segments of all the steel columns (31) to the corresponding vertical boreholes (6) in place, connect the steel columns (31), and connect all the steel columns (31) with the corresponding top longitudinal beams (11) and the corresponding bottom longitudinal beams (21) to form an integral force-bearing frame; Step 4: When the end wall (8) is constructed for force conversion, a waterproof layer is first pre-paved on the outer side of the end wall (8), and then the steel bars of the end wall (8) and the steel cage (33) of the wall column (3) are tied, and the steel bars of each steel cage (33) are inserted into the steel bar holes or waist-shaped holes of the corresponding steel columns (31) to form composite stirrups that meet the requirements of the wall column (3); Step 5: Concrete the pilasters (3) and the end walls (8) are poured simultaneously.

9. The construction method of the wall column structure of the underground excavation station according to claim 8 is characterized in that: The main reinforcement of the reinforcement cage (33) of the wall column (3) is fixed to the reinforcement in the corresponding top longitudinal beam (11) and the corresponding bottom longitudinal beam (21) by means of a reserved connector.

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