Tunnel type anchorage steel arch and manufacturing method thereof

Abstract

The present application relates to the field of civil engineering, in particular to a tunnel type anchorage steel arch and a manufacturing method thereof. The present application adopts a T-shaped steel frame with a wave-shaped web to form a cross-section bending stiffness with a large difference between the web trough and the web peak. The cross-section stiffness at the web trough is small, and it is easier to bend towards the flange plate side. Therefore, it is not necessary to use the inefficient on-site processing method such as fire roasting. On this basis, the reinforcing connecting piece in contact with the surrounding rock line is arranged on the top surface of the web. On the one hand, the web peak with small stiffness is connected to improve the overall stiffness of the T-shaped steel frame to meet the initial lining support stiffness requirement. On the other hand, it can effectively avoid the problem that the steel arch does not contact the initial shotcrete tightly, which is beneficial to effectively transfer the cohesion and friction of the interface to ensure the reliability of the anchor plug body against the pulling force.

Description

Technical Field

[0001] This invention relates to the field of civil engineering, and in particular to a tunnel-type anchor steel arch frame and its manufacturing method. Background Technology

[0002] The anchorage types for long-span suspension bridges in mountainous areas mainly include gravity anchorages, tunnel anchorages, and rock-hole anchorages. Tunnel anchorages are widely used due to their advantages such as smaller construction volume, economic efficiency, high safety factor, and land-saving and environmentally friendly characteristics. The construction structure of a tunnel anchorage (referred to as a tunnel anchor) differs from that of a typical tunnel and also from tunnel auxiliary tunnels and inclined shafts. In a tunnel anchorage, most of the tension in the main cable is directly transferred to the bedrock of the surrounding mountain. The surrounding rock mass and the anchor body together bear the tension transmitted by the main cable. A tunnel anchorage consists of an anchor chamber structure, an anchor plug structure, and a cable-stayed saddle support structure. The anchor plug section is generally constructed using the New Austrian Tunneling Method (NATM). After the structural cross-section is excavated, steel arch frames and shotcrete are required for initial support in a timely manner. Figure 1 After the tunnel excavation is completed, concrete is filled into the anchor plug section to form the anchor plug. The tunnel anchor mainly provides the main cable pull-out resistance through the self-weight of the anchor plug, the shear force of the contact surface, the friction force of the contact surface, and the wedge clamping effect; among them, the effective exertion of the shear force and friction force of the contact surface requires the anchor plug to be in close contact with the surrounding rock.

[0003] Currently, to ensure high structural rigidity of the surrounding rock support, the steel arch frame for the initial support during tunnel anchor excavation typically uses I22a I-beams. These steel frames are relatively large, such as... Figure 2 During on-site construction, such as Figure 3 As shown, the I-beam flange and the surrounding rock are in surface contact, which can easily lead to small gaps that cannot be filled tightly by shotcrete. This results in a loose contact between the steel arch and the surrounding rock, which in turn affects the shear resistance and friction of the contact surface and reduces the reliability of the anchor plug in resisting pull-out forces.

[0004] Furthermore, due to the characteristics of the tunnel anchor structure, such as large internal slope and frequent cross-sectional changes, the steel arch frame needs to be fabricated on-site according to the variable cross-sectional size based on the actual working conditions. I-beams have high bending stiffness, making on-site bending difficult. Methods such as fire annealing are usually required, resulting in low processing efficiency. Moreover, the outer flange is prone to deformation after bending, making it difficult to guarantee the accuracy of on-site steel frame fabrication. Summary of the Invention

[0005] The purpose of this invention is to address the problems in the prior art where the steel arch frame for the initial support of tunnel anchorages uses I-beams, which presents difficulties in on-site fabrication and results in poor contact between the steel arch frame and the surrounding rock. This invention provides a tunnel anchorage steel arch frame and its fabrication method.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A tunnel-type anchor steel arch frame includes a T-shaped steel frame and reinforcing connectors. The T-shaped steel frame includes a flange plate and a web plate vertically connected to the flange plate. The outer edge of the web plate (i.e., the side away from the flange plate) is wavy. The T-shaped steel frame is bent toward the flange plate side. The crests of the web plate are connected together by the reinforcing connectors. The reinforcing connectors are bent along the top surface of the web plate (i.e., at the outer edge of the crests) and are in contact with the surrounding rock line.

[0008] The aforementioned tunnel-type anchor steel arch frame, by employing a T-shaped steel frame with a corrugated web, can create a significant difference in cross-sectional bending stiffness between the web troughs and crests. The cross-sectional stiffness at the web troughs is low, making it easier to bend towards the flange side without resorting to inefficient on-site processing methods such as hot bending. Furthermore, by using reinforcing connectors that contact the surrounding rock line on the top surface of the web, the lower-stiffness crests on the web are connected together, improving the overall stiffness of the T-shaped steel frame to meet the initial lining support stiffness requirements. On the other hand, it effectively avoids the problem of poor contact between the steel arch frame and the initial shotcrete, facilitating the effective transfer of interfacial cohesion and friction, and ensuring the reliability of the anchor plug's resistance to pull-out forces.

[0009] As a preferred embodiment of the present invention, the reinforcing connector is made of reinforcing steel bars, which has high structural strength, is easy to process and has low cost; it can also adopt structural forms such as concrete steel pipes or downward-bent arc plates, which can also achieve contact with the surrounding rock line and ensure close contact between the steel arch frame and the initial shotcrete.

[0010] As an alternative implementation, instead of placing the reinforcing connector on the top surface of the web and having it in contact with the surrounding rock line, the reinforcing connector can be placed on the side of the web, with its height lower than the top surface of the web. This line contact between the web and the surrounding rock effectively avoids the problem of poor contact between the steel arch and the initial shotcrete, while also ensuring sufficient rigidity for initial support. The reinforcing connector placed on the side of the web can be linear, curved, rod-shaped, or plate-like.

[0011] As a preferred embodiment of the present invention, the steel arch frame further includes supporting stiffening ribs, which are symmetrically arranged on both sides of the web and integrally connected to the web and the flange. The supporting stiffening ribs can support the web to achieve its own stability; to prevent structural interference, the supporting stiffening ribs may be provided with corresponding notches to accommodate and support reinforcing bars.

[0012] As a preferred embodiment of the present invention, the aforementioned steel arch frame further includes perforated reinforcing bars. The flange plate has several openings, and the perforated reinforcing bars are welded to the supporting stiffening ribs and extend through the openings. The perforated reinforcing bars are used to strengthen the connection between the initial support and the secondary lining concrete.

[0013] This invention also provides a method for manufacturing a tunnel-type anchor steel arch frame, comprising the following steps:

[0014] S1. Cut the I-beam longitudinally into two halves in a wavy pattern at the web to form two T-shaped steel frames with multiple tooth-like structures;

[0015] S2. Bend the T-shaped steel frame toward the flange plate side;

[0016] S3. Weld a reinforcing connector to the top surface of the web, the reinforcing connector being able to contact the surrounding rock line, or weld a reinforcing connector to at least one side surface of the web; the reinforcing connector connecting a plurality of the toothed structures (i.e. crests) into one unit.

[0017] Using the above-mentioned method for manufacturing tunnel-type anchor steel arch frames, the cross-sectional stiffness at the trough of the web plate after cutting and forming is small, making it easy to bend and process. It eliminates the need for inefficient on-site processing methods such as fire-baking. The resulting steel arch frame can meet the stiffness requirements of the initial lining support and effectively avoid the problem of poor contact between the steel arch frame and the initial shotcrete.

[0018] In a preferred embodiment of the present invention, the reinforcing connector is continuously arranged along the T-shaped steel frame and bends in the same direction as the T-shaped steel frame. When the reinforcing connector is arranged on the side of the web, the reinforcing connector may also be discontinuous, for example, by overlapping, butt joint, or multi-segment interval arrangement, and may be a straight component or a curved component; when the reinforcing connector is arranged on the top surface of the web, it should be consistent with the bending amplitude of the T-shaped steel frame and adapted to the arch shape of the excavated cross-section.

[0019] As a preferred embodiment of the present invention, the reinforcing connector is a reinforcing steel bar.

[0020] As a preferred embodiment of the present invention, step S4 is further included: a plurality of supporting stiffening ribs are arranged longitudinally at intervals on the side of the web plate, the supporting stiffening ribs integrally connecting the web plate and the flange plate, which is beneficial to improving the construction stability of the steel arch frame. When the reinforcing connector is provided on the top surface of the web plate, the supporting stiffening ribs should be flush with the top position of the reinforcing connector; when the reinforcing connector is provided on the side of the web plate, the supporting stiffening ribs should be flush with the outer edge of the web plate.

[0021] As a preferred embodiment of the present invention, in step S4, the supporting stiffening ribs are symmetrically welded to both sides of the web, and the supporting stiffening ribs are respectively welded perpendicularly to the web and the flange, which has good stress performance. The supporting stiffening ribs are provided with notches to receive the reinforcing connectors, which is conducive to achieving a compact structure.

[0022] Furthermore, as a preferred embodiment of the present invention, the above manufacturing method further includes step S5: welding perforated steel bars on the supporting stiffening ribs, the perforated steel bars passing through the flange plate to strengthen the connection with the secondary lining concrete.

[0023] As a preferred embodiment of the present invention, when the reinforcing connector is disposed on the side of the web, the reinforcing connector is symmetrically disposed on both sides of the web, and the top of the reinforcing connector is disposed lower than the top surface of the web.

[0024] In summary, due to the adoption of the above technical solution, the beneficial effects of the present invention are:

[0025] 1. The tunnel-type anchor steel arch frame provided by this invention is easy to bend and process, has low on-site manufacturing difficulty, high manufacturing efficiency, and can effectively avoid the problem of poor contact between the steel arch frame and the initial shotcrete, which is conducive to effectively transferring the cohesion and friction of the interface, thereby ensuring the reliability of the anchor body against pull-out force.

[0026] 2. The tunnel-type anchor steel arch frame provided by this invention allows the shotcrete to fill the troughs of the corrugated web during construction, strengthening the connection between the steel arch frame and the shotcrete, which helps to improve the compressive strength of the steel arch frame and thus better support the convergence deformation of the surrounding rock.

[0027] 3. The method for manufacturing a tunnel-type anchor steel arch frame provided by this invention reduces the difficulty of on-site bending and processing of the steel arch frame, has high manufacturing efficiency, and the manufactured steel arch frame meets the rigidity requirements of the initial lining support. It can also effectively avoid the problem of poor contact between the steel arch frame and the initial shotcrete, which is conducive to effectively transmitting the cohesion and friction of the interface, thereby ensuring the reliability of the anchor plug body in resisting pull-out force. Attached Figure Description

[0028] Figure 1 This is a diagram showing the relationship between the I-beam steel frame commonly used in the initial support of tunnel anchor excavation and the surrounding rock structure.

[0029] Figure 2 yes Figure 1 Cross-sectional view of the I-beam steel arch frame;

[0030] Figure 3 This is a cross-sectional view of the installation of the I-beam steel arch frame;

[0031] Figure 4This is a schematic diagram of a tunnel-type anchor steel arch frame in Example 1;

[0032] Figure 5 yes Figure 4 Sectional view along the AA direction;

[0033] Figure 6 yes Figure 4 Cross-sectional view along the BB direction;

[0034] Figure 7 yes Figure 4 Cross-sectional structural diagram of the steel arch frame with supporting stiffening ribs;

[0035] Figure 8 This is a structural diagram showing the addition of perforated steel bars at the supporting stiffening ribs;

[0036] Figure 9 This is a cross-sectional view of the installation of a T-shaped web steel arch frame;

[0037] Figure 10 This is a cross-sectional view of the steel arch frame in Example 2, formed by welding two reinforcing bars at the serrated web.

[0038] Figure 11 This is a flowchart of a method for manufacturing a tunnel-type anchor steel arch frame, as shown in Example 3.

[0039] Icons: 1-I-beam; 11-web plate; 12-flange plate; 13-T-shaped steel frame; 2-cutting line; 3-reinforcing connector; 4-support stiffening rib; 5-perforated steel bar. Detailed Implementation

[0040] The present invention will now be described in detail with reference to the accompanying drawings.

[0041] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0042] Example 1

[0043] A type of tunnel-type anchor steel arch frame, such as Figures 4-8 As shown, it includes a T-shaped steel frame 13 and a reinforcing connector 3. The T-shaped steel frame 13 includes a flange plate 12 and a web plate 11 vertically connected to the flange plate 12. The outer edge of the web plate 11 (i.e. the side away from the flange plate 12) is wavy (or serrated). The T-shaped steel frame 13 is bent toward the flange plate side. The crests of the web plate 11 are connected together by the reinforcing connector 3. The reinforcing connector 3 is bent along the top surface of the web plate 11 (i.e. the outer edge of the crest) and is in contact with the surrounding rock line.

[0044] The aforementioned tunnel-type anchor steel arch frame, by employing a T-shaped steel frame 13 with a corrugated web 11, can form a cross-sectional bending stiffness with a significant difference between the troughs and crests of the web 11. The cross-sectional stiffness at the troughs of the web 11 is small, making it easier to bend towards the flange side without resorting to inefficient on-site processing methods such as hot bending. Furthermore, by using reinforcing connectors 3 that contact the surrounding rock line on the top surface of the web 11, the crests with lower stiffness on the web 11 are connected together, improving the overall stiffness of the T-shaped steel frame 13 to meet the initial lining support stiffness requirements. On the other hand, it effectively avoids the problem of poor contact between the steel arch frame and the initial shotcrete, facilitating the effective transfer of interfacial cohesion and friction, and ensuring the reliability of the anchor plug body against pull-out forces.

[0045] Specifically, in this embodiment, the reinforcing connector 3 is made of continuous reinforcing steel bars, which are bent as a whole, resulting in high structural strength, easy processing and low cost. As other possible implementation schemes, concrete steel pipes or downward-bent arc plates can also be used, which can also achieve contact with the surrounding rock line and ensure close contact between the steel arch frame and the initial shotcrete.

[0046] Furthermore, the steel arch frame also includes supporting stiffening ribs 4, which are symmetrically arranged on both sides of the web 11. The supporting stiffening ribs 4 are integrally connected to the web 11 and the flange plate to ensure the stability of the web 11 itself. To prevent structural interference, the supporting stiffening ribs 4 may be provided with corresponding notches to accommodate and support reinforcing bars.

[0047] The aforementioned steel arch frame also includes perforated steel bars 5. Several openings are provided on the flange plate 12, and the perforated steel bars 5 are welded to the supporting stiffening ribs 4 and pass through the openings. The perforated steel bars 5 are used to strengthen the connection between the initial support and the secondary lining concrete.

[0048] During construction, such as Figure 9 As shown, when the steel arch frame is erected on the anchor plug section, the reinforcing connector 3 set on the top surface of the web plate is in line contact with the surrounding rock, and the supporting stiffening ribs 4 on both sides of the web plate 11 are also in line contact with the surrounding rock. When spraying concrete, the top of the arch frame can be filled densely, and the quality is easier to guarantee.

[0049] Example 2

[0050] Based on Embodiment 1, this Embodiment 1 also provides a tunnel-type anchor steel arch frame, the main difference being: as follows Figure 10As shown, the reinforcing connector 3 is welded to the sides of the web 11 on both sides. The reinforcing connector 3 can be in the form of a straight line, a curved line, a rod, or a plate structure, and can be continuously installed or segmented. Line contact is achieved between the outer edge of the web 11 crest and the surrounding rock, effectively avoiding the problem of poor contact between the steel arch and the initial shotcrete. Simultaneously, the reinforcing connector 3 ensures that the steel arch has sufficient rigidity for initial support. The height of the reinforcing connectors 3 on both sides should be a certain distance from the height of the web crest to facilitate sufficient shotcreting. In this embodiment, they are preferably installed in the middle of the web crest.

[0051] Example 3

[0052] Based on Embodiment 1, the present invention also provides a method for manufacturing a tunnel-type anchorage steel arch frame, such as... Figure 11 As shown, it includes the following steps:

[0053] S1. Cut the I-beam 1 in half along the wavy cutting line 2 at the web 11 to form two T-shaped steel frames 13 with multiple tooth-like structures.

[0054] S2. Bend the T-shaped steel frame 13 toward the flange side.

[0055] S3. A reinforcing connector 3 is welded to the top surface of the web 11. The reinforcing connector 3 connects multiple toothed structures (i.e., the crests of the web 11) into one piece, and the reinforcing connector 3 can contact the surrounding rock line. In this embodiment, the reinforcing connector 3 is continuously arranged along the T-shaped steel frame 13 and bends in the same direction as the T-shaped steel frame 13. The bending amplitude of the reinforcing connector 3 is consistent with the bending amplitude of the T-shaped steel frame 13 and is adapted to the arch shape of the excavation with variable cross-section. In this embodiment, the reinforcing connector 3 preferably uses reinforcing steel bars with a diameter of 40mm.

[0056] After adding reinforcing steel bars, the overall stiffness is greater than that of I22a I-beam 1, and the material usage does not increase significantly, thus meeting the stiffness requirements of the initial lining support.

[0057] S4. Several supporting stiffening ribs 4 are longitudinally spaced along the sides of the web 11. These ribs support the surrounding rock, improving the construction stability of the steel arch frame. The supporting stiffening ribs 4 are symmetrically arranged on both sides of the web 11, using a double-sided welding method. Each supporting stiffening rib 4 is welded perpendicularly to both the web 11 and the flange 12. The supporting stiffening ribs 4 have notches for receiving reinforcing connectors 3. The top of the supporting stiffening ribs 4 should be flush with the top of the reinforcing connectors 3.

[0058] S5. Insert the perforated steel bar 5 through the flange plate 12, weld the perforated steel bar 5 to the supporting stiffening rib 4, and bend the free end of the perforated stiffness to strengthen the connection between the initial support and the secondary lining concrete.

[0059] Using the above-mentioned method for manufacturing tunnel-type anchor steel arch frames, the cross-sectional stiffness at the trough of the web plate 11 after cutting and forming is small, making it easy to bend and process. It is not necessary to use inefficient on-site processing methods such as fire-baking. The resulting steel arch frame can meet the stiffness requirements of the initial lining support and can also effectively avoid the problem of poor contact between the steel arch frame and the initial shotcrete.

[0060] Example 4

[0061] Based on Embodiment 2, the present invention also provides a method for manufacturing a tunnel-type anchor steel arch frame, comprising the following steps:

[0062] S1. Cut the I-beam 1 in half along the wavy cutting line 2 at the web 11 to form two T-shaped steel frames 13 with multiple tooth-like structures.

[0063] S2. Bend the T-shaped steel frame 13 toward the flange side.

[0064] S3. A reinforcing connector 3 is welded to at least one side of the web 11, the reinforcing connector 3 connecting multiple toothed structures (i.e., the crests of the web 11) into one piece. In this embodiment, the reinforcing connector 3 is symmetrically arranged on both sides of the web 11, using a double-sided welding method, and the top of the reinforcing connector 3 should be lower than the top surface of the web 11. The reinforcing connector 3 can be continuously arranged or discontinuously arranged, for example, by lap joints, butt joints, or multi-segment intervals, and can be a straight component or a curved component. In this embodiment, the reinforcing connector 3 preferably uses a reinforcing steel bar with a diameter of 32mm, continuously arranged along the T-shaped steel frame 13 and bent in the same direction as the T-shaped steel frame 13, which is convenient to process, widely available in materials, and low in cost.

[0065] After adding reinforcing steel bars, the overall stiffness is greater than that of I22a I-beam 1, and the material usage does not increase significantly, thus meeting the stiffness requirements of the initial lining support.

[0066] S4. Several supporting stiffening ribs 4 are longitudinally spaced along the sides of the web 11. These ribs support the surrounding rock, improving the construction stability of the steel arch frame. The supporting stiffening ribs 4 are symmetrically arranged on both sides of the web 11, using a double-sided welding method. Each supporting stiffening rib 4 is welded perpendicularly to both the web 11 and the flange plate 12. The supporting stiffening ribs 4 have notches for receiving reinforcing connectors 3. The top of the supporting stiffening ribs 4 should be flush with the outer edge of the web 11.

[0067] Using the above-mentioned method for manufacturing tunnel-type anchor steel arch frames, the cross-sectional stiffness at the trough of the web plate 11 after cutting and forming is small, making it easy to bend and process. This eliminates the need for inefficient on-site processing methods such as hot-bending. The resulting steel arch frame meets the stiffness requirements of the initial lining support and effectively avoids the problem of poor contact between the steel arch frame and the initial shotcrete. In the above embodiment, the T-shaped web steel arch frame can be bent in either an arc shape or a zigzag shape. The bending difficulty is low, and the steel arch frame has line contact with the surrounding rock, making it easy to fill and compact, and better transmitting the frictional force of the contact surface.

[0068] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A tunnel-type anchor steel arch frame, characterized in that, The device includes a T-shaped steel frame (13) and a reinforcing connector (3). The T-shaped steel frame (13) includes a flange plate (12) and a web plate (11) vertically connected to the flange plate (12). The outer edge of the web plate (11) is wavy. The T-shaped steel frame (13) is bent toward the flange plate. The crests of the web plate (11) are connected together by the reinforcing connector (3). The reinforcing connector (3) is bent along the top surface of the web plate (11). The reinforcing connector (3) is used to contact the surrounding rock line. The reinforcing connector (3) is a reinforcing steel bar, a steel pipe, or a downwardly bent arc plate. It also includes a support stiffening rib (4), which is arranged longitudinally at intervals on the side of the web (11). The support stiffening rib (4) is integrally connected to the web (11) and the flange plate (12). The support stiffening rib (4) is provided with a notch to receive the reinforcing connector (3). The support stiffening rib (4) is used to support the surrounding rock.

2. The tunnel-type anchor steel arch frame according to claim 1, characterized in that, The reinforcing connector (3) is placed on the top surface of the web (11). The reinforcing connector (3) is used to contact the surrounding rock line. The reinforcing connector (3) is a reinforcing steel bar, steel pipe or downward-bent arc plate. Alternatively, the reinforcing connector (3) is placed on the side of the web (11). The height of the reinforcing connector (3) is lower than the height of the top surface of the web (11).

3. A tunnel-type anchorage steel arch frame according to claim 1 or 2, characterized in that, The supporting stiffening ribs (4) are symmetrically arranged on both sides of the web (11).

4. A tunnel-type anchor steel arch frame according to claim 3, characterized in that, It also includes perforated steel bars (5), and the flange plate (12) has several openings. The perforated steel bars (5) are welded to the supporting stiffening ribs (4) and pass through the openings.

5. A method for manufacturing a tunnel-type anchorage steel arch frame, characterized in that, Includes the following steps: S1. Cut the I-beam (1) longitudinally into two halves in a wavy pattern at the web (11) to form two T-shaped steel frames (13) with multiple toothed structures. S2. Bend the T-shaped steel frame (13) toward the flange plate side; S3. A reinforcing connector (3) is welded to the top surface of the web (11). The reinforcing connector (3) is capable of contacting the surrounding rock line. The reinforcing connector (3) is a reinforcing steel bar, a steel pipe, or a downwardly bent arc plate. Alternatively, a reinforcing connector (3) can be welded to at least one side of the web (11) to achieve line contact with the surrounding rock through the outer edge of the crest of the web (11); The reinforcing connector (3) connects the multiple toothed structures into one piece; S4. Several supporting stiffening ribs (4) are arranged longitudinally on the side of the web (11). The supporting stiffening ribs (4) are integrally connected to the web (11) and the flange plate (12). The supporting stiffening ribs (4) are provided with notches to receive the reinforcing connectors (3). The supporting stiffening ribs (4) are used to support the surrounding rock.

6. The method for manufacturing a tunnel-type anchorage steel arch frame according to claim 5, characterized in that, The reinforcing connector (3) is continuously arranged along the T-shaped steel frame (13) and bends in the same direction as the T-shaped steel frame (13).

7. The method for manufacturing a tunnel-type anchorage steel arch frame according to claim 5, characterized in that, In step S4, the supporting stiffening ribs (4) are symmetrically welded to both sides of the web plate (11), and the supporting stiffening ribs (4) are perpendicularly welded to the web plate (11) and the flange plate (12) respectively. It also includes step S5: welding perforated steel bars (5) onto the supporting stiffening rib (4), the perforated steel bars (5) passing through the flange plate (12).

8. A method for manufacturing a tunnel-type anchorage steel arch frame according to any one of claims 5-7, characterized in that, When the reinforcing connector (3) is disposed on the side of the web (11), the reinforcing connector (3) is symmetrically disposed on both sides of the web (11), and the top of the reinforcing connector (3) is disposed lower than the top surface of the web (11).

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