Steel-concrete-steel truss composite beam structure with pre-applied hogging moment and construction method thereof

Abstract

The present application relates to the technical field of building structure, and provides a steel concrete-steel truss composite beam structure with pre-applied reverse bending moment and a construction method thereof, the structure comprising support columns, double-layer steel truss beams and steel concrete beams arranged between adjacent support columns, and the top of the double-layer steel truss beam being fixedly connected with the bottom of the steel concrete beam; pre-compression devices or pre-tension devices are arranged between several groups of adjacent nodes in the interlayer plane of the double-layer steel truss beam, the pre-compression devices being located in the negative bending moment area of the steel concrete beam, and the pre-tension devices being located in the positive bending moment area of the steel concrete beam; the pre-compression devices make the steel concrete beam generate pre-applied positive bending moment in the negative bending moment area thereof, and the pre-tension devices make the steel concrete beam generate pre-applied negative bending moment in the positive bending moment area thereof. The pre-applied reverse bending moment can offset part of the bending moment effect caused by the upper load, improve the bearing capacity and crack resistance of the steel concrete beam, improve the spanning capacity of the beam under the premise of ensuring small clear height, and also improve the seismic performance of the composite beam.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of building structures, and particularly relates to a steel-concrete-steel truss composite beam structure with pre-applied reverse bending moment and a construction method thereof. BACKGROUND

[0002] With the continuous development of public buildings and high-rise buildings in the direction of large span and heavy load, how to effectively improve the bearing capacity of the cross beam has become an important issue to be solved. Under normal circumstances, from the material point of view, high-strength concrete is often used to significantly improve the bearing capacity of the beam; from the perspective of structural design, the common method is to increase the vertical support between the spans, which reduces the calculation span of the beam, so as to better meet the design requirements of large span and heavy load, such as beam string structure, truss beam, steel-concrete composite beam, steel-concrete composite truss beam, cable-stayed beam, suspension beam, etc. are typical representatives. However, from the overall point of view, the various structural design schemes currently adopted are basically from the perspective of "passive" force, aiming to enhance the passive force bearing capacity of the beam structure to meet the design specification and safety requirements. However, these methods are difficult to apply to high-rise building floor systems, and it is also more difficult to meet the requirements under larger span and higher load conditions.

[0003] In addition, as a design method based on the concept of "active" force, prestressed technology can effectively improve the bearing capacity of the structure, but the construction technology is relatively complicated. In the design of large-span and heavy-load buildings, the conventional internal prestress has the problems of large friction loss, difficult control of grouting quality, easy to produce voids and cannot be detected and replaced, and also faces the risks of effective prestress attenuation along the span, corrosion difficult to detect and fatigue corrosion; although the external prestress has small friction loss and can be replaced, it is very dependent on the protection system, the exposed cable body is easy to be eroded by the environment, the stress concentration exists in the turning block, the fatigue vibration is significant, and the stiffness contribution is low, so additional fireproofing measures are needed, and therefore it is difficult to fully meet the growing demand for large-span and heavy-load building design.

[0004] In the existing structural system, the internal / external prestressed beam presents the characteristics of "high stiffness and low ductility" under the influence of high axial compression of prestress. Under the action of earthquake, if the compression zone concrete is crushed or the anchorage end fails, the prestressed steel strand will be unloaded instantly, which will cause the bearing capacity to drop sharply and easily cause brittle shear failure. The redundancy of the cable and strut of structures such as beam string structure and suspension beam is low, and once a single cable is relaxed or the anchor plate slips, the overall self-balancing system of the structure will collapse, and the redundancy is much lower than that of ordinary truss. The web member of the truss structure can play a role in energy dissipation under the action of earthquake, but the effect is limited.

[0005] Therefore, it is necessary to provide a steel-concrete-steel truss composite beam structure and a construction method thereof with pre-applied reverse bending moment to meet the requirements of large span, high bearing capacity and high seismic resistance. SUMMARY

[0006] The present application aims to provide a steel-concrete-steel truss composite beam structure and a construction method thereof with pre-applied reverse bending moment to solve one of the problems of insufficient bearing capacity, complicated prestress application with limited effect and insufficient seismic performance of the prior art under large-span and heavy-load working conditions.

[0007] To achieve the above-mentioned purpose, the present application adopts the following technical solutions:

[0008] In a first aspect, the present application provides a steel-concrete-steel truss composite beam structure with pre-applied reverse bending moment, comprising support columns, double-layer steel truss beams and steel-concrete beams arranged between adjacent support columns, the top of the double-layer steel truss beam being fixedly connected to the bottom of the steel-concrete beam, and pre-compression devices or pre-tension devices being arranged between several groups of adjacent nodes in the interlayer plane of the double-layer steel truss beam, wherein the pre-compression devices are located in the negative bending moment zone of the steel-concrete beam, and the pre-tension devices are located in the positive bending moment zone of the steel-concrete beam; the pre-compression devices cause the steel-concrete beam to generate pre-applied positive bending moment in its negative bending moment zone, and the pre-tension devices cause the steel-concrete beam to generate pre-applied negative bending moment in its positive bending moment zone.

[0009] Based on the above technical solutions, please refer to Figure 8 The pre-compression devices installed in the negative bending moment zone of the steel-concrete beam apply a pushing force to its adjacent nodes, which causes the steel-concrete beam to generate downward tension in its negative bending moment zone, the steel cross beam to generate upward tension, and the steel-concrete beam to generate downward deformation, and the pre-tension devices installed in the positive bending moment zone of the steel-concrete beam apply a pulling force to its adjacent nodes, which causes the steel-concrete beam to generate upward jacking force in its positive bending moment zone, and the steel-concrete beam to generate upward deformation, so that after the installation of the composite beam structure, the pre-compression devices or the pre-tension devices work together to apply a reverse arch effect (or reverse bending moment effect) to the steel-concrete beam, i.e. to pre-apply positive bending moment in the negative bending moment zone (side span area) and to pre-apply negative bending moment in the positive bending moment zone (mid-span area); when the upper load is applied, the upper load acts on the steel-concrete beam, and the pre-applied reverse bending moment effect can offset part of the bending moment effect brought by the upper load to the steel-concrete beam, thereby improving the bearing capacity and crack resistance of the steel-concrete beam and increasing the span capacity of the beam under the premise of ensuring small clear height.

[0010] As a preferred scheme of the present application, the pre-pressing device or the pre-pulling device comprises a first middle chord, one end of the first middle chord is fixedly installed on one node in the interlayer plane of the double-layer steel truss girder through a first flange, the other end of the first middle chord is installed on another adjacent node in the interlayer plane of the double-layer steel truss girder through a second flange, and the other end of the first middle chord can slide relative to the other node in the axial direction of the first middle chord; a spring is sleeved on the first middle chord, and both ends of the spring are fixedly installed on the first flange and the second flange respectively; for the pre-pressing device, the spring is a high-strength spring to which a pre-pressing force is applied, and for the pre-pulling device, the spring is a high-strength spring to which a pre-pulling force is applied.

[0011] Based on the above technical scheme, please refer to Figure 1 and Figure 3 (a) Figure 3 (d), before the upper load is applied, for the pre-pressing device, the spring is subjected to a pre-pressing force, one node (supposed to be A) and another node (supposed to be B) are subjected to a pushing force, the other end of the first middle chord is located at the right end of the node B (the right end of the sliding groove), the girder node E moves downward, the node G moves upward, because the sectional geometric parameters and the self weight of the steel cross beam 4 are lower than those of the steel-concrete beam, the upward displacement of the node G is greater than the downward displacement of the node E, at the same time, the steel cross beam 4 generates an upward turning angle at the node G, which drives the whole double-layer steel truss girder to present an upward reverse-arch deformation; before the upper load is applied, for the pre-pulling device, the spring is subjected to a pre-pulling force, one node (supposed to be C) and another node (supposed to be D) are subjected to a pulling force, the other end of the first middle chord is located at the right end of the node D (the right end of the sliding groove), the girder node F moves upward, a pre-added vertical force acting upward is applied to the steel-concrete beam, and the pre-added vertical force and the reverse-arch state generated by the pre-pressing device will generate a pre-added reverse bending moment acting on the steel-concrete beam; after the upper load is applied, the pre-added vertical force will resist the action load, the pre-added reverse bending moment will resist the load action bending moment, and the deformation of the beam is limited; after the upper load is applied, the girder node F moves downward, the node C and the node D move in the direction away from each other, for the pre-pulling device, the spring continues to be pulled, but the other end of the first middle chord slides to the left end of the node D (the left end of the sliding groove) to bear the pulling force; after the upper load is applied, the girder node E moves upward, the node B moves in the direction away from the node A, for the pre-pressing device, the spring restores the original length trend, and the other end of the first middle chord slides to the left end of the node B (the left end of the sliding groove) to bear the pulling force. Therefore, by using the pre-pressing device or the pre-pulling device, the pre-added reverse bending moment acting on the steel-concrete beam can be provided, the original stress form of the steel-concrete beam can be ensured, and the self weight of the composite beam and the influence on the clear height are reduced as much as possible.

[0012] As a preferred scheme of the present application, the double-layer steel truss girder comprises an upper truss and a lower truss, the lower truss comprises steel cross beams installed between adjacent support columns and a plurality of lower web members installed in a zigzag manner on the steel cross beams, the plurality of lower web members form a plurality of nodes in the interlayer plane of the double-layer steel truss girder, a plurality of upper web members are installed in a zigzag manner between the plurality of nodes and the steel-concrete girder, forming the upper truss; in addition to the pre-pressing device or the pre-pulling device being installed at some adjacent nodes, a second middle chord is also fixedly installed between other adjacent nodes.

[0013] Based on the above technical scheme, the double-layer steel truss girder is beneficial to the application of the pre-pressing device and the pre-pulling device to the inverted arch state, the structure of the double-layer steel truss girder significantly improves the stability and the bearing capacity of the structure through the cooperative work of the upper and lower trusses, and the zigzag force transmission path can optimize the load transmission. The second middle chord strengthens the node connection, thereby enhancing the stability and providing convenience for the maintenance of the structure and the re-application of the pre-pressing device and the pre-pulling device.

[0014] As a preferred scheme of the present application, the pre-pressing device or the pre-pulling device is staggered with the second middle chord in the length direction of the double-layer steel truss girder; that is, the pre-pressing device is staggered with the second middle chord in the side span area of the steel-concrete girder, and the pre-pulling device is staggered with the second middle chord in the mid-span area of the steel-concrete girder.

[0015] As a preferred scheme of the present application, a fixed steel plate is arranged at the node in the interlayer plane of the double-layer steel truss girder, the lower end of the fixed steel plate is connected with the lower web member, the upper end of the fixed steel plate is connected with the upper web member, and the middle part of the fixed steel plate is connected with the first flange, the second flange or the second middle chord.

[0016] As a preferred scheme of the present application, one end of the first middle chord is provided with an external thread, and the other end is provided with a sliding block with a size larger than the diameter of the first middle chord; one side of the first flange close to the spring is provided with a counterbore matched with the first middle chord, and the other side of the first flange away from the spring is provided with a first sliding groove matched with the sliding block, and the counterbore and the first sliding groove are in communication; the second flange is provided with a first screw hole matched with the external thread of the first middle chord.

[0017] As a preferred scheme of the present application, the fixed steel plate comprises a type I fixed steel plate provided with the first flange and a type II fixed steel plate provided with the second flange, the type I fixed steel plate is further provided with a second sliding groove coaxially butted with the first sliding groove, and the type II fixed steel plate is further provided with a second screw hole coaxially butted with the first screw hole.

[0018] As a preferred scheme of the present application, the first sliding groove and the second sliding groove are further provided with a rubber buffer pad.

[0019] As a preferred scheme of the present application, the spring outer periphery is further provided with a sleeve in the pre-pressing device or pre-pulling device to protect the spring from corrosion.

[0020] As a preferred scheme of the present application, the steel-concrete beam is a steel-concrete beam or a steel plate-concrete beam.

[0021] In a second aspect, the present application provides a construction method of a steel-concrete-steel truss composite beam structure with pre-applied reverse bending moment, comprising the following steps:

[0022] 1) installing support columns;

[0023] 2) constructing a steel-concrete beam between adjacent support columns;

[0024] 3) installing a double-layer steel truss beam below the steel-concrete beam between adjacent support columns; simultaneously, installing pre-pressing devices between a plurality of groups of adjacent nodes in the interlayer plane of the double-layer steel truss beam corresponding to the positive bending moment zone of the steel-concrete beam, and installing pre-pulling devices between a plurality of groups of adjacent nodes in the interlayer plane of the double-layer steel truss beam corresponding to the negative bending moment zone of the steel-concrete beam.

[0025] Compared with the prior art, the present application has the following beneficial technical effects:

[0026] 1) The present application uses a steel-concrete beam as the main load-bearing component and a double-layer steel truss beam as a component for supporting the steel-concrete beam to ensure its load-bearing performance, and the two components are combined to form a steel-concrete-steel truss composite beam; the present application further arranges a plurality of pre-pressing / pulling devices in the interlayer plane of the double-layer steel truss beam, wherein the pre-pressing devices are located in the negative bending moment zone of the steel-concrete beam, and the pre-pulling devices are located in the positive bending moment zone of the steel-concrete beam; the pre-pressing devices cause the steel-concrete beam to generate a pre-applied positive bending moment in its negative bending moment zone, and the pre-pulling devices cause the steel-concrete beam to generate a pre-applied negative bending moment in its positive bending moment zone, thereby providing a pre-applied reverse bending moment effect to the steel-concrete beam; after an upper load acts on the steel-concrete beam, the pre-applied reverse bending moment effect can offset part of the bending moment effect brought to the steel-concrete beam by the upper load, thereby improving the load-bearing capacity and crack resistance of the steel-concrete beam and improving the spanning capacity of the beam under the premise of ensuring a small clear height; in addition, the pre-pressing devices or pre-pulling devices are further provided with springs, and the sliding blocks at the ends of the first intermediate chords can also slide at the nodes and deform in coordination with the springs, thereby being able to absorb part of the energy generated by the seismic load and improving the seismic performance of the composite beam.

[0027] 2) The essence of the present application is to prestress the steel-concrete beam externally, and the double-layer steel truss beam is used as the prestress applying and transferring structure. Different from the traditional external prestress, the prestress member of the present application is divided into multiple small sections (i.e. the sections corresponding to the pre-pressing device and the pre-pulling device), which are uniformly distributed in the length direction of the composite beam. Not only the installation is convenient, but also when the prestress member (i.e. the pre-pressing device and the pre-pulling device) in each small section is damaged, the original prestress member can be replaced in time, and the maintenance is convenient. In addition, the pre-pulling and pre-pressing devices are set specifically, which can not only pre-apply the negative bending moment to offset the part of the positive bending moment borne by the steel-concrete beam in the middle region, but also pre-apply the positive bending moment to offset the part of the negative bending moment borne by the steel-concrete beam in the end region, so as to improve the bearing capacity and crack resistance of the steel-concrete beam in the whole section. BRIEF DESCRIPTION OF DRAWINGS

[0028] Figure 1 It is a whole schematic diagram of the steel plate-concrete-steel truss composite beam structure.

[0029] Figure 2 It is a structural schematic diagram of the steel plate-concrete beam.

[0030] Figure 3 It is a structural schematic diagram of the pre-pressing device or the pre-pulling device; wherein, Figure 3 (a) is a schematic diagram of the type II fixed steel plate, Figure 3 (b) is a front view of the pre-pressing device or the pre-pulling device, Figure 3 (c) is a schematic diagram of the type I fixed steel plate, Figure 3 (d) is a schematic diagram of the first middle chord.

[0031] Figure 4 It is a structural schematic diagram of the pre-pressing device installation at the end of the side span (node A-B); wherein, Figure 4 (a) is a schematic diagram of the node A position, Figure 4 (b) is a schematic diagram of the node B position, F1~F4 are the forces transferred by the web members respectively, N represents the pressure borne by the pre-pressing device.

[0032] Figure 5 It is a three-dimensional schematic diagram of the first flange and the second flange; wherein, Figure 5 (a) is the second flange, Figure 5 (b) is the first flange.

[0033] Figure 6 It is a structural schematic diagram (including the front view and the sectional view) of the first flange and the second flange; wherein, Figure 6 (a) is the second flange, Figure 6 (b) is the first flange.

[0034] Figure 7The figure shows the principle of the pre-pressing device or the pre-pulling device. Figure 7 (a) is a pre-pressing device, Figure 7 (b) is a pre-pulling device, T represents the pulling force of the pre-pulling device.

[0035] Figure 8 The figure shows the principle of the pre-pressing device or the pre-pulling device.

[0036] In the figure: 1, first support column; 2, second support column; 3, steel plate-concrete beam; 31, steel bottom plate; 32, concrete beam; 33, steel connection; 4, steel cross beam; 5A, pre-pressing device; 5B, pre-pulling device; 51, spring; 52, first middle chord; 52a, external thread; 52b, sliding block; 53, first flange; 53a, counterbore; 53b, first sliding groove; 53c, third screw hole; 53d, ear ring; 54, second flange; 54a, first screw hole; 54b, fourth screw hole; 6, double-layer steel truss beam; 61, upper web member; 62, lower web member; 63, second middle chord; 64, type II fixed steel plate; 64a, second screw hole; 65, type I fixed steel plate; 65a, second sliding groove. DETAILED DESCRIPTION

[0037] The technical solutions in the embodiments of the present application will be clearly and completely described in combination with the accompanying drawings in the embodiments of the present application.

[0038] In the description of the present application, it should be noted that the terms "upper", "lower", "outer", "one end", "the other end" and the like indicate the orientation or positional relationship shown in the drawings, and are only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the devices or elements referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as a limitation on the present application. In addition, the terms "first", "second" and the like are only for the purpose of description and cannot be understood as indicating or implying relative importance.

[0039] It should be noted that, under normal circumstances, the bending moment of the steel-concrete beam is manifested as a positive bending moment at the midspan and a negative bending moment at the side span, which is referred to as "positive bending moment action" in the present application. The purpose of the present application is to pre-apply a negative bending moment in the positive bending moment zone of the steel-concrete beam and to pre-apply a positive bending moment in the negative bending moment zone, which is referred to as "reverse bending moment action" in the present application, i.e. the pre-applied bending moment action is opposite to the normal load state. When the composite beam is installed, the pre-applied negative bending moment offsets part of the positive bending moment, and the pre-applied positive bending moment offsets part of the negative bending moment, thereby reducing the deflection of the steel-concrete beam or increasing the bearing capacity of the steel-concrete beam.

[0040] Example 1;

[0041] Please refer to Figures 1-8The application provides a steel-concrete-steel truss composite beam structure with pre-applied reverse bending moment, comprising support columns, double-layer steel truss beams 6 and steel-concrete beams arranged in sequence from bottom to top between adjacent support columns, and the top of the double-layer steel truss beams 6 is connected with the steel plate at the bottom of the steel-concrete beam; pre-compression devices 5A or pre-tension devices 5B are arranged between several groups of adjacent nodes in the interlayer plane of the double-layer steel truss beams 6, wherein the pre-compression devices 5A are located in the negative bending moment area of the steel-concrete beam, and the pre-tension devices 5B are located in the positive bending moment area of the steel-concrete beam; after installation, the pre-compression devices 5A apply a pushing force to the adjacent nodes, and the pre-tension devices 5B apply a pulling force to the adjacent nodes, and the pushing force and the pulling force are transmitted to the steel-concrete beam through the truss, so that the steel-concrete beam generates a pre-applied positive bending moment in the negative bending moment area and a pre-applied negative bending moment in the positive bending moment area.

[0042] The steel-concrete beam adopts a steel-concrete beam or a steel plate-concrete beam 3. The steel plate or the steel plate is located at the bottom of the concrete beam (i.e., a reinforced concrete beam) and is integrally poured or prefabricated with the concrete beam. The steel in the steel-concrete beam can be one of an I-beam, a channel steel, an angle steel, a T-shaped steel and an H-shaped steel, and the steel plate in the steel plate-concrete beam 3 can be one of a flat steel plate, a corrugated steel plate and an inverted T-shaped steel plate. In the steel-concrete beam, the connection between the steel plate and the truss can adopt welding or bolt connection. For the steel, welding is preferred. The concrete in the steel-concrete beam can adopt ordinary concrete or be specifically set according to other requirements, for example, in order to control the development of cracks, fiber cement-based composite concrete can be used in the negative bending moment area, and ordinary concrete or high-strength concrete can be poured in the midspan area.

[0043] Next, the steel plate-concrete beam 3 is taken as an example to describe the steel-concrete-steel truss composite beam structure with pre-applied reverse bending moment in detail. Please refer to Figure 2 The steel plate-concrete beam 3 comprises a steel bottom plate 31, a concrete beam 32 at the top and a steel-concrete connecting piece 33, the lower end of the steel-concrete connecting piece 33 is welded to the steel bottom plate 31, and the upper end is embedded in the concrete beam 32.

[0044] For convenience of description, as Figure 1 The support column on the left side of the composite beam is marked as the first support column 1, and the support column on the right side of the composite beam is marked as the second support column 2.

[0045] The pre-pressing device 5A or the pre-pulling device 5B comprises a first middle chord 52, one end of the first middle chord 52 is fixedly installed on one node in the interlayer plane of the double-layer steel truss beam 6 through a first flange 53, the other end of the first middle chord 52 is installed on another adjacent node in the interlayer plane of the double-layer steel truss beam 6 through a second flange 54, and the other end of the first middle chord 52 can slide relative to the other node in the axial direction of the first middle chord 52; a spring 51 is sleeved on the first middle chord 52, and both ends of the spring 51 are fixedly installed on the first flange 53 and the second flange 54 respectively; for the pre-pressing device 5A, the spring 51 is a high-strength spring that is pre-pressurized, and for the pre-pulling device 5B, the spring 51 is a high-strength spring that is pre-tensioned. The first middle chord 52 is made of round steel or steel pipe.

[0046] The pre-pressing device 5A and the pre-pulling device 5B have the same structure, and the difference is that, before the upper load acts (i.e. when the pre-stress is applied), the spring 51 in the pre-pressing device 5A is under compression, and the spring 51 in the pre-pulling device 5B is under tension; after the upper load acts, both springs 51 are under tension (the bending moment generated by the upper load is greater than the pre-applied reverse bending moment). The elastic coefficients of the two springs 51, as well as the pre-pressing force and the pre-tensioning force, are determined by design according to the size of the pre-applied bending moment required in the positive and negative bending moment regions.

[0047] The double-layer steel truss beam 6 comprises an upper truss and a lower truss, the lower truss comprises the steel cross beam 4 installed between adjacent support columns (the first support column 1 and the second support column 2) and a plurality of lower web members 62 installed in a zigzag shape on the steel cross beam (i.e. the first and last lower web members 62 are connected end to end), the plurality of lower web members 62 form a plurality of nodes in the interlayer plane of the double-layer steel truss beam 6, a plurality of upper web members 61 are installed in a zigzag shape between the plurality of nodes and the steel plate-concrete beam 3 (i.e. the first and last upper web members 61 are connected end to end), forming the upper truss; in addition to the pre-pressing device 5A or the pre-pulling device 5B installed between some adjacent nodes, a second middle chord 63 is also fixedly installed between other adjacent nodes; the second middle chord 63 belongs to both the upper truss and the lower truss. The upper web member 61 and the lower web member 62 are made of one of H-shaped steel, square steel pipe, round steel pipe, channel steel and double angle steel, and the second middle chord 63 is made of round steel or steel pipe.

[0048] A fixed steel plate is arranged at the node in the interlayer plane of the double-layer steel truss beam 6, the lower end of the fixed steel plate is connected with the lower web member 62, the upper end of the fixed steel plate is connected with the upper web member 61, and the middle part of the fixed steel plate is connected with the first flange 53 or the second flange 54 or the second middle chord 63. The use of the fixed steel plate is conducive to the installation of the pre-pressing device 5A or the pre-pulling device 5B and ensures the bearing capacity of the double-layer steel truss beam 6. The height of the fixed steel plate should not be too large, and the height thereof should be reduced as much as possible and the thickness thereof should be appropriately selected under the premise of meeting the installation of the web members and the flanges.

[0049] One end of the first middle chord 52 is provided with an external thread 52a, and the other end is provided with a sliding block 52b which has a larger diameter than the first middle chord 52. The first flange 53 is provided with a counterbore 53a which is adapted to (preferably, clearance fit) the first middle chord 52 on the side close to the spring 51, and is provided with a first sliding slot 53b which is adapted to (preferably, clearance fit) the sliding block 52b on the side away from the spring 51, and the counterbore 53a is in communication with the first sliding slot 53b. The first flange 53 is provided with a first screw hole 54a which is adapted to the external thread 52a of the first middle chord 52. The sliding block 52b has a larger size than the counterbore 53a, so that the sliding block 52b cannot slide out of the counterbore 53a of the first flange 53, and the position of the counterbore 53a of the first flange 53 cannot be damaged by tension. Thus, one end of the first middle chord 52 is detachably fixedly installed with the node, and the other end is detachably slidably installed with the node.

[0050] The steel beam 4 actually corresponds to the lower chord of the lower truss, the first middle chord 52 and the second middle chord 63 correspond to the upper chord of the lower truss, the lower web 62 is connected with the steel beam 4, the first middle chord 52 and the second middle chord 63, forming a complete lower truss structure. The first middle chord 52 and the second middle chord 63 correspond to the lower chord of the upper truss at the same time, the steel plate-concrete beam 3 corresponds to the upper chord of the upper truss, and the upper web 61 is connected with the steel plate-concrete beam 3, the first middle chord 52 and the second middle chord 63, forming a complete upper truss structure. The first middle chord 52 and the second middle chord 63 are common structures of the upper and lower trusses. An interlayer plane (theoretical plane) is formed between the upper and lower trusses, and the upper web 61 and the lower web 62 are connected to form multiple nodes in the interlayer plane. The first middle chord 52 and the second middle chord 63 are installed between adjacent nodes. In order to facilitate the installation of the pre-compression device 5A or the pre-tension device 5B based on the first middle chord 52, a fixed steel plate is arranged as a node component at the node position. The second middle chord 63 is a traditional rod with both ends fixed, and the first middle chord 52 is a rod with one end axially sliding at the node. In order to facilitate the force transmission of the web, the first middle chord 52 and the second middle chord 63 are staggered as much as possible. That is, in the length direction of the composite beam, the pre-compression device 5A or the pre-tension device 5B is staggered with the second middle chord 63. Further, in the negative moment area of the steel plate-concrete beam 3, the pre-compression device 5A is staggered with the second middle chord 63, and in the positive moment area of the steel plate-concrete beam 3, the pre-tension device 5B is staggered with the second middle chord 63.

[0051] As an improved scheme of the embodiment, the fixed steel plate includes a type I fixed steel plate 65 provided with the first flange 53 and a type II fixed steel plate 64 provided with the second flange 54, and the type I fixed steel plate 65 is further provided with a second sliding groove 65a coaxially connected with the first sliding groove 53b, and the type II fixed steel plate 64 is further provided with a second screw hole 64a coaxially connected with the first screw hole 54a. The first screw hole 54a and the second screw hole 64a are connected to form a longer screw hole, the screw hole for fixing the first middle chord 52 is lengthened, the bearing capacity of the node can be improved, the first sliding groove 53b and the second sliding groove 65a are connected to form a longer sliding groove, the anti-seismic deformation capacity of the double-layer steel truss beam 6 structure can be improved, and the first sliding groove 53b on the first flange 53 and the first screw hole 54a on the second flange 54 are prevented from being too short, or the first flange 53 and the second flange 54 are prevented from being too thick to increase the weight of the truss.

[0052] As an improved scheme of the embodiment, the first sliding groove 53b and the second sliding groove 65a are further provided with a rubber buffer pad, the first sliding groove 53b and the second sliding groove 65a are prevented from excessive friction with the sliding block 52b, and the effect of the pre-pressing device 5A or the pre-tensioning device 5B is affected. In addition, when the upper load is heavy, the first flange 53 and the type I fixed steel plate 65 can be welded, and the second flange 54 and the type II fixed steel plate 64 can be welded, so as to further strengthen the connection between the first middle chord 52 and the node.

[0053] The first flange 53 is further provided with a plurality of third screw holes 53c, and the first flange 53 is fixedly connected to the type I fixed steel plate 65 through high-strength bolts; the second flange 54 is further provided with a plurality of fourth screw holes 54b, and the second flange 54 is fixedly connected to the type II fixed steel plate 64 through high-strength bolts. The first flange 53 and the second flange 54 can be made of Q355, Q460 high-strength steel or 12Cr1MoV, 16Mn alloy steel and the like. The first flange 53 and the second flange 54 can meet the tensile requirements at the screw hole or counterbore position.

[0054] In the pre-pressing device 5A or the pre-tensioning device 5B, the outer periphery of the spring 51 is further provided with a sleeve for protecting the spring from corrosion; the sleeve is made of two half-round steel pipes connected through bolts.

[0055] As shown in FIGS. 1, 2 and 3, Figure 3 As shown in FIG. 2, the first middle chord 52 is not directly connected with the upper web member 61 and the lower web member 62, the member does not participate in force in the initial state, only bears axial force when a large deformation occurs and is pressed with the sliding groove end, and is mainly used for preventing the spring 51 from being laterally buckled and axially stretched and damaged. The spring 51 transmits force to the upper web member 61 and the lower web member 62 through the first flange 53, the second flange 54, the type II fixed steel plate 64 and the type I fixed steel plate 65.

[0056] As an improvement to this embodiment, the high-strength spring in the preloading device 5A can be replaced with a nitrogen spring. For example, a nitrogen spring with a nominal diameter of 200mm can stably provide a rated preload of 48t, thereby enabling the preloading device 5A and the pretensioning device 5B in the preloaded steel-concrete composite beam structure to achieve the pre-designed anti-arch shape.

[0057] The steel-concrete composite beam structure with steel truss in this embodiment has the following advantages:

[0058] 1) The present invention employs a preloading device 5A and a pretensioning device 5B, which apply an upward force to the upper crossbeam with the help of the web members, while also playing a supporting role, thereby improving the load-bearing capacity of the beam, meeting the requirements of larger spans and higher loads, and improving the traditional truss system by reducing the overall height of the beam.

[0059] 2) The preloading device 5A can generate an anti-arching effect in the steel plate-concrete beam 3, and will apply a pre-added positive bending moment in the negative bending moment region of the steel plate-concrete beam 3, thereby effectively weakening the unfavorable bending moment distribution during the service stage.

[0060] 3) The pre-tensioning device 5B can generate an upward pre-added vertical force at mid-span, which will generate a pre-added negative bending moment in the positive bending moment region of the steel-concrete beam. This stress state will further pre-apply a reverse bending moment to the steel-concrete beam and enhance the crack resistance and load-bearing capacity of the steel-concrete beam structure by resisting the external load through the pre-added vertical force.

[0061] 4) such as Figure 4 As shown in (a) and 4(b), under seismic load, the spring 51 in the preloading device 5A or pretensioning device 5B absorbs most of the energy generated by the dynamic load due to its elastic deformation capacity. The slider 52b at the end of the first middle chord 52 can slide in the groove, deforming in coordination with the spring 51 and also serving as a damping device. When a large displacement occurs, the rubber buffer layer inside the groove is compressed by the slider 52b, forming an energy-absorbing device together with the spring 51. When the load increases further, the slider 52b engages with the groove, forming a rigid limit to prevent excessive deformation of the structure and ensure seismic toughness.

[0062] 5) One end of the first middle chord 52 is in a sliding state relative to the node. On the one hand, when the preloading device 5A and the pretensioning device 5B apply force, it can be effectively transmitted to the upper web member 61 and the lower web member 62, so as to efficiently realize the application of preloaded vertical force and reverse bending moment. On the other hand, it can effectively avoid bearing large-scale alternating tensile and compressive stress under cyclic dynamic load, reduce its fatigue failure risk, and improve the stability and durability of the overall beam structure.

[0063] 6) Under the action of heavy moving load or seismic load, the beam body deflection makes the pre-pressing device 5A and the pre-tensioning device 5B stretch to participate in force; after the load action ends, the spring 51 generates a restoring force to pull back the upper and lower web plates 61 and 62, and the web plate pushes up the steel concrete beam upward in the resetting process, thereby recovering the deflection deflection, realizing the active rebound of the deflection deformation, increasing the overall elastic deformation capacity and self-resetting performance of the beam, and enabling the overall structure to have a "self-repairing" ability.

[0064] 7) The pre-pressing force or pre-tensioning force is convenient to apply: the pre-stress can be introduced through the spring 51, and the pre-tensioning force size can be controlled by adjusting the installation angle of the web plate; in the later maintenance or reinforcement, the pre-stress can also be quickly re-applied or adjusted, and good adjustability and maintainability are achieved.

[0065] 8) The entire device has the characteristics of being detachable and recyclable: only the spring 51 and the flange need to be removed to realize the disintegration of the structure, and the components can be reused, which meets the green construction and sustainable development concept.

[0066] The main technical idea of the application is to apply upward pre-stress to the steel concrete beam located on the upper side of the double-layer steel truss beam through a high-strength spring, thereby pre-applying a reverse arch effect, that is, a positive bending moment in the negative bending moment area (side span) and a negative bending moment in the positive bending moment area (midspan); under the action of heavy moving load or seismic load, the beam body deflection makes the spring device stretch to participate in force; after the load action ends, the high-strength spring generates a restoring force to pull back the web plate, and the web plate pushes up the steel concrete beam upward in the resetting process, thereby recovering the deflection deflection under the constant load, realizing the active rebound of the deflection deformation, increasing the overall elastic deformation capacity and self-resetting performance of the beam, and enabling the overall structure to have a "self-repairing" ability.

[0067] The composite beam of the application is essentially a structure form for reinforcing the span beam by using an external pre-stress method, which is an improvement based on the traditional external pre-stress method. Compared with the traditional external pre-stress method which applies pre-stress to the structure through pre-stressed cables or pre-stressed steel bars to generate a reverse bending moment, the application introduces a pre-pressing or pre-tensioning spring device from the perspective of external force to pre-apply a reverse load, that is, a vertical load and a reverse bending moment, thereby offsetting part of the upper load, improving the stress behavior of the structure, and improving the stability and carrying capacity of the structure. In addition, in view of the problem of insufficient ductility of the traditional pre-stress technology, the application provides an innovative external pre-stress application technology scheme, which improves the overall elastic capacity and recoverable deformation capacity of the structure, and improves the bending resistance and vibration resistance of the structure. This improvement enables the application to exhibit more excellent performance in dealing with complex working conditions and dynamic loads.

[0068] The present application is different from a steel-concrete composite truss beam, which is composed of a concrete or pure steel top plate, a steel web plate, a concrete bottom plate and internal and external prestressed beams, and the steel-concrete-steel truss composite beam structure in the present application is composed of a lower double-layer steel truss beam and an upper steel-concrete beam, the top of the double-layer steel truss beam is fixedly connected with the bottom of the steel-concrete beam, and a pre-pressing device or a pre-pulling device is arranged between a plurality of groups of adjacent nodes in the interlayer plane of the double-layer steel truss beam.

[0069] The present application is different from a steel-concrete composite truss beam, which is composed of a concrete or pure steel top plate, a steel web plate, a concrete bottom plate and internal and external prestressed beams, and the steel-concrete-steel truss composite beam structure in the present application is composed of a lower double-layer steel truss beam and an upper steel-concrete beam, the top of the double-layer steel truss beam is fixedly connected with the bottom of the steel-concrete beam, and a pre-pressing device or a pre-pulling device is arranged between a plurality of groups of adjacent nodes in the interlayer plane of the double-layer steel truss beam.

[0070] Embodiment 2;

[0071] Based on Embodiment 1, the present embodiment provides a construction method of the steel-concrete-steel truss composite beam structure with pre-applied reverse bending moment, which comprises the following steps:

[0072] 1) installing support columns;

[0073] 2) constructing a steel-concrete beam between adjacent support columns;

[0074] 3) installing a double-layer steel truss beam below the steel-concrete beam between adjacent support columns; at the same time, installing a pre-pressing device 5A between a plurality of groups of adjacent nodes in the interlayer plane of the double-layer steel truss beam and corresponding to the positive bending moment area of the steel-concrete beam, and installing a pre-pulling device 5B between a plurality of groups of adjacent nodes corresponding to the negative bending moment area of the steel-concrete beam.

[0075] Taking a steel plate-concrete beam 3 and site pouring as an example, the construction process is further described as follows:

[0076] 1) installing first and second support columns 1 and 2;

[0077] 2) installing a steel bottom plate 31 between the first and second support columns 1 and 2;

[0078] 3) installing upper web plates 61 in a zigzag shape (connected at the head and tail) between the steel bottom plate 31 and the truss nodes;

[0079] 4) erecting a formwork above the steel bottom plate 31, binding steel bars, pouring concrete, and obtaining a steel plate-concrete beam 3;

[0080] 5) Install steel cross beam 4 between first support column 1 and second support column 2, install several lower abdominal bars 62 in zigzag shape above steel cross beam 4 (connected at the ends), the bottom of lower abdominal bar 62 is welded to steel cross beam 4, the upper part of lower abdominal bar 62 is connected through fixed steel plate to form multiple nodes;

[0081] 6) After installing one node (from left to right or from right to left), install second mid-chord bar 63 or pre-compression device 5A or pre-tension device 5B between the current node and the next node to be installed, and then install the fixed steel plate of the next node;

[0082] Pre-compression device 5A is located in the negative moment area (side span) of steel plate-concrete beam 3, and pre-tension device 5B is located in the positive moment area (midspan) of steel plate-concrete beam 3; in the side span area, second mid-chord bar 63 is arranged alternately with pre-compression device 5A; in the midspan area, second mid-chord bar 63 is arranged alternately with pre-tension device 5B.

[0083] The schematic diagram of first mid-chord bar 52 is shown in Figure 3 (d), both sides of first mid-chord bar have threads, which are connected with type II fixed steel plate 64 and threaded second flange 54 through external threads 52a, and the other side is connected with sliding block 52b; first mid-chord bar 52 does not participate in initial stress, so that the pre-tensioning / compression force generated by spring 51 can be transmitted to upper abdominal bar 61 and lower abdominal bar 62; when bearing a larger load, sliding block 52b at the right end of first mid-chord bar 52 abuts against first sliding groove 53b to bear force cooperatively; the main role of first mid-chord bar 52 in normal times is to prevent out-of-plane buckling of spring 51.

[0084] The three-dimensional schematic diagram of first flange 53 is shown in Figure 5 (b), the front view and sectional view of first flange 53 are shown in Figure 6 (b), the three-dimensional schematic diagram of second flange 54 is shown in Figure 5 (a), the front view and sectional view of second flange 54 are shown in Figure 6 (a). Spring 51 is fixed at both ends of first flange 53 and second flange 54, first flange 53 and second flange 54 are fixed and installed on type II fixed steel plate 64 and type I fixed steel plate 65 through high-strength bolts, and type II fixed steel plate 64 and type I fixed steel plate 65 also fix pre-compression device 5A and pre-tension device 5B. Second flange 54 is provided with first screw hole 54a to connect first mid-chord bar 52; first flange 53 is slightly thick, provided with ear 53d for tensioning spring, one side is provided with first sliding groove 53b to provide sliding space for sliding block 52b of first flange 53; the other side is provided with counterbore 53a in communication with first sliding groove 53b, and counterbore 53a is used for passing through first mid-chord bar 52 and limiting sliding block from sliding out.

[0085] As shown in Figure 4(a), (b) shown, type II fixed steel plate 64 is provided with a first screw hole 54a, the first screw hole 54a is provided with internal thread, to connect the first middle chord 52; Type I fixed steel plate 65 has a second sliding groove 65a, to give the first middle chord 52 sliding block 52b has greater sliding space.

[0086] The first sliding groove 53b of the first flange 53 and the second sliding groove 65a of the type I fixed steel plate 65 have a rubber buffer layer at the bottom.

[0087] The spring 51 is preferably selected from heavy spiral spring, high strength disc spring and other high strength springs; the length of the spring 51 is longer than the first middle chord 52 in the compression section and shorter than the first middle chord 52 in the tension section.

[0088] Please refer to Figure 4 (a) and (b), the specific installation process of the pre-pressing device 5A is:

[0089] 1) The first middle chord 52 is sequentially inserted into the spring 51, the second flange 54 from the left side, and the second flange 54 is tightened, and the protruding part is screwed into the type II fixed steel plate 64 and fastened;

[0090] 2) The second flange 54 is provided with a fourth screw hole 54b, and the second flange 54 is fixed with the type II fixed steel plate 64 through the fourth screw hole 54b of the second flange 54;

[0091] 3) Spring pre-pressing: a pressurizing steel plate is sleeved near the first flange 53 in the inner cavity of the spring 51, and a hollow jack is used to compress the spring through the steel plate to the designed pre-pressing amount ΔL+2mm, and the oil pressure state is maintained;

[0092] 4) Maintain oil pressure, install the sliding block 52b to the exposed threaded section of the right end of the first middle chord 52;

[0093] 5) High-strength bolts are used to fix the first flange 53 and the type I fixed steel plate 65;

[0094] 6) Slowly release the pressure, remove the pressurizing steel plate and the hollow jack, then surround the edges of the first flange 53 and the type I fixed steel plate 65, the second flange 54 and the type II fixed steel plate 64 with welding reinforcement, and weld the lower abdominal rod 62 on the type I fixed steel plate 65 and the type II fixed steel plate 64;

[0095] 7) Finally, install the outer sleeve on the outside of the spring 51.

[0096] Please refer to Figure 4 (a) and (b), the specific installation process of the pre-pressing device 5A is:

[0097] 1) The first middle chord 52 is sequentially inserted into the spring 51, the second flange 54 from the left side, and the second flange 54 is tightened, and the protruding part is screwed into the type II fixed steel plate 64 and fastened;

[0098] 2) Through the fourth threaded hole 54b of the second flange 54, a high-strength bolt is used to fix it with the type II fixed steel plate 64;

[0099] 3) The right end of the first middle chord 52 is inserted into the counterbore 53a of the first flange 53 and extends into the first sliding groove 53b, and the sliding block 52b is connected to the right end of the first middle chord 52, so that the sliding block 52b is located in the first sliding groove 53b;

[0100] 4) Spring pre-tensioning: use a hand plate hoist to stretch the spring 51 to a design length ΔL+2mm through the ear ring 53d of the first flange 53;

[0101] 5) Maintain the hand hoist, connect the first flange 53 with the type I fixed steel plate 65 through a high-strength bolt;

[0102] 6) The edges of the first flange 53 and the type I fixed steel plate 65, the second flange 54 and the type II fixed steel plate 64 are surrounded by welding and reinforcement, and the lower abdominal rod 62 is welded on the type I fixed steel plate 65 and the type II fixed steel plate 64, and the hand hoist is removed;

[0103] 7) Finally, install the outer sleeve on the outside of the spring 51.

[0104] Example 3;

[0105] In this embodiment, the pre-compression device 5A and the pre-tensioning device 5B are made of shape memory alloy (SMA), and the two ends of the transverse connecting rod are also fixedly connected with the type II fixed steel plate 64 and the type I fixed steel plate 65. By applying a pre-compression force or a pre-tensioning force to the SMA transverse connecting rod, the combined beam structure can also be pre-loaded in a reverse arch stress state, and the subsequent stress and deformation requirements of the combined beam structure can also be met.

[0106] At this time, the construction process of the pre-compression device 5A and the pre-tensioning device 5B is as follows:

[0107] 1) Compress or tension the SMA transverse connecting rod to a preset deformation amount;

[0108] 2) Anchor the two ends of the SMA transverse connecting rod to the type II fixed steel plate 64 and the type I fixed steel plate 65;

[0109] 3) Activate the "shape memory effect" of the SMA through power heating or natural environmental heating;

[0110] 4) The SMA material generates a strong contraction force to return to the original length, and then applies a preset prestress to the combined beam structure.

[0111] In addition, it also needs to be explained that, in order to provide greater pre-load for the composite beam structure, multiple pre-pressing devices 5A and pre-pulling devices 5B can be arranged in parallel, that is, multiple pre-pressing devices 5A and pre-pulling devices 5B are arranged side by side in the direction perpendicular to the length direction of the composite beam structure, and the axis direction of the pre-pressing devices 5A and the pre-pulling devices 5B is the length direction of the composite beam structure, and the specific structure of the double-layer steel truss beam can be adjusted according to the arrangement requirement of the pre-pressing devices 5A and the pre-pulling devices 5B; or, by arranging multiple steel-concrete-steel truss composite beams, the parallel configuration of the pre-pressing devices 5A and the pre-pulling devices 5B on each beam is realized, and then the application of greater pre-applied reverse bending moment is realized. Those skilled in the art can adaptively adjust the specific number and arrangement form of the pre-pressing devices 5A and the pre-pulling devices 5B on the basis of the present application.

[0112] The above is only used to illustrate the technical solutions of the present application but not to limit the same, and the modification or replacement of the technical solutions by other technicians in the same field should be covered in the scope of claims of the present application as long as it does not deviate from the connotation of the technical solutions of the present application.

Claims

1. A pre-applied anti-bending moment steel-concrete composite beam structure, comprising supporting columns, characterized in that: A double-layer steel truss beam and a steel-concrete beam are provided between adjacent supporting columns. The top of the double-layer steel truss beam is fixedly connected to the bottom of the steel-concrete beam. A preloading device or a pretensioning device is provided between several groups of adjacent nodes in the interlayer plane of the double-layer steel truss beam. The preloading device is located in the negative bending moment zone of the steel-concrete beam, and the pretensioning device is located in the positive bending moment zone of the steel-concrete beam. The preloading device causes the steel-concrete beam to generate a preloaded positive bending moment in its negative bending moment zone and causes the steel truss to generate a preloaded anti-camber stress state. The pretensioning device causes the steel-concrete beam to generate a preloaded negative bending moment in its positive bending moment zone. The preloading or pretensioning device includes a first center chord, one end of which is fixedly installed on a node in the interlayer plane of the double-layer steel truss beam via a first flange, and the other end of which is installed on an adjacent node in the interlayer plane of the double-layer steel truss beam via a second flange. The other end of the first center chord can slide relative to the other node in the axial direction of the first center chord. A spring is sleeved on the first center chord, and both ends of the spring are fixedly installed on the first flange and the second flange, respectively. For the preloading device, the spring is a high-strength spring that has been preloaded with pressure, and for the pretensioning device, the spring is a high-strength spring that has been preloaded with tension.

2. The pre-applied anti-bending moment steel-concrete composite beam structure according to claim 1, characterized in that: The double-layer steel truss beam includes an upper truss and a lower truss. The lower truss includes steel crossbeams installed between adjacent support columns and a number of lower web members installed in a sawtooth shape on the steel crossbeams. The number of lower web members forms multiple nodes in the interlayer plane of the double-layer steel truss beam. A number of upper web members are installed in a sawtooth shape between the multiple nodes and the steel-concrete beam to form the upper truss. In addition to some adjacent nodes being equipped with preloading or pretensioning devices, a second middle chord is also fixedly installed between other adjacent nodes.

3. The pre-applied anti-bending moment steel-concrete composite beam structure according to claim 2, characterized in that: Along the length of the double-layer steel truss beam, the preloading device or pretensioning device is staggered with the second middle chord.

4. The pre-applied anti-bending moment steel-concrete composite beam structure according to claim 3, characterized in that: A fixed steel plate is provided at the node of the interlayer plane of the double-layer steel truss beam. The lower end of the fixed steel plate is connected to the lower web member, the upper end is connected to the upper web member, and the middle part is connected to the first flange, the second flange, or the second middle chord.

5. The pre-applied anti-bending moment steel-concrete composite beam structure according to claim 4, characterized in that: One end of the first middle chord is provided with an external thread, and the other end is provided with a slider whose size is larger than the diameter of the first middle chord; the first flange is provided with a countersunk hole adapted to the first middle chord on the side near the spring, and a first sliding groove adapted to the slider on the side away from the spring, and the countersunk hole is connected to the first sliding groove; the second flange is provided with a first threaded hole adapted to the external thread on the first middle chord.

6. The pre-applied anti-bending moment steel-concrete composite beam structure according to claim 5, characterized in that: The fixing steel plate includes a type I fixing steel plate with a first flange and a type II fixing steel plate with a second flange. The type I fixing steel plate is also provided with a second slide groove that is coaxially connected to the first slide groove, and the type II fixing steel plate is also provided with a second screw hole that is coaxially connected to the first screw hole.

7. The pre-applied anti-bending moment steel-concrete composite beam structure according to claim 6, characterized in that: In the pre-compression device or pre-tension device, a sleeve is also provided on the outer periphery of the spring.

8. The pre-applied anti-bending moment steel-concrete composite beam structure according to any one of claims 1-7, characterized in that: The steel-concrete composite beams are either steel-concrete beams or steel-plate-concrete beams.

9. The construction method of the pre-applied anti-bending moment steel-concrete composite beam structure according to claim 8, characterized in that... Includes the following steps: 1) Install support columns; 2) Construct steel-concrete beams between adjacent support columns; 3) Install double-layer steel truss beams between adjacent support columns and below the steel-concrete beams; at the same time, install preloading devices between several groups of adjacent nodes corresponding to the positive bending moment zone of the steel-concrete beams in the interlayer plane of the double-layer steel truss beams, and install pre-tensioning devices between several groups of adjacent nodes corresponding to the negative bending moment zone of the steel-concrete beams.

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