Study on Bending Performance of Tightly Spliced Truss-Reinforced Plate-Honeycomb Flat Beam

Abstract

The tightly spliced truss-reinforced plate-honeycomb flat beam is a novel type of composite beam, with the advantages of high rigidity, high bearing capacity, and ease of construction. In this study, on the basis of the performance in bending tests, the numerical analysis method is used to study the influence of the honeycomb hole–height ratio, and section height on the bearing capacity of the honeycomb composite flat beam. On this basis, the simplified method to calculate the ultimate bending capacity of the new honeycomb composite flat beam was proposed. The research results show the failure modes of the specimens are mainly divided into two states, including the deflection exceeding the limit and the concrete flange plate separating from the steel beam. The tightly spliced truss-reinforced plate-honeycomb flat beams have good ductility, of which the average value reaches 8.2. The simplified method proposed in this article for calculating this type of honeycomb composite beam has an error of less than 10% in terms of bending bearing capacity, which has advantages over the double T-shaped steel method. The calculation method and design suggestions proposed in this study provide a basis for the research and application of this type of composite flat beam.

1. Introduction

The steel–concrete composite structure has the outstanding advantages of high strength, convenient construction, and good overall performance, which can ensure the synergistic bearing of concrete and steel under reasonable combination and connection [1]. Among them, the steel–concrete composite beam utilizes the compressive properties of concrete and the tensile properties of steel, which is a high-performance flexural load-bearing component and widely used in floor structures [2]. The traditional steel–concrete composite beam is a T-shaped composite beam formed by placing a concrete slab on the upper flange of a steel beam or a concrete slab on the lower flange of a steel beam to form a composite flat beam, which can usually be divided into two forms of steel sections outside or inside the concrete, as shown in Figure 1. Among them, the steel beam is exposed in the T-shaped composite beam, which requires fireproof and anti-corrosion treatment, and the out-of-plane stability of the steel beam is not guaranteed when its cross-section is high, so extra out-of-plane support is added. In addition, although the T-shaped combination beam has better performance in bearing capacity, the large cross-sectional height of the component causes the overall height of the building to increase, resulting in an increase in construction cost.

In order to reduce the aforementioned defects, the combination flat beam places the precast concrete slab on the lower flange of the steel beam, connects the precast base plate with the steel beam as a whole through the cast-in-place layer, and wraps most of the area of the steel beam with concrete, as shown in Figure 2. Choi [3] proposed a new type of wide and large-span steel–concrete composite beam that can be used to reduce the height of the structure. The experimental results showed that as the steel thickness increased by 3 mm, the bending strength of the wide composite steel beam increased by about 20%. In related studies, Lee [4] conducted a flexural analysis of I-beam laminated plate composite beams and established a computational model for I-beam composite beams under vertical loads based on the theory of shear deformation beams. Yang [5] proposed a new type of composite flat beam composed of a wide flange section and laminated plate. The plane section assumption of the composite beam was verified by two concentrated forces loading on the specimen, and the calculation method of flexural bearing capacity was proposed. Cao [6] proposed a flat steel–concrete composite beam and tested its flexural performance. Through the traditional composite beam calculation theory, a simplified calculation model suitable for the bending stiffness and ultimate bending capacity of the model was proposed. Wang [7] established the analysis theory of steel–concrete composite beams considering interface slip and shear deformation in steel and deduced the calculation formulas of the ultimate flexural capacity of simply supported composite beams under uniform load and concentrated load in mid-span, respectively. Luo [8] studied the shear lag effect of steel–concrete composite beams under bending moment and proposed a method to predict the cracking site effect of steel–concrete composite beams. The effectiveness of the method was verified by experimental data. In summary, it can be seen that the flexural bearing capacity of the composite flat beam is mainly related to the effective plate width, the height of the composite flat beam, the beam spacing, and the thickness of the flange plate. The composite flat beam structure has reasonable structure, excellent bearing performance, and certain cost advantages. However, the integrity of the concrete composite plates on both sides is limited due to the form of the web, which is a certain restriction on the layout of the pipeline.

The honeycomb beam as the steel component in the combined beam provides a strong condition for solving the connection of the composite plate on both sides of the steel beam. Honeycomb beams are treated with holes in the beam webs to enhance the efficiency of steel utilization in the load-bearing process and to reduce the amount of steel used for cost control purposes [9]. The failure mode of composite flat beams is influenced by the combination form, with significant differences in failure modes, and has higher bending stiffness, bending moment bearing capacity, and ductility bearing capacity [6]. In addition, the honeycomb holes in the web of the steel beam can be used to pass through the layout of various pipelines as well as the internal reinforcement of the floor slab, which is conducive to the combined application with concrete slabs. Related studies have shown that the main factors affecting the load bearing of honeycomb beams include the honeycomb cavity rate, the shape of the openings, and the width-to-thickness ratio of the slab [10,11]. Deng [12] investigated the variation of the critical load of local buckling of the web of I-beams with different hole–height ratios and number of holes, with and without transverse stiffening ribs and with and without web openings. On the basis of this, Serror [13] supplemented to investigate the effects of parameters such as width-to-thickness ratio, opening shape, distance from the first opening to the end, hole size, and steel grade on the elastic and inelastic critical buckling stresses of honeycomb beams. Taufiq [14] investigated the optimum diameter and opening spacing of components at different spans. Chang [15] analyzed the effect of opening shape and expansion ratio on honeycomb beams by conducting four-point bending tests with octagonal hole honeycomb beams and studied the rise in the bearing capacity of honeycomb beams as the expansion ratio increased on the surface. Mehetre [16] found that the moment bearing capacity of honeycomb beams with round holes increased by 20.78% and deflection decreased by 11.51% compared to honeycomb beams with hexagonal openings due to avoiding the stress concentration phenomenon at the corners. Velrajkumar [17] investigated the flexural performance of hexagonal honeycomb beams using a combination of experimental and finite element methods, taking into account the effect of the location of the honeycomb hole openings. In addition, there are effects of beam length, section grade, and cross-section size on the buckling ultimate load and buckling mode of honeycomb steel beams [18,19,20]. Du [21] studied the influence of concrete strength on the load deflection, flexural bearing capacity, and ductility of high-speed steel–concrete composite beams. The simplified plastic method overestimated the ultimate flexural strength of high-speed steel–concrete composite beams, while nonlinear methods based on material constitutive models can accurately predict the bearing capacity of materials. In summary, the current research has conducted an in-depth analysis of the expansion ratio, span–height ratio, plate width–thickness ratio, and effective width of concrete flange plate, etc. Numerous experimental studies have been carried out for flexural load capacity, stiffness, buckling of honeycomb beams and their combination beams, and the corresponding design calculation equations for honeycomb beams and their combination beams have been proposed. However, there is limited research on the application of honeycomb beams to concrete composite floors.

The truss steel composited plate is a prefabricated steel truss formed by welding the lower part of the bent steel bars with the upper and lower longitudinal bars in the slab in the laminated layer and then tied to the upper layer of the reinforcing steel and cast-in-place concrete to form a concrete slab with overall stress [22]. Many webs of truss reinforcement are interspersed in the laminated layer and the cast-in-place layer, which facilitates a tight connection between the two layers of concrete. Due to the configuration of truss reinforcement in the precast base slab, the stiffness of the base slab is increased, so that it can bear greater construction loads during construction and reduce the deformation of the slab due to self-weight and construction loads before the hardening of the cast-in-place layer [23,24]. The reinforced truss precast concrete slab is shown in Figure 3. In order to solve the problem of traditional precast composited plates with reinforcement bars protruding from the sides, which affects the production and construction efficiency, the researchers propose to adopt the construction process of composited plates with tightly spliced joints to achieve standardized and automated production of composited plates and efficient construction. Ye [25] studied the flexural performance of composited plates with different splice forms and found that the load-carrying capacity of the plate depends on the splice section, while the splice reinforcement can control the development of cracks along the superimposed surface and improve the shear resistance of the specimen. Ding [26] studied the load-bearing mechanism and flexural performance of the slabs in terms of deformation, load-bearing capacity, and damage mode through static load tests on four types of reinforced concrete floor slabs, which showed that the stiffness of the close-packed composited plate was slightly lower than that of the cast-in-place slab, but the damage pattern was basically the same as that of the cast-in-place slab, and the strength of the close-packed composited plate could be improved by closing the reinforcement ring. The anchoring length of the transverse reinforcement at the bottom of the post-pouring zone has a significant impact on the shear performance of the composite concrete slab, especially when the anchoring length decreases to the critical value, and the ultimate bearing capacity and ductility of the test specimen decrease. In addition, the thickness of the composited plate, the depth of the splice recess, the reinforcement rate of the splice reinforcement, and the lap length of the splice reinforcement have obvious effects on the bearing capacity of the composited plate. The form of load transfer of dense splice is different from that of post-cast strip splice and monolithic cast-in-place slab, but the load-carrying capacity of the splice can be effectively ensured by enhancing additional reinforcement at the splice and encrypting reinforcement trusses so that it presents the force characteristics of the two-way slab.

This study proposes to arrange a truss-reinforced composite plate at the lower part of the upper flange of the honeycomb beam and strengthen the connection between the two through reasonable methods to form a new type of honeycomb composite flat beam structure. As the concrete slab is located within the height range of the steel beam, it maximizes the net height of the floor slab. In addition, laminated plates can limit the out-of-plane deformation of steel beams, thereby obtaining better stability and deformation capacity than ordinary T-shaped composite beams. On the basis of experimental research on the flexural bearing capacity of five composite flat beams, this article conducts numerical simulation analysis. Based on the analysis of failure modes and bearing capacity characteristics, the influence law of the honeycomb web on bearing capacity is established, and a simplified calculation method for this type of composite flat beam is proposed. Finally, design suggestions for honeycomb composite flat beams are proposed.

2. Experimental Programs

2.1. Test Specimens

2.1.1. Honeycomb Beam

The design and dimensional settings of the honeycomb steel beam used in this paper are shown in

Table 1. Dimensions of honeycomb beams.

Specimen Number	Honeycomb Beam Size	Diameter and Spacing of Honeycomb Holes	Span	H-Beam Shear Resistant Parts
DTCB1	200 × 100 × 125 × 6 × 8	Φ100@200	2800	H87 × 50 × 6 × 6
DTCB2	200 × 100 × 125 × 6 × 8	-	2800	H87 × 50 × 6 × 6
DTCB3	200 × 100 × 125 × 6 × 8	Φ120@200	2800	H87 × 50 × 6 × 6
DTCB4	220 × 100 × 125 × 6 × 8	Φ120@200	2800	H107 × 50 × 6 × 6
DTCB5	200 × 100 × 125 × 6 × 8	Φ100@200	2800	H87 × 50 × 6 × 6

, and the units are all in millimeters. The design of the steel beam specimen is shown schematically in Figure 4, and the support of the laminated plate is achieved by welding steel sheets on the webs of the honeycomb beam and the solid web steel beam.

2.1.2. Laminated Panels

The influence of concrete flange on the honeycomb composite flat beam cannot be ignored, not only because it can improve the flexural bearing capacity and stiffness of the honeycomb steel beam, but also has a good supporting effect for the steel beam in the out-of-plane lateral direction. However, the stress in the concrete flange plate gradually decreases with the increase in distance from the steel beam, there is a hysteresis effect, the concrete part away from the steel beam does not play a good role in strengthening the beam, the role of the concrete plate within the effective width should be considered in the design, and the width of the plate flange of the five combined flat beam specimens in this paper are taken as 940 mm.

The top surface of the cast-in-place layer of each honeycomb combined flat beam specimen was reserved 35 mm from the top surface of the upper flange of the steel beam to meet the construction of floor and roofing materials, so as to make full use of the space and reduce the floor height. The concrete of composited plate and cast-in-place layer is C30 fine stone concrete with ribbed reinforcement of HRB400 of 6 mm diameter. The effective flange width of the intercepted composite beam is 940 mm, and the whole span is made up of three laminated plates, of which the width of the central laminated plate and the laminated plate at the two end supports are 1200 mm and 800 mm, respectively, and the truss reinforcement is arranged perpendicular to the steel beam. The laminated layer is 30 mm thick, and the cast-in-place layer is 40 mm thick.

The Y-shaped groove is reserved on the upper surface of the composited plate at a spacing of 200 mm near the beam side for fixing and installing post-insertion ties (tie bars), and the splice joint reinforcement is provided at the splice joint of the composited plate, and the splice joint reinforcement adopts Φ6@200 reinforcement. In addition, in order to prevent the reinforcement from being pulled out during bending, two transverse bars are provided on each side of the splice, as shown in the reinforcement arrangement in Figure 4.

2.1.3. Close-Packed Truss Steel Composited Plate-Combined Honeycomb Flat Beam

The laminated plates on both sides of the honeycomb beam are anchored as a whole by means of tie bars, where specimens DTCB1, DTCB2, DTCB3, and DTCB4 are assembled laminated plates, and DTCB5 is cast-in-place laminated plate. The overall fabrication process of the specimens is shown in Figure 4.

2.2. Material Properties

The steels used in tests were of the grade of Q355, and the grade of the reinforcement was HRB400. The thickness of the honeycomb beam flange and web are 6 mm and 8 mm, and three test pieces are taken for each of the above two plate thicknesses, where the yield strength of 6 mm steel is 351.0 MPa and that of 8 mm steel is 367.6 MPa. In addition, the bars used in this test were all 6 mm diameter HRB400 bars, which had a yield strength of 415.8 MPa and an ultimate strength of 596.6 MPa.

The concrete used in the test was poured in two batches, mainly including the precast part of the truss-reinforced composited plate concrete and the concrete of the cast-in-place layer, both using the strength grade of C30 concrete. The strength of the 100 mm cube specimen cured for 28 days was tested. The standard value of the cube compressive strength of the laminated layer was 39.5 MPa and the standard value of the cube compressive strength of the cast-in-place layer was 39.9 MPa. The modulus of elasticity was 3.20 × 10⁴ MPa.

The main performance parameters of the materials in the composite beam are shown in

Table 2. Material properties of composite beams.

Component	Materials	Yield Strength	Ultimate Tensile Strength	Compressive Strength
Steel for beam 6 mm	Q355	351.0 MPa	507.3 MPa	/
Steel for beam 8 mm	Q355	367.6 MPa	524.4 MPa	/
Steel bar 6 mm	HRB400	415 MPa	596.6 MPa	/
Concrete for slab	C30	/	/	39.9 MPa

.

2.3. Test Set-Up

In this paper, the four-point bending loading was adopted for the specimens of densely spaced truss steel composited plate-combined honeycomb flat beams. A 500 kN jack is used for the test, and the reaction force is measured by setting the load sensor under the jack. The total length of the simply supported beam is 3000 mm, and the distance between the supports is 2800 mm. At 1000 mm from the supports at both ends, the load applied by the jack is applied to the two concentrated loading points through the distribution beam; the supports of the honeycomb flat beam specimen are fixed hinged supports at one end and movable hinged supports at the other end. The equipment used in the test cloth and its layout is shown in Figure 5.

The test loading system adopts the form of force-controlled graded loading, and the load applied to the specimen of dense truss steel laminated plate-honeycomb composite flat beam is 10 kN at each level. To ensure that the applied load is fully transferred to the specimen, the load is loaded for 5 min at each level, the load is held for 3 min, and the deformation and damage of the specimen are read and observed at the end of the load holding. When the specimen is close to the yield load or the first crack starts to appear at the bottom of the concrete flange plate, the loading rate is reduced, and the load is taken at half level until the specimen is damaged. The loading rate is reduced when the first crack begins to appear at the bottom of the concrete flange plate, and the loading is taken at half level until the specimen is damaged.

The measurement items of the test include the test load, the displacements at the support and loading points of the honeycomb composite flat beam and the medium part of the span, the vertical stress–strain distribution along the section of the purely bending section of the steel beam, the equivalent force distribution around the first hole of the bending and shearing section, the strain distribution in the compression zone of the concrete flange slab, the tensile strain distribution of the reinforcement in the splice of the composited plate, and the crack development during the stressing process of the concrete slab. The arrangement of displacement gauges and strain gauges is shown in Figure 6.

Among them, the specific arrangement of strain gauges is as follows:

(1)

One strain gauge is arranged on the upper flange and lower flange of the purely bending section of the specimen to detect the stress–strain of the upper and lower flange of the steel beam;

(2)

Three resistance strain gauges were arranged at the pier of the purely curved section of the honeycomb beam at a height spacing of 80 mm along the beam cross-section to check the development of vertical strain along the beam cross-section in the purely curved section;

(3)

Four resistance strain flowers were arranged around the first honeycomb hole in the bending and shearing section, i.e., 45° and 135° directions, to observe the change in equivalent force in the hole during the loading process;

(4)

Three strain gauges are set at 180 mm spacing transversely on the upper surface of the concrete slab to detect the compressive strain distribution on the upper surface of the concrete slab;

(5)

Strain gauges are installed at the joint of the composited plate to detect the development and distribution of tensile strains in the joint reinforcement at the joint of the composited plate.

3. Test Results and Discussions

3.1. Failure Mode

3.1.1. DTCB1

The specimen DTCB1 is in the elastic stage in the early stage, and the stiffness of the honeycomb combined flat beam specimen is large, and the deflection increases uniformly and slowly with the increase in the load, which is linear. When the load was loaded to 30 kN, cracks started to appear in the concrete below the bottom part of the concrete flange plate, and the location of the cracks was the contact surface between the stiffening ribs or the shear connection keys of the H-beam and the concrete, but no cracks appeared in the concrete flange plate at this time. When the load was loaded to 90 kN, cracks appeared for the first time at the bottom of the concrete flange slab in the middle of the span, which was the bottom of the composited plate, at this time there were no cracks or pulling bad at the joint, and the deflection of the specimen was 7.15 mm. After that, the specimen was loaded according to 5 kN per level, and new small cracks appeared under the concrete and the bottom of the concrete flange slab in the pure bending section one after another, but the size of the cracks developed slowly. When the load was loaded to 110 kN, cracks appeared for the first time on the north side of the specimen at the joint, and at this time, seven cracks had been produced at the bottom of the concrete flange in the span and extended upward along the side of the slab to the intersection of the laminated layer and the cast-in-place layer. The widest crack at the bottom of the slab was 0.06 mm, and the crack at the joint was 0.02 mm wide. When the load is loaded to 140 kN, transverse short cracks developed from the end of the crack at the bottom of the slab start to appear at the interface between the laminated layer and the cast-in-place layer on the side of the concrete flange slab, indicating that the shear force between the laminated layer and the cast-in-place layer increases gradually at this time, and small cracks appear in the places where the connection is not tight or there is no truss reinforcement. When the load is loaded to 160 kN, the span deflection of the specimen is 15.96 mm and the maximum crack width at the bottom of the concrete flange plate is 0.37 mm. After that, the development of deflection of the specimen is obviously faster and the crack development of the concrete under the plate and the concrete flange plate is obviously accelerated. When the load reached 195 kN, no new cracks appeared at the bottom of the concrete flange slab and at the joint, but the crack width developed faster, the widest being 1.16 mm. The crack at the bottom of the concrete flange slab crossed the interface between the laminated layer and the cast-in-place layer and extended further upward to the range of the cast-in-place layer. Although there were local short transverse cracks between the laminated plate and the cast-in-place layer, these short transverse cracks were not connected, and the two concrete slabs were not obviously misaligned and slipping. When the load was loaded to 215 kN, the span deflection of the specimen reached 141.05 mm (already more than 1/20 of the span), the specimen was completely damaged, the maximum crack width at the bottom of the span was 2.84 mm, and the crack width at the spandrel was 0.82 mm. The damage pattern of the partial loading stage of specimen DTCB1 is shown in Figure 7 below.

3.1.2. DTCB2

The specimen DTCB2 was in the elastic stage in the early stage, the specimen stiffness was large, and with the load increase the specimen deflection was uniform and slowly growing. When the load is loaded to 30 kN, the first small crack begins to appear on the contact surface of the shear connection key of the H-beam and concrete. At this time, the crack width develops slowly, the number gradually increases and is mostly concentrated in the pure bending section between the loading points. When the load was loaded to 100 kN, the first crack appeared at the bottom of the concrete flange plate in the span of the specimen, with a crack width of 0.04 mm, which penetrated along the width direction of the concrete flange plate, after which the loading level was changed to 5 kN per level for loading. When the load reached 110 kN, cracks appeared at both sides of the north and south joints at the same time, and at this time two cracks appeared at the bottom of the concrete flange plate in the span, and the maximum width of the cracks was still 0.04 mm. When the load reached 130 kN, the number of cracks in the span of the concrete flange plate increased rapidly and began to develop upward along the flange plate sides and reached the intersection of the laminated layer and the cast-in-place layer, but at this time there were no horizontal cracks along the intersection horizontal cracks along the intersection. When the load reaches 170 kN, the development of deflection of the specimen is obviously faster, and the crack width of the concrete under the slab and the concrete at the bottom of the slab develops rapidly, the maximum crack width at the bottom of the slab is 0.40 mm, and the span deflection of the specimen is 14.73 mm. When the load reaches 190 kN, a small horizontal crack appears at the intersection of the laminated layer and the cast-in-place layer, and the crack starts to develop from the end of the crack at the bottom of the composited plate, which is short and the crack is short and discontinuous, which means that the laminated layer and cast-in-place layer are still closely combined and in a common state of stress. When the load reached 210 kN, the concrete emitted a slight sound and the cracks on the side of the slab developed further upward and extended to the cast-in-place layer, and there was no obvious dislocation and slippage between the composited plate and the cast-in-place layer. The width of the cracks in the span developed rapidly, and the maximum width was 0.59 mm, and the specimen was obviously bent but no out-of-plane instability or local instability of the slab was found. When the load reached 220 kN, the span deflection of the specimen was 115.71 mm. The concrete slab made a thumping sound and the loading was stopped immediately, a large penetration crack appeared at the contact surface between the bottom of the composited plate and there was a shear connection key of the H-beam on the east side of the cellular combination flat beam span, a more obvious diagonal crack appeared in the concrete under the slab at the loading point, and a wider crack appeared along the web direction at the contact surface between the top surface of the cast-in-place concrete layer and the web of the steel beam. At this point, it can be considered that the middle-composited plate area of the concrete flange has failed to connect with the steel beam, and there is a relative slip between the two, but the top concrete is not crushed. The damage pattern of specimen DTCB2 in the partial loading stage is shown in Figure 8 below.

3.1.3. DTCB3

The specimen DTCB3 was in the elastic stage in the early stage with large initial stiffness, and the specimen deflection grew slowly with the increase in load. When the load was loaded to 30 kN, cracks appeared in the concrete under the slab at the junction of the span stiffening ribs and the shear connection keys of the H-beam with the concrete. In the elastic phase after that, the number of cracks in the concrete under the slab increased rapidly and was concentrated in the purely bending section between the loading points, but the crack width did not increase significantly. When the load reached 90 kN, cracks appeared for the first time at the bottom of the concrete flange plate in the span of the honeycomb composite flat beam specimen with a crack width of 0.04 mm, which penetrated along the width direction of the concrete flange plate, after which the loading level was changed to 5 kN per level. When the load reached 110 kN, cracks appeared at the north side of the splice, which were small and did not pass along the width of the plate; at this time, four cracks had appeared at the bottom of the concrete flange in the span, the maximum crack width was 0.12 mm, and the deflection of the specimen in the span was 10.37 mm. When the load reached 130 kN, the crack at the bottom of the slab began to develop along the concrete flange plate and reached the intersection of the laminated layer and the cast-in-place layer, but there was no horizontal crack along the intersection, indicating that the concrete connection between the cast-in-place layer and the laminated layer was good at this time.

When the load reached 150 kN, the specimen deflection developed significantly faster, and the span deflection was 17.42 mm, and the concrete crack under the slab and the concrete crack at the bottom of the slab developed rapidly, and the maximum crack width at the bottom of the slab was 0.34 mm. A number of short cracks along the intersection direction began to appear at the interface between the concrete laminated layer and the cast-in-place layer, but they were not continuous, and most of them started to develop at the end of the crack at the bottom of the composited plate. When the load reaches 180 kN, the crack at the bottom of the slab continues to develop upward, crossing the interface between the laminated layer and the cast-in-place layer and developing toward the cast-in-place layer. The cracks on the side of the slab show “Y”-shaped development and are mostly concentrated in the area of the pure bending section in the span. When the load is loaded to 200 kN, the span deflection of the specimen reaches 143.01 mm (more than 1/20 of the span of the combined flat beam), and the specimen is declared damaged and loaded. The maximum crack width at the bottom of the span was 1.90 mm, and the crack width at the spandrel was 1.33 mm, but the number of cracks at the bottom of the concrete flange of the specimen was less than that of specimen DTCB1. The concrete at the top of the concrete flange did not appear to be crushed. The damage pattern of specimen DTCB3 in the partial loading stage is shown in Figure 9 below.

3.1.4. DTCB4

Specimen DTCB4 has a large stiffness in the early stage, and the change in displacement is not obvious with the increase in load, and it is in the elastic stage. When the load was loaded to 30 kN, the first crack appeared in the concrete under the span area, and with the increase in load, the number of cracks gradually increased while the width remained basically the same, and it was concentrated in the pure bending section between the loading points. When the load reached 100 kN, cracks appeared for the first time at the bottom of the concrete flange plate in the span, and cracks also appeared at the north side of the spar near the outer edge of the plate, but at this time the cracks at the spar were short and did not penetrate the whole width of the plate. After that, the loading level was changed to 5 kN per level, and new small cracks appeared in the concrete under the slab and at the bottom of the concrete flange in the pure bending section, but the cracks developed slowly. When the load reached 120 kN, the span deflection of the specimen was 8.06 mm, the crack at the bottom of the concrete flange plate started to develop upward along the side of the plate and reached the intersection of the laminated plate and the cast-in-place plate, and the maximum width of the crack at the bottom of the plate was 0.06 mm. When the load reached 130 kN, short transverse discontinuous cracks appeared at the intersection of the laminated plate and the cast-in-place plate, but they did not penetrate along the intersection, and the concrete flange plate still had good integrity. When the load reached 170 kN, the deflection deformation and crack width of the specimen were accelerated significantly, the deflection in the span of the specimen was 13.81 mm, and the maximum crack width at the bottom of the slab was 0.46 mm. When the load reached 195 kN, the crack of the concrete slab continued to develop upward and crossed the interface between the laminated layer and the cast-in-place layer, and entered the cast-in-place layer. However, the transverse crack at the interface between the composited plate and the cast-in-place layer did not develop significantly compared with that at 130 kN and did not penetrate along the interface, indicating that the laminated layer and the cast-in-place layer were still closely combined and jointly stressed. When the load reached 230 kN, the span deflection of the specimen reached 143.72 mm (more than 1/20 of the span of the combined flat beam) and the specimen was damaged. At this time, a slight local instability appeared on the upper flange of the steel beam, and the maximum crack width at the bottom of the span was 1.613 mm. The distribution of cracks at the bottom of the specimen showed that the number of cracks at the bottom of the plate was more than that of the first three specimens. The damage pattern of specimen DTCB4 in the partial loading stage is shown in Figure 10 below.

3.1.5. DTCB5

The specimen DTCB5 is a cast-in-place composite beam control group with a monolithic cast-in-place formed concrete flange slab, and there is no intersection between the composited plate and the cast-in-place layer, nor is there a splice joint between the composited plate and the composited plate splice. In the early stage in the elastic stage, the initial stiffness of the specimen is large, and the deflection increases uniformly and slowly with the increase in the load, which is linear. When the load was loaded to 30 kN, cracks appeared at the intersection of the concrete and H-beam shear connection keys and stiffening ribs under the DTCB5 plate of the specimen. When the load was loaded to 110 kN, the deflection of the specimen was 11.14 mm and cracks started to appear at the bottom of the concrete flange plate in the span. When the load was loaded to 120 kN, the concrete cracks continued to develop, forming through-length cracks along the width of the slab, and at the same time, there were also signs of cracks developing upward on the side of the slab, and at this time, the number of cracks was more than that of the composited plate combination flat beam and the distribution was more uniform, and the maximum crack width was 0.03 mm. When the load was loaded to 165 kN, the span deflection of the specimen was 20.94 mm, and the development of the specimen deflection became significantly faster. The development of concrete cracks under the slab and concrete at the bottom of the slab is obviously accelerated, and the maximum crack width is 0.34 mm, and the specimen enters the yielding stage. As the specimen was formed by the fully cast-in-place concrete slab, there were no transverse cracks similar to those at the junction of the composited plate and the cast-in-place layer during the test, and all cracks developed vertically upward from the bottom of the concrete flange slab. When the load was loaded to 210 kN, the deflection in the span of the specimen reached 143.88 mm (more than 1/20 of the span of the combined flat beam), and the specimen selected the damage to end the loading. At this time, the maximum width of the crack was 1.26 mm, and a slight local buckling phenomenon appeared on the upper flange of the steel beam. The damage pattern of specimen DTCB1 in the partial loading stage is shown in Figure 11 below.

In summary, the analysis of the damage mode of the combined flat beam shows that the specimens are in the elastic stage in the early loading period, the initial stiffness is large, the number of cracks increases rapidly with the load, and the crack width and the span deflection of the specimens develop slowly. When the specimens yielded, the crack width of the concrete flange plate increased faster and gradually developed along the width through and to the upper part of the concrete plate. The cracks at the splice in the laminated plate composite beam generally appear later than the cracks at the bottom of the concrete flange slab in the span. On the one hand, the reinforcement at the splice of the laminated plate has a good force transfer effect to effectively inhibit the development of cracks at the splice of the laminated plate, and on the other hand, the design layout of the laminated plate avoids the appearance of the splice in the area of maximum bending moment. Because the concrete flange restrains the lateral displacement of the honeycomb steel beam and provides large out-of-plane stiffness, all specimens did not show overall instability outside the loading plane, and only some specimens showed slight local instability near the concentrated loading point on the upper flange. The ultimate displacement of the specimens reached 8.2 times the yielding displacement on average, indicating that the honeycomb composite flat beam specimens have good ductility, and the plastic hinge is formed in the pure bending section at the mid-span after yielding. If it is applied to the frame, it can give full play to its plastic deformation and has good energy dissipation capacity.

From the overall damage pattern of the specimens, there are two main types: specimen deflection exceeding the limit and concrete flange plate and steel beam detachment. The overall damage patterns of specimens DTCB1, DTCB3, DTCB4, and DTCB5 are similar, with the rapid development of cracks at the bottom of the concrete flange plate and rapid increase in mid-span deflection after yielding of the honeycomb composite flat beam, which is damaged by exceeding the limit value. The specimen DTCB2 was damaged in advance due to the large penetration crack at the contact surface of the bottom of the composited plate and the shear connection key of the H-beam in the east span, and the more obvious diagonal cracks in the concrete under the slab at the loading point of the concentrated force on both sides, while the top surface of the cast-in-place layer of concrete slipped from the contact surface of the steel beam web and did not reach the maximum deflection value of the other specimens loaded. It can be seen that the honeycomb hole breaks the separation of the web for both sides of the concrete flange plate, and the concrete through the honeycomb hole, the post-inserted tie bars, and the transverse force reinforcement at the top of the plate can effectively tie the concrete flange plate on both sides of the web of the steel beam to prevent the failure of the connection between the concrete flange plate and the steel beam in the process of stressing, resulting in the failure of the steel and concrete flange plate in the combined flat beam to carry the load in a better coordinated way.

By comparing the crack distribution and damage of the cast-in-place slab composite flat beam specimen DTCB5 with the other four composited plate composite flat beam specimens, the cracks at the bottom of both slabs are concentrated in the pure bending section in the middle of the span between the loading points, but the number of cracks at the bottom of the cast-in-place slab is more and more uniformly distributed, which indicates that the stress distribution of the cast-in-place slab is more uniform than that of the composited plate. This is because the transverse small cracks at the interface between the laminated layer and the cast-in-place layer make the composited plate and the cast-in-place layer not completely close together, and the stress transfer efficiency is reduced in the local area with insufficient shear strength, resulting in relatively concentrated cracks and wider width at the bottom of the composited plate. Through the overall bending test, it can be seen that although cracks appear locally on the contact surface of the laminated plate and the cast-in-place layer, from the beginning of the specimen loading until the destruction of the specimen, there is no large relative slip between the two, and the overall state of stress can be maintained. The cracks on the side of the cast-in-place slab develop vertically upward, while the cracks on the side of the composited plate go through the stage of transverse development along the intersection of the laminated layer and the cast-in-place layer when they develop to the intersection and continue to develop inside the cast-in-place layer only with the increase in the load, finally forming “Y”-type cracks on the side of the slab.

3.2. Bearing Capacity Analysis

In this paper, the spanwise bending moment at cracking of concrete flange slab $M_{bt}$, spanwise bending moment at cracking of composited plate joints $M_{ct}$, spanwise bending moment at yielding of specimen $M_y$, spanwise bending moment at the end of loading of specimen $M_u$, spanwise deflection at yielding of specimen ${\delta }_y$, spanwise deflection at the end of loading of specimen ${\delta }_u$, measured in the test procedure of five specimens with two concentrated forces are detailed in

Table 3. Main characteristic loads of honeycomb composite flat beams.

Eigenvalue	Unit	DTCB1	DTCB2	DTCB3	DTCB4	DTCB5
$M_{bt}$	kN·m	45	50	45	50	55
$M_{ct}$	kN·m	55	55	55	50	—
$M_y$	kN·m	81.3	83.1	75.7	86.4	82.6
$M_u$	kN·m	107.5	110	100	115	105
${\delta }_y$	mm	17.1	14.1	18.0	14.6	21.3
${\delta }_u$	mm	141.05	115.71	143.01	143.72	143.88
$M_y/M_u$	/	75.63%	75.55%	75.70%	75.13%	78.67%
${\delta }_u/{\delta }_y$	/	8.25	8.21	7.95	9.84	6.75
$M_y/{\delta }_y$	/	4.75	5.89	4.21	5.92	3.88

.

The yield moment and yield displacement of the specimen are obtained by the tangent method, i.e., the horizontal coordinate of the intersection of the initial tangent line of the load–deflection curve of the specimen and the horizontal line passing through the ultimate load point of the specimen is the yield displacement of the specimen, and the corresponding load obtained by the linear interpolation method is the yield load of the specimen. Since the stiffness of the specimen remains basically unchanged in the elastic stage, the initial tangent of the load–deflection curve is obtained by taking the data of the first cracking of the bottom surface of the concrete flange of the specimen in order to prevent the accidental error of the first few loading levels of the specimen from being too large.

Comparing the four specimens with the composited plate composite flat beam, the cracking load of the concrete flange slab in the middle of the span and the cracking load of the splice are basically the same. All four specimens have the concrete flange cracking in the middle of the span earlier than the cracking at the splice, which indicates that the splice reinforcement measures and the splice arrangement of the composited plate can avoid premature damage at the splice better. The cracking load at the bottom of the cast-in-place slab specimen DTCB5 is slightly larger than the other four specimens, the crack distribution at the bottom of the slab is more uniform, and the crack width is smaller than that of the combined flat beam of the composited plate. This is due to the localization of horizontal cracks along the intersection of the composited plate and the cast-in-place layer (such as at the place without truss reinforcement or at the place where the cast-in-place layer is not filled), which affects the transfer of stress between the concrete on both sides and leads to the uneven stress inside the composited plate.

As can be seen from Table 3, when the bending moment of the specimen reaches 0.75 times the ultimate flexural load bearing capacity around the honeycomb composite flat beam specimen begins to yield, the specimen deflection can reach 8.2 times the yield displacement on average at the time of damage, and the ductility of the specimen has a good performance. It is found that the flexural load capacity and flexural stiffness of the specimens slightly decrease with the increase in the hole–height ratio of the honeycomb holes in the web of the steel beam, and the amount of steel used in DTCB2 is 1.05 times that of DTCB4, but the yield moment and ultimate bending moment of DTCB4 are 4.0% and 4.5% higher than that of DTCB2, respectively. This indicates that the honeycomb beam can increase the cross-sectional moment of inertia while reducing the amount of steel used, improve the use of steel, and increase the flexural capacity of the specimen, and also proves that the honeycomb composite flat beam is a more reasonable structural form than the solid web composite flat beam. The yield load and limit of specimen DTCB1 and DTCB5 are basically the same, but the stiffness of the specimen is slightly increased because the truss reinforcement in specimen DTCB1 can ensure the tight bond between the laminated layer and the cast-in-place layer.

The load–deflection curves of the five combined flat beam specimens in the span are shown in Figure 12. All five specimens mainly experienced two main stages, the elastic stage and the plastic deformation. Before the load reaches 0.75 times the ultimate flexural capacity, the specimens are basically in the elastic stage. The load–deflection curve of the honeycomb composite flat beam specimen in this stage shows an approximately linear relationship, the initial stiffness of the specimen is large, and the deflection grows slowly with the increase in load. When the load reaches 0.75 times the ultimate flexural load-carrying capacity, the stiffness of the combined beam decreases and the deflection development accelerates due to the gradual yielding of the upper and lower flanges of the steel beam, which leads to the redistribution of its cross-sectional stress. Except for specimen DTCB2, the deflection of other specimens after yielding keeps growing until it reaches 1/20 of the span, indicating that the honeycomb composite flat beam specimen has good ductility and can be approximated to have formed a plastic hinge at the part of the pure bending section in the span after yielding. The concrete flange plate on both sides of specimen DTCB2 is completely separated from the steel beam web, and there is no other tensile effect except for the bond between the concrete and steel surface, which leads to the detachment of the concrete plate from the steel beam in the late loading period. However, the final deflection of specimen DTCB2 also reached 1/25 of the beam span, and its ductility also met the requirements.

From the displacement–load curves of DTCB1, DTCB2, and DTCB3, it can be seen that the initial stiffness and yield load of the three specimens show a decreasing trend with the increase in the honeycomb hole–height ratio in the elastic stage, and the yield displacement increases. After entering the yielding stage, the yield load and ultimate load of the combined beam specimens decreased significantly with the increase in the honeycomb hole, the stiffness of all three specimens decreased significantly compared with the elastic stage, and the slope of the load–displacement curve was basically the same. Comparing the load–displacement curves of specimen DTCB3 with those of specimens DTCB2 and DTCB4, it can be found that filling the honeycomb holes and increasing the section height can increase the initial stiffness of the specimens, and the honeycomb beam has better load-bearing characteristics and coordinated deformation with the concrete under the conditions of similar steel consumption. The deflection–load curves of specimens DTCB1 and DTCB5 show that there is no significant difference between them, the dense truss steel composited plate can achieve the same load capacity as cast-in-place, and the initial stiffness and ultimate flexural load capacity of specimen DTCB1 are slightly higher than those of DTCB5 due to the influence of truss steel in the composited plate.

3.3. Strain Characteristics

3.3.1. Strain Distribution in Steel Beams

Five strain gauges were distributed symmetrically along the height at the solid web of the purely curved section of the beam to examine the strain distribution along the vertical direction of the beam section. The data of DTCB1 specimen 5 overflowed after 205 kN, DTCB4 specimen 4 was damaged after loading to 90 kN, and DTCB5 specimen 5 overflowed after 195 kN, and the overflowed or inaccurate data were discarded.

The load–strain trend along the height of the purely bending section of the steel beam in the combined beam and the strain distribution along the height of the purely bending section of the steel beam are shown in Figure 13. At the beginning of loading, the steel beam in the composite beam is in an elastic state, the strain distribution along the height of the purely bending section of the steel beam is close to a straight line, and the strain of the section is basically in accordance with the assumption of the flat section. When the load increases to about 0.75 P_u, the component enters the yielding stage, the rate of strain increase in the upper and lower flange of the steel beam is significantly accelerated, and the load–strain curve begins to show nonlinear development. The specimens were the first to yield at the upper and lower flanges during loading, followed by the yielding of the steel plate at 80 mm above and below the web from the midline, while the strain at the midline was small and there was a slightly tensile strain after the specimens yielded. Except for specimen DTCB2, the compressive strain on the upper flange of the specimen was significantly smaller than the tensile strain on the lower flange. Since the concrete flange plate was close to the upper flange of the steel beam, the initial neutral axis of the combined beam was located above the midline of the steel beam section, and the neutral axis showed a small upward shift when the combined beam specimens yielded. Although the strains of both upper and lower flanges of the steel beam accelerate after yielding, the strains along the height of specimen DTCB2 after yielding are relatively closer to a straight line than other specimens, and the change trend is more uniform. It indicates that the honeycomb holes in the web of the combined beam have a relatively small effect on the stress distribution in the vertical section of the specimen in the elastic stage and exacerbates the uneven strain distribution in the section after the specimen yields, which has a greater influence on the strain development, and the larger the honeycomb holes are, the greater the influence. Comparing DTCB3 and specimen DTCB4, it can be seen that with the same size of honeycomb holes, the strain inhomogeneity of the cross-section will be improved with a larger moment of inertia for a larger cross-section height.

3.3.2. Distribution of Transverse Compressive Strain in Mid-Span Concrete Flange Plate

Three strain gauges were set in the span direction along the width of the upper surface of the concrete slab of the composite beam specimen, and the distances to the center line of the composite beam were 80 mm, 260 mm, and 440 mm, and the strains on the upper surface of the concrete flange slab in the span of the composite beam are shown in Figure 14 below. In the elastic stage, the compressive strain of the concrete flange plate is small and uniformly distributed, and there is no obvious stress reduction with the increase in the distance to the center line. When the specimen yielded, the stress in the compression zone of the concrete flange plate increased rapidly, and the longitudinal compressive strain along the width direction showed a more obvious shear hysteresis phenomenon, i.e., the internal force of the concrete flange plate was reduced due to the shear hysteresis, and the stress gradually decreased with the increase in the distance to the center line of the beam. Specimens DTCB1, DTCB3, DTCB4, and DTCB5 showed significantly higher compressive strains at the concrete near the central axis than at the concrete flange edge at the end of the test, and since the concrete slab of specimen DTCB4 was located more upward, the compressive strains on its concrete upper surface should be larger than those of the other specimens. Except for specimen DTCB2, the distribution trend of longitudinal strain on the upper surface of the concrete flange slab in the span of the other four combined beam specimens is the same along the beam width direction, and the strain varies little with distance, but the maximum value of compressive strain on the top surface of concrete flange slab increases with the increase in section height. The presence of honeycomb holes can strengthen the concrete connection on both sides of the web of the steel beam and strengthen the connection between concrete and steel beam, so that the combined flat beam specimen is less likely to have the damage phenomenon similar to the concrete and steel beam detachment in specimen DTCB2, but it does not have much effect on the distribution of concrete longitudinal compressive strain with the width direction. The compressive strain at the top surface of the concrete flange plate of specimen DTCB2 was more uniformly distributed in the early stage, and the concrete compressive strain showed a slightly decreasing trend with the increase in the distance to the center of the beam. However, at the later stage of loading, the rate of increase in concrete strain near the center decreased significantly due to cracks and slippage at the contact surface between the steel beam and concrete of specimen DTCB2, resulting in the edge concrete bearing more stress compared to the other specimens, and the longitudinal strain diagram at the top surface of its span concrete flange plate showed an upward trend at the later stage.

3.3.3. Strain Distribution Law of Steel Bars in Laminated Plate Joint Connection

In this test, three steel strain gauges were set at the location of the splice to monitor the longitudinal strain distribution along the width of the connection reinforcement of the composited plate, the longitudinal reinforcement strain on the upper surface of the composite beam composited plate splice is shown in Figure 15 below. From the initial strain of the joint reinforcement of the composited plate at the loading of each specimen, it can be seen that the starting position of the neutral axis is at the intersection of the composited plate and the cast-in-place slab, when the specimen is in the elastic stage, the joint reinforcement is not yet under tension and is only anchored in the concrete. When the loading load of the specimen reaches the yield load, the neutral axis of the combined beam specimen starts to move up, the joint reinforcement of the composited plate starts to be in the tensile position, and its tensile stress starts to increase, which plays the role of transferring the stress of the adjacent composited plate and prevents the composited plate from cracking excessively due to the joint, resulting in the concrete flange plate to quit the work in advance. As shown in the longitudinal strain distribution diagram of the composited plate of the composite beam, there is also a stress lag along the width of the composited plate, i.e., the closer to the central axis of the beam, the greater the stress on the reinforcement. As the concrete slab of specimen DTCB4 is closer to the upper part making the position of the connecting reinforcement relatively upward, the connecting reinforcement is in the compressive stage at the early stage of loading and does not bear the tensile stress until the neutral axis moves up after the specimen yields, and the tensile stress of the reinforcement is also smaller than that of specimen DTCB1 and specimen DTCB3 at the end of the test.

3.3.4. Stress Distribution of Honeycomb Holes in Shear Section

Four three-way strain flowers were arranged around the first hole in the bending and shearing section of the combined flat beam specimen. The values of the strain flowers should be converted into equivalent forces according to the Mises yield criterion, and the yielding of the steel at this test point occurs when the equivalent force reaches the yield strength, calculated as follows:

$${\sigma }_1^3=\frac{E_s}{2}\left[\frac{{\mathsf{\epsilon }}_1+{\mathsf{\epsilon }}_3}{1-v}\pm \frac{1}{1+v}\sqrt{2{\left({\mathsf{\epsilon }}_1-{\mathsf{\epsilon }}_2\right)}^2+2{\left({\mathsf{\epsilon }}_2-{\mathsf{\epsilon }}_3\right)}^3}\right]$$

(1)

$$\mathsf{\theta }=\frac{1}{2}arctg\left(\frac{2{\mathsf{\epsilon }}_2-{\mathsf{\epsilon }}_1-{\mathsf{\epsilon }}_3}{{\mathsf{\epsilon }}_1-{\mathsf{\epsilon }}_3}\right)$$

(2)

Among them:

${\mathsf{\epsilon }}_1$, ${\mathsf{\epsilon }}_2$, ${\mathsf{\epsilon }}_3$—0°, 45°, 90° directional strain values;

$v$—Poisson’s ratio of the steel;

$\mathsf{\theta }$—The angle between the direction of the principal stress at the strain flower collection point and the positive direction of the x-axis;

${\mathsf{\sigma }}_1$, ${\mathsf{\sigma }}_2$, ${\mathsf{\sigma }}_3$—The three principal stresses at the location of the measurement point, where ${\mathsf{\sigma }}_1$ is the maximum principal stress, ${\mathsf{\sigma }}_2$ = 0, and ${\mathsf{\sigma }}_3$ is the minimum principal stress.

According to the fourth strength theory of material mechanics, it is known that:

$${\mathsf{\sigma }}_y=\sqrt{\frac{1}{2}\left[{\left({\mathsf{\sigma }}_1-{\mathsf{\sigma }}_2\right)}^2+{\left({\mathsf{\sigma }}_2-{\mathsf{\sigma }}_3\right)}^2+{\left({\mathsf{\sigma }}_3-{\mathsf{\sigma }}_1\right)}^2\right]}$$

(3)

Figure 16 shows the load–stress trends around the first honeycomb hole in the bending and shearing section of the composite flat beam specimen near the centralized force loading point. After the strains measured by the strain flowers around the first honeycomb hole in the bending and shearing section of the composite flat beam are converted into Mises stresses, it can be seen that the stresses at the locations of the No.6 and No.9 strain flowers are the largest, i.e., the stress changes are more obvious near the loading end of the concentrated force. The stress in the No.9 strain pattern, which is in the tension zone in the middle of the beam and below the axis, is the highest, and the area around the honeycomb hole in the web of the beam near the loading end of the concentrated force is the first to yield during loading. The equivalent stresses at the locations of strain pattern No.6 and No.7 in the specimen are smaller than those at the locations of strain pattern No.8 and No.9, and the stresses in the honeycomb holes in the tensile zone are larger than the stresses in the honeycomb holes in the compressive zone in the composite flat beam specimen. All specimens except DTCB4 started to yield after the combined flat beam started to yield, and the equivalent force at strain pattern No.6 and No.9 started to increase rapidly, and the yielding around the honeycomb holes was slightly later than the yielding of the upper and lower flange parts. After yielding the holes near the concentration side, the stresses at the locations of strain blossoms No.7 and No.8 slowed down due to the redistribution of stresses around the specimens and the honeycomb holes, and none of them exceeded the yield stress value of the steel.

3.4. Finite Element Analysis

3.4.1. Establishment and Validation of the Model

ABAQUS was used for numerical analysis, in which the models were established based on the dimensions and reinforcement in the experiment. The plastic damage constitutive relationship model is used for concrete materials, and the ideal elastic–plastic model is used for steel. The concrete and steel sections use the C3D8R element, while the steel bars use the T3D2 element. The overall size of the element in the honeycomb beam is 30 mm × 30 mm, appropriately densified around the honeycomb hole. The element size of the concrete is 20 mm × 60 mm. In addition, the steel bars were meshed in the size of 60 mm.

In terms of contact properties, the contact between steel and concrete adopts a coulomb friction contact model, where the normal direction is hard to contact, and the tangential friction coefficient is a penalty function contact of 0.3. The embedded property is used between steel bars and concrete. The boundary conditions for loading are consistent with the experiment.

On this basis, numerical analysis was conducted on specimens DTCB1, DTCB2, DTCB3, DTCB4, and DTCB5. Taking DTCB1 as an example, the compressive damage and state of the concrete and the Mises stress distribution of the honeycomb beam are shown in the following Figure 17. The distribution of damage stress in each specimen and the yield characteristics of the steel beam are consistent with the experiment.

The test results and finite element analysis results of the bending bearing capacity of composite flat beam specimens are shown in

Table 4. Comparison between finite element simulation value and test value of flexural capacity.

Specimens	Mys (kN·m)	Mym (kN·m)	Error	Mus (kN·m)	Mum (kN·m)	Error
DTCB1	81.3	80.8	−0.62%	107.5	94.5	−12.09%
DTCB2	83.1	86.6	4.21%	110.0	98.5	−10.45%
DTCB3	75.7	78.8	4.10%	100.0	91.9	−8.10%
DTCB4	86.4	88.7	2.66%	115.0	105.6	−8.17%
DTCB5	82.6	80.4	−2.66%	105.0	92.5	−11.90%
Average	-	-	1.54%	-	-	−10.14%

, where M_ys is the measured value of the midspan bending moment test when the specimens yield, M_ym is the calculated value of midspan bending moment finite element analysis results when the specimens yield, M_us is the measured value of midspan bending moment test when the specimens fail, and M_um is the calculated value of midspan bending moment finite element analysis results when the specimens fail.

The error between the simulated calculation value of yield bending moment and the measured value in the experiment is within ±5%, with an average error of 1.54% and a mean square error of 0.0273. The simulated values of the ultimate bending moment of the specimen are generally lower than the measured values, with a maximum error of −12.09%, an average error of −10.14%, and a mean square error of 0.0173. The simulated calculation results of the yield bending moment and initial stiffness value of the specimen are basically consistent with the physical test results, indicating that the model has good consistency with the test in the elastic stage. When the test piece enters the yield stage because the test piece can fully bear the force and fully develop its plastic deformation capacity during the finite element analysis, the overall stiffness of the yield section in the physical test is greater than the value calculated by the finite element method, and the result calculated by the finite element model is less than the result measured by the test. At the same time, it can be seen from the failure process and morphology of the specimen that there are slight differences between the local interactions of H-shaped steel shear keys, stiffeners, and concrete in the finite element model and actual experiments, resulting in the final calculated results being smaller than the experimental values. From the comparison between the bearing capacity of the finite element calculation model and the bearing capacity of the test piece in the physical experiment, it can be seen that the elastic stage of the model is relatively more accurate, and the error after yielding increases, but its error value is within the acceptable range, and the simulation result of the model has a certain stability from the mean square deviation of the error.

3.4.2. Parametric Analysis

Based on the validated numerical analysis model mentioned above, parameterized analysis was conducted, mainly considering factors such as hole-to-height ratio and, where the range of hole-to-height ratio is 0.45–0.75 and taking into account the bearing characteristics of honeycomb beams with a cross-sectional height of 160–230 mm at a hole-to-height ratio of 0.5. A total of 16 specimens were selected, and their variation parameters and bending bearing capacity calculation values are shown in Table 4.

Among them, research has found that the reduction coefficient of bearing capacity exhibits a nonlinear relationship under different hole–height ratios, which can be used for subsequent simplified calculation and analysis, as shown in Figure 18.

4. Theoretical Analysis and Design Method

4.1. The Basic Assumptions

The holes in the web of honeycomb composite flat beam led to its local weakening, which changes the longitudinal and vertical continuity of the section of steel web, forming a special kind of variable section beam, resulting in the calculation of the bearing capacity of the composite beam in the process of stressing becomes complicated. The honeycomb composite flat beam cross-sectional combination form is different from the common T-shaped composite beam, and the design formula of the T-shaped composite beam cannot be applied directly. The honeycomb composite flat beam component is formed by a honeycomb steel beam and concrete flange plate together, but the honeycomb composite flat beam can refer to the design calculation method of solid web steel composite flat beam in the calculation process because the concrete flange plate is located on both sides of the web, and the honeycomb hole is basically filled intact. The following assumptions should be met when calculating the flexural load-carrying capacity of the honeycomb composite flat beam:

(1)

Honeycomb beam holes, stiffening ribs, and H-beam shear connection keys can ensure that the connection between steel and concrete resists sufficient longitudinal shear to form a synergistic force-bearing whole.

(2)

The concrete in the tensile zone withdraws from the work after cracking, and the concrete tensile stress is much smaller than its compressive stress, so the concrete in the tensile zone is not involved in the force analysis. Since the concrete flange plate below the bottom part of the concrete is basically in tension, its enhancement of the combined flat beam flexural bearing capacity is not considered.

(3)

Since the cross-sectional area of the reinforcement in the concrete flange slab is much smaller than the cross-sectional area of the steel beam, the enhancement effect of the reinforcement on the flexural bearing capacity of the combined flat beam specimen is not considered.

(4)

The steel and reinforcement exhibit ideal elasticity and can reach the strength design value when in tension or compression.

(5)

The cross-section of the honeycomb composite flat beam specimen can maintain the plane state during the load-bearing process, i.e., the assumption of flat section is satisfied, without considering the uneven distribution of stresses in the concrete flange.

(6)

Since the concrete is located on both sides of the steel beam to have a stabilizing effect on it, and the steel beam section grade is S1, the overall buckling or local buckling of the specimen is not considered.

(7)

The effect of the shear connection keys of H-beams on the lower flange of the honeycomb beam and the stiffening ribs is not considered.

(8)

The concrete flange plate is treated as a whole.

(9)

In the bending process of the specimen, the concrete compressive part of the stress is actually not rectangular distribution, but for the sake of calculation simplicity, this paper adopts the equivalent rectangular stress method to convert the concrete irregular compressive stress into rectangular stress equivalently. The magnitude of the equivalent rectangular stress should be equal to the magnitude of the actual stress, and the location of the point of action of the joint force should also be the same, then the height of the equivalent compressive zone of concrete $x=\mathsf{\beta }\left(y_c-c\right)$, the equivalent compressive stress value is $\mathsf{\alpha }{f}_c$.

Where $\mathsf{\beta }$ is the ratio of the height of the rectangular stress compression zone $x$ and the height of the plastic neutral axis of the combined flat beam section from the top surface of the concrete flange $y_c-c$, $y_c$ is the distance of the plastic neutral axis of the combined flat beam section from the top surface of the steel beam, $c$ is the distance of the top surface of the concrete flange from the top surface of the steel beam, and the coefficient $\mathsf{\alpha }$ is the ratio of the equivalent stress value of the concrete in the compression zone to the design value of the concrete axial compressive strength.

4.2. Calculate the Ultimate Bending Capacity by Using Double T-Section Method

4.2.1. Cross-Section Type

In this paper, the equivalent rectangular stress method is used to equate the height of the theoretical compression zone of the concrete flange plate to the actual compression height of the concrete flange plate, so that it can better calculate the location of the plastic neutral axis of the cross-section of the honeycomb composite flat beam specimen. Since the honeycomb composite flat beam specimen is weakest at the honeycomb hole section, the ultimate flexural load capacity of the composite flat beam is calculated mainly by using the honeycomb hole section as the control section in this section. The combined flat beam is divided into two T-shaped sections above and below the honeycomb hole and the concrete flange plate for calculation, referred to as the double T-shaped steel method. Referring to the classification standard of the cross-section in a T-shaped reinforced concrete beam, the combined flat beam can be divided into two categories according to the position of the neutral axis: the first category of cross-section when the neutral axis is located within the thickness of concrete flange plate; the second category of cross-section when the neutral axis is located below the bottom surface of concrete flange plate. One type of cross-section can be distinguished according to the position of the plastic neutral axis relative to the honeycomb hole, ${\mathsf{{\rm I}}}_a$ cross-section when the plastic neutral axis is located at the web above the honeycomb hole, and ${\mathsf{{\rm I}}}_b$ cross-section when the plastic neutral axis is located within the height of the honeycomb hole opening, as shown in Figure 19.

When the plastic neutral axis is located at the web above the honeycomb aperture when the section belongs to the ${\mathit{{\rm I}}}_a$ class of sections:

$$b_1{h}_1{f}_y+b_w\left(\frac{h-h_e}{2}-h_1\right)f_y+\alpha f_c\beta b_f\left(\frac{h-h_e}{2}-c\right)\ge b_2{h}_2{f}_y+b_w\left(\frac{h-h_e}{2}-h_2\right)f_y$$

(4)

when the plastic neutral axis is located below the concrete flange slab, the section is a Class II section:

$$b_1{h}_1{f}_y+\left(\frac{h-h_e}{2}-h_1\right)b_w{f}_y+{\mathsf{\alpha }}_1{f}_c\mathsf{\beta }{b}_f{h}_f\le b_2{h}_2{f}_y+\left(\frac{h-h_e}{2}-h_2\right)b_w{f}_y$$

(5)

Among them:

$b_1$, $b_2$, $b_w$, $b_f$—The widths of the plates of the upper flange, lower flange, and web of the steel beam as well as the concrete flange plate;

$h_1$, $h_2$, $h_w$, $h_f$—Plate thicknesses of the upper flange, lower flange, and web of the steel beam as well as the concrete flange plate;

h—Height of the cross-section of the honeycomb combined flat beam;

$h_e$—Honeycomb hole diameter;

$f_y$—The yield stress of the steel section;

$f_c$—Peak concrete stress.

When none of the specimen sections satisfy Equations (4) and (5), the plastic neutral axis of the combined flat beam section is located within the opening of the honeycomb hole and is a $I_b$ type section.

4.2.2. Bearing Capacity Calculation Formula

Due to the relative position between the plastic neutral axis of the composite specimen and the plastic neutral axis of the honeycomb beam, there are differences in the stress characteristics of the composite specimen. For example, the neutral axis of the composite specimen is located at position a, and the plastic neutral axis of the honeycomb steel beam is located at position d as shown in Figure 19a. The equivalent stress distribution of the composite specimen is shown in Figure 19b.

According to the cross-sectional tensile equilibrium equation of the combined flat beam specimen [27,28], Equation (6), the distance of the cross-sectional plastic neutral axis from the top of the combined flat beam can be found at $y_c$.

$${\mathsf{\alpha }}_1{f}_c\mathsf{\beta }{b}_f\left(y_c-c\right)+b_1{h}_1{f}_y+b_w\left(y_c-h_1\right)f_y=b_2{h}_2{f}_y+b_w\left(h-y_c-h_e-h_1\right)f_y$$

(6)

The cross-sectional equivalent force method decomposes the cross-sectional bending moment of the combined flat beam into two parts: the flexural load capacity of the steel beam around its own plastic neutral axis and the concrete equivalent compression zone combined force on the equivalent steel beam tension zone combined force after taking distance.

$$M=M_s+b_f\mathsf{\alpha }{f}_c\mathsf{\beta }\left(y_c-c\right)y$$

(7)

$$M_s=W_{px}{f}_y$$

(8)

where $W_{px}$ is the resistance distance of the shaped section of the honeycomb steel beam around its own plastic neutral axis. From Equation (9), the distance between the plastic neutral axis of the honeycomb steel beam and the top of the beam can be found at $y_s$:

$$h_1{b}_1+\left(y_s-h_e-h_1\right)b_w=h_2{b}_2+\left(h-y_s-h_2\right)b_w$$

(9)

From Equation (10), the distance y between the combined force in the compression zone of the concrete to the combined force in the tension zone of the steel beam after equivalence is found:

$$y=y_1+y_c-c-\frac{x}{2}$$

(10)

where $y_1$ is the equivalent combined force distance of the honeycomb beam in tension from the plastic neutral axis of the combined section.

$$y_1=\frac{\left(y_s-\frac{h+h_e}{2}\right)\left(\frac{y_s}{2}+\frac{h+h_e}{4}-y_c\right)+\frac{1}{2}{\left(\frac{h+h_e}{2}-y_c\right)}^2}{y_s-h_e-y_c}$$

(11)

Similarly, when the plastic neutral axis of the composite specimen is located at position b in Figure 19, and the plastic neutral axis of the honeycomb steel beam is located at position d in Figure 19, its bending capacity is:

$$M=W_{px}{f}_y+b_f\mathsf{\alpha }{f}_c\mathsf{\beta }\left(y_c-c\right)\left(\frac{y_s}{2}+\frac{h+h_e}{4}-c-\frac{x}{2}\right)$$

(12)

When the plastic neutral axis of the composite specimen is located at position c in Figure 19, and the plastic neutral axis of the honeycomb steel beam is located at position d in Figure 19, its bending capacity is:

$$M=W_{px}{f}_y+b_f\mathsf{\alpha }{f}_c\mathsf{\beta }{h}_{f}y$$

(13)

$$y=\frac{y_s}{2}+\frac{h+h_e}{4}-c-\frac{\mathsf{\beta }{h}_f}{2}$$

(14)

4.3. Simplified Calculation of Ultimate Bending Capacity

Using the equivalent rectangular stress method of double T-beams in the above section, the calculation is based on the section where the honeycomb hole is located. Although the flexural load capacity of the weakest section can be calculated, the calculation decomposes the cross-section of the combined flat beam into the upper and lower parts of unconnected T-beams and concrete flange plate and does not consider the role of the web at the longitudinal solid web of the honeycomb beam, which leads to conservative calculation results. This method also requires the identification of the relative relationship between the plastic neutral axis of the combined beam and the honeycomb hole before calculation, which makes the calculation process a bit tedious. Therefore, this section simplifies the formula for calculating the flexural load capacity of the honeycomb composite flat beam specimen on the basis of the equivalent rectangular stress method.

Under the condition that the previous assumptions hold, the effect of honeycomb holes on the calculation is ignored first, the formula for calculating the flexural load capacity of the solid web combination flat beam is derived, and the fitting function of the honeycomb hole–height ratio and the load capacity reduction coefficient fitted by the integrated finite element calculation results are derived for the flexural load capacity of the honeycomb combination beam.

In Figure 20, a′, b′, and c′ represent the plastic neutral axis positions at different positions. When the neutral axis of the composite specimen is located at position a’ and the plastic neutral axis of the honeycomb steel beam is located at position c’, the section stress distribution of the first type of solid web composite beam is shown in Figure 20b. Similarly, when the neutral axis of the composite specimen is located at position b’ and the plastic neutral axis of the honeycomb steel beam is located at position c’, the section stress distribution of the second type of solid web composite beam is shown in Figure 20c.

According to the cross-sectional tensile equilibrium equation of the combined flat beam specimen, i.e., Equation (15), the distance of the cross-sectional plastic neutral axis from the top of the combined flat beam can be found at $y_c$.

$${\mathsf{\alpha }}_1{f}_c\mathsf{\beta }{b}_f\left(y_c-c\right)+b_1{h}_1{f}_y+b_w\left(y_c-h_1\right)f_y=b_2{h}_2{f}_y+b_w\left(h-y_c-h_2\right)f_y$$

(15)

The cross-sectional equivalent method decomposes the cross-sectional bending moment of the combined flat beam into two parts: the bending capacity of the steel beam around its own plastic neutral axis and the concrete equivalent compression zone combined force on the equivalent steel beam tension zone combined force after taking distance. Substitute Equations (7) and (15) into Equation (8) to find out the flexural load-carrying capacity of solid web type combined flat beam. Where y is the distance from the concrete compressive zone combined force to the steel beam tensile zone combined force, which can be found by the following formula:

$$y=\frac{y_s-y_c}{2}+y_c-c-\frac{x}{2}$$

(16)

The bending capacity of the solid web combined flat beam is calculated as shown in Equation (17):

$$M=M_s+b_f\mathsf{\alpha }{f}_c\mathsf{\beta }\left(y_c-c\right)\left(\frac{y_s-y_c}{2}+y_c-c-\frac{x}{2}\right)$$

(17)

When the plastic neutral axis is located below the bottom plate of the concrete flange plate, the stress diagram shown in Figure 20c is used.

According to the cross-sectional tensile equilibrium equation of the combined flat beam specimen, i.e., Equation (18), the distance between the plastic neutral axis of the cross-section and the top of the combined flat beam can be found at y_c.

$${\mathsf{\alpha }}_1{f}_c\mathsf{\beta }{b}_f{h}_f+b_1{h}_1{f}_y+b_w\left(y_c-h_1\right)f_y=b_2{h}_2{f}_y+b_w\left(h-y_c-h_2\right)f_y$$

(18)

Substitute Equations (8), (16) and (18) into Equation (7) to find the bending capacity of the solid web type combined flat beam:

$$M=M_s+b_f\mathsf{\alpha }{f}_c\mathsf{\beta }{h}_f\left(\frac{y_s-y_c}{2}+y_c-c-\frac{x}{2}\right)$$

(19)

After finding out the above two types of solid web type combined flat beam specimen flexural load bearing capacity, in order to consider the honeycomb hole on the honeycomb combined flat beam flexural load bearing capacity reduction effect, in this introduction of the finite element calculation results of the honeycomb combined flat beam flexural load bearing capacity reduction coefficient with the function of the honeycomb hole–height ratio, then the honeycomb combined flat beam specimen ultimate flexural load bearing capacity calculation formula is as follows:

$$M_u^{\prime }=\left[1+0.07\frac{h_e}{h}-0.3{\left(\frac{h_e}{h}\right)}^2\right]M$$

(20)

4.4. Verification of Ultimate Bending Capacity Calculation

In order to verify the applicability of the formula and simplified calculation formula for calculating the ultimate bending bearing capacity of specimens using the double T-shaped steel method, the bending bearing capacity of honeycomb composite flat beams under different section heights and hole–height ratios was explored by comparing the finite element model calculation results in the previous text.

Table 5. Verification of ultimate bending capacity of honeycomb composite flat beams.

Specimen	Height of Section (mm)	Hole–Height Ratio	DT (kN)	RWP (kN)	FE (kN)	Error of DT	Error of RWP
ZB1	200	0.45	77.79	87.01	95.85	18.84%	9.22%
ZB2	200	0.50	76.63	86.05	94.43	18.85%	8.87%
ZB3	200	0.55	75.45	84.95	93.15	19.00%	8.80%
ZB4	200	0.60	74.24	83.73	91.93	19.24%	8.92%
ZB5	200	0.65	72.95	82.37	90.36	19.27%	8.84%
ZB6	200	0.70	71.56	80.88	88.84	19.45%	8.96%
ZB7	200	0.75	70.07	79.25	87.15	19.60%	9.06%
ZB8	200	0	87.17	89.68	98.50	11.50%	8.95%
ZB9	160	0.5	57.10	62.59	70.51	19.02%	11.23%
ZB10	170	0.5	62.29	68.18	75.85	17.88%	10.11%
ZB11	180	0.5	66.97	73.96	81.56	17.89%	9.32%
ZB12	190	0.5	71.75	79.91	87.07	17.60%	8.22%
ZB14	210	0.5	81.61	92.36	99.31	17.82%	7.00%
ZB15	220	0.5	86.68	98.86	107.54	19.40%	8.07%
ZB16	230	0.5	91.85	105.54	114.55	19.82%	7.87%

presents the results and comparative analysis data of three different calculation methods, including the double T-shaped steel calculation method (DT method), web reduction calculation method (RWP method), and finite element method (FE method).

The ultimate bending bearing capacity of the specimen calculated by the equivalent stress method is less than the finite element simulation value, indicating that the calculation of the bending bearing capacity of the specimen according to the formula given in this chapter is conservative and has sufficient safety. The main reasons why the calculated value of the formula is less than that of the finite element method are: (1) The formula ignores the tensile strength of concrete and the reinforcement in the concrete flange plate for the improvement of the bending bearing capacity of the specimen; (2) When calculating the formula, the effect of the H-shaped steel shear connection key and stiffener in the composite specimen was not considered. The H-shaped steel shear connection key and stiffener appear relative compression with the concrete during bending, which suppresses the deformation of the specimen and improves the flexural bearing capacity of the specimen. The error of formula A is basically between 17% and 20%, while the error of the RWP method is basically between 7% and 12%, indicating that the accuracy of the simplified calculation formula is higher than that of the double T-shaped steel method calculation formula.

When calculating using the double T-shaped steel method, the connection effect of the web plate at the honeycomb steel beam web plate pier on the upper and lower T-shaped steel sections was not considered. The contribution of the upper and lower separated T-shaped steel to the bearing capacity of the composite flat beam was calculated separately, and the formula calculation value was generally lower than the finite element simulation result. From Table 5, it can be seen that the error of the DT method increases with the increase in the honeycomb hole–height ratio. When the hole–height ratio is 0, the formula calculation error is only 11.50%, which is significantly reduced compared to other hole–height ratio errors. The calculation results of the DT method for solid web composite flat beams are significantly better than those of honeycomb composite flat beams, and this formula takes into account the weakening effect of honeycomb holes too conservatively. The RWP method shows a relatively stable error in the formula calculation value when the cross-sectional height of the composite flat beam remains constant, but the error tends to decrease with the increase in the cross-sectional height of the honeycomb composite flat beam. It shows that the calculation of the bending bearing capacity of specimens with the same section height and different hole–height ratios has relatively stable results, but there is still about a 10% safety margin relative to the finite element method.

In summary, both the DT method and RWP method proposed for the bending bearing capacity of honeycomb composite flat beams can effectively calculate the bending bearing capacity of specimens, and the formula calculation values are generally smaller than the finite element simulation values, leaving a certain safety margin for specimen design. The RWP method simplifies the calculation process by using the reduction factor of the hole–height ratio on the basis of the solid web steel composite flat beam. Compared with the results of the finite element method, the RWP method has a smaller relative error and is more stable.

4.5. Design Suggestion Method of Honeycomb Composite Flat Beam

(1) In the process of bending, the steel components may have local buckling of the cross-sectional plates, which may cause early damage to the specimens and reduce the strength and stiffness of the combined flat beam specimens, thus affecting the bearing capacity and deformation capacity of the combined flat beam. It is recommended to fill the honeycomb holes and set up supporting stiffening ribs at the support and upper flange of the beam where the load is concentrated by a large, fixed load. When the web width-to-thickness ratio of the honeycomb beam is greater than 80 ${\mathsf{\epsilon }}_k$, transverse stiffening ribs should be provided at places with high local stress.

(2) The load location has a certain influence on the critical load of the steel beam. The floor load of the honeycomb composite flat beam proposed in this paper is transferred to the lower flange of the steel beam through the floor slab and to the H-beam anti-kinematic connection key, and this load transfer path can reduce the beam torsion as well as prevent beam instability. At the same time, since the concrete flange plate is located on both sides of the honeycomb beam web and basically fills the honeycomb holes, it provides effective lateral support to the honeycomb steel beam, so it is not necessary to verify the overall stability of the honeycomb composite beam when the overall stability of the honeycomb steel beam meets the requirements. The overall stability of the honeycomb beam element can be calculated by discounting the overall stability of the solid web flexural element by the corresponding formula, and the discount factor is related to the span-to-height ratio, the opening rate, and the hole density of the beam [29]. Therefore, the honeycomb steel beam can be subjected to the overall stability test according to the following equation:

$$KM{b}_0{}_{max}$$

(21)

where:

$M_{max}$—The maximum bending moment acting around the strong axis;

${\mathsf{\phi }}_b$—The overall stability coefficient of the beam;

$W_0$—Section modulus of the bridge section of the cellular beam;

$f$—Standard values of steel strength;

$K$—Cellular hole discount factor, $K=0.24e^{0.16\mathsf{\rho }+2.3\mathsf{\lambda }+0.01D}$, which can be approximated as 1.1;

$D$—The span-to-height ratio of the beam;

$\mathsf{\lambda }$—Opening rate;

$\mathsf{\rho }$—Hole density.

(3) The component strength of the honeycomb combination flat beam and other load-bearing limit states are generally designed using the plastic design method, that is, using Equations (7), (14), (15) or Equation (20) ultimate flexural load-bearing capacity calculation formula proposed in this chapter, with a certain safety factor. The honeycomb combined flat beam directly subjected to power should adopt the elastic design method, and the load should be combined according to the short-term effect.

(4) The calculation of the deflection of cellular combination beams is mainly divided into the estimation method, comparison method, and finite element method, where the estimation method is based on the calculation of the deflection of web-type combination flat beams by introducing an increase factor for the corresponding calculation. The calculation of deflection of solid web combination beams in China’s steel structure code is based on the standard combination of loads and the larger calculated deflection of quasi-permanent combination, and the combination beam subject to positive moment only should have its stiffness discounted accordingly:

$$B=\frac{E{I}_{eq}}{1+\mathsf{\xi }}$$

(22)

where:

E—Modulus of elasticity of the steel;

$I_{eq}$—Composite beam commutation section moment of inertia;

$\mathsf{\xi }$—Stiffness reduction factor.

According to the relevant literature, the deflection calculation value of the cellular composite flat beam is obtained by appropriately scaling up the deflection calculation value on the basis of the solid web composite flat beam.

$$f=\mathsf{\eta }{f}_{sm}$$

(23)

where:

f_sm—The bending deflection of a solid web combined flat beam of equal section;

η—The deflection increase factor, when the expansion ratio is less than 1.5 and can be taken according to the value of

Table 6. Coefficient of deflection increase.

Beam Span-to-Height Ratio	1/40	1/32	1/27	1/23	1/20	1/18
η	1.1	1.15	1.2	1.25	1.35	1.4

.

(5) The connection between the honeycomb steel beam and the concrete flange slab is mainly realized through the honeycomb holes, the slab bearing reinforcement through the honeycomb holes, the post-inserted tie bars, and the H-beam shear connection keys. From the test results, it can be seen that the solid web-type composite flat beam only relies on the bond stress of concrete to a steel beam, and the shear connection key of the H-beam may produce the damage state of the concrete flange plate detached from the steel beam during bending. The equally spaced honeycomb holes and the reinforcement and concrete through the honeycomb holes connect the two sides of the flange plate, which makes it have better integrity and can meet the requirements of its anti-slip in the normal use stage. Therefore, the H-beam shear connection key provides shear strength on the one hand and lies in supporting the composited plate on the other hand, making the construction process simpler. For the post-inserted tie bars according to the honeycomb hole arrangement, it is appropriate to meet the requirements of C6@200, and the anchorage length of the two ends extending into the end of the laminated plate using 135° bend hooks can be 0.6 times the basic anchorage length ξ_ab, the inner diameter of the bent hook is not less than 4d, and the length of the straight section after bending is not less than 5d.

(6) For the honeycomb combination flat beam with higher fire protection requirements, the exposed steel beam part should be designed for fire protection in accordance with the requirements of structure type, working environment, and fire resistance time limit, such as using fireproof paint. Steel beams within the contact range of steel beams and concrete should not be coated to prevent slippage and misalignment between concrete and steel beams. Steel structure corrosion should be based on the importance of the building, corrosive environment, and other factors to determine a reasonable anti-corrosion design life can use anti-corrosion coatings, zinc or aluminum, and other metal protective layers, cathodic protection measures, the use of weathering steel, and other measures for corrosion. For the parts that endanger personal safety and maintenance difficulties, important load-bearing structures should be strengthened to protect them.

5. Conclusions

In this study, based on the experimental research on the bending capacity of the tightly spliced truss-reinforced plate-honeycomb flat beam, the effects of the size of the opening of the H-shaped steel beam, the spacing of the opening, and the height of the web are explored. The difference between the splicing of composite plates and cast-in-place was compared and studied. The validated numerical model was used for parametric analysis. The calculation formula for the bending capacity of this type of composite flat beam was proposed and the design method was suggested. The following conclusions were obtained:

(1)

The failure of the specimen is mainly divided into two states: the deflection exceeds the limit, and the concrete flange plate is separated from the steel beam. The honeycomb holes can effectively connect the concrete flange plates on both sides of the steel beam web by passing through the steel bars and concrete inside and can better disperse the stress of the concrete in the middle of the span so that the stress distribution of the concrete flange plates is more uniform. The concrete flange plate in the honeycomb composite flat beam, including its steel bars, has a shear lag effect along the width direction after the specimen yields.

(2)

The process of the honeycomb composite flat beam under bending load is mainly divided into two parts: the elastic stage and the plastic deformation stage. When the bearing capacity reaches 0.75 times the ultimate bearing capacity, the structure exhibits significant damage and degradation. The pure bending section of the specimen forms a plastic hinge in the plastic stage, and the average ductility of the specimen reaches 8.2, which has good ductility.

(3)

The simplified method proposed in this article for calculating this type of honeycomb composite beam has an error of less than 10% in terms of bending bearing capacity, which has advantages over the DT method. The calculation method and design suggestions proposed in this study provide a basis for the research and application of this type of composite flat beam.