Study on Mechanical Properties of Steel-Strengthened Bamboo Beams with Webbing Opening

Abstract

Bamboo beams are often reinforced with built-in steel sections to enhance their strength and load-bearing capacity. In this paper, we studied the effect of different parameters, including the location of the hole, the hole size, and the thicknesses of the steel and bamboo, on the mechanical properties of reinforced beams. The damage patterns, deformation characteristics, and force-transfer mechanisms, as well as the mechanical properties of reinforced beams with different hole shapes, underwent non-linear finite element analysis. The damage sustained by the reinforced bamboo beam differed from that of the traditional bamboo beam; two diagonal points formed a plastic hinge, mainly during the process of shear damage to the hole. It was determined that the hole size and the thickness of the bamboo have the greatest influence on the mechanical properties of the reinforced beam. The damage characteristics of the composited beams with different holes are similar; the bearing capacity of reinforced beams with open square holes is reduced by 10%–25%compared with circular holes.

1. Introduction

Bamboo is a cheap, strong, tough, and environmentally friendly resource. It is renewable, quick to grow, and produces minimal carbon; as such, its use in building structures is beneficial to the environment and allows an ecological balance to be sustained [1,2,3]. Bamboo bundles are bonded with phenolic resin and then compressed and compacted to create laminated composite bamboo veneer, which has superior physical and mechanical properties compared to single-layer bamboo and laminated timber, with improved bending and tensile strength. Its use facilitates the sustainable development of the environment. This and other green, low-carbon, energy-saving, environmentally friendly building concepts are steadily increasing in value [4,5,6]. When designing high-rise structures, boring holes in webbed plates and fitting piping equipment through said holes can lower the height of the floor, increase the building space, and reduce the structural deadweight, providing both economic and social benefits. As such, this strategy has excellent prospects for use in real-life applications. Chung et al. [7,8] suggested creating large holes in webbed plates and provided the corresponding design rules. They introduced a method for boring holes of different shapes and sizes in the webbed plate of steel beams. Liu et al. [9] analysed the damage characteristics of web plates with holes of different shapes. Wang et al. [10,11] found that webbed open-hole steel–concrete composite beams exhibit typical shear damage characteristics, a good performance, and ductility. Du et al. [12] found reinforcing holes with longitudinal stiffeners can significantly improve the ultimate load-carrying capacity of composite beams and, thus, the shear capacity of each hole. Due to natural defects and environmental changes, reinforcements are required to ensure new environmental standards are met and to extend the service life of these materials. Borri et al. [13] used fibre to reinforce timber, which has major advantages in terms of energy and production cost reductions but does not lead to significant strength gains. E. McConnell’s [14] method of reinforcing glued laminated timber structures using steel post-tensioning improves their bending strength and stiffness. At present, most scholars use FRP, CFRP, and GFRP to strengthen bamboo and wood beams, with some studies indicating that the ultimate load-carrying capacity and flexural strength of strengthened beams increased significantly and the traditional bamboo and wood beams sustained different types of damage at the bottom. Therefore, FRP and other composite materials could replace traditional reinforcement and repair apparatus [15,16,17,18,19].

Steel sections are strong, lightweight, small, and attractive but, in spite of their excellent mechanical properties, they are susceptible to local flexural instability. Researchers [20,21] theorise that a combination of steel profiles and bamboo glue boards could overcome this shortcoming and optimise the advantages of both materials, forming a strong composite cross-section and improving the overall stability of the structure. Li et al. [22,23] described steel–bamboo composite floor slabs and walls, whose excellent mechanical properties meet modern building requirements. Steel–bamboo composite walls have excellent thermal insulation properties, which can reduce buildings’ energy expenditure. Zhang et al. [24] research shows that steel–bamboo composite frame structures release 36.4 percent less emissions throughout their life cycle than steel–concrete structures. In previous studies [25,26,27], some scholars designed a series of steel–bamboo composite I-beams and tested their shearing and bending behaviours. They found that the overall working performance of the composite beams was excellent, making them highly suitable for building structures; however, I-section composite beams are prone to transverse instability in the cross-section, as well as local buckling behaviour. Ding et al. [28,29] proposed a steel–bamboo composite single-box and double-chamber beam for a shear and bending performance study. The results show that the two materials, steel and bamboo, have good synergy. Although it is prone to compression and tensile failure, the box beam can effectively avoid lateral instability and local buckling because the bamboo glue boards can bend, ensuring the stability of the cross-section. Thus, the steel enters the plastic phase.

We used a new method of reinforcement—built-in steel sections—to strengthen an open bamboo box girder, as shown in Figure 1a. We aimed to study the shear behaviour of the reinforced beams; observe and analyse the damage mode, load-carrying capacity, and deformation; and explore the effects of different parameters on the mechanical properties of the reinforced beams. Finally, nonlinear finite element analysis was carried out on reinforced beams with different hole shapes to study their effects on load-carrying capacity.

2. Material Properties and Manufacturing

To form a box structure, first, two U-shaped steels were connected to the bamboo plywood through structural adhesive and placed opposite to each other. Then, the bamboo plywood was connected to the upper and lower flanges with structural adhesive again. The bamboo plywood surrounds the section steel to form a box-shaped structure. As shown in Figure 1b, the thickness of the structural adhesive was 2 mm and the thickness of the bamboo plywood was 20 mm.

When preparing the materials, first, a grinder was used to remove the zinc coating and impurities from the steel surface. Dust was removed with sandpaper. Then, alcohol was coated on the steel surface. The surface of the reconstituted bamboo was sanded with a grinder and sandpaper to increase friction. Structural adhesive was evenly applied to the surface of the section steel. It was bonded to the bamboo plywood. It was fixed in place with clamps and pressure was applied with a heavy object. The curing time is 7 days to ensure that there are no gaps between the bamboo plywood and the steel and that the adhesive is firm.

To determine the mechanical properties of the bamboo glue board, reference was made to the (GB/T 1927.14-2022, GB/T 1927.11-2022) [30,31]. Twelve of the following bamboo plywood with smooth and transverse grain directions were acquired: axial tensile, compression, flexural, and compression specimens. All the specimens were obtained from Nanhuang Bamboo Industry Co., Ltd., Lu’An, China. The test results are shown in

Table 1. Mechanical properties of bamboo plywood.

EX/MPa	EY/MPa	EZ/MPa	VX	VY	VZ	GX/MPa	GY/MPa	GZ/MPa
5240.5(123) (0.234)	630.7(29.05) (0.046)	630.7(29.05) (0.046)	0.213(0.007) (0.032)	0.213(0.007) (0.032)	0.115(0.01) (0.086)	1190(70.65) (0.059)	1190(70.65) (0.059)	347(30.22) (0.087)

X, Y, and Z denote the direction of bamboo plywood in the longitudinal, radial, and tangential directions, E denotes the modulus of elasticity of bamboo plywood, G denotes the shear modulus of bamboo plywood, and V denotes the Poisson’s ratio. The values in parentheses are the standard deviations and coefficient of variations.

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The materials were divided into smooth-grained, radial, and chordal bamboo according to the stress–strain curve results, as shown in Figure 2. Ultimately, the average values were plotted along the red line of the stress–strain curve for the bamboo adhesive board. Bamboo plywood showed the same trend in the transverse grain compression and tension directions. This constitutive relationship can be simplified as a two-stage linear model, the parallel tension relationship can be simplified to a single linear model, and the conforming grain compression constitutive model can be simplified to a three-stage linear model.

2.1. Steel Sections

This test profile is made of a Q235-grade galvanised steel plate and it is 1.5 mm and 2 mm thick. The axial tensile test was carried out according to the (GB/T228.1-2010) [32], in which six steel samples of 1.5 mm and 2 mm were used. The steel sections fractured in the middle. The results of the specimens are summarised in

Table 2. Material properties of thin-walled steel sections.

Specimen Thickness	Yield Strength fy/MPa	Tensile Strength fu/MPa	Modulus of Elasticity Es/MPa	Poisson’s Ratio μ
1.5 mm	247.25 (3.17) (0.012)	306.75 (7.54) (0.024)	2.03 × 105 MPa	0.3
2 mm	316.84 (2.95) (0.009)	338.33 (6.57) (0.019)	2.05 × 105 MPa	0.3

. A tensile intrinsic curve of the steel section is plotted using the tensile test results. The steel did not pull off in the combined beam loading test, so this paper adopted the triple-folding model (without considering the descending section). The values in parentheses are the standard deviations and coefficient of variations.

2.2. Structural Adhesives

The adhesive used in the test is epoxy resin adhesive; epoxy resin adhesive belongs to the type of thermosetting adhesive, produced by Shanghai Rubber Products Co., Ltd., Shanghai, China, which has good insulation performance, water and oil resistance, high strength, and normal temperature curing. Its elastic modulus is 1100 MPa, tensile strength is 45 MPa, and shear strength is 12 MPa; its hardness should be at least Shore D70.

3. Overview of the Experiment

3.1. Specimen Design and Fabrication

To investigate the stress performance of the reinforced beams, the specimens were subjected to six tests, as shown in Figure 1c, with a length of 2000 mm and a clear span of 1800 mm. The holes of specimens L-1~L-3 were located along the combined beams at 1/4, 1/5, and 1/3 of the length, the hole sizes and thin-walled and webbing section thicknesses of specimens L-4~L-6 varied, and the openings were all at 1/4. The basic parameters of the specimen are shown in

Table 3. Basic parameters of specimens.

Specimen Number	L/mm	Hole Size/mm	hfb/mm	tfb /mm	tws/mm	bzy /mm	tzy/mm	Hole Location
L-1	2000	125 × 200	150	20	1.5	100	20	1/4
L-2	2000	125 × 200	150	20	1.5	100	20	1/5
L-3	2000	125 × 200	150	20	1.5	100	20	1/3
L-4	2000	100 × 160	150	20	1.5	100	20	1/4
L-5	2000	125 × 200	150	15	1.5	90	20	1/4
L-6	2000	125 × 200	150	20	2	100	20	1/4

h_fb is the height of the bamboo web, t_fb is the width of the bamboo web, t_ws is for the thickness of steel sections, b_zy is the width of the bamboo flange, and t_zy is the thickness of the bamboo flange.

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3.2. Loading Device and Measurement Point Arrangement

The test specimen was placed under the reaction frame and loaded with a hydraulic jack using the three-point loading method, with a concentrated load applied directly at the centre of the span of the specimen. Rigid shims were arranged at the support to prevent local damage to the structure, as shown in Figure 3. The test was carried out using graded loading, starting with 1 kN, and the specimen was preloaded before formal loading to eliminate residual stresses. Then, the load was held for 3 min until it was loaded with 1/2 of the predicted ultimate load and, thereafter, every 3 kN load, the load was held for 3 min; it was used to observe the change in the specimen under stable load. The manually controlled loading speed is 0.1 kN/s [10].

To measure the deflection change in the test beam in response to loading, one electronic displacement gauge was placed at 50 mm at the left and right ends of each opening, while displacement gauges were arranged in the middle of the test specimen and at 300 mm equidistant positions. To measure the strain changes, strain gauges were uniformly arranged along the beam, around the hole, on the upper and lower flanges, and at the four corner points of the window of the test beam, as shown in Figure 4 [28,29].

Before the test, scattering spots were positioned in the holes and at the wing edge. During the loading process, the deformation of the specimen was monitored using the XTDIC test system and, after the test was over, the displacement and strain fields on the surface of the specimen were measured using images of the scattering spots.

4. Test Results and Analyses

4.1. Destruction Characteristics

The damage characteristics of specimens L1~L6 are similar. Take specimen L-1 as an example: when the load was increased to 50 kN, the bamboo audibly debonded; when it increased again to 66 kN, transverse gaps formed at the upper-right and lower-left corner points of the holes in the bamboo web. Transverse cracks appeared at the connection between the bamboo web and flange above the holes, and the gaps continued to increase in size with continued loading until they encompassed the two sides, as shown in Figure 5a,b. The thin-walled sections buckled at the diagonal corner points of the holes, as shown in Figure 5d, and the cracks continued to expand, showing signs of penetration. The final result upon termination of the test is depicted in Figure 5c. In conclusion, when 2/3 of the ultimate load was applied, the tensile stress at the two intersections of the upper-right and lower-left holes was greater than that observed for the bamboo material. The bamboo material tore at the two corners, but those in the upper-left and lower-right areas did not tear due to compression; the corner holes on the bamboo webbed plate accumulated stress, and a plastic hinge formed as a result. Thus, the bamboo material broke visibly. Specimen L-4 had a larger bearing capacity, as the holes were smaller. The test beam underwent further deformation, and the thin-walled section buckled, as shown in Figure 5e. Specimen L-6 was thicker and stiffer, so no buckling was observed; see Figure 5f.

4.2. Load–Deflection Curve

In the mid-span load–displacement curves of specimens L1~L6, the performance of the specimens can be divided into three stages characterised by the increase in load. The first stage was the elastic phase and, before 2/3 of the ultimate load was applied, L1~L6 underwent this phase. The specimens deformed linearly as the load increased, and the bamboo audibly debonded during the loading process. No obvious damage was sustained, as the steel–bamboo combination proved successful, and the load was borne by the bamboo-adhered panel profiles. In the second phase, the elastic–plastic stage, more than 2/3 of the ultimate load was applied. The deflection growth rate of the test beam was nonlinear with the increase in load, the slope of the curve decreased, and shear deformation occurred in the holes of the web. The corner holes were the first to enter the plastic stage due to the concentration of stress, and cracks appeared in the two corners that were subjected to the highest stress, which gradually spread towards the ends of the beam as the load continued to increase. The final phase was the destruction stage. When the ultimate load was applied to the test beam girder, the transducer value decreased rapidly, the corner hole of the bamboo material yielded, and larger cracks developed. Plastic hinge appeared at the lower-left and upper-right corners of the holes; the shear deformation increased significantly; and, finally, the test beam lost its load-bearing capacity due to shear damage.

According to the literature [33], f ≤ L/250 represents the deflection control value of a normal specimen, and the mid-span deflection of the reinforced beam specimen is 7.2 mm. The corresponding load value represents the normal load-bearing capacity, and the main test results are shown in

Table 4. Test results for the reinforced beams.

Specimen Number	Normal Service Limit Mid-Span Deflection	Normal Use Ultimate Load Capacity	Ultimate Load Test Value	Ultimate Load Mid-Span Deflection	Ultimate Load Test Value/Normal Use Ultimate Load Capacity
L-1	7.2	70	95	14.2	1.35
L-2	7.2	65	102	16.6	1.56
L-3	7.2	68	87	11.8	1.27
L-4	7.2	78	130	22.0	1.66
L-5	7.2	67	81	11.6	1.20
L-6	7.2	67	104	15.4	1.55

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4.3. Parametric Studies

4.3.1. Effect of Hole Location

The hole locations in specimens L1~L3 varied, but the rest of the parameters were the same. As can be seen from Table 4, the ultimate load-carrying capacity of L-1 increased by 9.2% compared with that of L-3. The ultimate load-carrying capacity of L-2 increased by 17.2% compared with that of L-3. The closer the location of the holes was to the support region of the test beams, the better the load-carrying capacity of the test beams and the higher the change in deflection. The closer the hole is to the loading point, the larger the bending moment and the same for shear force, so the bearing capacity is small.

4.3.2. Effect of Hole Size

Specimens L1 and L4 had different hole sizes. That of specimen L4 was 0.8 times larger than that of L1, but the rest of the parameters were the same, as can be seen from Table 4. The load-carrying capacity of L4 increased by 36.8%, and the deformation-bearing capacity increased by 54.9%. It can be seen that the change in hole size of the reinforced beam has a greater impact on the bearing capacity of the beam. The reason for this is that, with an increase in the size of the hole in the web, smaller bamboo and steel sections remain in the web, weakening the capacity to resist bending moments and shear forces, resulting in a significant decrease in the moment of inertia. The stiffer the reinforced beam, the greater the shear sub-moment and the greater the hole angle in the shear and shear sub-moment of the action of the early emergence of plastic hinges; thus, the reduction in the load-carrying capacity is clear.

4.3.3. Effect of Bamboo Web Thickness

Compared with specimens L1 and L5, the thickness of the bamboo web is reduced by 25%, and its flange width is also reduced by 10%, as shown in Table 4. Its load-carrying capacity is reduced by 17.2% and the deformation-bearing capacity is reduced by 22.4%. It can be seen that the load-carrying capacity of the reinforced beams increases with the increase in the thickness of the bamboo web, which also improves the deformation-bearing capacity.

4.3.4. Influence of the Thickness of Thin-Walled Steel Sections

Specimen L1 was 33% thicker than specimen L6, but the other parameters were the same. Its load-carrying capacity increased by only 9%, while its deformation-bearing capacity increased by 8%. Increasing the thickness of the solid steel sections reinforced the beam and marginally improved its deformation-bearing capacity. During the destruction of the reinforced beam, the holes at the four corners of the cross-section widened. The holes on the left side of the upper and lower cross-sections of the sub-moment were subjected to negative moments, while the holes on the right side of the upper and lower cross-sections of the sub-moment were subjected to positive moments. These were located on the top and bottom of the holes on the bamboo boards and steel plates.

4.4. Deflection Distribution Along the Beam Length

The distribution of deflections along the lengths of specimens L1~L3 under different loads is shown in Figure 6. When the load is small, the deflection curve of the reinforced beam is symmetrically distributed. The deflection change in the reinforced beam increases linearly with the increase in load but, in the same position, it is small. When a 2/3Pu load is applied, the deflection distribution is no longer linear, but the deflection in the reinforced beam hole region changes abruptly. The deflection closest to the mid-span loading point underwent the largest change, which is similar to the deflection along the reinforced beam. Finally, deformation occurred in the region of the reinforced beam hole that sustained damage.

Figure 7 shows the XTDIC image displacement calculation results. This image shows the same trend for specimen displacement from the left hole to the right hole (near the loading point), increasing step by step. The right hole displacement is larger. It is more consistent with the specimen deformation characteristics.

4.5. Strain Distribution in the Hole Section

The strain distribution in the vicinity of the reinforced beam hole is illustrated in Figure 8, using L-1 as an example. After the opening of the web, the webbed section significantly weakened and there was an obvious stress concentration phenomenon. However, a larger shear deformation occurred in the section, so the longitudinal strain in the hole region is roughly an S-shaped distribution along the height of the hole, which no longer satisfies the flat section assumption. As can be seen from Figure 8a, in the left hole, the bamboo flange is compressed and the bamboo web is tensile. The right-end hole is exactly the opposite of the left end section, as shown in Figure 8c. For specimen L-1, when a 2/3Pu load was applied, the hole began to yield at the upper-right and lower-left two corner points. There was an intense stress concentration with the increase in the load. In the diagonal direction, deformation is obvious and a plastic hinge gradually formed, indicating specimen damage. Figure 9 shows the XTDIC strain calculation results of specimens L1~L6. It can be seen that the strain change (Figure 7) coincides with this event. Obviously, the strain in the diagonal two corner points is the largest. Shear damage occurred in this specimen first. The hole region was the first to enter the plastic stage; plastic deformation spread toward the solid web cross-section and the flange edge. The loading point of the bending moment is large, and the corner point of the weakest specimen leading to stress concentration first produced deformation.

5. Finite Element Analysis

5.1. Mechanical Properties of Materials

To investigate the effect of different parameters on the force performance of box beams, nonlinear finite element analysis calculations of the combined beams were carried out using abaqus 2021 (France) software.

5.1.1. Steel Sections

The mechanical properties of the steel sections were measured as described above; the specific values are listed in Table 2. Since Q235 steel is an isotropic material, during the test, it yielded without strengthening; as such, the steel section in this numerical simulation can be used as an ideal elastic–plastic model, adhering to the von Mises yielding criterion.

5.1.2. Bamboo Plywood

Bamboo plywood is created by combining resin with three layers of bamboo bundles. Bamboo plywood is a nonhomogeneous anisotropic material. A finite element simulation was applied using the Hill yield criterion. The mechanical properties of bamboo plywood and the ontological relationship were measured, as shown in Table 1, and the measurements were repeated during the plastic stage, as shown in

Table 5. The value of the potential function in the plastic phase.

R11	R22	R33	R12	R13	R23
0.744	0.313	0.313	0.25	0.25	0.05

. R indicates the strength of bamboo plywood in different directions and its tensile and compressive strength ratios. The anisotropic elastoplasticity model was employed in this simulation.

5.1.3. Structural Adhesive

The cohesive model is used to simulate the adhesive layer between the bamboo adhesive plate and the steel section to simplify the interaction forces at the steel–bamboo interface and simulate the failure of the structural adhesive and the damage of the adhesive layer through measuring the functional relationship at the interface stress and relative displacement. Referring to the study by Xu et al. [34] on the performance of the adhesive layer at the interface of combined steel–bamboo box girders, Quads Damage is adopted as the initial damage criterion and the B-K Damage Criterion is adopted to describe the damage evolution in this paper. A bilinear cohesion model is also adopted to simulate the steel–bamboo interface to satisfy the computational accuracy (see Figure 10). In the initial stage, cohesive unit stress increases with relative displacement. When the displacement is ${\delta }_0$, cohesive stress reaches the maximum value and, at this point, bamboo damage begins to occur. When the relative displacement reaches ${\delta }_{\mathrm{m}}$, complete cracks develop and the adhesive layer fails completely. The cohesion parameter is shown in

Table 6. Cohesive model basic properties.

E /MPa	G1 /MPa	G2 /MPa	σnmax /MPa	τsmax /MPa	τtmax /MPa	GN /(J·mm−2)	GS /(J·mm−2)	GT /(J·mm−2)
1500	1500	1500	13.6	13.7	13.7	0.32	0.41	0.41

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5.2. Cell Types and Meshing

In this paper, an eight-node cell (C3D8R) is used for the steel profile. The bamboo glue board adopts a 20-node secondary hexahedral unit (C3D20R) in order to meet the requirements of calculation accuracy. This model adopts a structured mesh, the grid size of this paper is 10 mm, the hole area is encrypted with a grid size of 5 mm, and the mesh is divided, as shown in Figure 11.

5.3. Comparison Between Test Results and Finite Element Results

5.3.1. Comparison of Damage Characteristics

Taking specimen L-1 as an example, Figure 12 depicts a local damage diagram created using FEA. The results of the simulation are consistent with the test results. The stress was mainly concentrated in the hole corner, beginning during the plastic phase and then spreading around the corner. The initial stress in the corner was too great, and the entire reinforced beam underwent shear deformation. The damage to the hole area is obvious, as is the shear damage to the corner. As shown in Figure 12, the finite element simulation and test results are consistent, showcasing the reliability of the finite element analysis.

5.3.2. Comparison of Load–Deflection Curves

The finite element analysis results of each specimen agree well with the test results, especially in the elastic phase when there is a high degree of overlap in the linear growth curve. In the plastic phase, the material was ideal and, thus, the simulation failed to reflect the impact of cracks in the bamboo. The final stress value in the finite element calculations was too large for the material to withstand, so the experimental results contain an approximate 10% error to meet the accuracy requirements. The margin for error is approximately 10%, which meets the accuracy requirements and verifies the accuracy of the finite element analysis results.

5.4. Basic Parameters of Finite Element Analysis

To analyse the effect of the different-shaped holes on the stress performance of the reinforced beams and the damage modes and stress distribution around said holes, five reinforced beams with different holes (A2~A6) were designed based on specimen L-4. Rectangular, square, whole hexagonal, circular, elliptical, and lapped circular holes were included, and the area of each hole was equal to A1 = 16,000 mm. An intact beam was provided to act as the control. The other geometric parameters are shown in Figure 13.

5.5. Analysis of Simulation Results

Load–Deflection Curves

The presence of holes weakens the webbed cross-section, significantly reducing the stiffness and ultimate bearing capacity of the specimen, as shown in Figure 14. The circular holes have the highest bearing capacity, followed by the elliptical holes, the square–hexagonal holes, the rectangular holes, the spliced circular holes, and the square holes. The different hole shapes have a significant impact on the load-carrying capacity of the reinforced beams. The deformation-bearing capacity of the specimens increases with the increase in load, indicating that different hole shapes also influence the deformation-bearing capacity of the reinforced beams. When the load is small, the deflection of each specimen does not differ much. The deflection increases linearly with the load, the slope of the curve decreases, the stiffness of the specimen decreases, the deformation shows a clear acceleration, and displacement increases continuously with a very small increase in load until the specimen shows signs of damage.

As shown in Figure 15, all the specimens initially showed a stress concentration at the corner point of the hole, and the stress then spread from the corner of the hole to the flange, with a greater concentration near the hole. The rectangular, square, hexagonal, and spliced circular holes had sharp corners, resulting in an excessive shear sub-bending moment and the most severe stress concentration, and all the polygonal specimens formed plastic hinges at the corners of the holes. Following the same trend, the shear strain is greater compared to that in the plastic hinges and, thus, the load-carrying capacity and deformation-bearing capacity are poor. There are no sharp corners around the circular and elliptical holes, and the connection forms a rounded excess; the stress concentration phenomenon is unclear. Additionally, the shear sub-moment is also smaller and, therefore, the higher the ultimate load, the greater the deflection. As shown in

Table 7. Simulation results of different hole shapes.

Specimen Number	Hole Shape	Ultimate Carrying Capacity	Ultimate Deflection
A1	rectangular	120.14	22.41
A2	square	107.92	20.78
A3	regular hexagon	114.44	21.10
A4	circular	145.41	27.11
A5	elliptical	132.13	24.75
A6	lapped circle,	127.77	23.88
A7	unhole	182.23	43.21

, the load-carrying capacity of the circular-hole-reinforced beams increased by 34.7% compared with that of the square-hole-reinforced beams. Since A7 had no holes, it avoided concentrated stress, improving both its load-bearing capacity and its deformation capacity. The load-carrying capacity of the reinforced beams with no holes increased by 68.8% compared with that of the square-hole-reinforced beams, and the deformation capacity doubled. According to the finite element analysis, the damage of the reinforced beams without any holes mainly occurred at the loading point and the support area, since the bearing capacity was too large. Local extrusion resulted in an increase in the relative displacement, the adhesive layer reached the critical degradation value, and the cohesion unit reached the damage threshold, ultimately leading to the total failure of the adhesive layer.

6. Conclusions

In this paper, a type of reinforced open bamboo box girder member with a built-in steel profile is proposed. The following conclusions are drawn from tests and finite element analyses.

Under the joint action of bending and shear force, the reinforced beam can maintain good integrity and the structural adhesive can effectively bond the bamboo veneer and the steel section. During the entire loading process, no obvious interfacial bond damage occurs between the steel section and the bamboo veneer and the reinforcing effect is good; it is a very good building material for the future.

The damage form of the new reinforcement is different from that of a traditional bamboo beam, mainly consisting of shear damage in the hole area and the formation of plastic hinges at the corners of the holes. The appearance of the holes weakened the effective cross-section capable of resisting bending moments and shear forces, resulting in a significant decrease in the moment of inertia and the stiffness of the reinforced beam, and the hole area became the weakest part.

The factors that have the greatest influence on the load-carrying capacity of reinforced beams are, in order, the hole size, the bamboo webbing thickness, the hole location, and the steel section thickness. The closer the hole location was to the loading point of the reinforced beam, the smaller the specimen load-bearing capacity was.

Different opening shapes have a greater impact on the shear capacity of the reinforced beams, in which the stress concentration phenomenon of the reinforced bamboo box beams with circular holes appeared later and was not obvious compared with that of square holes. In addition, the bearing capacity of reinforced beams with open square holes is reduced by 10%–25%compared with circular holes.