Study on bearing capacity of LVL asymmetric truss of lightweight poplar wood

Wan, Z., Zhang, Y., Liu, Y., Fang, X., and Gong, M. (2024). “Study on bearing capacity of LVL asymmetric truss of lightweight poplar wood,” BioResources 19(1), 716-731.

Abstract

Compared with natural wood, laminated veneer lumber has the characteristics of high strength, flexible specifications, excellent stability, and good economy. In order to study the bearing capacity of LVL trusses, the mechanical properties of LVL materials were tested. The static load test was carried out by using 3 pieces of LVL truss, and the load-deflection relationship, load-strain relationship, bearing capacity and failure mode of LVL trusses were studied. Based on the simplified joint analysis method, the metal plate connection and bolted truss were analyzed, and the bearing capacity calculation formula was developed. The results show that the upper chord instability is the main failure mode for large-span light LVL truss. A simplified formula for bearing capacity of LVL truss was proposed, and the predicted results were in good agreement with the experimental results. Finally, an application example of LVL truss engineering design and construction is briefly introduced. The research can provide technical support for the promotion and application of LVL truss of lightweight poplar wood.

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Study on Bearing Capacity of LVL Asymmetric Truss of Lightweight Poplar Wood

Zeqing Wan,^a,b Yu Zhang,^a Yan Liu,^a,* Xiuyuan Fang,^a and Meng Gong ^c

DOI: 10.15376/biores.19.1.716-731

Keywords: Laminated veneer lumber; Truss; Experimental study; Bearing capacity; Deflection

Contact information: a: College of Civil Science and Engineering, Yangzhou University, Yangzhou 225127, China; b: State Key Laboratory of Green Building in Western China, Xi’an University of Architecture & Technology, Xi’an 710055, China; c: Wood Science and Technology Center, University of New Brunswick, Fredericton, New Brunswick E3C 2G6 Canada; *Corresponding author: liuyan@yzu.edu.cn

INTRODUCTION

Early Chinese buildings were mainly built in the form of wooden structures. The discovery of wooden structures at the Hemudu site confirms that China had advanced mortise and tenon joint technology a long time ago, indicating that wooden structures had already experienced a long-term development process. In recent decades, wooden structures have experienced rapid development in China, with a large number of wooden houses, particularly lightweight wooden structures, being built. This article introduces a new type of structural material: laminated veneer lumber (LVL). LVL has the characteristics of high strength, flexible specifications, good stability, excellent seismic performance, good economy, and better fire resistance compared with natural wood. The dimensions of this material are not limited by the size of logs or the specifications of individual veneers, allowing for standardized and serialized production and application (Zhou et al. 2013). During the production process, veneers are graded according to certain standards, resulting in standard products of different quality grades. This material fully meets the needs of large-span beams, vehicles, and ships, with flexible and variable specifications that can be freely chosen.

Concrete and steel are widely used in modern structures, but they cause pollution during production and construction. Once concrete structures are formed, they become irreversible, and their demolition results in a large amount of construction waste. LVL is an engineering wood product made of veneers laid in the same direction and bonded with phenolic adhesive. Compared with solid wood, this kind of engineering wood not only can improve the utilization rate of raw materials, but it also can disperse the defects such as knots and cracks of logs to reduce the variability of materials. LVL can be classified into structural LVL (load-bearing components) and non-structural LVL (non-load-bearing components) based on its usage. Extensive research has been conducted on the mechanical properties of LVL materials both domestically and internationally (Tenorio et al. 2011; Wei et al. 2013; Li et al. 2019; Zhang et al. 2020; Mohammad et al. 2022), but their structural applications are limited (Ali et al. 2019; Francisco et al. 2020). Study on the structural performance of LVL components is needed (Rescalvo et al. 2021).

There are many studies on light wood trusses with different connection methods and different truss types (Xiao et al. 2014; Sagara et al. 2017; Pratiwi and Tjondro 2018). Barbari et al. (2014) introduced a new connection system for traditional timber trusses and conducted the finite element analysis on bearing capacities, discovering that the bearing capacity of the developed connection systems was four times higher than the design value. Osama et al. (2022) introduced potential sustainability solutions for building structures, and comparisons were made between two load-bearing columns with different building materials – glued laminated timber and concrete – regarding structural design, economic consequences, and the emission of greenhouse gases.

The mechanical properties of joint connections are an important aspect of wood structure design and research. Metal-plate-connection wood trusses are widely used in civil buildings as roof and floor systems. Ling et al. (2022) explored the simulation model and analysis software for predicting the performance of metal-plate-connected wood joints. Yeoh et al. (2004) investigated the effect of metal plate connected joints on strength properties of rubberwood LVL. The study showed no negative effect on the strength of LVL as compared to their equivalent species of sawn timber in metal plate connected joints. Su et al. (2022) tested and calculated the mechanical performance of 12 polar LVL truss joint specimens connected by bolts and tooth plates to provide a necessary basis for the application of polar LVL in light wood truss.

Currently, LVL structural materials have matured in developed countries such as the United States, Japan, and Europe, while China is still in the development stage (Masaeli et al. 2022). The research and production of LVL mainly has focused on non-structural boards, with limited studies on the application of structural materials. Developed countries have extensive experience and expertise in wood roof systems, especially in Canada, where long-term experiments and research have been conducted on wood truss roof systems. However, China’s modern wood structures are in the stage of revival, and there is relatively limited accumulated experience in wood truss roof systems (Li et al. 2022). In practical projects, the use of asymmetric trusses may be less common, but they are nevertheless indispensable. Such instances may arise from architectural aesthetics requirements or in structural retrofitting projects where the new structure’s span differs from the existing one, but in order to maintain the consistency of the roof ridge line, the asymmetric trusses are essential. This study conducted static load tests on three pieces of LVL trusses with span of 6.5 m to investigate their load-deflection relationship, load-strain relationship, bearing capacity, and failure pattern. Emphasis was placed on the bearing capacity of the asymmetric truss. The research results have practical significance for the protection, utilization, and inheritance of wood structure buildings, and the research and development of LVL material for building structures not only can achieve considerable economic benefits, but also, they can provide a new choice for wood structure buildings.

EXPERIMENTAL

Element Design

Mechanical properties test of materials

The material tests mainly refer to the Chinese specification GB/T 1928-2009 (2009) and GB/T 20241-2021 (2021). The materials used in this test were from the same batch as those used in the truss bearing capacity test. The dimensions of the material test specimens were determined according to American specifications ASTM D143-2014 (2014). A total of 15 specimens were processed for the wood compressive strength test, including 5 specimens for parallel to grain compression, 5 specimens for diagonals to grain compression, and 5 specimens for perpendicular to grain compression. The schematic diagram of compressive strength test specimen is shown in Fig. 1. The specific dimensions were as follows, 40×40×200 mm for parallel to grain compression and perpendicular to grain compression, 40×40×150 mm for diagonals to grain compression. The loading instrument used was the electronic universal testing machine WDW100.

Fig. 1. Compressive strength specimen

In the mechanical properties test of LVL, 5 specimens were used to test the tensile strength parallel to the grain. The tensile specimen size was 15×15×200 mm, as shown in Fig. 2. The test is based on the Chinese specification “Method of testing strength parallel to grain of wood (GB/T 1938-2009)” (2009).

Fig. 2. Tensile strength specimen parallel to grain

Static test of light LVL truss

There are different types of connections of wooden trusses, such as mortise-tenon connection, nail connection, metal plate connection, bolt connection and so on (Ling et al. 2022). The bolt connection is the most common connection method in modern wood structure. Meanwhile, metal plate connection is widely used in the field of light wood structure. So, two types of trusses, bolted and metal-plate connection, were tested. The truss specimens used in Fig. 3 are the asymmetric frame connected by metal plate, the symmetrical truss connected by metal plate and the asymmetric truss connected by bolt, named HJ1, HJ2, and HJ3 in turn. The section size of the truss rod was 40×90 mm according to the technical specifications for light wood trusses. The metal plate used in the specimen was TR-1, which was made of Q235 carbon steel with tooth plate size of 75×114 mm.

Fig. 3. Truss specimens

Experimental Loading Programme

The processes for manufacturing LVL metal plate joint trusses mainly include construction preparation, measurement and layout, rod cutting, truss assembly, static pressure metal plate installation, and finished product maintenance. The process for manufacturing LVL bolt joint trusses mainly includes construction preparation, measurement and layout, rod cutting, hole drilling (for steel plates and truss members), hole cleaning, and truss assembly (Mendis et al. 2019).

A self-balancing test device was used, as shown in Fig. 4. The specimens are shown in Fig. 5, and the arrangement of measurement points is shown in Fig. 6. To ensure the lateral stability of the LVL truss, lateral supports were set at the indicated positions and the H-beam acted as a reaction beam. The truss, lateral supports, and H-beam form a self-balancing system and the truss was connected by knife hinges on both sides. The tested parameters include deflection and strain. The load was only applied to the upper chord joints of the truss. According to the standard method, the design load at the upper chord joints of the truss was 1200 N. The experiment adopted graded loading method, with 1/6 of the design load as one loading level.

Fig. 4. Test device

Fig. 5. Truss test piece

Fig. 6. Strain gauges layout points

RESULTS AND DISCUSSION

The Properties of LVL Materials

The compressive strength of wood f_c, which obtained by dividing the final failure load by the actual sectional area of specimen, is expressed as follows,

(1)

where f_c is compressive strength of specimen with moisture content of w% (MPa), P_uc is fracture load (N), A is the cross-sectional area of specimen (mm²), according to the equation A=b×h, b and h are the sectional width and thickness of specimen (mm), respectively.

The formula for compressive modulus of elasticity (E_c) is expressed as follows,

(2)

where E_c is compressive elastic modulus of wood parallel to grain (MPa), A is the sectional area of specimen tested (mm²), ΔF_c is the compression load increment below the proportional limit (N), and Δε_c is the compressive strain increment under the compression load increment ΔF_c.

The experimental data of Poisson’s ratio, elastic modulus, and compressive strength parallel to grain, diagonals to grain and perpendicular to grain are shown in Tables 1, 2, and 3, respectively.

Table 1. Poisson’s Ratio, Modulus of Elasticity and Compressive Strength Parallel to Grain of Polar LVL Specimens

Table 2. Poisson’s Ratio, Modulus of Elasticity and Compressive Strength Diagonals to Grain of Polar LVL Specimens

Table 3. Poisson’s Ratio, Modulus of Elasticity and Compressive Strength Perpendicular to Grain of Polar LVL Specimens

The ultimate tensile strength f_t obtained by dividing the maximum tensile load by the cross-sectional area, can be expressed as follows,

(3)

where f_t is tensile strength of specimen with moisture content of w% (MPa), P_ut is fracture load (N).

The tensile elastic modulus of LVL along grain direction is calculated according to the following formula.

(4)

where E_t is tensile elastic modulus of wood parallel to grain (MPa), ΔF_t is the tension load increment below the proportional limit (N), and Δε_t is the tensile strain increment under the tension load increment ΔF_t. The tensile elastic modulus and ultimate tensile stress of LVL are shown in Table 4.

Table 4. Tensile Elastic Modulus and Strength of LVL

Figure 7 exhibits the LVL compression/tensile stress-strain curves. As can be seen from the curves, the general shape and smoothness of the compressive stress-strain curve are similar. LVL undergoes three stages during compression: 1) in the initial stage of loading, the specimen is primarily in the elastic working stage, and when the load is less than the critical load, the stress-strain curve is essentially linear; 2) in the intermediate loading stage, the specimen gradually enters the elasto-plastic working stage, where the stress-strain relationship is no longer linear and the strain growth exceeds the stress growth, resulting in significant plastic deformation of the specimens; 3) in the later stage of loading, the deformation of the specimens increases while the stress suddenly decreases, leading to a compression failure.

The tensile stress-strain curve along the grain direction does not exhibit a yield stage or a descending stage, which indicates that tension failure of LVL is brittle. In the process of tension, the specimen fractures at the weakest point in the fiber tensile strength, resulting in small microcracks. Then, multiple parts of the specimen’s cross-section reach the ultimate tensile strength simultaneously. Finally, the cracks propagate along the weak points of the fiber tensile strength until the specimen is pulled apart. Additionally, through the analysis of the data from the transverse shear test of LVL, it can be concluded that LVL exhibits a brittle behavior when it fails under shear, and the shear strength is relatively low. Therefore, shear loading is a very unfavorable condition for LVL.

Fig. 7. LVL compression/tensile stress-strain curves

LVL Truss

Testing phenomenon

For the LVL truss, there was no obvious deformation phenomenon after incremental loading to design load. When the reading stabilized, the deflection and strain values at various points were recorded. The final failure pattern is shown in Fig. 8, and the results of truss test are summarized in Table 5.

Table 5. Summary of Test Results of LVL Trusses

Specimens HJ1, HJ2, and HJ3 failed after being loaded to 5.0, 4.8, and 5.6 kN, respectively. The failure was mainly due to the instability of components, with little damage at the joints, indicating that instability failure is the main failure pattern in large-span lightweight rotary-cut veneer lumber truss structures.

Fig. 8. LVL truss failure diagram

Load-joint deflection curve

Figure 9 shows the load-nodes deflection curves of specimens tested. Specimen HJ1 failed with an ultimate load of 5.0 kN and a maximum deflection of 20.36 mm at point G in the long span. Specimen HJ2 failed with an ultimate load of 4.8 kN and a maximum deflection of 21.12 mm at point B in the mid-span. Specimen HJ3 fractured with an ultimate load of 5.6 kN and a maximum deflection of 17.96 mm at point H in the long span. It can be seen that the deformation capacity of the three trusses was different. Under the design load, the maximum deflection at each measuring point in specimen HJ3 was 2.73 mm, which was greater than the maximum deflections in specimen HJ1 and HJ2, indicating that specimen HJ3 not only had a greater load-carrying capacity, but also stiffness and deformation capacity, when compared with those of specimen HJ1 and HJ2. This demonstrates that the bolted steel plate connection was more effective than the metal plate connection in enhancing the stability of the truss.

The load-displacement curves illustrate that from being loading to the design load, the load-deflection curve remained linear. From the design load onwards up to twice the design load, the nonlinearity of load-deflection curve was still not significant. After loading at twice the design load, the load-deflection curve showed significant nonlinearity, especially in the later stages of loading, the deflection increased rapidly, resulting in truss instability.

Fig. 9. Load-nodes deflection curves

Load-rod axial strain curve

As shown in Fig. 10, each truss is in a basic elastic working state from loading to design load, and the curve shows a linear relationship. With continuously load to twice the design load, slight nonlinearity begins to appear. The truss enters the elastic-plastic working stage, and the slope of the load-strain curve starts to increase. In this stage, the truss exhibits elastic-plastic deformation. From loading twice the design load to failure of truss, nonlinearity becomes more obvious. Under the same load, component HJ2 shows a greater strain than component HJ1, indicating the asymmetric truss has a higher stiffness than the symmetric truss.

Fig. 10. Specimen rods load-axial strain curves

Theoretical Analysis

Basic assumptions

The support nodes, web members, and ridge joints of the LVL truss were considered as hinged. The web rod acted as a two-force rod, while the top and bottom chords acted as continuous members spanning multiple supports. The bolt connections between the web members and chord members, as well as the metal plate connections, were set as semi-hinged. The right end of the truss was supported by a pinned support, while the left end was supported by a fixed hinge support. The dimensions of the structural calculation model components are the same as the original truss.

Calculation diagram

Figure 11 illustrates the simplified structural calculation diagrams of the trusses tested. The difference between the tension and compression elastic modulus of LVL is small. When the deformation is small, the cross-sectional strain of LVL rod conforms to the assumption of plane section. Therefore, the following basic assumptions are adopted in the simplified calculation. The LVL conforms to Hooke’s law in the elastic stage, all the load acts on the joints, and the cross section keeps plane before and after loading.

Fig. 11. Simplified structural calculation diagram

Bearing Capacity

Calculation model and yield mode of bolt connection

The double cover plates are used for bolt connections. The formula for bearing capacity of each shear surface of bolt is expressed by Eq. 5, where the bolt connection is subjected to force along the grain direction,

(5)

where V is the shear capacity of the bolt each shear surface (N); f_c is the compressive strength of LVL along the grain direction (MPa); d is the diameter of the bolt (mm); k_v is a coefficient related to bearing capacity, and k_v is 7.5 for bolt.

The shear capacity of bolts should be checked by the following formula,

(6)

where N is the axial force (in the direction of rod length) sustained by the bolts (N); n_b is the number of bolts in one side of the connector; and n_v is the number of shear surface for each bolt.

Bolt connection of wood to steel

The bearing capacity of the bolt connection between wood and steel should be calculated according to the compression of pin slot and the bending of bolt.

Calculated according to the compression failure of pin slot

(7)

where V_s is the bearing capacity per shear surface of each bolt (N); a is the thickness of side rod (mm); d is the diameter of bolt (mm); and f_es is the compressive strength of pin slot inside rod (MPa).

2) Calculated according to bolt bending damage

For symmetrical double-shear connections of bilateral steel plates, bolt bending failure is calculated according to two hinge yield mode.

(8)

where V_max is the bearing capacity per shear surface of each bolt (N); f_bs is the bending strength of bolt (MPa); f_ec is the compressive strength of wood pin slot (MPa); k_max is the model coefficient when the bolt has two plastic hinges, and k_max=0.443 for a symmetrical double-shear connection with bilateral steel plates.

The theoretical calculation shows that the bearing capacity of each shear surface is 9.6 kN, the bolts far from reaching the bearing capacity, the joints are not damaged, and the trusses are destabilized.

Stability calculation of axially loaded chord

By calculation, the minimum slenderness ratio of the upper is greater than the flexibility _p. The upper chord of the truss belongs to the slender axial compression rod with both hinged ends, and Fig. 12 shows its calculation model.

According to the material mechanics, the critical force P_cr of the axial compression rod is affected by the constraint of the rod ends. The stronger the constraint, the greater the bending resistance, the higher the critical force. The Euler formula for compression rod with both hinged ends can be expressed as follows,

(9)

where EI is the bending stiffness of the rod; l is the length of the rod; and m is a factor of length, which is related to the rod end constraints. Here, m=1 for the case of hinge at both ends. According to the above formula, the critical force P_cr is 16.9 kN, and the calculated bearing capacity of the truss is 4.1 kN, while the tested bearing capacity of the asymmetric truss is 5.6 kN.

Fig. 12. Stability calculation model for compression rod

Engineering Application Case

Fig. 13. Joint construction diagram

This project is a wood engineered structure with a building area of 520 m². In this project, the structure consists of a LVL frame system. The frame beams and columns were both made of LVL. The joint form adopted in the truss is shown in Fig.13, with bolt joints used for supports and ridge connections, and metal plate joints for other connections. To ensure the harmonious appearance of the building and the consistent linearity of the roof ridge, the symmetrical and asymmetrical LVL trusses tested above were adopted in the roof truss.

The engineering project site is shown in Fig. 14. When the truss span reaches 6.5 m, the truss is prone to buckling failure. It is recommended to control the truss span appropriately or take measures to enhance the stiffness of the top chord. For LVL roof systems in China, it is suggested to widen the spacing between trusses to 1.2 m. If larger spacing is needed, the bearing capacity needs to be checked.

Fig. 14. As-built drawings

CONCLUSIONS

In this paper, three laminated veneer lumber (LVL) trusses with a span of 6.5 m were tested under static loading. Through this testing, the load-deflection relationship, load-strain relationship, and failure mode of LVL truss were obtained. There are many factors affecting the bearing capacity of lightweight LVL trusses, mainly including the physical and mechanical properties of LVL, the performance of metal plates and bolt connections, and the dimensions of LVL trusses. The uncertainty of these factors results in a certain degree of discreteness of bearing capacity of LVL trusses. In view of the popularization of this material in truss structures, the related problems need to be further studied. Aspects needing more attention include optimization of the size of structural members to be stressed and additional analysis of flexural changes as a potential vulnerable aspect. According to the experimental research and theoretical analysis, the principal results can be summarized as follows:

From the initial loading to the design load, LVL trusses are in a basically elastic working state, and the load-strain curve shows a linear relationship. From the design load to twice the design load, slight nonlinearity begins to appear. The trusses enter the elastic-plastic working stage, and the slope of load-strain curve starts to increase. In this stage, the trusses undergo elastic-plastic deformation. Nonlinearity becomes evident as the trusses are loaded from twice the design load to failure.
Bolted truss joints have high stiffness and good load transmission ability, so stability is the main factor affecting the bearing capacity of bolted truss joints.
For large-span lightweight LVL trusses, the main failure mode is upper chord instability. Meanwhile, the ultimate load of truss is far from the bearing capacity of the joint failure.

ACKNOWLEDGMENTS

The authors are grateful for the supports of the National Natural Science Foundation of China (Grant No.51878590), and The Opening Fund of State Key Laboratory of Green Building in Western China (No.LSKF202324), and Yangzhou Government-Yangzhou University Cooperative Platform Project for Science and Technology Innovation(No.YZZP202206).

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Article submitted: October 3, 2023; Peer review completed: October 28, 2023; Revised version received and accepted: November 7, 2023; Published: December 6, 2023.

DOI: 10.15376/biores.19.1.716-731