Abstract
Glued laminated timber beams are vulnerable to brittle tensile failure in the tension zone, which limits their flexural efficiency and application in modern timber structures. This study investigated two reinforcement strategies for such beams: external steel reinforcement and bottom steel plate reinforcement connected with self-tapping screws. Unreinforced beams were tested as controls. Full-scale four-point bending tests were conducted to evaluate failure mode, load-deflection response, load-strain behaviour, bearing capacity, stiffness, and ductility. A three-dimensional finite element model was then developed and validated against the experimental results. The control beams failed in brittle tension, with an average ultimate load of 21.6 kN. The externally reinforced beams showed no effective improvement in flexural behaviour because prestress transfer and steel-timber interaction were insufficient. In contrast, the steel plate reinforcement restored 76% of the original load-bearing capacity of damaged beams and improved deformation capacity, although failure was still governed by timber delamination and screw shear. The validated numerical model reproduced the full loading-to-failure process and can support the design and optimization of reinforced timber members.
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Effect of Different Reinforcement Methods on the Flexural Performance of Glued Laminated Timber Beams
Yaoyao Du,* Biqing Shu, Chen Li, Xudong Zhu, and Xuewen Zhang
Glued laminated timber beams are vulnerable to brittle tensile failure in the tension zone, which limits their flexural efficiency and application in modern timber structures. This study investigated two reinforcement strategies for such beams: external steel reinforcement and bottom steel plate reinforcement connected with self-tapping screws. Unreinforced beams were tested as controls. Full-scale four-point bending tests were conducted to evaluate failure mode, load-deflection response, load-strain behaviour, bearing capacity, stiffness, and ductility. A three-dimensional finite element model was then developed and validated against the experimental results. The control beams failed in brittle tension, with an average ultimate load of 21.6 kN. The externally reinforced beams showed no effective improvement in flexural behaviour because prestress transfer and steel-timber interaction were insufficient. In contrast, the steel plate reinforcement restored 76% of the original load-bearing capacity of damaged beams and improved deformation capacity, although failure was still governed by timber delamination and screw shear. The validated numerical model reproduced the full loading-to-failure process and can support the design and optimization of reinforced timber members.
DOI: 10.15376/biores.21.3.7220-7246
Keywords: Glulam beams; Flexural performance; Steel reinforcement; Failure mode; Finite element modeling
Contact information: School of Civil Engineering and Transportation, Yangzhou Polytechnic Institute, Yangzhou 225127, China; *Corresponding author: 1569864882@qq.com
INTRODUCTION
The transition of the construction industry towards a green economy has become an irreversible trend within the global macro-strategic framework. This is due to the need to address climate change and advance sustainable development (Tam et al. 2017). In order to achieve the strategic objectives of “carbon peaking and carbon neutrality,” the construction sector is compelled to move away from its conventional model, which is characterised by high energy consumption and high greenhouse gas emissions. This transition necessitates a fundamental shift toward low-carbon, environmentally friendly, and energy-efficient construction practices. In the context of the construction industry, timber is being re-evaluated and increasingly recognised for its critical value. This is because timber is a natural, renewable, bio-based material with significant carbon sequestration capacity. It is evident that timber structures satisfy the fundamental criteria for sustainability by virtue of their superior ecological performance. Moreover, they are commensurate with contemporary aspirations for green and healthy living environments. The inherent natural affinity of these materials, in conjunction with their ability to create comfortable indoor spaces and their excellent physical and mechanical properties, serves to further enhance their appeal (De Araujo 2023; Wang et al. 2024a). Among various engineered wood products, glued laminated timber (Glulam) occupies a central role in modern timber construction. The primary factors contributing to this phenomenon are its high material efficiency, stringent quality control measures, adaptable dimensional design capabilities, and exceptional mechanical strength. Glulam is particularly well-suited for large-span, expansive, and architecturally complex projects (Shu et al. 2020; Zhao et al. 2024; Liu et al. 2025). Such projects include, but are not limited to, stadiums, convention centers, transportation hubs, and cultural landmarks. Glulam offers important advantages and considerable application potential in such projects.
The mechanical performance of glulam beams is of particular significance within the context of timber structural systems, given its role as the most crucial bending element. Nevertheless, the mechanical behaviour of glulam beams is subject to certain intrinsic limitations. Although the tensile strength parallel to grain of small clear wood specimens may exceed the compressive strength parallel to grain, tensile failure of glulam beams in bending is typically brittle and is highly sensitive to knots, finger joints, local defects, lamination interfaces, and size effects. Consequently, flexural failure in glulam beams often initiates in the tension zone on the underside of the beam, where localized tensile rupture or splitting can develop suddenly (Yang et al. 2016b). Such failures often occur without noticeable warning, presenting a significant risk to structural safety. Moreover, prevailing design standards stipulate that the deflection of glulam beams must adhere to serviceability limit state criteria. Consequently, the design of glulam beams is predominantly governed by deformation control, or stiffness control (GB/T 50708 2012; EN 14080 2013). This design philosophy frequently results in the selection of cross-sectional dimensions that exceed the requirements based solely on load-bearing capacity calculations. Consequently, the high compressive strength of the timber in the upper compression zone is largely underutilised. This not only constitutes a considerable inefficiency in terms of material properties, but it also restricts the economic feasibility and broader application of glulam in structures with larger spans. Given the increasing demand in modern architecture for greater spatial spans, addressing key technical challenges has become imperative. These include enhancing the bending load-bearing capacity of glulam beams, improving their failure modes to avoid sudden brittle fractures, and optimizing the utilization efficiency of the cross-sectional area. Achieving these advancements is essential for overcoming current limitations and realizing the full potential of glulam in advanced timber engineering applications (Yang et al. 2016a; Wang et al. 2024b; Li et al. 2025).
To address the inadequate tensile performance at the bottom of glulam beams and the associated risk of brittle failure, researchers have undertaken extensive and pioneering studies, introducing a variety of structural reinforcement and strengthening methodologies. Among these approaches, fibre-reinforced polymer (FRP) reinforcement technology has emerged as the most extensively utilized and rigorously investigated method, because of its outstanding mechanical properties. There is a substantial body of experimental evidence (Gunasekaran et al. 2026) that attests to the efficacy of bonded composite materials when applied to the tension zone surface of glulam beams. This efficacy is further compounded by the incorporation of carbon fibre reinforced polymer (CFRP) (Fossetti et al. 2015; Glišović et al. 2016; He et al. 2022; Karagöz İşleyen et al. 2023), glass fibre reinforced polymer (GFRP) (Guan et al. 2005; Raftery and Whelan 2014; Thorhallsson et al. 2017), and fabrics (Uzel et al. 2018; Di et al. 2023) embedded within the tension zone. These measures have been shown to result in a substantial enhancement of the flexural strength of beams, stiffness, and deformation capacity. These polymer-based composites offer notable advantages, such as low weight, high tensile strength, and superior durability, while also establishing an effective synergistic stress transfer mechanism with the wooden matrix. In addition to FRP-based techniques, steel reinforcement strategies have also proven to be highly effective (De Luca et al. 2012; Soriano et al. 2016). Conventional approaches, including the embedding of steel rebars or the bonding of steel plates within the tension zone, not only facilitate a more uniform distribution of tensile stresses but also significantly enhance the overall ultimate load-carrying capacity of the structure. These methods concurrently improve the ductility of the beam, shifting the failure mode away from the typically brittle fracture characteristic of wood to a more controlled and predictable sequence: initial yielding of the steel reinforcement, followed by compressive failure of the wood (Kliger et al. 2007). Further notable advancements in this field include the integration of high-strength compressed timber elements during the lamination process (Anshari et al. 2012; Anshari et al. 2017), as well as the development of hybrid reinforcement systems that capitalize on the synergistic effects between steel or concrete and composite materials (Ferrier et al. 2012). These multifaceted technical strategies work together to optimize the mechanical performance of glulam beams across various structural dimensions. Collectively, these research findings not only contribute to the refinement of structural design theories specific to glulam beams but also offer dependable engineering guidelines for their practical application. They unequivocally demonstrate the technical viability and real-world applicability of mitigating the inherent tensile limitations of timber through the use of external reinforcement methods.
While the reinforcement methods discussed above have demonstrated substantial improvements in the performance of glulam beams, a more detailed examination of their timing and application contexts reveals that these techniques can be broadly classified into two conceptual categories: “design-stage integrated reinforcement” (or “pre-failure reinforcement”) and “post-construction remedial reinforcement” (or “post-failure reinforcement”). Each category presents unique technical challenges and practical limitations during implementation. The concept of “integrated reinforcement” entails the incorporation of reinforcement materials, such as steel bars or FRP rods, within the wood laminations during the design and manufacturing phases of glulam beams. This process effectively bonds the reinforcement materials into a cohesive structural element, thereby enhancing the overall strength and durability of the glulam beams. The efficacy of this approach is predicated on the robust and integral connection between the reinforcement and the timber substrate, thereby facilitating clear and efficient load transfer pathways that optimise the synergistic interaction among all components. However, it is important to note that this approach is not without significant technical challenges. Firstly, it is important to consider the differential deformation characteristics of reinforcement materials (steel, FRP) and wood under varying temperature and moisture conditions. This includes thermal expansion and contraction, as well as hygroscopic swelling and shrinkage. It is notable that such conditions can induce significant interfacial stresses. It is hypothesised that, over time, these stresses may result in the formation of cracks in the adhesive layer or delamination at the interface. This, in turn, could potentially compromise the long-term structural durability of the material (Chen et al. 2022). Secondly, the manufacturing process is inherently more complex, demanding higher precision and reliability from production equipment and stringent quality control systems. This, in turn, raises production costs and increases technical implementation challenges. Thirdly, due to the reinforcement being incorporated during the initial fabrication stage, subsequent modifications or replacements of the internal reinforcement scheme are rendered largely impractical. This has a detrimental effect on design adaptability and future retrofit options. Conversely, the term “retrofit reinforcement” is employed to denote the process of augmenting the structural integrity of existing or damaged structures through the external application of methods, such as the adhesion of FRP fabrics or steel plates to the surface. These techniques offer considerable flexibility, enabling tailored solutions based on specific structural conditions and observed damage.
The purpose of this study was not to demonstrate that the proposed reinforcement schemes outperform well-developed bonded FRP or embedded steel-reinforcement systems. Rather, the study evaluated two practical and easily fabricated reinforcement/ repair configurations and identified their effectiveness and limitations. This study employed a combined experimental and numerical simulation approach. Full-scale four-point bending tests were designed and conducted, using unreinforced glued laminated timber beams as the control group. A systematic analysis was conducted to assess the damage evolution, ultimate failure modes, load-deformation/strain responses, and core mechanical properties of glulam beams under both reinforcement methods. This analysis encompassed a comprehensive range of parameters, including load-carrying capacity, stiffness, and ductility. The testing process involved rigorous and systematic testing to ensure the reliability of the results. Concurrently, a three-dimensional refined finite element model validated by experiments was established to deeply reveal the internal stress distribution and load transfer mechanisms within glulam beams under different reinforcement schemes. This study aimed to quantitatively evaluate the enhancement and restoration efficacy of the two reinforcement methods on the mechanical properties of glulam beams, elucidate their reinforcement principles, and identify technical limitations. It provided experimental evidence for future optimization and validation.
EXPERIMENTAL
Material Properties
This experiment used Douglas fir as the glulam species of choice. The glulam timber used in the study had a smooth, flat surface with no noticeable defects or imperfections. It demonstrated notable mechanical properties, including high hardness and strength, as well as excellent dimensional stability and strong corrosion resistance. To ensure a standardised evaluation, the density and moisture content of wood were measured in accordance with Chinese national standards (GB/T 1927.5 2021) and (GB/T 1927.4 2021), respectively. For mechanical testing, a series of specimens were prepared in accordance with the relevant standards. All test specimens were prepared in six copies. Different specimen geometries were used because the compression-strength test and the modulus-of-elasticity test follow different Chinese standards and require different gauge lengths and loading configurations. The 30 mm × 20 mm × 20 mm specimens were used for compression strength parallel to grain according to GB/T 1927.11 (2022), whereas the 60 mm × 20 mm × 20 mm specimen was used for modulus-of-elasticity measurement according to GB/T 15777 (2017), which provides a longer gauge length for deformation measurement. All specimens were cut from the same Douglas fir glulam batch and aligned with the longitudinal grain direction to minimize material variability. Tensile strength parallel to grain was determined according to GB/T 1927.14 (2022). Figure 1 shows the exact dimensions of all prepared specimens and the testing equipment used. The detailed test results regarding the mechanical properties of the wood are presented in Table 1.
Fig. 1. Geometric parameters and test diagram for material properties testing
The bolts utilized in this study were hexagon head bolts with a strength grade of 8.8, characterized by a nominal diameter of 8 mm and a total length of 80 mm. The steel plates were fabricated from Q345 grade structural steel. For fastening, TP30 double countersunk self-drilling wood screws, supplied by Shanghai Meigu Chengfan Fastener Co., Ltd., were employed; these screws feature an internal thread diameter of 4 mm and a length of 60 mm. The mechanical properties of all steel components were assessed in accordance with the procedures specified in the national standard GB/T 228.1 (2022). The 8.8-grade high-strength structural bolts demonstrate a yield strength of 635 MPa and an ultimate tensile strength of 840 MPa. For the TP30 screws and Q345B steel plates, both materials possess an identical modulus of elasticity of 210,000 MPa. Their respective yield strengths are 489 MPa for the TP30 screws and 463 MPa for the Q345B steel plates, while the ultimate tensile strengths are 610 MPa for the TP30 screws and 664 MPa for the Q345B steel plates.
Specimens Preparation
The study systematically investigated the impact of two reinforcement techniques for enhancing the flexural capacity of glulam beams. The reinforcement techniques in question were external steel reinforcement and bottom steel plate reinforcement. Three specimen groups were designed and fabricated for testing: the control group comprised ordinary glulam beams (A1 to A3); the second group consisted of externally reinforced glulam beams (B1 to B3); and the third group included steel plate-reinforced glulam beams (C1 to C3). To maintain uniform testing conditions, the span-to-depth ratio of all beams was kept within the range 1/12 to 1/18. All beam specimens were fabricated with identical cross-sectional dimensions: 1800 mm in length, 60 mm in width, and 100 mm in height. The source specimens used for the C-series beams were tracked during the retrofit process. Specimen C1 was prepared from the previously failed specimen A1, C2 from A2, and C3 from A3. No separate destructive residual-capacity test was conducted before retrofitting, because additional loading would have further altered the damage state of the already failed beams. The residual condition was therefore characterized by visual inspection of crack propagation, delamination, and anchorage/connection damage.
The externally reinforced glulam timber beam specimen was composed of three main components: a metal fixture (refer to Fig. 2 (b)), steel supports (refer to Fig. 2 (c)), and the glulam beam itself. For the B-series beams, the external reinforcement consisted of 12-mm steel bars with a strength grade of Q345, yield strength of 345MPa, and ultimate strength of 630 MPa. The bars were arranged below the beam and anchored by the end steel fixtures. Before formal loading, a pre-tightening force of 5 kN was introduced into each steel bar by tightening the end anchorage system and was checked using torque-wrench calibration. This relatively low pre-tightening level was intentionally selected to avoid excessive local embedment of the end bolts into the timber and premature crushing around the anchorage region. Therefore, the B-series reinforcement should be interpreted as an externally anchored steel-reinforcement scheme with limited pre-tightening, rather than as a fully developed prestressed glulam system. The fully assembled specimen, incorporating all these elements, is illustrated in Fig. 2 (a). The beam-bottom steel plate reinforcement test specimen is designed to achieve full and intimate contact between the steel plate and the glulam beam. This is accomplished by driving self-tapping screws into the bottom surface of the glulam beam. The detailed dimensions of this reinforcement scheme are shown in Figs. 2 (d) and (e). The number and spacing of self-tapping screws were selected as a preliminary repair configuration by considering the beam size, steel-plate dimensions, constructability, minimum edge/end distances, and the need to distribute the clamping force along the damaged tension zone. An 80 mm spacing was adopted to provide a relatively uniform connection along the steel plate while reducing the risk of splitting caused by an excessively dense screw arrangement. However, this layout was not intended to represent an optimized screw configuration. The test results indicate that screw shear and local connection failure governed the later-stage response; therefore, the spacing, diameter, penetration depth, and arrangement of self-tapping screws should be further optimized in future parametric studies.
Fig. 2. Geometric dimensions and reinforcement configuration of test beams: (a) schematic of the overall assembly of externally reinforced glulam beam; (b) dimension diagram of fixture; (c) dimension drawing of support device; (d) overall dimensions drawing for steel plate reinforced glulam beam; and (e) dimension drawing of steel plate
The main machining procedures for reinforced glulam beam specimens consist of several critical steps. Initially, precise positioning and layout are conducted to establish the exact locations where machining will take place. Following this, a sequence of operations is performed, which includes drilling the components, grinding the surfaces, and cleaning the drilled holes. The final step involves the attachment of strain gauges to the prepared specimens. Detailed descriptions of the specific operational methods for these processes are illustrated in Fig. 3.
Fig. 3. Reinforcement process: (a) B series and (b) C series
Loading Protocol and Measurement Arrangement
The four-point bending test was performed utilizing an electro-hydraulic servo-controlled loading system. The setup included a reaction frame, load and displacement sensors, a force-distribution beam, and rigid loading pads. A hydraulic actuator with a maximum capacity of 200 kN was employed to apply load in stages. Vertical force from the actuator was transferred through the force-distribution beam to produce two symmetric point loads on the specimen. Prior to formal testing, a preloading cycle between 2 and 5 kN was applied and then fully removed to minimize system compliance and seating errors, ensuring that the initial conditions approximated actual service loading. During the linear-elastic response phase of the material, the load was increased in increments of 2 kN per stage. After the specimen behaviour deviated from linear elasticity, the increment was reduced to 1 kN per stage until ultimate failure was reached. Each load stage was held constant for 120 s to allow for stabilization, during which all monitoring data were recorded synchronously. The test monitored mid-span deflection, strain distribution across the beam section at mid-span, and strain in the reinforcing steel plate. Instrumentation consisted of five linear variable displacement transducers (LVDTs), designated as W1 through W5, installed at both support locations, under each loading point, and at the mid-span. For the unreinforced control glulam beams and externally reinforced glulam beams, five electrical resistance strain gauges (S1 to S5) were mounted at equal vertical intervals along the side face at mid-span to capture the strain profile over the beam depth. For the steel-plate-reinforced glulam beam, an additional strain gauge (S6) was attached to the tension face of the steel plate at the mid-span section. The detailed loading protocol and instrumentation layout are illustrated in Fig. 4.
Fig. 4. Schematic diagram showing the layout of the loading equipment and measurement points
RESULTS AND DISCUSSION
Typical Failure Modes
Unreinforced glulam beams
As illustrated in Fig. 5, the unreinforced control glulam beam exhibited the typical mechanical behaviour of glulam beams when subjected to loading. The primary failure mode observed in this type of beam consists of fiber-tearing failure at the bottom of the beam and shear failure within the wood material. Additionally, the presence of knots introduces certain variations and shifts in the failure behaviour of the beam. As illustrated by specimen A1, the relationship between the applied load and beam displacement during the initial phase of loading was predominantly linear.
Fig. 5. Typical failure of unreinforced glulam beams
Upon reaching a load of approximately 10 kN, the beam began to emit faint acoustic signals. As the load was increased to approximately 16.7 kN, the beam began to emit continuous cracking sounds, and initial cracks aligned with the wood grain became visible (see Fig. 5 (b)). As the loading continued, distinctive crackling sounds indicative of tearing were audible, and cracks located proximally to the knots began to propagate more noticeably (see Fig. 5 (c)). When the applied load reached 23.79 kN, the beam abruptly emitted a loud cracking noise, followed by severe splitting along the bottom surface (Fig. 5 (d)). This was immediately followed by a brittle tensile failure of the beam (Fig. 5 (e)), which marked the conclusion of the test.
Externally reinforced glulam beams
For externally reinforced glulam beams, the incorporation of prestressed steel bars at the bottom altered the typical failure mechanism. However, as the prestressing elements were confined to the beam ends, their synergistic interaction and reinforcing efficiency did not fully exhibit the expected performance. Consider specimen B1 as a representative case: In the early stages of loading, the load-displacement response of the timber beam demonstrated a predominantly linear behaviour.
At a load of approximately 11 kN, minor flexural deformation was observed, accompanied by faint acoustic emissions. Upon reaching about 15 kN, continuous splitting noises emerged, and distinct cracks became visible along the grain direction. With further loading, localized wrinkling developed near the point of load application (refer to Fig. 6 (b) and (c)), concurrent with persistent crackling sounds indicative of wood fiber separation. Cracks progressively widened at the beam’s knot locations, bolt holes within the end metal connectors exhibited slippage, and noticeable bending deformation occurred in the tensile region beneath the beam.
Notably, the reinforcing bars had not yet transitioned into an effective stress state at this phase. When the load attained 23.5 kN, the wooden beam experienced sudden failure accompanied by a pronounced cracking sound. Extensive longitudinal cracks propagated along the beam bottom (Fig. 6(d)). At the end anchorage region, local indentation and relative movement between the external steel bar/fixture system and the timber were observed after peak loading, indicating that the external reinforcement did not remain fully engaged with the timber member.