Abstract
This review explores the evolution of prestressed timber/bamboo structures across component, connection, and structural levels, and it examines the corresponding performance of prestressed specimens. Firstly, the utilization of prestress in beams is achieved through either pre-bending methods or the incorporation of additional components. Subsequently, prestress in timber/bamboo columns often appears in the form of lateral confinement, which improves the compressive performance of the columns. On the structural level, prestress is applied in self-centering structural systems and large-span string timber/bamboo structures. Detailed schematic diagrams illustrate the application methods and underlying principles of prestress in timber/bamboo components and structures. Based on the current state of research, the future research needs and development directions are outlined. The research aims to promote the broader application of prestressed timber/bamboo structures in practical engineering, contributing to the advancement of sustainable building practices.
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Prestressed Wood or Bamboo Structures: Historical Overview and State-of-the-Art
Qingfang Lv, Juewen Wang, and Yujie Lu *
This review explores the evolution of prestressed timber/bamboo structures across component, connection, and structural levels, and it examines the corresponding performance of prestressed specimens. Firstly, the utilization of prestress in beams is achieved through either pre-bending methods or the incorporation of additional components. Subsequently, prestress in timber/bamboo columns often appears in the form of lateral confinement, which improves the compressive performance of the columns. On the structural level, prestress is applied in self-centering structural systems and large-span string timber/bamboo structures. Detailed schematic diagrams illustrate the application methods and underlying principles of prestress in timber/bamboo components and structures. Based on the current state of research, the future research needs and development directions are outlined. The research aims to promote the broader application of prestressed timber/bamboo structures in practical engineering, contributing to the advancement of sustainable building practices.
DOI: 10.15376/biores.19.4.Lv
Keywords: Timber/Bamboo structures; Prestress; Beams; Columns; Self-centering systems; String structures
Contact information: Key Laboratory of Concrete and Prestressed Concrete Structures of the Ministry of Education, Southeast University, Nanjing 210096, China; *Corresponding author: 230218117@seu.edu.cn
GRAPHICAL ABSTRACT
INTRODUCTION
Timber/bamboo structures, as an ancient architectural form, have played an essential role in construction since primitive societies and continue to exert influence to this day. The 67-meter Yingxian Wooden Pagoda, completed in 1056, has been preserved for thousands of years and has served as a representative example of ancient timber construction techniques (Liu and Yang 2019). The Forbidden City in Beijing, one of the most emblematic collections of ancient timber structures, employed exquisite mortise, tenon joints, and bracketing systems (Paliszewska-Mojsiuk 2018). This configuration ingeniously alleviated impacts caused by earthquakes, thereby ensuring the structure’s stability and safety (Gustafson et al. 2008). In the 1840s, Chicago, USA, witnessed the emergence of lightweight timber frame houses constructed using 38 mm thick timber, assembled with long nails (De Geetere and Ingelaere 2014). This lightweight timber frame structure has progressively become one of the most predominant architectural forms for low-rise housing, dormitories, and hotels (Caniato et al. 2017). The Dai’s bamboo house in China represents a typical example of traditional bamboo architecture. The connection method involves the tying of bamboo rafters, characterized by the use of local materials, effective ventilation and moisture prevention, and high spatial efficiency (Sattar 1995). In contrast, the Naman Conference Center in Da Nang, Vietnam, integrates bamboo with modern materials including glass and steel, offering new solutions to current environmental challenges (Thi et al. 2017).
In the latter half of the 19th century, the demand for large-scale infrastructure facilitated the emergence and utilization of steel and reinforced concrete (Yeoh et al. 2011). Due to the limited industrial technological capabilities of timber/bamboo structures at that time, they gradually receded from the purview of construction engineering (Omenzetter et al. 2011). Nevertheless, since 1950, timber/bamboo materials have experienced a resurgence in construction practices (Aloisio et al. 2023b). The use of bamboo in foundations, doors, windows, and walls has progressively increased, enabling nearly all parts of a house, to be constructed using bamboo except fireplaces and chimneys (McClure 1956). The distribution of timber architecture in England was summarized by Smith (1965), highlighting the distinctions and connections among different regional forms of timber structures. Timber planks can be widely applied in farm buildings such as sheds, barns, silos, and granaries (Banks 1972). Timber arch bridges and timber truss bridges also became increasingly common starting from the 1980s (Duwadi and Ritter 1997; Totman 1983; Verna et al. 1984). The reasons for this revival can be attributed to the following two aspects:
- The rise of the green and sustainable development concept
In recent years, the green and sustainable development concept has prompted a reassessment of traditional materials’ environmental impact in construction engineering (Abdulhameed et al. 2023). Studies on timber/bamboo materials have shown that they are associated with low carbon emissions and energy consumption during growth and processing (Lim et al. 2023). Additionally, buildings with timber/bamboo materials exhibit excellent insulation properties, resulting in outstanding lifecycle carbon emissions and energy utilization efficiency (Dodoo et al. 2014; Robati and Oldfield 2022). These advantages have driven the application of timber/bamboo structures in green buildings.
(2) Development of modern bio-materials
Natural wood is susceptible to decay, with irregular shapes and varying mechanical properties. Its mechanical performance is significantly impacted by environmental factors such as temperature and humidity, making it unsuitable for use in some aspects of construction (Bartlett et al. 2019; Cui et al. 2024). Against this background, engineered timber/bamboo materials have gradually emerged, such as glulam-laminated timber (GLT), cross-laminated timber (CLT), and laminated veneer lumber (LVL). GLT, which dates back to 1870, involves bonding solid timber sections together in the same direction through gluing (Kim et al. 2011). Each layer of timber in CLT is glued perpendicularly to the adjacent layers, providing uniform strength in both directions (Huang et al. 2022; Bai et al. 2024). LVL is a single-layer board with the grain oriented in the same direction, which has a better mechanical performance than common GLT (Zhu et al. 2007; (Ardalany et al. 2011). Given the scarcity of timber and the abundance of bamboo resources in some areas, various engineered bamboo materials, such as glulam-laminated bamboo (GLB), cross-laminated bamboo (CLB), and laminated veneer bamboo (LVB), have been proposed (Lee et al. 1998; Liu et al. 2016; Chen et al. 2022). Due to the superior mechanical properties of bamboo fibers, engineered bamboo materials manufactured from them exhibit higher density, elasticity, and strength compared to timber (Chen et al. 2022; Tian et al. 2023), and the bending strength of GLB is higher than that of CLB (Sinha et al. 2014).
Although engineered timber/bamboo materials are economical and practical, effectively addressing the defects of natural wood, the promotion of timber/bamboo structures still faces challenges. Firstly, the fire resistance of bamboo and timber is directly related to the safety of occupants in buildings. Published research has employed the charring rate to measure the fire resistance of timber/bamboo materials (Schmid et al. 2014; Xu et al. 2015; Zhang et al. 2019). Based on the Reduced Cross Section Method (RCSM), novel fire design models for glulam timber/bamboo have been proposed to provide significant references for modern bamboo and timber engineering (Xu et al. 2018; Cui et al. 2023).
Furthermore, compared with steel or concrete materials, the relatively lower mechanical performance and brittle failure mode of timber/bamboo materials limit the development of timber/bamboo structures towards long spans, high-rise, and large-scale applications (Li et al. 2019; Wang et al. 2024). Existing research has explored additional components, such as FRP plates (Canning and Luke 2008; Colombi and Fava 2015; Stratford and Cadei 2006), strips (Motavalli et al. 2010; Khedmatgozar Dolati and Mehrabi 2022), bars (Cheng et al. 2018; Hadhood et al. 2021; Wdowiak-Postulak et al. 2023), or steel strands (Ranzi and Ostinelli 2017; Zhang et al. 2022) for reinforcing timber/bamboo elements, but their combined efficiency still requires improvement.
Additionally, despite the high strength-to-weight ratio of timber/bamboo materials, which offer higher utilization efficiency compared to low-carbon steel and ordinary concrete (Crocetti 2016), many experimental investigations have shown that failures in timber/bamboo structures are mainly concentrated in the connection areas. The connection methods, joint stiffness, and bearing capacity directly impact the overall performance of prefabricated timber/bamboo structures. Although many new modern connection methods have been proposed, such as nail connections (Gattesco and Boem 2016; Ruan et al. 2022), screw connections (Hossain et al. 2016; Schiro et al. 2018), bolted connections (Lam et al. 2010; Quenneville and Mohammad 2000; Song et al. 2017; Cui et al. 2024), and self-tapping wood screws (Li et al. 2017; Petrycki et al. 2020; Cui et al. 2022), the most suitable connection for improving the brittle characteristics of timber/bamboo structures still needs to be studied.
To solve the issues mentioned above, prestress technology, which has served as a main reinforcement approach for concrete structures (Akl et al. 2017; Gao et al. 2017), is introduced into timber/bamboo structures for the enhancement of structural components. The introduction of prestress can significantly enhance the fire resistance of glulam timber or bamboo, with the degree of enhancement positively correlated with the magnitude of prestress (Quiquero et al. 2020; Zhang et al. 2024). In the research on mechanical properties, extended research has demonstrated the effectiveness of prestress in improving strength and stiffness, reducing deflection, and optimizing tress distribution. Additionally, prestress technology is also utilized in the connections of timber /bamboo structures to improve overall stress and deformation patterns, enhancing the vertical load-bearing and seismic performance of structures. This advancement promotes the development of large-span and high-rise buildings.
This study provides an overview of the methods for applying prestressing technology in timber/bamboo structures at the component, connection, and structural levels and the corresponding performance of the prestressed specimens. By summarizing the characteristics of existing research, this paper identifies the directions to be explored, particularly in the lack of research on efficient reinforcement for timber/bamboo materials, unestablished prestress loss standards, and the actual seismic performance. This review points out future research directions, providing theoretical references for engineering applications.
THE ORIGIN OF PRESTRESS IN TIMBER/BAMBOO STRUCTURES
A prestress condition exists early in the growth process of trees. The stress associated with the expansion of cross-sections during germination is referred to as growth stress. The causes and distribution characteristics of growth stress in trees were first discussed by Münch (1938) and Jacobs (1938). Growth stress was observed to be unevenly distributed radially along the cross-section, with compressive prestress generated at the center and tensile prestress at the outmost edges, as shown in Fig. 1(a). Due to the susceptibility of buckling under compression for wood fibers (Boyd 1950), the compressive strength along the grain is typically only about half of its tensile strength. Commonly, wind or seismic loads result in a bending response in trees, causing tension on one side and compression on the other side of the cross-section, as illustrated in Fig. 1(b). After the superimposition of growth and bending stress states, compressive stress on the compression side is mitigated while tensile stress on the tension side increases, as illustrated in Fig. 1(c). The utilization of the mechanical properties and overall load-bearing capacity are improved.
In ancient times, tents constructed with tensioned bent slender branches to resist wind loads exemplified an early application of prestress in timber/bamboo structures (Gasparini 2006). Wedge connection was a significant traditional structure connection form (Gustafson et al. 2008). A wedge was a small wooden peg with a thick top and sharp bottom. Its tip was inserted into a gap in the connection, and the flat end was struck with a heavy object driving the wedge deeper (Yang et al. 1999), as shown in Fig. 2(a). The gaps expanded and prestressed to achieve fixing and connection. Grubenman combined wedges with iron straps to construct the first laminated timber arch (James 2017). The Wettingen Bridge, built in 1765, was based on this principle, as shown in Fig. 2(b) (Caldenby 2018). The basic wedges were improved by incorporating iron straps, with additional tensile strength and stability added to the bridge.
Fig. 2. Evolution of prestressed timber bridges
In the 19th century, the rapid expansion of American railroads drove the further development of prestressed timber bridges (Wipf et al. 2000). It began with Lon’s patent for a timber truss bridge (Long 1830), as shown in Fig. 2(c), where counterbraces applied prestress using wedges (Gasparini and Simmons 1997a,b; Gasparini 2006; Gasparini et al. 2006). This method incorporated engineering mathematical principles to distribute loads and stresses within the structure better. However, it was soon superseded by Howe’s new patent (Howe 1840), which introduced pre-tensioned iron tendons into the truss, as illustrated in Fig. 2(d) (Sutherland 2016). The introduction of iron tendons also facilitated the construction of more standardized and reliable bridges.
Timber/bamboo structures have gradually evolved into modern large-span structural systems represented by glued laminated timber structures (D’Aveni and D’Agata 2017; Li et al. 2019; Jia 2022). The development of prestress in timber/bamboo structures is exhibited in Fig. 3. The review of timber/bamboo structures in the following sections is divided into the following main sections in order from components to structures: (1) Prestressed timber/bamboo beams; (2) Prestressed timber/bamboo composite columns; (3) Timber/bamboo lateral resistance structural systems adopting prestress technology; and (4) Prestressed technology in large-span timber structural systems. The methods of prestress applied and the effects it contributed to in timber/bamboo components, connections, and structures are discussed in detail.
Fig. 3. Development of prestress in timber/bamboo structures
PRESTRESSED TIMBER/BAMBOO BEAMS
Applying Prestress through Pre-arching
The elastic modulus and strength of timber/bamboo materials are relatively lower compared with steel and concrete, making timber/bamboo beams prone to deformation and brittle bending failure (Zheng et al. 2021; Arafat and Imam 2022). Therefore, various prestressing techniques have been introduced to enhance the mechanical performance of timber/bamboo beams or other bending components. One common method is applying external loads to achieve pre-arching, as shown in Fig. 4(a). After flipping, the pre-arched timber beam experiences tension in the upper flange and compression in the lower flange, resulting in improved flexural load capacity (De Luca and Marano 2012).
In 2005, Borri et al. (2005) attained prestress in beams through the three-point bending test, with the stress intensity corresponding to about 25 to 35% of the ultimate timber flexure strength. The Preflex process was first implemented in prestressed concrete beams, applying symmetric four-point bending loads to achieve a more uniform stress distribution in flexural members. The Preflex cambering method for timber flexure members was proposed by Morano and Mannini (2006). In this method, the bottom of the beam was turned upwards before a two-point load was applied. The end anchors were tightened when the deformation of the GLT beam reached the predicted value and a pre-arched beam was achieved after rotating. Moreover, Lehmann (2015) and Negrão (2016) achieved beam cambering using an adjustable support that can move up and down, creating a triangular moment distribution in the beam, as shown in Fig. 4(b). However, due to the excessive support force and bearing displacement required, it was not suitable for large-size beams.
Fig. 4. Main forms of pre-arched timber/bamboo beams
Anshari and associates (Anshari et al. 2012; Anshari and Guan 2014; Anshari 2015) conducted a series of research on inducing camber in timber beams using the moisture-dependent swelling nature of compressed wooden (CW) blocks since 2012. The researchers cut rectangular holes at the top of GLT beams and inserted CW blocks with low moisture content to induce camber through hygroscopic swelling, as shown in Fig. 4(c). Significant initial tensile stress at the top and compressive stress at the bottom of the beam was generated. Compared with non-prestressed beams, beams with three 45mm CW blocks had flexural stiffness increased by 19% and the load-bearing capacity increased by 14%. In 2017, Anshari and Guan (2017) further validated the effects of the thickness and depth of CW blocks on initial flexural stiffness and the ultimate load-bearing capacity of beams through finite element analysis. The results illustrated that increasing the thickness and insertion depth of CW blocks could enhance the initial flexural stiffness of the beam by an average of 20%, but the improvement in ultimate flexural load capacity was relatively limited. Müller and associates (Müller 2020; Müller et al. 2021) applied Anshari’s method to timber-concrete composite components to reduce the buckling of timber elements caused by cast-in-place concrete.
In 2023, Zhang et al. (2023) presented a pioneering concept that utilized the laminated interaction between layered timber/bamboo components with different curvatures to generate prestress, as shown in Fig. 4(d). A single layer of bamboo was glued and bent in a curved arc-shaped mold and then bonded with a straight glulam beam to form a composite component. In this process, the pre-bent laminated bamboo layer was straightened, and prestress was stored within it. The study found that compared with non-prestressed beams, the bending performance of sandwich beams with top and bottom laminated bamboo layers was obviously improved, showing a shift in failure mode from brittle tensile to ductile failure.
Applying Prestress through Additional Components
In addition to the aforementioned cambering methods, using additional prestressed components to reinforce timber/bamboo beams is also a widely cited effective method (Garas and Chalmers 1980; Gessner et al. 2019). The forms of additional prestressed components include prestressed steel bars (Chen et al. 1998; De Luca and Marano 2012), steel strands (Guo et al. 2021; Zhang et al. 2022), FRP materials (Achintha and Balan 2019; Breveglieri and Czaderski 2021; Custódio and Cabral-Fonseca 2023), etc. The methods of combining prestressed components with main load-bearing beams include internal unbonded or bonded methods (Al-Emrani and Kliger 2006; Capozucca 1998; Yi et al. 2015), and the Near Surface Mounted (NSM) method (Al-Saadi et al. 2017; Rocha et al. 2023), etc.
Post-tensioned steel bars or wires
Fig. 5. Post-tensioned steel bars or wires strengthen bamboo/timber beams
In 1962, Bohannan first proposed the idea of using prestressed high-strength steel bars or strands in timber/bamboo structures to improve mechanical performance. Peterson (1965) used epoxy resin to bond prestressed steel bars on the tension side of timber beams in 1965. Song et al. (2002) drilled holes at the end of timber beams and placed 4 mm diameter round steel bars at the bottom edge, then applied prestress by tightening nuts, as shown in Fig. 5(a). De Luca and Marano (2012) used mechanical tensioning devices to apply a pre-tension force of 18 kN to steel bars, then bonded the bars with adhesive into the bottom grooves of beams as external prestress reinforcement, as shown in Fig. 5(b). The stiffness and ultimate load capacity of the beams increased by 37.9% and 40.2% respectively, and the ductility increased by 79.1% compared to unreinforced components.
Negrão et al. (2016) discussed the effects of pre-tensioning and post-tensioning methods on beams reinforced with prestressed bars. The adhesive was required in the pre-tensioning method, while the steel bars were anchored at the beam ends using mechanical devices before applying prestress in the post-tensioning method. The long-term fatigue risk of the adhesive interface in the pre-tensioning method made post-tensioned bars more suitable for practical engineering applications. Liu et al. (2008) discovered that setting post-tensioned bars in the tension section improved the ultimate load capacity significantly, whereas the improvement in stiffness was imitated. Wei et al. (2020) and Tian et al. (2021, 2023) found that embedding post-tensioned steel bars in bamboo beams enhanced stiffness, load-bearing capacity, and material efficiency. However, the influence of prestress level on ultimate load capacity was limited under the same reinforcement ratio.
The selection of bonded and unbonded prestressed tendons directly affects the overall performance of flexural members. Bonded prestressed tendons form an integral whole with beams by using adhesive (Al-Emrani and Kliger 2006; Hahn et al. 2019), while unbonded prestressed tendons transmit prestress directly through end anchorage, avoiding potential negative impacts from the adhesive (Bedriñana et al. 2021; Bu and Wu 2018). McConnel et al. (2014) studied the flexural performance of bonded and unbonded linear prestressed glulam beams, finding that unbonded prestressed timber beams contributed to a 17.6% and 8.1% rise in load-bearing capacity and stiffness respectively. Meanwhile, bonded prestressed increased about 40.1% in load-bearing capacity and 30.0% in stiffness, showing a more significant improvement in beam performance. Christoforo et al. (2022) conducted similar experiments, finding that failure mode changed from tension fiber rupture to compression fiber buckling and wrinkling.
Steel strands possess higher tensile strength compared to steel bars, and they are also widely used in the external prestress reinforcement of bending components, as shown in Fig. 5(c). Yang et al. (2016) applied prestress to steel strands using a jack. The GLT beams reinforced with externally prestressed steel strands showed significant improvements in ultimate load-bearing capacity and flexural stiffness. Prestressed steel strands also changed the brittle tensile failure mode into ductile compressive yielding failure mode. Guo et al. (2018) studied the short-term and long-term bending deflections of bamboo-timber composite beams reinforced with prestressed steel strands. Under the same external load level, prestress could reduce costs effectively by saving 13.3% to 52.0% on bamboo-timber composite materials. Additionally, the application of prestress effectively reduced the long-term deflection of composite beams. Zhang et al. (2022) proposed a new steel-bamboo composite beam strengthened with externally prestressed steel strands tensioned by cross-core jacks. Test results indicated that two-point prestressed specimens exhibited better deformation performance and higher loading-bearing capacity than the one-point prestressed specimens.
FRP materials
Fiber Reinforced Polymer (FRP) materials, characterized by high strength, good ductility, lightweight, and corrosion resistance, have been widely used in the reinforcement of building structures. With successful applications of prestressed FRP materials in concrete, attempts have been made to reinforce timber/bamboo structures with FRP materials. The common reinforcement practice was achieved by gluing (usually epoxy resin) sheets, plates, bars, etc., to timber beams.
According to the bonding position, FRP reinforcement of bamboo-timber beams can be categorized into five forms, as shown in Fig. 6. The method (a) in Fig. 6 directly performs in-situ flexural reinforcement (Johnsson et al. 2007), which is relatively convenient for construction. Based on method (a), FRP can also be fixed inside the beam, and this modified method (b) in Fig. 6 can be directly incorporated into the production process of glued-laminated timber. Method (c) utilizes thin FRP sheets to wrap the entire timber beam, allowing reinforcement in cases of partial timber degradation and cracking. In methods (d) and (e), the bottom of the timber beam is slotted and FRP bars are inserted.
Fig. 6. Types of strengthening beams with FRP
Numerous studies have employed pre-tensioned FRP bars to reinforce timber/bamboo components. Johnsson et al. (2007) adhered carbon fiber reinforced polymer (CFRP) bars to the bottom grooves of beams with epoxy adhesive, validating the Near Surface Mounted (NSM) method for timber components. Ahmad (2010) applied prestress to GFRP bars by mechanical tensioning. The bending performance of timber beams reinforced with bonded glass fiber (GFRP) bars was studied, showing enhancements in both load-bearing capacity and stiffness after reinforcement. Lv et al. (2019) proposed a one-step forming method suitable for large-scale factory production to produce prestressed bamboo beam specimens. The one-step forming method combined pre-tensioned basalt fiber-reinforced polymer (BFRP) bars with bamboo through compression molding. The BFRP-bamboo composite beams exhibited better flexural performance. Wdowiak-Postulak and associates (Wdowiak-Postulak 2023; Wdowiak-Postulak et al. 2023, 2024) conducted four-point bending tests on timber beams enhanced by different post-tensioned bars, including steel, glass, and basalt bars. The results indicated that prestressed steel bars provided the most significant improvements in ultimate strength and stiffness.
In terms of FRP sheets or plates, researchers mainly have adopted three methods to apply prestress in timber/bamboo structures (Halicka and Slosarz 2021, 2022). The first method involved adhering FRP sheets to the bottom surface after cambering the beam. Triantafillou and Deskovic (1992) first established the method of FRP-sheet reinforcement for timber/bamboo structures. CFRP sheets were adhered to the tension surface of the timber beam. Wang et al. (2016) also conducted similar experiments, proposing that this method was suitable for reinforcing traditional timber structure beams. Borri et al. (2005) adhered CFRP sheets to the bottom surface of timber beams after the pre-bending process with three-point loading. It was observed that three layers of CFRP sheets could enhance the flexural capacity by 60.3% compared with unreinforced beams.
In the second method, the strips were pre-tensioned and then adhered to the bottom surface of the beam with adhesive. Dolan et al. (2001, 2016) used hydraulic jacks to introduce kevlar fiber-reinforced polymer (KFRP) fabric and GFRP rods to reinforce GLT beams. It was observed that the stiffness increased by 25% and 70%, and the ultimate flexural strength increased by 25% and 110%, respectively. Ä°ÅŸleyen et al. (2021, 2023) conducted analyses of pre-tensioned CFRP-reinforced timber beams, finding that the flexural performance of damaged timber beams could be restored to an undamaged state after reinforcement. Halicka and Slosarz (2021, 2022) used hydraulic actuators to tension FRP sheets, which were then adhered to the surface of timber beams. The advantages of pre-tensioned CFRP strips lie in reducing beam deflection and shifting the failure mode from flexural failure to delamination failure.
Step-wise pre-stressing was the third method. This method was first proposed by Stöcklin and Meier (2003) and was initially applied to prestressing concrete structures at EMPA (Swiss Federal Laboratories for Material Testing and Research). In this method, a pre-tensioned section was bonded in the middle of the FRP strip at mid-span first. An electric heating system was used to accelerate the curing of the adhesive. After the middle part of the FRP strips was firmly bonded to the mid-span of the beam, the prestress value was reduced and the FRP was adhered to both sides of the beam gradually. These steps were repeated multiple times until the entire strip was bonded. Brunner and Schnüriger (2005) used the step-wise pre-stressing method to introduce prestress to FRP plates reinforcing timber beams. Epoxy adhesive is applied to one side of the FRP plate, and heating activates the adhesive in that area, attaching the FRP to the bottom mid-span of the beam. Starting from the mid-span, the process gradually moves towards the supports while reducing the prestress. Dagher and Altimore (2005) suggested that the load-bearing capacity of prestressed GFRP reinforced timber beams using the EPMA method increased by about 95% compared with non-reinforced beams.
Prestress Loss
Prestress loss is inevitable in prestressed flexural members. The loss can be classified into immediate losses and long-term losses based on the time the losses occurred. Immediate loss includes friction loss, anchorage loss, and sequential tensioning loss in post-tensioning (He et al. 2022). Long-term losses include creep, prestress relaxation, temperature differential loss, and loss due to elastic deformation, all of which develop over time (Davies and Fragiacomo 2011; Riccadonna et al. 2020).
Scholars have described the changes in prestress over time in timber/bamboo beams. Quenneville and Vandalen (1994a,b) proposed rheological models and equations for prestress relaxation in timber beams under different environmental humidity conditions. Fragiacomo and associates (Fragiacomo and Davies 2011; Fragiacomo et al. 2011; Davies and Fragiacomo 2011) derived the prestress loss of LVL timber beams under long-term loading. Palermo et al. (2011) extended this model to other cable profiles such as catenary or parabolic and multi-span beams, providing a unified design procedure. Chen and Feng (2013) derived and validated the initial prestress formula for pre-tensioned FRP-strengthened timber beams with 3 CFRP-reinforced timber beams. Lv et al. (2019a,b) studied a novel BFRP-reinforced bamboo beam, categorizing prestress loss into 4 parts and proposing a method for effective prestress calculation. Based on experimental results, FojtÃk et al. (2023) established an effective prestress expression for prestressed glued laminated timber beams. The prestress loss calculation methods summarized in each study are depicted in Table 1.
Table 1. Prestress Loss Calculation Model