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Changing Failure Modes of Cross-Laminated Timber
Abstract
Cross-laminated timber relies on the adhesive layer between adjacent timber plies to provide composite action between the lamella for increased member strength and stiffness. Previous research has shown that adhesive loses normal and shear stiffness at elevated temperatures increasing the slip between adjacent timber plies. Slip in the bond layer results in reduced composite action increased deflections and a potential loss in ultimate strength in the CLT member. In order to study the effects of temperature on the flexural behavior of CLT, two series of tests were conducted. The first series focused on identifying the changing failure modes while the second series established conditions that led to those failure modes in large CLT beams. The results clearly showed the failure mode of CLT changes from timber failure to failure in the adhesive as a function of the in-depth temperatures. The adhesive failure yielded larger deflections and a loss in stiffness and ultimate strength.
... (1) the failure within the timber (lamellae), (2) through the loss of cohesion within the adhesive, and (3)loss of adhesion between the timber and adhesive at the bonding interface. These mechanisms are presented visually in Figure 2. The terminology used to describe this phenomenon varies greatly among researchers, and often terms such as delamination [1,7,[29][30][31][32]11,21,[23][24][25][26][27][28], char fall-off [6,[33][34][35][36], or loss of stickability [36,37] are used interchangeably to describe the fall-off of both, completely or partially charred lamellae. Figure 2 shows the difference between the two concepts of debonding, char fall-off and delamination, and the classification used in this study. ...
... Polyurethanes ductile, high fracture energy [1] [51], -the long-term durability of PUR adhesives is not well known [1], -resistance to heat poorer than for formaldehydes [23], -structural performance highly reliant on its chemical composition, manufacturer and fillers [22,44,46,47], and the methodology used to test the specimen [46,47], -if well adapted it can reach the PRF adhesive strength and WFP [22], -based on the small-scale shear tests, increased elongation, WFP reduction, and temperature induced creep is expected from 80°C to 150°C [21,26,108], -low heat flux as 6 kW/m 2 can also cause 1-C-PUR adhesive failure [24], -char fall-off observed in several full-scale tests [11,13,30,31,38], -"Non-delaminating'' PUR [11,30,31] could be used to avoid CLT delamination in furnace tests, ...
... Failure mode change in the beam tested in three-point test. Timber failure at ambient temperatures (left) and bond line failure at elevated temperatures (65-80C, right)[26] Since wood is an orthotropic material (different properties in three mutually perpendicular directions), the shear strength of timber varies in different directions depending on the application of load relative to the grain; in-plane (wall) or out-of-plane (slab). As presented inFigure 11, there are three types of shear present in EWP: in direction parallel to the grain as radial-longitudinal and tangential-longitudinal shear, and in direction perpendicular to the grain as rolling and shear. ...
Samples were simultaneously exposed to a structural load (shear) and a thermal load (radiant panel) to study debonding (char fall off and delamination). Small scale tests (30) were performed on CLT blocks (3-lamellae), from three different European manufacturers. Two different types of one-component polyurethane (1-C-PUR), and one type of melamine-urea formaldehyde (MUF) were used. The analysed variables are change of structural load, bond line temperature at failure, adhesive type, and moisture content.
... The increasing demand from practitioners to create resilient critical infrastructure that can resist thermal loading has created a need for steel manufacturers to improve their connection design technologies. Current design can be overly conservative, or even potentially unsafe, in some nontypical cases rather than using a performance-based solution, which optimize the amount of fire protection (Emberley et al. 2016). Previous collapses of steel structures have shown that connections are critical to structural integrity. ...
The behaviour of steel beam-to-column connections under thermal loading is understudied. When exposed to a fire, the members within steel structures expand during heating and contract during cooling, inducing potentially large connection forces. Within Canadian design, there exists flexibility for designers to consider more advanced computational practices that can optimize the fire protection design and allow for safer and more resilient design. A series of fire tests were undertaken, with three different methanol pool fire durations, to understand the deformation behaviour of steel beam-to-column connections when a localized thermal exposure is applied to the centre span of the beam. It provides a preliminary understanding of how the forces and heat are dissipated into the connections. Preliminary recommendations for the development of the relevant Canadian design code, Annex K within CSA S16-19, have been outlined.
... The bond line is exposed to a moving moisture front emanating from the heated surface, thermal degradation due to thermal penetration, and mechanical normal and/or shear stresses which may increase due to thermal expansion or dehydration shrinkage. Such loss of composite action can occur before a visual failure [4], or as a visual failure where the bonded lamellae pieces detach before the char front has propagated to the bond line. Detached lamella fragments consist of both virgin and charred timber [3] and therefore HID differs from only char fall off [5]. ...
Heat induced delamination (HID) is a phenomenon in engineered wood products which may result in the detachment of the bonded timber lamellae before charring propagates to the bond line interphase. This study aims to correlate the thermal degradation of two 1-c-polyurethane adhesives at the microscale to the thermo-hydro-mechanical behaviour of tension shear lap cross laminated timber at an intermediate scale. The performance criteria studied are the failure mode, time to failure, and bond line temperature at failure. The variables considered are the adhesive type (standard adhesive (Loctite HB S) and an adhesive with an improved performance fire (Loctite HB X), initial moisture content, and severity of thermal exposure. Results indicate that both adhesive types exhibited HID, with temperatures at failure being influenced by heat flux severity and initial moisture content variations. The study also correlates the responses with microscale analyses using differential scanning calorimetry (DSC) and dynamic thermo-mechanical analysis (DTMA).
... However, this is an ongoing research area, and caution should be exercised accordingly. Recent research has begun exploring the effect of adhesive deterioration beyond the char layer (Emberley 2016;Quiquero 2017;Otto 2017), which may lead to a further understanding of the load carrying capabilities of the non-charred wood regions. This research is still too premature to specify an acceptable criterion as a limiting temperature or affected depth. ...
This chapter reviews how temperature affects the mechanical properties of steel, concrete, masonry, and wood. Many tests have characterized the mechanical properties of reinforcing steels at elevated temperatures. The chapter covers some fundamental concepts about the effects of temperature on materials’ mechanical properties and provides references to the most widely accepted material models found in design codes and the research literature. All mechanical properties used for timber design derive from extensive sampling and analysis procedures, and these properties are represented as either minimum strength or average stiffness. Because the strength and stiffness of the char layer are nearly zero, the properties of wood from room temperature to 300°C are of most interest to structural fire engineering design.
... However, in transient conditions the temperature at which the specimen fails remains unknown. Some researchers [9,32,41] also condition (dry) their samples to around 103°C before testing to reduce the testing time, excluding in this way the impact of moisture movement, or of long preheating times on the bond line performance [13]. In the current study, to represent a real case scenario, the aim was to develop a transient thermal profile in the sample, with the application of constant structural (shearing) load along the bond line between lamellae. ...
... Studying the adhesive performance under elevated temperature alone is not enough to determine the overall performance of a composite material such as engineered timber. Several studies [4,149,150] showed the importance of bond length and the resulting interfacial stresses in the overall performance of a bond. Nicolaidis et al. [150] showed that bond length coupled with temperature effects on the adhesive is important for ultimate strength. ...
... The results in this test series were highly variable, but similarly found that the performance of various PUR adhesives were diverse with some losing thermal stability around 70°C while others remained stable until 150~200°C. The most recent testing that specifically referenced to adhesive performance was a twopart test series by Nicolaidis et al. (2016) and Emberley et al. (2016) on glued single lap samples exposed to environmental chamber heating and CLT beams exposed to radiant heating, respectively. Pine and spruce wood were used, respectively, with a one-component PUR. ...
As engineered timber such as Glulam is seeing increasing use in tall timber buildings, building codes are adapting to allow for this. In order for this material to be used confidently and safely in one of these applications, there is a need to understand the effects that fire can have on an engineered timber structural member. The post-fire resilience aspect of glulam is studied herein. Two sets of experiments are performed to consider the validity of zero strength guidance with respect to short duration fire exposure on thin glulam members. Small scale samples were heated in a cone calorimeter to different fire severities. These samples illustrated significant strength loss but high variability despite controlled quantification of char layers. Large scale samples were heated locally using a controlled fuel fire in shear and moment locations along the length of the beam respectively. Additionally, reduced cross section samples were created by mechanically carving a way an area of cross section equal to the area lost to char on the heated beams. All of the samples were then loaded to failure in four-point (laterally restrained) bending tests. The beams that have been burnt in the shear region were observed as having a reduction in strength of up to 34.5% from the control beams. These test samples displayed relatively little variability, apart from beams that displayed material defects. The suite of testing indicated that zero strength guidance may be under conservative and may require increasing from 7 mm up to as much as 23 mm.
... For standard fire resistance tests, the temperature gradient in the timber is assumed to be sufficiently steep so as to ensure that glue line temperatures are sufficiently low that debonding or loss of composite action can be avoided before charring of the affected timber occurs [61]. However, for prolonged heating with lower heat fluxes and shallow temperature gradients (e.g. during a decay phase) it has been shown from lap shear elevated temperature bond experiments that debonding may occur as a failure mode [62]; it has also been shown that local debonding can cause stress concentrations, and that these could develop into rolling shear failures [63]. A bond line temperature of 150°C has been identified as critical by Craft for the weakening of PUR adhesives [64], however different formulations of adhesives can show considerable variation in thermal performance [65]. ...
In compartment fires with boundaries consisting of exposed mass timber surfaces – for example in compartments with exposed cross-laminated timber (CLT) walls or floors – the thermal penetration depth, i.e. the depth of timber heated to temperatures significantly above ambient behind the char-timber interface, during fire exposure may have a significant influence on the load bearing capacity of structural mass timber buildings, particularly in the decay phase of a real fire. This paper presents in-depth timber temperature measurements obtained during a series of full-scale fire experiments in compartments with partially exposed CLT boundaries, including decay phases. During experiments in which the timber surfaces achieved auto-extinction after consumption of the compartment fuel load, the thermal penetration depth continued to increase for more than one hour, whilst the progression of the in-depth charring front effectively halted at extinction. A simple calculation model is presented to demonstrate that this ongoing progression of thermal penetration continues to reduce the structural load bearing capacity of the CLT elements, thereby increasing the potential for structural collapse during the decay phase of the fire. This issue is considered to be most important for timber compression elements. Currently utilised structural fire design methods for mass timber generally assume a fixed ‘zero strength layer’ depth to account for thermally affected timber behind the char line; however they make no explicit attempt to account for these decay-phase effects.
... This highlights the importance of understanding the adhesive behaviour, since a glue line failure will significantly weaken the structure, induce stress concentrations, and may result in sudden shear failure. Conversely, the occurrence of rolling shear could also lead to propagated debonding along nearby glue lines, as observed in previous experimental [13] and computational work [14]. From Figures 3 and 4 it is observed that at higher temperatures (135 °C in the case of Figure 4), the shear strain is concentrated along the glue-line, which indicates that the bond between the two plies is weakened, leading to a failure along the ply interface. ...
Modern engineered timber products have led to a renewed interest in mass timber as a construction material in mid-and high-rise buildings. Cross-laminated timber (CLT) is produced from timber board layers in alternating directions that are glued together to form slabs or panels. Shear failures along the tangential-radial direction of the cross layers, denoted as rolling shear failures, have been identified as an important ambient temperature failure mode for CLT subjected to out of plane loading, and must therefore be considered in structural design of such CLT elements. However, no studies are currently available on the effects of heating on the rolling shear capacity of timber, creating a knowledge gap for the safe design of CLT structures in case of fire. This paper presents results and analysis from a series of experiments on CLT samples tested in rolling shear at temperatures between 20 and 200 °C. The samples were heated to steady state thermal conditions within an environmental chamber before displacement controlled loading was applied until failure. An approximately bilinear reduction in rolling shear capacity was observed with increasing temperatures, although this reduction was less severe than that currently suggested by available design guidance. The observed reduction in rolling shear capacity with temperature is used for an example calculation, demonstrating that, in fire, rolling shear failure is unlikely to occur. This is because failure due to exceedance of the normal strength (in bending) governs.
... While delamination has previously been reported as an important issue to consider for CLT floor slabs in fire [15,16], it has been suggested that it is less likely in furnace tests of vertically oriented elements, presumably due to reduced separation forces from gravity [17,18]. Delamination should not be confused with debonding, which describes loss of composite mechanical action between lamellae, with both plies still theoretically capable of performing a significant load bearing function [19]. ...
A set of novel structural fire tests on axially loaded cross-laminated timber (CLT) compression elements (walls), locally exposed to thermal radiation sufficient to cause sustained flaming combustion, are presented and discussed. Test specimens were subjected to a sustained compressive load, equivalent to 10% or 20% of their nominal ambient axial compressive capacity. The walls were then locally exposed to a nominal constant incident heat flux of 50 kW/m² over their mid height area until failure occurred. The axial and lateral deformations of the walls were measured and compared against predictions calculated using a finite Bernoulli beam element analysis, to shed light on the fundamental mechanics and needs for rational structural design of CLT compression elements in fire. For the walls tested herein, failure at both ambient and elevated temperature was due to global buckling. At high temperature failure results from excessive lateral deflections and second order flexural effects due to reductions the walls' effective cross-section and flexural rigidity, as well as a shift of the effective neutral axis in bending during fire. Measured average one-dimensional charring rates ranged between 0.82 and 1.0 mm/min in these tests. As expected, the lamellae configuration greatly influenced the walls' deformation responses and times to failure; with 3-ply walls failing earlier than those with 5-plies. The walls' deformation response during heating suggests that, if a conventional reduced cross section method (RCSM), zero strength layer analysis were undertaken, the required zero strength layer depths would range between 15.2 mm and 21.8 mm. Deflection paths further suggest that the concept of a zero strength layer is inadequate for properly capturing the mechanical response of fire-exposed CLT compression elements.
With architectural styles changing and the knowledge of fire behaviour constantly evolving, it is important to continue advancing the field of fire safety engineering to ensure that the existing and expanding infrastructure is safe and resilient. Within the National Building Code of Canada and subsequent provincial and material design standards, there exists flexibility for designers to consider more advanced computational practices that can optimize the fire protection design. These clauses permit alternative design solutions to be used when they can be proven to be equivalent or superior to the prescriptive design. This, however, can be hard to implement regarding structural fire designs as the Authorities Having Jurisdiction (AHJs) typically do not have the fire education and resources to evaluate and compare a design. Alternative solutions, especially for structural fire design, allow for economic and material savings. A performance-based solution is able to check for all possible scenarios and optimize the fire protection, reducing the environmental impact of the design by reducing the need for excess fire protection, which can be toxic and have negative life cycle analysis impacts. The connections are known as the most vulnerable part of a steel-framed building construction. A preliminary series of fire tests were undertaken at York University’s Fire Resiliency Lab, with different methanol pool fire durations, to understand the deformation behaviour of a simple steel post-and-beam frame and how the forces and heat are dissipated into the connections. With an accurate understanding of the thermal forces created by a localized fire, the design of connections would be able to dissipate the large forces that occur through ductile connections. The tests demonstrated how connections and the remaining structure behave intrinsically when exposed to thermal forces, such as displacements and rotations.KeywordsSteel beam-to-column connectionFire design
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