In-plane stability of truss chords with application to wood trusses

Stability analysis of truss chords loaded by a combination of axial and lateral forces is studied. A simple model based on beam-column theory is proposed and analyzed. The continuous truss chord is replaced by a beam column loaded by end moments replacing the effect of adjacent members. The solution yields deflections and moments allowing one to compute the stress at any point of the truss chord. Analytical and numerical solutions are applied to an example of a wood truss with a continuous top chord. The numerical solution for the deformations converges rapidly. Non-converging results indicate instability. A new design procedure for wood truss chords subjected to possible buckling is proposed. The procedure is based on the analysis of a beam-column model and a mechanics of materials approach

Bending behavior of adhesively-bonded engineered wood-concrete composite decks

Four-point bending tests were conducted on five medium-sized (i.e., 2300 mm in length and 215 mm in width) engineered timber (laminated veneer lumber (LVL) and cross-laminated timber (CLT)) - concrete (wood chip concrete and plain concrete) composite decks. The concrete was glued to the wood substrate with epoxy and polyurethane adhesives. The observed failure modes of the composite decks were concrete crushing or wood failure in tension or shear. No failure of the adhesive interface was observed and the decks behaved linearly until failure. In the subsequent analysis, the authors quantified the shear flexibility of transverse layers (stressed perpendicular to the fiber direction) in CLT and LVL boards and its effect on the bending stiffness on the composite decks using γ-method described in EN 1995-1-1 (EC5). The analytical predictions of the effective bending stiffness were verified via experiments, showing consistently good agreement

A nonlinear three-dimensional finite-element model of a light-frame wood structure

The light-frame wood structure is an assemblage of several components such as walls, floors and roof connected by intercomponent connections such as nails or metal plates. The behavior of the full-structure is determined by the behavior of the individual components and connections. Whereas individual substructures were investigated both experimentally and analytically, there is a lack of research aimed to incorporate individual components of the light-frame wood building into the full-structure model. This research provides an analytical tool to investigate the behavior of light-frame wood structures loaded by static loads. Special attention is given to load sharing among wall components. A one story, 16- by 32-ft (4.88- by 9.75-m) wood-frame building was tested under cyclic quasi-static loads. Results of the experiment were used to verify a nonlinear finite-element model of the full building. Concepts of superelements and substructuring are applied to the finite-element problem. A special quasi-superelement energetically equivalent to a three-dimensional finite-element model of the full substructure was developed to represent the walls. Intercomponent connections were transformed into one-dimensional nonlinear elements, which had properties obtained from experiments and detailed finite-element analyses. The full structure was an assemblage of the superelements representing floor and roof, and quasi-superelements, which represented walls and intercomponent connections. Boundary conditions and loads used in the experiment were applied to the model, and deformations and reaction forces were compared. A sensitivity study of the model was performed, and the influences of the properties of substructures and intercomponent connections on the load sharing capability of the model were investigated. The response of the three-dimensional model of the full-structure to the static wind loads was studied and compared with currently used analytical models. Linear and nonlinear analytical models for computing reaction forces in the shear walls were proposed and their sensitivity studied. Stress analysis of the three-dimensional substructure was performed when the full model was loaded by a combination of dead, snow and wind load. Use of tensorial strength criteria as a part of the postprocessing procedure was demonstrated on evaluation of stresses in plywood sheathing

Interfacial bond behavior of adhesively-bonded timber/cast in situ concrete (wet bond process)

The goal of this research was to study the strength of the interfacial bond between cast-in-situ concrete and engineered timber (cross-laminated timber (CLT)). Double lap specimens were manufactured using fresh concrete that was cast between two CLT blocks. Polyurethane and epoxy adhesives were used to bond the wet concrete with the CLT blocks. The shear strength of wet-bond specimens was compared with the specimens prepared under dry conditions (prefabricated concrete cube glued to CLT blocks). The statistical analysis (T-test) of bond strength showed that the shear strengths of wet- and dry-bond specimens using epoxy and polyutrthane adhesives were no significantly different for the tested C25 plain concrete and the CLT. The failure mode of dry-bond specimens were concrete failure near the interface, however, debonding at interface was the dominant failure for the wet-bondspecimens

Multivariate regression models in estimating the behavior of FRP tube encased recycled aggregate concrete

This study applied newly developed multivariate statistical models to estimating the mechanical properties of recycled aggregate concrete cylinder encased by fiber reinforced polymer (FRP). Two different types of RFPs were applied, namely flax FRP and polyester FRP. Ten independent variables were predefined including the FRP type and cylinder size. It was found that several mixed models outperformed the traditional linear regression approach, based on the accuracy and residual value distribution. Individual factor analysis indicated that the fiber thickness and layer number had more significant impacts on the strength and strain of FRP-encased concrete’s transitional point, compared to their impacts at the ultimate state

Thermal Stability, Fire Performance, and Mechanical Properties of Natural Fibre Fabric-Reinforced Polymer Composites with Different Fire Retardants

In this study, ammonium polyphosphate (APP) and aluminum hydroxide (ALH) with different mass contents were used as fire retardants (FRs) on plant-based natural flax fabric-reinforced polymer (FFRP) composites. Thermogravimetric analysis (TGA), limited oxygen index (LOI), and the Underwriters Laboratories (UL)-94 horizontal and vertical tests were carried out for evaluating the effectiveness of these FR treatments. Flat-coupon tensile test was performed to evaluate the effects of FR treatment on the mechanical properties of the FFRP composites. For both fire retardants, the results showed that the temperature of the thermal decomposition and the LOI values of the composites increased as the FR content increases. Under the UL-94 vertical test, the FFRP composites with 20% and 30% APP (i.e., by mass content of epoxy polymer matrix) were self-extinguished within 30 and 10 s following the removal of the flame without any burning drops, respectively. However, the mechanical tensile tests showed that the APP treated FFRP composites reduced their elastic modulus and strength up to 24% and 18%, respectively. Scanning electronic microscopic (SEM) for morphology examination showed an effective coating of the flax fibres with the FRs, which improved the flame retardancy of the treated composites

Flax, Basalt, E-Glass FRP and Their Hybrid FRP Strengthened Wood Beams: An Experimental Study

In this study, the structural behavior of small-scale wood beams externally strengthened with various fiber strengthened polymer (FRP) composites (i.e., flax FRP (FFRP), basalt FRP (BFRP), E-glass FRP ("E" stands for electrical resistance, GFRP) and their hybrid FRP composites (HFRP) with different fiber configurations) were investigated. FRP strengthened wood specimens were tested under bending and the effects of different fiber materials, thicknesses and the layer arrangements of the FRP on the flexural behavior of strengthened wood beams were discussed. The beams strengthened with flax FRP showed a higher flexural loading capacity in comparison to the beams with basalt FRP. Flax FRP provided a comparable enhancement in the maximum load with beams strengthened with glass FRP at the same number of FRP layers. In addition, all the hybrid FRPs (i.e., a combination of flax, basalt and E-glass FRP) in this study exhibited no significant enhancement in load carrying capacity but larger maximum deflection than the single type of FRP composite. It was also found that the failure modes of FRP strengthened beams changed from tensile failure to FRP debonding as their maximum bending load increased

Thermal Behavior of Adhesively Bonded Timber-Concrete Composite Slabs Subjected to Standard Fire Exposure

peer reviewedFire tests were performed for the first time on adhesively bonded timber-concrete composite slabs. The two medium-scale (1.8 x 1.25 m) slabs were produced by gluing an 80-mm thick three-layer cross-laminated timber (CLT) board to a 50 mm thick prefabricated reinforced concrete (RC) slab with epoxy and polyurethane (PUR) adhesives, respectively. The behavior of the composite slabs under elevated temperature was monitored by (1) observing the burning behavior of the used CLT, for example, charring and delamination and (2) measuring the temperature development at different locations of the CLT slabs, in the adhesive bond between concrete and timber boards, and in RC slabs. It was found that employing a one-dimensional charring model for pure softwood, as prescribed by Eurocode 5-1-2, underestimated the charring depth of CLT due to the delamination effects. Measurements revealed that the average charring rates in the middle layer of CLT panels were approximately 0.65 mm/min, suggesting that the presence of concrete does not significantly affect the thermal behavior of the CLT panel. Delamination within the CLT was observed when its adhesive temperature was around 230°C. It was followed by the free-fall of delaminated wood plies, which progressed slowly and lasted until the end of the test. At 90 min into the test, the temperatures of epoxy at the nine locations ranged between 55°C and 130°C, while that of PUR between 60°C and 100°C. The adhesive between concrete and CLT could lose stiffness significantly along the rising of temperature after surpassing of glass transition temperature (58°C for epoxy and 23°C for PUR in this study). The results indicated a high risk of weakening the composite action between the concrete slab and timber board. The measured temperatures of steel rebar were lower than 50°C. However, the concrete temperature reached about 120°C and the concrete cracked due to the distinct thermal expansions between concrete and timber and the rigid constraint of adhesive bond.11. Sustainable cities and communitie

Compressive performance of fiber reinforced polymer encased recycled concrete with nanoparticles

Nanomaterials have been used in improving the performance of construction materials due to their compacting micro-structure effect and accelerating cement hydration reaction. Considering the brittle characteristic of fiber reinforced polymer (termed as FRP) tube encased concrete and inferior properties of recycled concrete, nanoparticles were used in FRP tube encased recycled aggregate concrete. The axial compressive performance of FRP tube used in recycled concrete treated with nanoparticles strengthening, termed as FRP-NPRC, were investigated by axial compression experiments and theoretical analysis. Five experimental variables were considered including (1) the dosages and (2) varieties of nanoparticles (i.e. 1% and 2% of nanoSiO2, 1% and 2% of nanoCaCO3), (3) replacement ratios of recycled coarse aggregates (termed as RCAs) (0%, 50%, 70% and 100%) the RCAs were mainly produced from the waste cracked bricks, (4) the number of glass FRP (GFRP) tube layers (2, 4 and 6-layer) and (5) the mixing methods of concrete. Results indicate that the combination of FRP confinement and nanoparticle modification in recycled concrete exhibited up to 76.2% increase in compressive strength and 7.62 times ductility improvement. Furthermore, a design-oriented stress–strain model on the basis of the ultimate condition analysis were executed to evaluate the stress–strain property of this strengthened component

Can Plant-Based Natural Flax Replace Basalt and E-Glass for Fiber-Reinforced Polymer Tubular Energy Absorbers? A Comparative Study on Quasi-Static Axial Crushing

Using plant-based natural fibers to substitute glass fibers as reinforcement of composite materials is of particular interest due to their economic, technical, and environmental significance. One potential application of plant-based natural fiber reinforced polymer (FRP) composites is in automotive engineering as crushable energy absorbers. Current study experimentally investigated and compared the energy absorption efficiency of plant-based natural flax, mineral-based basalt, and glass FRP (GFRP) composite tubular energy absorbers subjected to quasi-static axial crushing. The effects of number of flax fabric layer, the use of foam filler and the type of fiber materials on the crashworthiness characteristics, and energy absorption capacities were discussed. In addition, the failure mechanisms of the hollow and foam-filled flax, basalt, and GFRP tubes in quasi-static axial crushing were analyzed and compared. The test results showed that the energy absorption capabilities of both hollow and foam-filled energy absorbers made of flax were superior to the corresponding energy absorbers made of basalt and were close to energy absorbers made of glass. This study, therefore, indicated that flax fiber has the great potential to be suitable replacement of basalt and glass fibers for crushable energy absorber application