The research presented in this paper examines the shear resistance performance of self-tapping screws (STS) in three-ply cross-laminated timber (CLT) panels. Specifically, the feasibility of using innovative STS assemblies with double inclination of fasteners was investigated for the shear connection of CLT panels. The specimens (1.5×1.5 m) were subjected to quasi-static and reversed-cyclic loading. The tests were set up to approximate pure shear loading, with three-panel CLT assemblies connected with STS. The resulting load-displacement and hysteretic curves were used to determine an equivalent energy elastic-plastic curve to estimate assembly capacity, yield load, yield displacement, ductility ratio, stiffness, and damping. Excellent structural performance in terms of capacity and stiffness was obtained while still providing the required ductility for the system to be used in seismic applications. The average static and cyclic yield loads were 6.0 kN/screw and 5.9 kN/screw, respectively. Average static and cyclic and ductility ratios were 7.7 and 4.1, respectively, allowing the connection to be classified as highly ductile under quasi-static loading and moderately ductile under reversed cyclic loading. The data obtained allow engineers to specify an innovative connection assembly with double inclination of fasteners for lateral load–resisting systems of CLT structures.
Effective preservative treatments for Canadian glulam products are needed to maintain markets for mass timber on building facades, access markets with significant termite hazards, and expand markets for wood bridges. For all three applications, borate-treatment of lamina before gluing would be preferred as it would lead to maximum preservative penetration. However, the need to plane after treatment and prior to gluing removes the best-treated part of the wood, and creates a disposal issue for treated planer shavings. The present research evaluates the block shear resistance of glulam prepared from untreated and borate-treated lamina with a polyurethane adhesive. Borate treatment was associated with a small but statistically significant loss in median shear strength when evaluated dry; however, there was no difference between the performance of untreated and borate-treated samples when exposed to the vacuum-pressure soak/dry or the boil-dry-freeze/dry procedures. Further work is needed to modify the composition or application of the resin to improve shear strength for glulam applications and ensure consistent performance. However, overall, these data indicate that samples prepared from borate-treated lamina perform similarly in terms of block shear resistance to those prepared from untreated lamina.
International Conference on Structural Health Assessment of Timber Structures
Numerous large-span, load-bearing structures, e.g. for public venues, sports halls or industrial facilities, utilize glued-laminated timber (glulam) due to its versatility and aesthetic appearance. Since glulam is an organic and composite material consisting of wooden lamellas joined with glue, mistakes during planning, fabrication and use of the structure can lead to deficiencies or even damages. Following a visual inspection suitable to detect surface deterioration, further holistic investigation of the glulam material is necessary to appraise structural safety or estimate the need for restoration. Besides the integrity of the wooden lamellas, the glue bond of the lamellas is vital for the operational reliability of glulam. A currently practiced, semi-destructive assessment method to revalue the condition of the glue lines consults a shear test on drill core samples with included glue line. The presented paper links this method to medium-scale shear tests and large-scale 3-Point bending-shear tests representing a practical loading situation. To provoke shear failure during bending, reinforcements of the specimens at critical tension and compression zones included glued-on beech veneer lamellas and self-tapping screws, respectively. Executed on the same sample material, shear resistance was determined for all three testing formats. The sampling included aged and new glulam. Published values derived from comparable test programs augmented the database. Based on the evaluated test results of the drill cores under shear loading, suggestions regarding the drill core extraction and the implementation of the valuation method of EN 14080 are outlined. After incorporating a size effect to account for varying dimensions of the bendingshear specimen, their shear resistance values correlated well with the obtained values from the drill core tests. This adumbrates the possibility to derive the shear resistance of structural members from shear values of drill core samples taken from an existing glulam structure.
International Council for Research and Innovation in Building and Construction, Working Commission W18 - Timber Structures
Cross laminated timber (CLT) has become a well-known and widely applied two-dimensional, engineered timber product worldwide. It constitutes a rigid composite of an odd number of orthogonal and glued layers. Focusing on a single glued node loaded in plane in shear and composed of two crossed board segments and the adhesive layer in-between, in principle three types of shear mechanisms can be distinguished: mechanism I "net-shear" (shearing perpendicular to grain), mechanism II "torsion" and mechanism III "gross-shear" (shearing parallel to grain). In fact, while having generally accepted values for the resistance against mechanism II and good estimates for mechanism III the resistance against "net-shear" (mechanism I) is still in discussion. In spite of numerous investigations on nodes and on whole CLT elements in the past, a common sense concerning the test procedure, the consideration and handling of distinct influencing parameters and the quantification of the shear strength are open. We focus on the in plane shear resistance of single nodes according to mechanism I. We (i) propose a test configuration for reliable determination of the shear strength, (ii) determine the shear resistance in case of shear loads perpendicular to grain, (iii) discuss influences of some parameters on the shear strength of single nodes, and (iv) give a brief outlook concerning the resistance of CLT elements against shear loads in plane.