On the failure modes and strength of steel-wood-steel bolted timber connections loaded parallel-to-grain

Abstract

The current Canadian code provisions for the design of timber bolted connections were essentially developed based on connections showing a ductile behavior and then further modified to account for situations where connections fail in a brittle way. An experimental study was undertaken to evaluate the strength of bolted connections specifically experiencing a brittle mode of failure. Specimens consisting of steel-wood-steel connections with either 19.1 mm or 12.7 mm bolts were tested in tension. Test variables included end distance, bolt spacing, row spacing, number of bolts per row, number of rows, thickness and species of wood member, glulam or sawn lumber members. Connections were tested to the ultimate to observe possible modes of failure as variables were changed. Results show that the current Canadian standard approach to evaluate the resistance of timber bolted connections is not optimal although conservative. Brittle modes of failure such as row shear-out, group tear-out, and splitting were observed. The resistances calculated using the O86.1 design provisions are as little as a third as compared to tested results. Also, the design equations do not allow the designer to take advantage of the increases in strength as a result of increases in row spacing, as observed in tests. Analysis of the results show that the longitudinal shear stress at failure is related to a parameter which is a function of the smaller distance (end distance or bolt spacing) and the specimen thickness. This relation was used to formulate design equations to predict the row shear-out and group tear-out strengths of glulam specimens using the specified strength values listed in O86.1. As well, it was found that Mode I of the European yield model is the only ductile ultimate failure mode and that other equations for bearing failure can be neglected. In this paper, the research program is described, results are presented, and an alternate design approach is proposed to predict the failure mode and the ultimate strength of steel-wood-steel bolted connection groups.Résumé : Les clauses du présent code canadien pour la conception de connexions boulonnées de madriers ont été essentiellement développées sur la base de connexions montrant un comportement ductile et par la suite modifiées pour considérer des situations où les connexions défaillent de façon fragile. Une étude expérimentale a été entreprise pour évaluer la résistance de connexions boulonnées subissant spécifiquement un mode de rupture fragile. Des spécimens consistant en des connexions acier-bois-acier avec des boulons soit de 19,1 ou 12,7 mm ont été testés en tension. Les variables des tests ont inclus la distance à l'extrémité, l'ecartement entre les boulons, l'ecartement entre les rangées, le nombre de boulons par rangées, le nombre de rangées, l'épaisseur et l'espèce de bois du membre, et si le membre est de bois lamellé et collé ou scié. Les connexions ont été testées jusqu'à l'extrême afin d'observer les modes de ruptures possibles en fonction des variables. Les résultats m...

Ductile Behavior of Timber Structures under Strong Dynamic Loads

“…Joints with dowel-type fasteners are expected to be ductile due to the highly nonlinear behavior of the wood under embedding stresses and the plastic behavior of the steel fasteners in bending [44]. Nevertheless, they can sometimes be affected by sudden and brittle failures like block shear or splitting [45]. Ten different types of failures (six in single shear and four in double shear) are considered by the European standards for dowel-type timber connections [46].…”

Reinforcement of dowel type connections

“…The failure mode (a) is ductile because it is characterised by the embedment of the fasteners into the wood, the others are not [8,[10][11][12]. Timber may show a tendency to split in the connection area before the embedding strength is reached according to timber thickness, diameter and number of dowels, load to the grain angle, spacing as well as the end and edge distances of the dowels.…”

Performance of heavy timber connections in fire

“…As a result, the probability of brittle failure modes increases. Brittle failure modes have often been observed in tensile tests (Masse 1988;Quenneville 2000;Mohammad 2001). However, these failures can not be predicted by the EYM.…”

“…To overcome the drawbacks of the EYM not accounting for the brittle failure modes occurring in timber connections, modification factors were introduced in previous version of the Canadian design standard (CAN/CSA-O86-01 2005). However, this effort resulted in a very conservative situation and the connection strengths were much underestimated (Quenneville 2000;Mohammad 2001).…”

“…As mentioned in Chapter 3, only WSW Group #1 was tested using the furnace loading system prior to the fire-resistance tests. For other groups in WSW and SWS, the tensile tests were conducted by Mohammad (2001) and Quenneville (2000). All tensile tests were carried out in compliance with the ASTM Standard D1761-88 (2000).…”