Load carrying capacity of multiple fastener steel to bamboo joints loaded parallel to the fibre direction

Abstract

Laminated bamboo could be a promising material when it comes to large span roofing structures where often truss structures are adopted. It is assumed that connection design used for timber trusses can also be applied in laminated bamboo trusses. For this research truss nodes with slotted-in steel plates are considered. This research will focus itself on the effect of using multiple fasteners in a row in laminated bamboo connections with a single slotted-in steel plate. As a starting point, the possible failure modes and methods for predicting the load-bearing capacity of a single fastener timber connection are assumed to be also applicable to laminated bamboo. Timber can show ductile and brittle failure modes. Ductile failure modes are described by the European Yield Model (EYM) which was first published by Johansen in 1949. Three different failure modes can be distinguished in the EYM, where mode 1 represents pure embedment failure, mode 2 is failure due to one plastic hinge in the fastener per shear plane and mode 3 shows two plastic hinges per shear plane. The brittle failure modes that can be distinguished are splitting, shear plug, tension failure of the net section and group tear-out. The capacity of a multiple fastener connection might not equal the capacity of a single fastener connection multiplied by the number of fasteners. The main parameters that might influence the load-carrying capacity of multiple fastener joints are the inaccuracies in manufacturing, fastener spacing and end distances, dowel slenderness ratio and the plastic behaviour of the fastener. The testing methods for determining the strength en deformation characteristics of laterally loaded connections with dowels in load-bearing timber structures are used for this research. The objective is to study the influence of the number of fasteners in one row on the load carrying capacity of a steel to laminated bamboo connection. The design of the test pieces is based on the European Yield Model (EYM) where three different failure modes can be distinguished. In order to accomplish this, 9 test series are performed with a variation in the number of fasteners in one row (i.e. 1, 2 and 3) and a variation in thickness of the laminated bamboo members (i.e. 15 mm, 43 mm and 90 mm). The test results show that laminated bamboo connections with a slotted-in steel plate and thin bamboo members have limited deformation capacity and tend to show premature brittle failure due to a shear plug. A clear plastic branch in the load slip diagram of the test series where a single fastener and a member thickness of 15 mm is used, indicates that there is some amount of embedment prior to fracture. In case of two dowels in a row this plastic branch shortens and in case of three dowels in a row no plastic behaviour was observed. The limited deformation capacity of these type of joints (especially when multiple dowels in a row are used) makes the applicability of the EYM for a laminated bamboo connection where failure mode 1 is governing questionable. Plastic hinges did develop in the dowels in the test variants where failure mode 2 and 3 was expected. the EYM can quite accurately predict the capacity of a double shear laminated bamboo joint with a central steel plate where the dowel slenderness ratio is sufficient to achieve failure mode 2 or 3. The effective number of fasteners in a laminated bamboo connection was found to be higher than predicted with the equation given in EC5 irrespective of the observed failure mode. It is because of the decrease in deformation capacity that the load per dowel in a multiple fastener connection is lower than in a connection with a single fastener. The nef for laminated bamboo and several wood species was found to be in the same order of magnitude for a connection with three dowels in a row. The joint stiffness per dowel of a laminated bamboo connection with a slotted-in steel plate was found to decrease with increasing number of fasteners in a row. It also seems that the joint stiffness is higher in case of failure mode 2 and 3 than in case of failure mode 1.