Birch plywood in timber connections

QC 20250429

Abstract

Earlier studies have demonstrated that birch plywood is a viable substitute for steel plates in timber connections, thanks to its excellent mechanical properties, workability, and sustainability. The mechanical properties of birch plywood have been thoroughly investigated concerning key parameters, such as face grain angle and moisture content. Furthermore, its potential use as gusset plates in timber connections has been studied. However, these investigations were mostly limited to uniaxial tension and rather small-scale compared to real applications. Therefore, additional research is needed to further understand the mechanical behaviour of birch plywood, thus ensuring a safe design of such connections. This thesis aims to gain new knowledge on connections with birch plywood plates. The results of this study will then be used to improve the design of such connections, both in terms of safety and structural efficiency.

The influence of the connector type (i.e. smooth dowel or fully threaded screw) was investigated concerning the shear capacity of connections using birch plywood with varying thickness. The results showed that, when utilizing screws, the analytical estimations according to EN1995-1 Eurocode 5 (EC5) significantly underestimate the shear capacity. This is mainly due to EC5’s conservative estimation of the so-called “rope effect” contribution to the connection’s shear capacity. Moreover, when the number of shear planes was increased from two to four, the discrepancy between EC5’s estimation and experimental results substantially increased regardless of the type of fastener. This substantial discrepancy was associated with the failure mode in the inner shear planes.

Furthermore, both glued and mechanical connections were investigated by performing tests on full-scale glulam trusses joined through birch plywood gusset plates. Failure was designed to occur in the plywood plates subjected to a complex stress state. In this investigation, the thickness of these plywood plates was varied between 9, 12 and 21 mm, testing only one specimen for each configuration. The experimental results showed that glued trusses possessed higher load-carrying capacity and almost twice the stiffness of those with mechanical fasteners. Failure occurred in the plywood plates for all trusses with mechanical fasteners and the glued truss with thinner plywood plates (9 mm). However, the trusses with thicker plates (i.e. 12 and 21 mm) showed glue line failure between the plywood plate and the glulam beam. The analytical estimations for specimens that failed in the plywood plate generally showed good agreement with the experimental capacity, although they exhibited a slight overestimation. This was expected and was attributed to the size effect, as the input strength values of plywood adopted in the analytical estimations were obtained from small-scale tests. 

Furthermore, based on the size of the glued area compared to previous studies, bonding strength values were estimated and used in the analytical calculations of the bonding capacity for glued connections. However, these strength values should be further investigated to calibrate the analytical model and obtain more realistic estimations.

In addition, two-dimensional numerical models demonstrated good agreement in predicting the global stiffness of the tested trusses.

For future work, it is suggested to investigate the size effect on the mechanical properties of birch plywood and bonding strength, respectively, to further calibrate and validate the analytical models. Furthermore, more advanced numerical simulations of the truss specimens are suggested.

urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-362577