Lateral Torsional Buckling Design for Multi-Ply Structural Composite Lumber Beams

Abstract

The objectives of this study were to investigate the lateral torsional buckling performance of multi-ply structural composite beams to develop experimental procedures to understand their flexural and torsional rigidity that can be used to develop design guidance capable of reliably accounting for their composite capacity. Three 44.5 mm×457 mm×14.8 mm (1.75 in.×18 in.×48.3 ft) laminated veneer lumber beams were nondestructively tested more than 140 times in this investigation using a variety of single and multi-ply member configurations to determine their member stiffness about each primary orientation and joist orientation buckling controlled moment capacity. To allow a wide range of member slenderness to be evaluated, the moment capacity tests used Southwell techniques and single-span bending configurations with two-point loads to determine the critical buckling moment capacity (Mtest′) and torsional rigidity for two different member depths, two different spans, two different relative load point positions, and with up to three beam plies. Multi-ply headers were attached together with nails using a scheme that approximated the manufacturer’s minimum recommendations for applications without significant side loads. This study demonstrated that the measured lateral torsional buckling moment capacities were overpredicted where the multi-ply members were assumed to behave with the rigidity of a solid section and underpredicted where the members were assumed to behave as a collection of unattached plies. For the multi-ply beam loading conditions, nail attachment intensities, and bracing methods tested to near their full-scale buckling capacity in this study, Mtest′ was reasonably estimated using predictive calculations that explicitly accounted for torsional rigidity and assumed that the between-ply fastenings provided partial composite action with empirically derived efficiencies of 6% for torsional (GKJ) and 25% for plank orientation flexural (EpIp) rigidity. It was also found that the degree of multi-ply flexural EpIp composite action measured in the plank orientation flexural stiffness tests conducted under different loading and boundary conditions proved to be a poor predictor of relative lateral torsional buckling rigidity in the beam tests and was not correlated to the measured buckling resistance.