A Design Perspective on the Stability of Timber Beams Under Lateral Torsional Buckling

Abstract

Lateral torsional buckling (LTB) of timber beams is an important out-of-plane behavior mode due to large depth/width ratios and long span lengths, as in steel beams. A beam that is subjected to a lateral torsional buckling failure would deform under an applied load or moment until the critical moment (Mcr) is reached, after that, a slight increase in the applied moment will create large out-of- plane deformations, and the beam will buckle by deflecting laterally and twisting. If the compression zone of the beam is braced by using out-of-plane nodal lateral bracing, these deformations can be prevented. In timber structures, specifications of the nodal bracing for timber beams are not clear enough to provide easy-to-use calculations in Eurocode 5. This study aims to investigate the optimum level of the bracing stiffness using the brace force - brace stiffness relationship, so that bracing calculations are easier to be applied. As the nature of nodal bracing, it is expected to observe lesser brace forces as the brace stiffness increases. However, this effect balanced itself at optimum brace stiffness. After the ideal stiffness value is reached, any brace stiffness greater than this value does not increase substantially the buckling capacity of the beam. In this paper, the study is conducted using a validation of Finite Element Analysis (FEA) coupled with experimental studies from the literature. Also, different sizes of the specimens were generated using the FEA procedure to conduct a matrix of cases that includes both brace stiffnesses and sizes of timber beams. These matrices are then presented in graphs that demonstrate the optimum level to stabilize the beams. The Eurocode 5 effective length design concept was used for the calculations. This study provides easy-to-use bracing calculations to reduce the effect of LTB on timber beams.