Duration of load tests for shear strength

Abstract

The strength of wood falls with time under load, and in current design codes the short-term strength of wood is reduced by about 40% to account for duration of load effects. This figure is based on tests made on small bending specimens. In this paper are described tests made on wooden torque tubes to investigate the effect of duration of load on shear strength. A control sample was tested to establish a curve for short-term strength, and four groups of 80 specimens each were then tested under various levels of constant long-term load. Stress ratio at failure was estimated by assuming that the short-term strength of each group could be represented by the control curve, and that under long-term loading specimens would fail in the order of their short-term strength. In each group the stress ratio at failure fell with time under load, and this reduction appears to be related to that predicted by a viscoelastic plastic model. It is concluded that the Madison curve presently used to predict duration of load effects may be conservative at normal levels of applied stress. Key words: wood, shear, long-term loading, duration of load.