Load‐Duration Effects in Structural Lumber: Strain Energy Approach

Abstract

A critical strain‐energy‐density failure criterion is introduced to describe the load‐duration (creep‐rupture) behavior of structural lumber. From extensive ramp, constant, and step‐constant load tests in various constant and cyclic environments, a critical strain‐energy density is identified, which forecasts impending failure. Impending failure is defined as the initiation of member failure, excluding partial failures that do not influence subsequent member behavior. Most previous load‐duration research defined failure as the complete collapse of a test member and modeled the response using cumulative damage theory. Select structural and no. 2 grade Douglas‐fir nominal 2 in. by 4 in. lumber are used here. The critical strain‐energy density is found to be invariant with respect to load history, load level, grade, and hygrothermal condition. A model is developed and used to predict the observed load‐duration behavior of the Douglas‐fir lumber sample. A comparison between the damage accumulation approach to modeling load‐duration effects and the developed strain energy approach is also presented.