Hygrothermal Effects on Load‐Duration Behavior of Structural Lumber

Abstract

The effect of environmental histories on the load‐duration behavior of structural lumber is presented. Select Structural Douglas‐fir nominal 2‐by‐4 lumber was tested in bending for this investigation. Constant load tests were conducted in a variety of constant and cyclic environments to define environmental effects on the load‐duration behavior and determine model constants. Constant environments were found not to have any substantial effect on the load‐duration behavior if their effect on short‐term strength is accounted for properly. However, cyclic moisture conditions caused a marked decrease in time‐to‐failure. That is, additional damage associated with so‐called mechano‐sorptive creep strain often observed in wood is present in the load‐duration behavior. A cumulative damage modeling approach was used to model the effect of mechanical, thermal, and moisture loadings on the load‐duration behavior. A damage parameter is introduced to account for the additional damage associated with cyclic moisture histories.