HYGROTHERMAL EFFECTS ON LOAD-DURATION BEHAVIOR OF STRUCTURAL LUMBER

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.