MODELING THE RELIABILITY OF WOOD TENSION MEMBERS EXPOSED TO ELEVATED-TEMPERATURES

The merit of approaching fire safety design from the standpoint of rel iability is the impetus of this paper. Reliability, a direct function of time to failure, is a measure of performance that falls naturally u nder a performance-based code. The objectives of this study focus on a dvancing our understanding of the structural behavior of light-frame w ood members subject to tension and elevated temperatures, and on the t ime to failure under a given stress and temperature history. A model b ased on linear damage accumulation theory was developed to predict the time to failure. This model is based on a kinetic theory for strength as a function of temperature and stress, coupled with a kinetic term, to express the pyrolytic process as a form of damage. The model, whic h requires the short-term strength as an input, fits well to experimen tal data on nominal 2x4 structural lumber tested at three different ra tes of tension loading, and at 150, 200, and 250 degrees C, and room t emperature. The model also predicts, with reasonable accuracy, the beh avior of lumber under constant-load at 250 degrees C. It predicts that lower-grade material generally has a lower reliability index; however , those differences are insignificant as far as current design practic es are concerned. The reliability is sensitive to variability in tempe rature but not to variability in stress.