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Behaviour and reliability of wood tension members exposed to elevated temperatures Lau, Peter Wing Cheong

Abstract

The merit of approaching fire safety design from the standpoint of reliability is the impetus of this thesis. Reliability, a direct function of time to failure, is a measure of performance which falls naturally under a performance-based code. The objectives of this study focus on advancing our understanding of the structural behaviour of light-frame wood members subject to tension and elevated temperatures, on actual strengths of brittle materials and how these strengths are affected by time at elevated temperatures, and on the time to failure under a given stress-and-temperature history. A model based on linear damage accumulation theory was developed to predict the time to failure. This model is based on a kinetic theory for strength loss as a function of temperature and stress histories, coupled with a kinetic term, to express the pyrolytic process as a form of damage. The model has four independent parameters and requires the short-term strength of the member as an input. This model fits well to experimental data on nominal 2x4 structural lumber tested at three different rates of loading under tension, and at four temperatures, 150, 200 and 250°C, and room temperature. The model also predicts, with reasonable accuracy, the behaviour of lumber under constant load at 250°C. In analyzing the rate-of-loading effect on lumber at elevated temperatures and the reliability as affected by lumber grade, the model predicts that the rate-of-loading effect becomes increasingly significant as temperature is increased. It also predicts that lower-grade material, when subject to a constant tension stress, has a lower reliability index at elevated temperatures up to 250°C. The differences, however, are expected to be insignificant based on current design practices. Temperature effects on strength are approximately linear at temperatures up to 150°C. Above 150°C durations becomes as dominant a factor as temperature. Based on a comparison with existing data, lumber at 9-11% moisture content is more adversely affected by temperature than small, dry, clear specimens.

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