Young, Scott (2000) Structural modelling of plasterboard-clad, light timber framed walls in fire. PhD thesis, Victoria University of Technology.

Abstract

The building regulations in Australia and other countries are being reformed from a prescriptive basis of designing fire safety system requirements to a performance basis, which requires the application of engineering principles. Under the prescriptive regulations, fire resistant construction elements were tested using a standard fire resistance test heating regime, which is invariably quite different from the characteristics of real fires. The engineered approach requires the ability to predict the performance of elements of construction that may have different details those tested under the standard heating regime and under different heating regimes. This thesis comprises part of a comprehensive research program undertaken at the Centre for Environmental Safety and Risk Engineering, Victoria University of Technology to develop a comprehensive framework to determine the time dependent probability of failure of timber-framed assemblies subjected to real fires. The objective of the thesis is to determine the structural response of plasterboard-clad, timber framed walls subject to fire and is a key component that is incorporated into the framework to determine the probability of failure of timber-framed assemblies subjected to fire. The structural model simulates dominant phenomena, which have conventionally been ignored in the determination of the structural response of timber-framed walls in fire. The phenomena considered include the degradation in mechanical properties of materials due to thermal effects, non-linear mechanical and geometric effects caused through buckling, partial composite interaction with the plasterboard sheeting, thermal expansion and shrinkage and varied end restraint conditions. These phenomena have been modelled using common frame analysis methods to provide a computationally efficient and robust model. A transient, second-order direct stiffness approach has been utilised, with specific elements devised to consider the partial composite interaction. In conjunction with the development of the model, a comprehensive experimental program has been undertaken, to provide a means of comparison of the model predictions, and to obtain data found to be lacking in the literature. A series of full-scale experiments were conducted on timber framed assemblies under ambient and elevated temperature conditions. The variability, end restraint conditions and contribution of the sheeting were carefully controlled and examined in the experiments. The literature review identified that there was a lack of data detailing the reduction in the modulus of elasticity in compression with temperature. This was considered the most critical mechanical property in determining the time-to-failure of a load bearing timber-framed wall and so a series of experiments was undertaken on short lengths of timber to determine the reduction in the mechanical properties in compression with temperature. The full-scale experiments demonstrated the significance of end restraint and the influence of plasterboard sheeting on the time-to-failure. The full-scale experiments also showed an apparent rapid drop in the apparent stiffness of the timber framing as it approached a mean temperature of approximately 1 OODC through the cross-section. The reduction in the mechanical properties of radiata pine in compression due to elevated temperature exposure was determined directly from tests. It was determined that there was a significant reduction in the mechanical properties in compression of moist timber specimens heated to 70DC, compared with dry specimens. This finding was consistent with results from the full-scale experiments. The structural response model was successfully validated against closed form and finite element solutions and the predictions of the model compared closely with results from a series of full-scale experiments undertaken as part of the research. The deformation induced in the full-scale experiments was irrecoverable and may have been associated with a mechano-sorptive type creep phenomenon, although further research is required to study this phenomenon. This is one several areas requiring future research that has been identified by the author.

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