The Fire Performance of Post-Tensioned Timber Buildings

Post-tensioned timber buildings utilise a new construction technique developed largely as part of research undertaken at the University of Canterbury. Timber buildings are constructed using an engineered timber product, such as laminated veneer lumber (LVL), and then stressed with post-tensioned unbonded high-strength steel tendons. The tendons apply a compressive stress to timber members to create a ductile moment resisting connection between adjacent timber members. The major benefit of post-tensioned timber buildings is a significantly improved structural performance. As timber is a combustible material there is a perceived high fire risk in timber buildings. While timber buildings can be designed to perform very well in fire, a design guide for the fire safety design of post-tensioned timber buildings has not been previously developed. Furthermore, previous research has found that post-tensioned timber box beams may be susceptible to shear failure in fire conditions. This research investigated the fire performance of post-tensioned timber buildings. A design strategy for the fire performance of post-tensioned timber buildings was developed in conjunction with a simplified calculation method for determining the fire resistance of post-tensioned timber structural members. The fire performance and failure behaviour of post-tensioned timber box beam was also specifically investigated, with special focus given to the shear performance of box beams. A full scale furnace test of a LVL post-tensioned LVL box beam was conducted at the Building Research Association of New Zealand (BRANZ). Four further full scale tests of LVL box beams were conducted at ambient temperature at the University of Canterbury structural laboratory. Through this research two distinct strategies for the fire design of post-tensioned timber structures were developed. The first strategy is to rely on the residual timber of the members only. The second strategy considers specific fire protection of the post-tensioning system, which can then be used to contribute to the fire resistance of the member. The results of the full scale tests showed good agreement with the proposed the simplified calculation method. It was also determined that shear failure does not need to be specifically considered other than performing strength checks as for other design actions