The Reduced Cross-Section Method for the Design of Timber Structures Exposed to Fire-Background, Limitations and New Developments

摘要

The load resistance of timber members exposed to fire is determined from the uncharred residual cross section. Owing to elevated temperatures in parts of the residual cross section, the strength and stiffness properties are lower than under normal conditions. The effective residual cross-section model provides a simplified user-friendly design concept to account for the reduced properties of timber exposed to fire. A fictitious zero-strength layer is removed from the residual cross section obtained after removal of the char layer, and the remaining cross section is assumed to have normal strength and stiffness properties. The method is implemented in Eurocode 5 as the reduced cross-section method. This paper deals with the background of this method, originally developed for rectangular cross sections of glued laminated timber, and shows extensions to other types of cross sections such as solid timber frame members and I-joists. While the thickness of the zero-strength layer was originally given as 7,6 mm, the results of simulations presented here show that the thickness of the zero-strength layer depends on a number of parameters, such as the dimensions and geometry of the cross section, the stress conditions (compression or tension) of the fire-exposed side(s), the load ratio and the duration of fire exposure. It is concluded that the assumption of a fixed value of 7,6 mm is often non-conservative.