Three-Dimensional Analysis of Steel Frames and Subframes in Fire

Abstract

The aim of the present work is to develop a sophisticated analytical model for columns within three-dimensional assemblies in fire conditions. A preliminary investigation into this problem resulted in the development of a simplified approach for the analysis of isolated columns in fire. This model is based on the Perry-Robertson approach to defining critical loads of imperfect columns at ambient temperature. It takes into account uniform and gradient temperature distributions across the section of an isolated pin-ended column. It also accounts for initial out-of-straightness, load eccentricity and equal end-moments. A three-dimensional finite element model has subsequently been developed for the analysis of frames in fire conditions. This model is based on a beam finite element with a single node at each end of the element. At each node eight degrees of freedom are introduced. The finite element solution of the problem is obtained using an incremental iterative procedure based on the Newton-Raphson method, adapted to account for elevated temperature effects. The developed procedure offers a unique treatment of the thermal effects which allows solutions to be arrived at regardless of the problem's boundary conditions. The finite element formulation takes into consideration geometrical and material nonlinearities, initial out-of-straightness and residual stresses. It allows for virtually any temperature distribution across and along the structural members, and the analysis can handle any three-dimensional skeletal steel structure. The developed model allows the material mechanical properties to be expressed either as trilinear or continuous functions which vary with temperature. A computer program, 3DFIRE, has been developed based on the above-mentioned formulation and validated extensively against a wide range of previous analytical and experimental work. This program has then been used to perform parametric studies to establish the most prominent features of column behaviour in fire whether as isolated members or as part of structural assemblies. These studies have yielded a large amount of data from which generalised conclusions have been made. The analysis has been extended recently to include composite beams within the structural assembly. This development was undertaken to enable analytical studies on the test building at Cardington, in which fire tests are planned to take place in the near future.