Three-dimensional analysis of steel frames and subframes in fire.

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.