The deformation behaviour of unprotected steel framed structures exposed to real fire conditions
has been studied experimentally and theoretically. A total of three tests on steel frames
under different load level and different heating process are conducted, all the tested frames
are single span and single story. In order to simulate a real fire condition, heating is attained
by a specially built gas furnace. Behaviour of each tested frame in fire is described and comparisons
are made between fire test results and results calculated by a 2D finite element computer
program—NASFAF developed in the paper. The secant stiffness matrix which relating
the total element displacements to the corresponding total element nodal forces is used in
developing the program and the effects of geometrical non-linearity, “softening” of material
at elevated temperature, gradual penetration of inelastic zone both along and across each member,
non-uniform distribution of temperature are all taken into consideration. Creep effect is
considered in an approximate way in the temperature-dependent stress–strain curves of the
material.