Response of long-span and high-strength steel beams to nonuniform temperature fields induced by fire

Abstract

This thesis explores the effects of nonuniform temperature fields on the behaviour of long-span structures and compares the responses of members made of mild and high-strength steel (HSS). Advanced nonlinear analyses of structural members using the finite element (FE) method and fire simulations using computational fluid dynamics were conducted. Comprehensive steady and transient state tensile tests at elevated temperatures were carried out to determine the mechanical properties of S700 MC grade HSS, which were used to establish its material model for numerical analyses. The FE calculations were carried out using the Abaqus programme, while the Fire Dynamics Simulator (FDS) programme was used for fire simulations. The coupling between FE and FDS analyses was based on adiabatic surface temperatures and was carried out using the FDS2FEM programme. The study indicates that the EN 1993-1-2 material model with reduction factors based on the results of transient state tests reasonably describes the S700 MC material. Moreover, its strain hardening at large strains can be modelled using the Ramberg-Osgood equations with the reduction factors derived from steady state test results. Responses of beams and frames exposed to the nominal fire indicate that the critical temperatures of structures made of HSS are higher than those of structures made of mild steel with similar loads. However, at equal load ratios, the use of HSS can only be promoted if the displacements are allowed to be larger for its structures. The developed five-stage mechanism indicated that heating-cooling cycles of travelling fire and higher strength of HSS beams increased fluctuations in the axial force response. At load ratio of 0.5, these fluctuations activated catenary action at a much lower temperature than the critical temperatures based on the EN 1363-1. This means that the catenary action temperatures should be considered in structural design. The analyses of 31 m span truss beams pointed out that the orientation of the beams with respect to the travel path of fire affects their temperature fields and mechanical responses. Depending on the orientation, the structural failure may occur due to large local or global deflections. The simulation of fire intervention with a duration of 360 s decreased the temperature and overall deformation of the truss beam locally. However, with insufficient water, the fire intervention transferred the flames to one of the ends of the beam span. The results point out that efficient fire-fighting tactics can be developed with the methods used in the study. Fire and fire intervention cause three-dimensional and time-dependent temperature fields in long-span steel structures. The structural fire-safety design of these structures should be based on methods capable of calculating highly nonlinear responses. The study also highlights the need for experimental investigations of the mechanical properties of HSS at elevated temperatures during and after fires considering the effects of different heating and cooling rates.