Seismic Behavior Investigation of Rigid Steel Frames after Fire

There is a large number of situations which expose structures to the fire. Most of the studies have focused on the safety of structures after fire, or the effect of fire that would take place after earthquakes. Observations show that there have been structures that experienced fire and still remained stable, and only by modifying the appearance, have continued to serviceability without structural retrofitting. Due to the risk of earthquakes, it is necessary to determine whether the seismic resistance of the structure to tackle against lateral load is enough or not. In this study, therefore, the seismic behaviour of three steel moment frames after fire were assessed and compared with a building which have not experienced fire. For this purpose, these models with different stories have been selected and designed. The structural models have been designed according to the Iranian seismic code 2800 (4th edition). For performing the nonlinear static analysis, ABAQUS software was implemented. Because most of the existing steel structures are not insulated against the fire with fire-retardant cover, it is assumed that the investigated frames do not equipped with any fire-resistant substances such as fire-fighting foams or fire-retardant gels. For these frames, two symmetrical fire scenarios were considered: 1) each corner spans in the ground floor were exposed to fire; 2) the middle spans in the ground floor were exposed to fire. The process of fire consists of two stages. The first stage illustrates the heating process causing by ignition, and the second stage illustrates the effects of cooling process in fire extinguishing duration. In structural analysis, according to the Eurocode, the parametric curve has been used to apply the first stage, and a linear curve has been used for cooling stage. Having been heated frames according to parametric time-temperature curve of the fire, in order to obtain the maximum time and the corresponding temperature, a failure criteria was determined. By determining these parameters and the fire compartment’s properties, the second stage of the fire behaviour (the cooling curve) was obtained. The seismic behavior of frames, by imposing the complete curve (fire-cooling) to the structure, through a nonlinear static analysis were examined. In the process of the analysis, the maximum relative displacement specified by the Iranian seismic code 2800 has been applied to each story, then the response of the structure has been evaluated. The illustrated results of this research show that in a similar base shear, the nodal displacements of the structure exposing to fire and cooling are more than that of a structure which have never experienced fire. In addition, in the structures that were exposed to fire, the limited drift was not satisfied above the story where the fire was applied. Besides, the maximum drift in the first scenario in which the fire was happened in the corner spans, was significantly higher than that of the one in the second scenario (fire in the middle spans).