Performance Assessment of Special and Intermediate Steel Moment Resisting Frames Under Post-Earthquake Fire

Severe earthquakes may cause extensive fires in the structures. In the aftermath of earthquake, factors such as the occurrence of residual displacement in the structural stories (geometrical damage), increased fire intensity, and damage to the fireproofing (local damage) reduce structural resistance rating to fire. The methods and requirements of seismic design of structures can have a great impact on their fire resistance rating. In this paper, the performance of intermediate and special moment frames in post-earthquake fire is compared.
For this purpose, two 5 and 10 story frames are designed with two special and intermediate moment frame systems assuming they are in a high risk seismic zone. It is supposed that the beams and columns of the frames were protected by cement vermiculite spray fireproof material and accordingly the required thickness of the fireproofing is designed for two hours of fire resistance time. The 2D-frames model in Abaqus software for pushover- heat transfer- and thermal-mechanical analyses. The compartment fire scenario is assumed to start from the second floor and extend to two-fifths the height of the frames. For seismic loading, two hazard levels including the Design Base Earthquake (DBE) and the Maximum Credible Earthquake (MCE) are considered. The results of seismic and thermal-mechanical analyses are combined to accurately consider the behavior of seismically-loaded structures under fire loads. The fire resistance time of two sample steel moment frames for pre- and post-earthquake situations is compared to specify the reduction of fire resistance time due to the seismic effects. Two types of temperature-time curves for natural fire in pre- and post-earthquake conditions are considered in relation to Eurocode 3 and fire extinguishing coefficients in post-earthquake conditions. To determine the seismic damage (geometric and local damages) in the frames, the pushover method is employed in which the frames were first pushed by the first mode loading pattern to reach the target displacement of each hazard levels and then unloaded to return their elastic displacements and stay plastic displacements in the frame’s stories as the permanent displacements. The location of the plastic hinges in frame’s members at the target roof displacement is determined for each hazard levels. It is assumed that the fireproofing delaminate at these points as long as the depth of the beam cross section and directly exposed to the fire. The collapse of the structural systems under fire loads mainly occurs in three different modes: (i) yield of beam members, (ii) buckling of columns, and (iii) side-way collapse. The occurrence of the mentioned collapse mechanisms depends on the factors such as structural characteristics, fireproofing and geometric damage. The resistance time of the frames is the time at which any of the abovementioned collapse modes are detected.
The results show that the failure mode of the undamaged and damaged special moment frames (SMF) under fire scenarios is yield of beam members. The fire resistant rating for undamaged SMF is about 21% less than damaged one. The reason for the low resistance time reduction is due to the redistribution forces in the damaged beam at high temperatures so that by eliminating the fireproofing at the two ends of the beam, the temperature at these points increases more rapidly than the beam center. By reducing the stiffness of the two ends of the beam, due to the redistribution of forces, the moment at these points will be reduced and added to the mid-beam moment. Therefore, some of the lower two end resistance of the beam is compensated by its middle. The fire resistance time of Intermediate Moment Frames (IMF) after Design Base Earthquakes (DBE) is estimated to be 19% less than undamaged one.
However, Maximum Credible Earthquakes (MCE) reduce the resistance time of these frames by 75% since at this hazard level the plastic hinges occurrence in some columns of such frames that results in damage to the column’s fireproofing. Also, the results of natural fire analyses show that after MCE earthquakes, the intermediate moment frames may collapse under natural fire, while special moment frames are able to withstand against the natural fire in post-earthquake conditions.