مهندسی عمران شریف (Aug 2021)
Modification of base shear of moment steel frame to ensure the level of life safety in post-earthquake fire
Abstract
In this paper, post-earthquake fire (PEF) modeling in 3- and 9- story structures of moment steel frame is discussed. In this modeling, different levels of ground motion intensity and several time intervals for extinguishing the fire during PEF are considered. The structures are modeled using OpenSees software and tested to the performance level of life safety. First, the structures are subjected to a scale of earthquake accelerograph, then, assuming 60 seconds of free vibration until the structure is damped, the thermal load is applied as a 9-point thermal gradient to the beams and columns exposed to heat in this software. These structures were examined in different durations of post-earthquake fire based on the ISO 834 standard fire curve and fire scenarios in 3-story structure are considered as fire in 2 lower floors and 2 upper floors and in 9-story structure as fire in 3 lower and 3 upper floors. Having a maximum drift of the floors under PEF and the maximum allowable drift for the life safety level of the moment steel frame, which is 0.025 (according to FEMA356 Standard), Sa(T1) a scale of accelerograph that the structure under earthquake alone will reach the maximum values of drift under PEF can be calculated. According to the direct relationship between the base shear and the pseudo-acceleration spectrum component at the time of period of the first mode of the structure Sa (T1) the base shear of the structure can be modified. The results of the study show that these structures are more sensitive to general fire scenarios in the lower floors and the maximum relative displacement between the floors increases under these scenarios. Also, in the 3- and 9-story structures, according to the fire scenarios considered, the resistance of the structure to the level of life safety can be increased by 160 seconds by increasing the base shear by 4.4% and 8.3%, respectively. As a result, according to the time required to extinguish the post-earthquake fire, the structure can be designed by modifying the base shear for the performance level of fire safety under PEF.
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