Citation: Dehcheshmeh, E.M.; Rashed,P.; Broujerdian, V.; Shakouri, A.; Aslani, F. Predicting Seismic Collapse Safety of Post-Fire Steel Moment Frames. Buildings 2023, 13, 1091. https://doi.org/10.3390/ buildings13041091 Academic Editors: Xavier Romão and Hugo Rodrigues Received: 20 March 2023 Revised: 5 April 2023 Accepted: 18 April 2023 Published: 20 April 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). buildings Article Predicting Seismic Collapse Safety of Post-Fire Steel Moment Frames Esmaeil Mohammadi Dehcheshmeh 1 , Parya Rashed 1 , Vahid Broujerdian 1, * , Ayoub Shakouri 1 and Farhad Aslani 2,3, * 1 School of Civil Engineering, Iran University of Science and Technology, Tehran 16846-13114, Iran; esmhd.dehcheshmeh@gmail.com (E.M.D.); p_rashed@alumni.iust.ac.ir (P.R.); m_shakoori@alumni.iust.ac.ir (A.S.) 2 Materials and Structures Innovation Group, School of Engineering, University of Western Australia, Perth, WA 6009, Australia 3 School of Engineering, Edith Cowan University, Joondalup, WA 6027, Australia * Correspondence: broujerdian@iust.ac.ir (V.B.); farhad.aslani@uwa.edu.au (F.A.) Abstract: This paper summarizes a study focused on evaluating the post-fire performance of steel Intermediate Moment Frames (IMFs) following earthquakes. To this aim, archetypes comprising 3-bay IMFs with three different heights were seismically designed, and their two-dimensional finite element models were created in OpenSees software. The post-fire mechanical properties of steel were inserted into the models based on 64 different fire scenarios. The effects of different cooling methods are scrutinized at system level. To develop seismic fragility curves, Incremental Dynamic Analysis (IDA) was performed using 50 suites of far-field and near-field records, according to FEMA-P695. Then, the Collapse Margin Ratio (CMR) of each model was calculated based on the data from the fragility analysis. The results show that the seismic resistance of structures that experienced fire declines to some extent. In addition, the lowest safety level was observed when the structures were subjected to pulse-like near-field records. Keywords: post-fire earthquake; intermediate moment frame; fire scenario; fragility curves; collapse margin ratio 1. Introduction Steel structures are inherently prone to fire incidents. Although structural steel is noncombustible, its strength and stiffness can be compromised at high temperatures, followed by permanent large deformations. In most cases, the post-fire condition alone may not lead the structure to collapse, but problems arise when the structure sustains seismic loading after experiencing fire. Therefore, it is essential to gain comprehensive knowledge on the seismic performance level (PL) of steel structures after the fire incident which results in the degradation of material properties. Of course, whether a fire-exposed structure can be reliably reused or not is important in the first place [1]. The structural safety of steel structures in case of extreme circumstances such as fire is often addressed by many codes in qualitative, yet ambiguous ways. Therefore, structural engineers cannot achieve structural integrity by referring to specific code-based methodologies [2]. The integrity of steel structures in a post-fire condition is mainly influenced by the degradation of the steel material properties. Previous studies suggest that although the mechanical properties of steel decrease at elevated temperatures, the structure can completely or partially regain its initial properties after cooling down and returning to ambient temperature [3–8]. However, there are some major factors that affect this recovery process, namely the method of cooling down the metal and its rate, the maximum temperature of steel, the chemical composition of the metal, etc. [9]. When the steel members are rapidly cooled down from an elevated temperature in water or oil, the Buildings 2023, 13, 1091. https://doi.org/10.3390/buildings13041091 https://www.mdpi.com/journal/buildings