Evaluation of the Post-fire Seismic Response of Reinforced Concrete Frames by Finite Element Method

Document Type : Original Research

Authors
M. Kamalvand
A. Massumi
A. Barimani
Kharazmi University
10.22034/23.2.9
Abstract
Accidental fire can be a catastrophe for engineering constructions, especially in building structures. Among structures made of various engineering materials exposed to fire, the reinforced concrete (RC) structures show better performance against fire, due to lower relative thermal conductivity, higher specific heat capacity of concrete, and slower reduction of concrete mechanical characteristics compared with other types of the structure materials. However, in case of severe fire exposure, the RC structures may experience serious structural damage due to the explosive concrete spalling resulting in a high-temperature rise in the reinforcing rebars and relatively large deformation with very limited residual bearing capacity. Although the explosive spalling and significant loss of the cross-sectional area of RC structural elements is a sign of severe damage to these elements, the reduction of mechanical properties of the materials and the performance level of the structure due to chemical reactions such as C-S-H gel dehydration caused by penetration of high temperature in the interior layers of the element cross-section may not be easily visible and evaluated.

A building that has experienced a fire, cannot be exploited for immediate reuse, even when the fire is completely extinguished until load bearing capacity of its members is determined. Therefore, it is necessary to determine the residual capacity of structural elements through logical engineering methods to facilitate the re-operation or development of strengthening methods in the fired RC structures.

Due to the importance of recognizing the behavior and residual seismic capacity of the structures exposed to fire, in this paper, a numerical study based on the nonlinear finite element method has been performed on RC frames. In the first step of the research, the process of heat distribution in the frames located in the furnace based on the previous experimental study is simulated by heat transfer analysis. All three modes of heat transfer including convection, radiation, and conduction were considered in this analysis and the effect of moisture content and emissivity coefficient was evaluated. In the second step, using the residual mechanical properties of materials (reinforcing steel rebar and concrete) based on the maximum heat experienced in the previous step, the seismic behavior of RC frames is evaluated using the pushover analysis. The experimental RC specimens used to validate the proposed numerical model consist of two frames with various beam/column bending capacity ratios in two cases, at room temperature and after being exposed to fire. Due to the different relationships available to determine the residual compressive strength of concrete, the seismic response of the frame was investigated based on three common relations Shi, Lie, and Schneider. The results showed that the proposed numerical analysis method has good accuracy in both steps of analysis and different models for estimating the residual compressive strength, despite some differences, have the ability to predict the post-fire performance of RC frames. It was also shown that for the RC frame specimen with the strong beam-weak column, the ratio of reduced post-fire load bearing capacity and energy absorption is higher.

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Modares Civil Engineering journal
Volume 23, Issue 2
May and June 2023
Pages 135-150