POST EARTHQUAKE FIRE RESISTANCE OF RC FRAME

Abstract

Fire following an earthquake is an important factor causing damage to buildings and life-line structures. Therefore, besides satisfying structural design requirements for normal loads, such as dead and live loads, including the seismic hazard, buildings should also be designed to withstand the fire following earthquake for a certain minimum duration as required for a desired level of performance. Calculating structural response to fire after earthquake is a few step process: modeling the structure including nonlinear analysis options; choice for earthquake analysis scenario; seismic nonlinear analysis: pushover or dynamic time history; fire hazards analysis to identify all possible fire scenarios; thermal analysis to calculate temperature history in each member; structural analysis to determine forces, stresses and deformations to estimate whether local or global collapse would occur during any of the fire hazard scenarios. To evaluate the seismic damage in a structure, first, the seismic hazard level is determined from the seismic hazard spectrum for the given site, followed by the selection of appropriate ground motion records and structural analysis. The seismic excitation induces damage and lateral deformation provoking additional stresses in the frame due to the moment caused by the P-D effect. Structural members and joints are also weakened by the cyclic inelastic deformation, causing stiffness and strength degradation. Once the earthquake-induced damage in the structure is determined, the damaged structure is subjected to a post earthquake fire (PEF) scenario, which involves fire hazard analysis to determine the time history of fire growth and spread and stress and collapse analysis of the structure but also to analyze no-collapse conditions and cooling after fire. The behavior of a particular reinforced concrete structure that was fire exposed after seismic action is presented in this paper. The seismic response of the structure is evaluated using a pushover analysis, while the displacement demand under the corresponding seismic event is determined using the recommendations implemented in Eurocode 8. The earthquake-induced damage in the structure is determined and, as next step, the structure is exposed to Standard fire ISO 834. For that purpose the program FIRE, based on FEM, is used. The program FIRE carries out the nonlinear transient heat flow analysis and nonlinear stress-strain response associated with fire. The solution technique used in FIRE is a finite element method coupled with time step integration. The computer modulus FIRE-T solves the governing differential equation of heat transfer in conduction. The response of a reinforced concrete elements and plane frame structures exposed to fire is predicted by modulus FIRE-S. This modulus accounts for: dimensional changes caused by temperature differences, changes in mechanical properties of materials with changes in temperature, degradation of sections by cracking and/or crushing and acceleration of shrinkage and creep with an increase of temperature. Pushover analysis, representing dynamic effects of an earthquake via static nonlinear procedure, is incorporated in the program as an option that precedes the postearthquake fire analysis.