SEISMIC PERFORMANCE EVALUATION OF A FIRE-DAMAGED HIGH-RISE BUILDING

Abstract

Concrete is widely used all over the world for the construction of buildings and infrastructures that may remain stable under fire. It shows some damage and deformations that requires maintenance. Although rehabilitation and restoration procedures of firedamaged buildings are usually time-consuming and costly, the post-fire resilience of the structure is very important to understand the structure’s capacity and situations. In the seismic zone, the buildings must satisfy the seismic demand after the potential repair and retrofitting of the fire-damaged building. A simple verified simulation procedure for the post-fire seismic analysis is necessary to advance the understanding of the post-fire seismic performance of RC structures. However, individual computer programs which can perform well in the both thermal and seismic analysis may help structural engineers taking decisions. Damage identification is the first step to predict the resilience of RC structures after fire events. This study investigates the performance assessment of different structural elements (column, beam, shear wall) within a code-designed RC frame building and thus compares pre and post-fire structural strength and serviceability parameters. Fire occurred at the 7th, 8th and 9th floors of a twenty-three-storied reinforced concrete building with two basements. The building can be categorized as a dual framing system which is located in Dhaka. Primarily, a damage assessment has been performed following some field investigation and testing techniques. A series of non-destructive tests such as UPV, carbonation concrete core, and rebar tensile strength tests have been carried out in the field and at the laboratory on the collected samples for damage classification and identification. Damage intensity of eighth-floor roof was found (8%) to be more than the other two (Seventh and ninth) floors (5%) whereas concrete strength reduction of vertical elements was found up to 33% (in the columns and shear wall) and 23% in the horizontal elements (beams and slab). Based on the maximum gas temperature, a parametric fire curve has been constructed to simulate the thermal stress of fire-affected (seventh, eighth, and ninth) floors that influence the capacity of the materials by FIN EC software. The simulated material strength is compared with the equivalent concrete compressive strength obtained from the test data. The entire building is then modeled using finite element based computer program to assess the structural performance at pre and post-fire conditions. Initially, linear static analysis has been performed to obtain the force-based structural capacity of the building at iii different performance levels such as maximum considered earthquake (MCE), design basis earthquake (DBE), and service level earthquake (SLE) and the capacity was compared between pre and post-fire conditions. The mechanical properties of concrete after fire exposure, such as residual compressive strength, stiffness, and constitutive responses were reviewed. Finally, a nonlinear dynamic time history analysis using ten different ground motions is performed to observe the actual response of the structure. The results are presented in terms of storey drift and displacement, stiffness, and column demand /capacity ratios and they are compared for pre and post-fire exposure at different performance levels, i.e. SLE, DBE & MCE. The results showed that the inter-storey drifts and demand /capacity ratio in the few cases particularly at the fire-damaged floors exceeds the allowable limit. The displacement increases and storey stiffness decreases considerably due to stability loss by elevated temperature. In most of the cases, it fulfills the requirement of strength and serviceability of structure in terms of strength, displacement, and drifts. The maximum concrete and rebar strain developed in the RC elements is found within the allowable limit of performance criteria for all service levels. The proposed assessment procedure is capable of capturing the actual scenario of a fire-damaged building at different performance levels that may help the stakeholders to take further strategic decisions on the damaged structure.