Performance Assessment of Reinforced Concrete Planar Frame Using Nonlinear Analysis

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Date

2015-10

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Addis Ababa University

Abstract

Ground excitation is the major cause of failure in reinforced concrete structures constructed in seismically active zone. This can be due to either the unpredictable nature of the seismic excitation or due to lack of proper structural design, detailing and construction. During seismic excitation the response of structures should be estimated appropriately. For response determination of structures, codes fail to predict the real time response of structures. Hence existing performance assessment should be done using a reliable and easily applicable analysis procedure. The revised Ethiopian building code (EBCS-EN-2014) shows a major change on the expected peak ground acceleration (from 0.05g to 0.1g) for Addis Ababa. For the past several years large numbers of buildings are designed and constructed using the previous building code. The performance of these structures is highly affected by this change. There for it is mandatory to assess the performance of the existing structures using provisions of revised code. This study focuses on condominium buildings as they are constructed in large scale all over the country and as they are areas where people are congregated. The study is done under the assumption that premature failures are not expected to happen. A planar frame was taken from the representative 3D case study building. The capacity demand comparison was done for each member of the frame using EURO CODE and ACI provision for stiffness reduction factor. The frame fails to satisfy minimum code requirements even after the consideration of moment redistribution. In the aim of saving the structure load reduction was performed as engineering solution. The solution improve the structural performance but still it is unsafe. A mechanistic solution was also proposed to utilize the reserve capacity of the structure. The solution being pushover analysis with fiber based modeling which capture the nonlinear properties. Seismostruct and SAP2000 software were used for analysis. Element level verification was done using both software followed by frame level verification which will address effect of redundancy. At relatively low loads prediction obtained from both software give consistent result with the experimentally obtained data At a relatively high load the result obtained from Seismostruct gives a better prediction of the experimental result. With the aim of investigating the stiffness reduction factor recommended by EURO CODE and ACI extent and progress of crack checked. For this particular frame it is found out that the columns reduction factor is closer to ACI provision while the beam reduction factor is closer to EURO CODE provision. Finally the case study building was assessed for its performance level, for soil class B the result shows that the structure‟s performance level do not satisfies drift requirements for operational and immediate occupancy but it is adequate for life safety and collapse prevention. From the study it is found that the use of pushover analysis results in 25.6% increment in shear capacity from Seismostruct and 16% increment in shear capacity from SAP2000 for this particular frame. The higher increment from Seismostruct is expected due to the plastic zone consideration which is a plastic hinge in SAP2000. The advantage obtained by using the pushover analysis may increase or decrease depending on the rate of progressive hinge formation, length of the plastic hinge and other parameters.

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Keywords

seismic excitation, base shear, pushover, performance level, plastic hinge, plastic zone, fiber based modeling

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