Evaluation and Strengthening of Existing RC Buildings Using FEMA P695 Methodology and Calculation of CMR for Quick Assessment

Abstract

Investigating the seismic performance of existing structures that have been designed and constructed in accordance with previous codes is one of the existing challenges. Retrofitting techniques must be practical and comprehensive. Reinforced concrete (RC) buildings in Iran comprise a large part of residential buildings. Four editions of the Iranian Code of Practice for Seismic Resistant Design of Buildings (Standard no. 2800) have been published since 1986. Noticeable changes have been made between the second and third editions regarding the seismic design of RC buildings. Firstly, this study focuses on evaluating the design reliability and seismic performance factors presented in Standard No. 2800-99 (second edition) and Standard No. 2800-05 (third edition). A set of 51 reinforced concrete archetypes (1 to 12 stories) and 2244 incremental dynamic analyses were performed. For the first time, the concept of Adjusted Collapse Margin Ratio or ACMR is used for seismic evaluation of stock buildings. ACMR is obtained from the results of pushover analysis, incremental dynamic analyses, and fragility analysis based on the FEMA P695 methodology. The results reveal that the structural systems designed in accordance with Standard No. 2800-05 fulfilled the requirements for moderately intense earthquakes. In contrast, the vulnerability of low and mid-height building stocks designed based on the second edition are significant and are prone to incur dramatic damage in the event of far-field earthquakes. Secondly, the possibility of adding concentric steel braces with different configurations was investigated to strengthen vulnerable archetypes. In this study, the concept of the collapse probability of a structure due to a set of far-field earthquake records, has been investigated in order to find the optimal design point in strengthening the vulnerable stock RC structures. In addition, appropriate seismic design coefficients have been investigated and introduced using the probability of collapse. Sixty archetypes were defined in 20 performance groups and 15800 non-linear dynamic analyses were performed. The probability of collapse of all possible total uncertainties (0.3 to 0.9) was investigated. Using the trial and error method, the best seismic design coefficients of steel braces were found. Thirdly, a set of analyses including Pushover Analysis, Incremental Dynamic Analysis (IDA), along with Fragility Analysis, which are usually time-consuming and computationally unaffordable, must be performed to obtain CMR. Artificial Neural Network (ANN), Response Surface Method (RSM), and Adaptive Neuro-Fuzzy Inference System (ANFIS) are also introduced to solve this issue and to achieve a quick and efficient method to estimate the CMR. So, almost 5016 IDA analyses were used based on 114 archetypes. In this regard, five parameters were used as independent and desired entries of the system. In addition, CMR is considered the output of the systems. In the RSM method, ANOVA analysis is also applied to obtain significant parameters. However, ANFIS offered the best efficiency and accuracy with the least number of errors. In comparison, the ANN method was found to be more effective than the RSM and has a higher regression coefficient and fewer statistical errors. Keywords: Incremental dynamic analysis, Pushover analysis, Steel bracing, Collapse probability, FEMA P695 methodology, Soft computing