Comparison of cost between earthquake resistant design and retrofitting for torsional irregularity of buildings

Abstract

Many buildings in the present day scenario have irregular configurations in both plan and elevation. Past earthquake studies show that buildings suffer severe damage during earthquakes due to torsional irregularity. Response spectrum method is the most commonly used tool in the seismic analysis of structures with torsional irregularity. Number of retrofitting works have been increased in recent years because it reduces the vulnerability of damage of an existing irregular structure in the event of a future earthquake. The study is conducted considering planar irregularity in ordinary moment resisting frame (OMRF) only. Many structures with planar irregularity were designed without considering torsional effects, require retrofitting. Considering this fact, the study assessed design of structures by response spectrum method of seismic analysis, design of retrofit schemes and estimates for these designs using PWD rate schedule. In this study, 4-storied, 6-storied, 8-storied and 10-storied buildings with same architectural plan are considered in soil type S1, S2 & S3 in seismic zone-2. Equivalent static load method and response spectrum method have been adopted. These irregular plans were modeled using Finite Element Method (FEM). BNBC code is used to analyze and design structures. FEMA guidelines and methodology are used for retrofitting works. The analysis was performed considering concrete strength 3500 psi, reinforcing steel strength 60 ksi, live load 63 psf and peripheral brick wall load. From the study it is found that structures with torsional irregularity considered in this study which is designed by equivalent static load method is vulnerable for response spectrum analysis and retrofit is needed for them. Variations of design output for two design methods increase with increasing building story height for three types of soils. For a building of specific height outputs of these methods vary for different soil types. However, soil variation effect is little for 4-storied building. It also has been observed that almost two times extra cost is needed for retrofit compared to the extra cost for earthquake resistant design of new buildings.