Back-of-the-Envelope analysis of plan-asymmetric confined masonry buildings for force-based seismic design

Location of centers of rigidity (CR) in a confined masonry building is sensitive to the aspect ratio of constituting walls and wall-piers (in case of wall with openings). These centers, generally, do not lie on one vertical line. In contrast, the mass centers (MC) are nearly vertically aligned for a regular confined masonry building. Assuming an average vertical line for the CRs (elastic axis), this paper first presents a simplified framework to arrive at the design force resultants for the force-based seismic design. The framework uses a set of 2D Finite Element (FE) analyses (that can be performed using any commercial software) and some nominal spread sheet calculations. Case study on a recently constructed confined masonry building at Indian Institute of Technology Gandhinagar (India) campus is considered for the purpose of illustration. This framework leads to the design member force resultants close to that calculated using a detailed 3D FE analysis. Next, the framework is converted into a Back-of-the-Envelope method that does not require any FE analysis and yet leads to the member force resultants acceptable for all practical purposes. This is particularly suitable for routine seismic design, especially at the preliminary stage, provided none of the constituting walls comprises of a squat wall-pier.