Natural Period Of Setback Buildings -Assessment Of Is 1893: 2016 Guidelines

Abstract

The motion of the ground during earthquake do not damage the building by impact or by any external force, rather it impacts the building by creating an internal inertial forces which is due to vibration of building mass. The magnitude of lateral force due to an earthquake depends mainly on inertial mass, ground acceleration and the dynamic characteristics of the building. To characterize the ground motion and structural behaviour, design codes provide a Response spectrum. Response spectrum conveniently describes the peak responses of structure as a function of natural vibration period. Therefore, it is necessary to study of natural vibration period of building to understand the seismic response of building. The performance of a multi-storey framed structure during earthquake motions totally depends upon stiffness, distribution of masses and strength in both the vertical and horizontal planes of the structure. In multi- storeyed framed structure, damage from earthquake ground motion generally starts at locations of structural weaknesses present in the lateral load resisting frames. Sometimes weakness might be created by discontinuities in strength, stiffness or mass between the adjacent storey. Such discontinuities between storeys are often associated with sudden variations in the frame geometry along the height. There are various examples of collapse of buildings in past earthquakes due to such vertical discontinuities. A common type of vertical geometrical irregularity in building structures arises from abrupt reduction of the lateral dimension of the building at specific levels of the elevation. This type of building is defined as setback. Setback building frame with geometric irregularity (both in elevation and plan) are now increasingly encountered in modern urban construction. changes in stiffness with height and mass render the dynamic characteristics of these buildings different from the ‘regular’ building. Many investigations have been performed to understand the behaviour of irregular structures as well as setback structures and to ascertain method of improving their performance. This study presents the design code perspective of this building category. Almost all the major international design codes recommend dynamic analysis for design of setback buildings with scaled up base shear corresponding to the fundamental period as per the code specified empirical formula. However, the empirical equations of fundamental period given in these codes are a function of building height, which is ambiguous for a setback building. It has been seen from the analysis that the fundamental period of a setback building changes when the configuration of the building changes, even if the overall height remains the same. Based on modal analysis of setback buildings with varying irregularity and height, the goal of this research is to investigate the accuracy of new empirical formula given by code and if required any modification then give a modification factor.

This study shows that it is difficult to quantify the irregularity in a setback building with any single parameter.

Fundamental time periods of setback frames are found to be less than that of regular frame. In the code it is given that fundamental time period of framed structure without infill stiffness only depends on height of building but in actual condition it depends on bay width, irregularity of frame and other structural and geometric parameters.