Importance of Dynamic Analysis in Earthquake Safety of Tall Buildings

Abstract

Urbanization is rapidly increasing in almost every city in India. Huge infrastructure developmental plans have been laid by government and private organizations. Large advertising boards of tall buildings (up to 30 or 40 floors)already started attracting people to invest in these infrastructure plans. However, from the point of view of seismic hazard prevailing in the country, “Will these buildings survive during future earthquakes?” is a question to be answered before proceeding for construction. Especially, in the last 25 years, the country has witnessed several moderate earthquakes (Bihar-Nepal border (M6.4) in 1988, Uttarkashi (M6.6) in 1991, Killari (M6.3) in 1993, Jabalpur (M6.0) in 1997, Chamoli (M6.8) in 1999, Bhuj (M6.9) in 2001, Sumatra (M8.9), Kashmir (M7.6) in 2005, Sikkim (M5.7) in 2006 and Sikkim (M6.8) in 2011 caused around 40,000 fatalities due to collapse of buildings. These earthquakes have clearly exposed the lack of understanding of seismic hazard of the country. Sometimes, even when the hazard is understood, the lack of knowledge is exposed on earthquake resistant design and construction practice of reinforced concrete structures. The professionals involved in building construction should be more concerned with the safety of building infrastructure during future earthquake events. Given below are some important aspects which need not be ignored while considering the earthquake safety of tall buildings. As per seismic zone map given in the latest revision of IS 1893, India is divided into 4 seismic zones (Zone II, III, IV and V). However, the guidelines given in this code are useful for regular and relatively small, low-rise buildings. When it comes to tall buildings, every structure is special. The main structural design issues considered in this building are Static analysis, Dynamic Analysis, Wind Analysis and Stability Analysis. Initial static and dynamic analysis can be conducted in SAP/STAAD or other sophisticated packages. In case of earthquake analysis of tall buildings, site specific parameters need to develop for every structure and the building behaviour needs to be understood. Dynamic analysis must be performed for building using response spectrum method or time history analysis methods depending on the availability of acceleration records. Safety of building for appropriate Peak Ground Acceleration (PGA) must be ensured. In addition to it, safety against toppling and falling hazards in all the floors of tall buildings must also be ensured. For performing wind analysis, wind tunnel experiments may be expensive. However, these days CFD tool of Ansys can be used to obtain wind forces on tall buildings. After CFD analysis the wind forces on the building model are extracted in Ansys CFX-Post and imported to the structural model in SAP for the structural analysis.