SEISMIC ANALYSIS OF UNREINFORCED MASONRY WALLS

Jaya Prakash Vemuri, Subramaniam Kolluru

Abstract


The Alpine-Himalayan belt which covers the entire Himalayan range of India is the second most active seismic belt in the world. In the Himalayan range, the Indian plate is in an on-going collision with the Eurasian plate. The consequent interlocking between the two plates causes a release of accumulated strain at several faults in this collision zone and leads to frequent inter-plate earthquakes. This region has a potential for high seismic activity in the future as well. Unreinforced, non-engineered brick masonry structures comprise a large percentage of buildings in the Himalayan region. Recent earthquakes have exposed the seismic vulnerability of such structures, which have been either severely damaged or have completely collapsed. Due to the high seismic hazard of the region and the inherent vulnerability of non-engineered masonry structures, a seismic assessment of masonry construction in this region is imperative.
A suite of strong ground motions is developed using data obtained from major Himalayan earthquakes. The response spectra and frequency plots for ground motions are examined. Ground motions are categorized using parameters such as peak ground acceleration, peak ground velocity, mean frequency, predominant frequency, peak ground A/V ratio, strong motion duration etc. Under seismic excitation, masonry exhibits significant displacement into the plastic range. Its hysteric behaviour is characterized by strength and stiffness degradation. In this study, Nonlinear Time History Analyses (NTHA) are performed on a Single Degree of Freedom (SDOF) model of an unreinforced masonry wall whose behaviour is simulated using the Modified-Takeda model. It is observed that the storey drifts exceeds the prescribed drift limits under some severe ground motions. Further, the effect of various ground motion parameters on structural response of masonry walls is examined. The Peak Ground Acceleration is observed to be the most important parameter influencing the Storey Drift. No clear correlation was observed between storey drift and frequency content.

Keywords


unreinforced masonry; seismic assessment; nonlinear time history analyses; drift; peak ground acceleration, himalayan region

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