Natural period and vertical distribution of base shear in confined masonry buildings using ambient vibration test

摘要

In semi-urban setting where availability of land can afford construction of low-rise buildings, confined masonry may compete with other alternatives of seismic resilient system provided well-articulated design standards and construction guidelines are available. Most seismic standards do not make explicit recommendations on the natural period and vertical distribution of base shear for the design of confined masonry buildings. In such a case, one of the two alternatives, such as (1) reinforced concrete (RC) frame building with masonry infill walls and (2) RC frame building with structural walls, is tacitly extrapolated. This paper is first aimed to explore the possible recommendations from the ambient vibration testing of a class of confined masonry building stock. Nine (G + 3) confined masonry hostel buildings are considered for Ambient Vibration Testing (AVT). Recorded signatures are processed and modal characteristics (primarily restricted to the first triplet of fundamental modes) are extracted. Each building is modelled numerically and fine-tuned followed by a comparison of natural frequencies and mode shapes in numerical model and experimental results. The fine-tuned numerical models are analysed against a set of recorded ground motions. Possible design recommendations for natural period and distribution of base shear along the height are the key contributions. Empirical equation for natural periods is derived from the seismic code recommendation on that of reinforced concrete (RC) buildings but removing the bias contributed from the height shorter than one storey while using the experimental results. Distribution of the base shear along the height follows a parabolic profile with an exponent close to 0.4. Results of AVT indicate the inherent damping ratio on an average of about 5% which, however, may not be directly used for seismic excitation. The building stock used for AVT in this paper does not include considerable variations in height and different varieties of confined masonry constructions. Therefore, recommendations of this paper should be verified against a larger size of dispersed building stock.