Seismic Design of Multi-Storied RCC Buildings

Reinforced concrete multi-storied buildings are very complex to model as structural systems for analysis. Usually, they are modeled as two-dimensional or three-dimensional frame systems using finite beam elements. For evaluating stiffness matrix of beam elements, the properties required are – area of section, effective area in shear, and moment of inertia about the axis of bending and for three-dimensional analysis, in addition, torsional moment of inertia. Also, the modulus of elasticity and rigidity are to be estimated. However, no guidelines are available for the rational computation of sectional properties incorporating the effects of reinforcements in concrete members and the analysis is full of approximations. A case history of a RC structure, where the first author was involved in the study, is briefly cited in the paper.

The current version of the IS: 1893 – 2002 requires that practically all multistoried buildings be analyzed as three-dimensional systems. This is due to the fact that the buildings have generally irregularities in plan or elevation or in both. Further, seismic intensities have been upgraded in weaker zones as compared to the last version IS: 1893-1984. It has now indirectly become mandatory to analyze all multistoried buildings in the country for seismic forces. This paper appraises briefly the significant changes in the current version of the code compared to the previous version.

Some of the poor planning and construction practices of multistoried buildings in Peninsular India in particular, which lead to irregularities in plan and elevation of the buildings are also discussed in this paper. At present, there is too wide a variation in the modeling of buildings. This paper emphasises the need for guidelines in order to limit the range of assumptions to a narrow range. This is necessary to certify the analysis and design, or in case legal disputes arise later regarding the procedure adopted.
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