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Fundamental natural period of vibration T of the building is an important parameter for evaluation of seismic base shear. Empirical equations given in the Indian seismic code for the calculation of the fundamental period of a framed structure, primarily as a function of height, do not consider the effect of stiffness of the structure, base dimensions of the structure, number of panels in both the directions, amount of infill and properties of the infill. The fundamental period can be evaluated using simplified expressions found in codes, which are based on earthquake recordings in existing buildings, laboratory tests, numerical or analytical computations. These technical codes provide expressions which depend on basic parameters such as building height or number of stories. Building periods predicted by these expressions are widely used in practice although it has been observed that there is scope for further improvement in these equations since the height alone is inadequate to explain period variability. It is also known that the period of a RC frame structure differs depending on whether the longitudinal or transverse direction of the structure is considered. The aim of this study is to find the effects of building base width in both the directions, stiffness of the structure etc. and to predict the fundamental period of vibration of reinforced concrete buildings with moment resisting frames (MRF). A few examples of dynamic analysis are presented in this study to show the effect of base dimensions and stiffness of the structure in calculating the time period of the structure. And it is recommended to be incorporated in the formula for evaluating the natural period of vibration of structures.

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Reinforced concrete frame buildings are most common type of construction in urban India, which is subjected to several types of forces during their life time such as static forces and dynamic forces due to wind and earthquakes. The static loads are constant with time, while dynamic loads are time varying, causing considerable inertia effects .It depends mainly on location of building, importance of its use and size of the building. Its consideration in analysis makes the solution more complicated and time consuming and its negligence may sometimes becomes the cause of disaster during earthquake. So it is growing interest in the process of designing civil engineering structures capable to withstand dynamic loads. The behavior of building under dynamic forces depends upon its mass and stiffness properties, whereas the static behavior is solely dependent upon the stiffness characteristics.