Ambient vibration, fundamental time period, Fast Fourier Transform(FFT), geophone, RC building

Abstract

For the proper design of any structure, its fundamental properties should be known. The fundamental time period is a primary consideration for seismic design. Generally empirical relations given by code are used in design of building structure, where time period is the function of height and lateral dimension in most cases. But, these empirical relations given by codes are not verified in the context of Nepal. For existing structures, in addition to code formulas and available analytical tools, such as modal analyses, various methods of testing, including ambient and forced vibration testing procedures may be used to determine fundamental time period. In this study, the fundamental period of the 31 RC infill buildings located in Kathmandu valley was identified using ambient motions recorded at each building. Ambient vibration measurements of buildings have been performed by using geophone. Fundamental time periods evaluated experimentally and calculated by different codal formulas were compared. Single variable regression analysis was done, and time period in relation with height is evaluated. In this analysis, codal time period was found higher than experimental one. Multi-variable regression analysis was also done, and the relation between time period, height and lateral dimension was formulated. From multi variable regression formulation, it was concluded that the effect of base dimension of building to fundamental time period is very less. It can be concluded that the fundamental time period of RC building in our current practice can be better correlated by height only relation (T = CHa ) than by height and base-dimension relation (T = C ு √஽ ) given by code. Fundamental time period calculated from codal formula for RC building with infill and from experiment was found near in most cases.