Effect of randomly distributed infills on the vibration characteristics of reinforced concrete frames

Abstract

Conventional modeling of reinforced concrete structures by FE method results in too long and unrealistic value of period of buildings. A reason of this phenomenon is that a structure usually contains many secondary elements likeI infills, which are generally ignored to have any structural contribution. lAs such the numerical models become too flexible resulting in longer time period. For this reason, code provisions usually impose a limit on the period value obtained from numerical analysis with conventional FE modeling. In this thesis, an extensive computational study has been conducted to determine the fundamental period of buildings considering the structural effect of infills. A numerical investigation has been performed to calculate the fundamental period of a series of regular building frames under various conditions. Variation of number of floors, floor height, number of span, number of bays, floor panel size, percentage of infills etc. are considered during the investigation. The infills are modeled as equivalent diagonal strut. Beams and columns are modeled using three-dimensional frame element and floor slab is modeled using shell element. The period of the same buildings are also evaluated using empirical code equations and a comparison with the results of FE analysis are made. On the basis of the investigation it has been found that the period found by modal analysis for models without infill are much higher than the same found by code equations and from modal analysis with infill. It has been found that the code equations generally produce shorter values of periods when compared to the periods obtained from modal analysis with infill, although the difference is not as high as with the models without infill. Three parameters namely (i) number of stories or total height of the building, (ii)floor panel size and (iii)amount of infilled panels present in the building have been identified to have significant influence the building period. Incorporating the effects of these parameters, a few correction factors have been proposed which may be used to further refine the period value predicted by code equations. The period of a few arbitrary examples are determined on the basis of the proposal and are compared with modal analysis. It has been found that the periods determined by applying the suggested correction factors match better with modal analysis than existing code equations.