تحلیل تاریخچه زمانی چارچوب ساختمان های بتنی تقویت شده با طبقه های نرم Time-History Analysis of Reinforced Concrete Frame Buildings with Soft Storeys

1 Introduction A large number of moment-resisting frame buildings have been or are being constructed. In addition, more are being planned to be constructed all over the world. These types of buildings have various social and functional uses such as parking garages, reception lobbies and any other open air spaces which have no infill masonry walls and called soft or weak storey. Although multi-storey reinforced concrete buildings with open spaces are highly vulnerable to collapse under the effect of lateral earthquake loads, they have become an unavoidable feature for the most of the newly constructed reinforced concrete framed buildings. This may be due to the essential needs of such open spaces particularly in big cities with limitations in land availability. Figure 1 shows a cross section through a frame building with soft storey. These types of buildings are generally designed considering walls as nonstructural elements without regard to the masonry infill wall action. Some of the modern seismic codes and the conventional practices as well neglect the effect of masonry infill wall based on the assumption that it may lead to some conservative results [1]. Some other codes (e.g. IS 1893–2002 [2], IBC2003 [3]) provide a factor to magnify the induced straining actions in terms of bending moments and shear forces. In order to propose a magnification factor for the induced shear at base for a building with soft storey, Scarlet [4] performed an analysis based on two extreme situations in which uniform structures as well as rigid structures with soft storey have been used. Most of structural design practices in Egypt treat masonry infill walls as non-structural elements, and consequently the contribution from stiffness and strength of such elements to the building is neglected during the analysis. Actually, the presence of such infill walls significantly changes the frame action behaviour and results in changing lateral load transfer mechanism. A building with an open storey or sometimes called soft storey is the one that has a stiffness discontinuity due to the significant flexibility of the open storey compared with the adjacent storeys. Several codes defined the stiffness discontinuity in a building storey as the one with lateral stiffness less than 70% of the lateral stiffness of the storey above or less than 80% of the average stiffness of the three storeys above [3,5,6]. Dolsek and Fajfar [7] attempted to explain the reason behind occurrence of soft storey effect in uniformly infilled frames as well as when this phenomenon occurs. Structural models designed according to the Eurocode 2008 together with structures designed with limited strength and ductility according to previous codes have been utilized to perform the analysis. In 2002, Demir and Sivri [8] studied the seismic response of reinforced concrete structures with different configurations of masonry infill in order to show the effects of non-structural masonry infill walls on the induced building’s response.