Sway characteristics of brick infilled reinforced concrete building frame under lateral loading

Abstract

Frame structure is most common in tall buildings due to simplicity in design and construction. In Bangladesh brick walls are used in frame structures as exterior walls and partitions. Present code of practice does not include provision of taking into consideration the structural effect of infill in multistoried building construction. The presence of infill increases the lateral stiffness of the structure and reduces the amount of deflection. If this effect is taken into consideration, the resulting structure will be economic. This study has been undertaken to investigate the sway characteristics of common reinforced concrete frames in presence of brick masonry infill subjected to lateral loads. Here, a seven story and an eleven story building frames are taken as reference structures. Finite element modeling is used to model the frame. Common three dimcnsional frame elements are used to model the beams and columns. The inplane stiffness of brick wall contributing to the stiffness of the frame against lateral load is modelled by the equivalent strut model proposed by Saneinejad and Hobbs (1995). Their model consisted of a bi-linear force-displacement relationship with an initial stiffness called Ko. In this study, instead of the bi-linear relationship, a nonlinear constitutive relation is proposed and used to simulate the force vs. displacement behavior of the diagonal strut as infill. In the FE model the struts are incorporated using special link element having only axial stiffness. Lateral loads (wind load) are calculated as described in BNBC, 1993. A parametric study was performed using various parameters of frame and infill to see their effects on stiffness properties of infill and deflection characteristics of the frame. A few parametcrs are found to have significant effect on the infill characteristics while the other parameters do not have any appreciable effect. While studying the deflection characteristics, it is observed that deflection in frames with infill is much smaller than that with frames without infil!. On the average, lateral sway is reduced by about thirty percent if only one third of the panels of the frames contain infil!. It proves that the sway predicted by ordinary frame analysis overestimates the sway by a significant amount. A suggestion is thus made that after computing sway by a ordinary frame analysis, it may be reduced by at least thirty percent before it is compared with the maximum allowable sway specified by code.