THREE DIMENSIONAL BEHAVIOUR OF MULTISTOREYED BUILDINGS WITH VARIOUS LATERAL LOAD RESISTING SYSTEMS

Abstract

There are many lateral load resisting systems for multistoreyed buildings. The selection of a suitable, stable and economical system Is still a matter of research. In reinforced concrete tall buildings, the lateral loads are commonly resisted by specially provided shear walls. The satisfactory performance of shear walls depend on the stiffening effect of floor diaphragms, which prevents buckling of walls. Shear panels (infills) and bracings are also used as effective lateral load resisting elements in tall buildings. Very recently, the concept of providing storey deep and bay wide discrete panels to resist lateral loads has been introduced. Till now only a little work has been done as regard to the three dimensional behaviour of these lateral systems. Three dimensional action develops mainly because of the high degree of continuity at joints, lack of symmetry in a structural system, eccentricity of lateral or gravity forces, differential settlement of foundations, temperature changes, etc. Based on the displacement matrix method, sub structure techniques can make the analysis possible on micro computers which in-corporate the techniques of reducing the number of unknowns from the stiffness equations. Static condensation, co-ordinate transformation, etc. are such supporting techniques available. This approach „can thus be advantageous over the finite element method, saving much in computer storage and time. The present study is, an attempt to investigate the general behaviour of a variety of suitable and effective lateral load (v) resisting systems using flexural panels, pure shear panels (infills), RC bracings, combination of panels and bracings, etc. in addition to conventional shear walls. The analytical procedure adopted in the program is based on the assumption that floors are rigid in their own planes thus reducing the degree of freedom to three at each floor level. The above lateral load resisting systems have been used in three different arrangements-vertical, zig-zag and diagonal; and their relative behaviour in terms of critical member forces and storey deflections has been reported. The results for all these systems have been compared to bring out the difference in the behaviour amongst them with a view to establish the more appropriate ones. Besides, the effect of arrangement of panels and bracings has also been studied. It is concluded that the disposition of panels and bracings in zig-zag and diagonal fashion in the plane of a frame causes a considerable reduction in lateral deflections In comparision with the conventional vertical arrangements. Fifteen storey buildings with three different asymmetric floor plan shapes have been considered. \ Significant reduction in the shear wall/panel base moments is achieved __Oich is a desirable feature in the design of the foundation for the shear walls. Also, the column forces such as moments and axial forces are reduced in comparision with the conventional shear wall or vertical panel systems. It is also observed that the pure shear panels and RC bracings behave in a similar manner and hence pure shear panels may be replaced by RC bracings and vice-versa as per the necessity of a particular system. Bracings of all the arrangements have shown their superiority in resisting large torsion caused in asymmetric buildings.