STUDIES IN SHEAR WALL PANEL - RC FRAME INTERACTION

Abstract

Tall buildings have become one of the impressive reflections of today's civilization. There are three major factors to consider in the design of such structures, strength, stability and rigidity. The strength requirement is the dominant factor in the design of low-height structures. However, as height increases, the rigidity requirement becomes a dominant factor in the design. There are basically two ways to satisfy this requirement in a structure. The first one is to increase the size of the members beyond the required strength. However, this approach has its own limits, beyond which it becomes either impractical or uneconomical. The second and more elegant approach is to change the form of the structure into something rigid and stable to limit the deformations (drift) and increase the stability. Tall buildings are usually provided with walls extending from the foundation to roof to resist lateral loads. The effectiveness of structural walls in resisting lateral forces in reinforced concrete buildings has been demonstrated repeatedly. Structural walls are wide and relatively thin vertical members. For low-rise buildings ranging from three to five stories, the role of structural walls is to carry a large part of horizontal shear by means of their high in-plane strength and stiffness. In case of medium to high-rise buildings, the role of these walls is not so clear, particularly for walls with boundary columns. The lateral stiffness of walls relative to frames decreases as the building height increases thus reducing their effectiveness to some extent. In fact, the role of structural walls in medium-to high-rise buildings is not primarily for carrying the horizontal shear, but more for ensuring the principal mode of deflections to be the fundamental mode. Anordinary reinforced concrete wall has a large lateral load bearing capacity, but its deformation capacity is less than 0.004 radian and collapses suddenly in brittle manner. In order to improve this shortcoming associated with these walls it is suggested to have the walls either slitted or bounded by boundary columns. The latter technique offers the advantage that it prevents the instability problem associated with these walls under the in vertical (gravity) loading. Besides, this technique provides further advantage of additional stiffness and hence the natural frequency of vibration of the structure increases without any change in the lateral deformation mode. There are many lateral load-resisting systems used for tall buildings. The selection of suitable and structurally efficient system is still a matter of research. In reinforced concrete tall buildings, especially provided shear wall commonly resist the lateral load. The performance of shear walls/wall panels depends upon the stiffening effect of floor diaphragms, which prevents buckling of the walls. For architectural, structural or functional reasons it may be necessary sometimes to omit the wall in ground floor or discontinue it in £, some intermediate storeys. It may not be always feasible to make these walls as continuous walls, which results in large horizontal deformations even in the case of moderately tall buildings. In such a situation it might be advantageous to systematically staggered bay-wide storey-deep shear wall panels in the plane of the frame and achieve a system functional or structurally preferred to the continuous wall-frame system. The concept of storey-deep and bay-wide discrete shear wall panels bounded by boundary columns is an attractive solution that has been taken up for detailed investigation in the present study. Besides the use of these panels in a single bay, to form a continuous vertical wall, the staggered form ofthis type ofarrangements within the structure can also be exploited for structural advantage. This aspect has been a specific aim of the present study. The shear wall panels are staggered in such amanner that only single wall panel is placed in each storey covering different bays all along the height of a building frame. Five different basic patterns for arrangements of these shear wall panels have been taken for investigation in the present study. These are (i) Conventional Vertical Shear Wall (SW), (ii) Core Shear Wall (CSW), (iii) Scattered Shear Wall Panels (SSWP), (iv) Diagonal Shear Wall Panels (DSWP) and (v) Zigzag Shear Wall Panels (ZZSWP). Out of the various existing theories, the one employing the frame-shear element seems to be the most appropriate for analysis ofthe proposed structural system. This, in its general form, is a hybrid element, which has been adopted, in the present study. The frame- IV shear element can be used for the shear wall panels as well as the boundary columns. As an attempt for possible simplification, the use of pure shear element, frame element, frame- shear element to represent the shear wall panels and the frame-shear element, frame element to represent the boundary columns has been investigated. Thus the suitability of simpler elements has been explored. Further, five combinations of these elements have been made, giving rise to five mathematical models. In three of the models (I, II & III) the boundary columns have been treated as frame-shear elements with seven degree of freedom and the shear wall panels either as pure shear elements with four degree of freedom, frame-shear elements with eight degree of freedom or frame element with six degree of freedom. In the other two models (IV & V) the boundary columns have been treated as frame elements with six degree of freedom and the shear wall panels either as pure-shear elements with four degree of freedom or frame elements with six degree of freedom. Direct stiffness matrix method has been employed in the analytical approach used. A general three-dimensional computer programme with the provision to take on any arrangements of the shear wall panels within the frame of the building and to incorporate any of the five proposed mathematical model has been developed using Matlab to analyze the static, non-linear static and dynamic response of tall building with shear wall panels. The frame-shear element and the application of its different variants to represent the various components of the frame and shear wall panels building are relatively new and not very well proven. Therefore, it was decided to undertake an experimental programme to validate the analytical results, Accordingly, four physical models were made from perspex sheets, each 24-storey in height and having four-bays in shorter direction and five-bays in longer direction with the different shear wall panels arrangement. These models have been tested in the elastic range. The experimental investigation was carried out in two parts: (i) static tests, and (ii) free vibration tests. Test was also conducted to evaluate the elastic constants of perspex used for making the models. The Young's modulus of elasticity and Poisson's ratio were determined. To understand the behaviour of the structural systems with different shear wall panels arrangements (staggered and non-staggered), a parametric study has been carried out for the various loading cases including the dynamic earthquake loads. Tall-framed shear wall panels buildings ranging from 5-storeyed to 60-storeyed with an interval of 5-storeys have been analyzed under linear static, non-linear static and dynamic analysis. The most appropriate mathematical models (Model-II for all staggered forms and Model- IV for non-staggered panel arrangement) have been used in the analysis. Results for the static and dynamic responses were presumed and compared. Location of the shear walls/wall panels in a tall building has a significant effect on the stiffness of the structural system and the distribution of forces within the structure, particularly under lateral loadings. The arrangements pattern of the shear wall panels has an important role in the behaviour of the building under lateral toads. The structural walls/panels may behave in two distinct ways; in flexure and/or in shear, depending upon their aspect ratio. These structural actions the flexural and the shear may be economically mobilized by the wayof arrangements of these walls/panels within the structure. The concept of staggering and bounding of shear wall panels by boundary columns within the frame of building has been investigated in the present study. The same has been critically assessed for its feasibility and advantages as compared to non-staggered (continuous and core) shear wall arrangement in building frames. Lastly, the effectiveness of various arrangements of shear wall panels within the building frames has been reported. From the study, it can be observed that staggered system of construction could be an attractive technique to limit the deformations of structures and it may be superior to conventional and core wall system with respect to member forces. In the sixth chapter of this thesis, an effort has been made to see the effects of location and size ofthe conventional shear wall in a building frame, subjected to gravity and lateral loadings for static and dynamic analysis. Various combinations of shear wall and frames have been compared for deformation and member forces. The lateral deflection and interstorey drift has been taken for comparison. Taking into account the combined effect of axial force and moments, requirement of reinforcement for various combinations for different column have been compared. VI >