BEHAVIOUR OF R.C. FRAMED MULTISTOREYED BUILDINGS WITH BOUNDED WALL PANELS

Abstract

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, 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 ofstructural 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. An ordinary R.C. 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 (i) either slitted or (ii) be bounded by boundary columns. The latter technique offers the advantage that it prevents the instability problem associated with these walls under the vertical (gravity) loading. Besides, this technique provides further advantage, that is, additional stiffness and hence the natural frequency of vibration of the structure increases without any change in the lateral deformation mode. 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. Thus 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 stagger bay-wide storey-deep wall panels in the plane of the frame and achieve a system functional or structurally preferred to the continuous wall-frame system. Theconcept of storey-deep and bay-wide discrete structural wall panels bounded by boundary columns is an attractive solution which has been introduced recently and taken up for detailed study in this work. Besides the use of these panels in a single bay, to form a 'continuous' vertical wall, the staggered form of this type of construction within the structure can also be exploited for structural advantage. This aspect has been a specific aim of the present study. The wall panels are staggered in such a manner that only single wall panel is (HI) placed in each storey covering different bays all along the height of a building frame. Four different basic patterns for dispositioning of these wall panels have been taken for investigation in this study. These are (i) diagonal, (ii) zig-zag, (iii) scattered, and (iv) vertical. Out of the various existing theories, the one employing the frame-shear element seems to be the most appropriate for analysis of the proposed structural system. This, in its general form, is a 2-D hybrid element which has been adopted in the present study. The frame-shear element can be used for the wall panels as well as the boundary columns. As an attempt for possible simplification, the use of 'pure shear element' to represent the wall panels and the 'stringer element' to represent the boundary columns has been investigated. Thus the suitability of simpler elements has been explored. Further, four combinations of these elements have been made, giving rise to four mathematical models. In two of the models (-1 & -IV) the boundary columns have been treated as frame-shear elements with 7-DOF and the wall panels either as pure shear elements with 4-DOF or frame-shear elements with 8-DOF. In the other two models (-II & -III) the boundary columns have been treated as stringer elements with 3-DOF and the wall panels either as pure-shear elements with 4-DOF or frame- shear elements, with 8-DOF. Direct stiffness matrix method has been employed in the analytical approach used. A general 2-D computer programme with the provision to take on any dispositioning of the wall panels within the frame of the building and to incorporate any of the four proposed mathematical model has been developed independently (without team work) to analyze the static and dynamic response of multistorey framed-wall panel buildings. The frame-shear element and the application of its different variants to represent the various components of the frame-and-wall- building is relatively new and not very well proven. Therefore, it was decided to undertake an experimental programme to verify the analytical results. Accordingly, two physical models were made from perspex sheets, each 9 storey high and having 3 bays - the vertical and the staggered diagonal arrangement cases. 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. Tests were 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 thus determined. To understand the behaviour of the structural systems with different panel dispositioning (staggered and non-staggered), a parametric study has been carried out for the various loading cases including the dynamic earthquake loads. Multistorey framedwall panel buildings having 3 bays and 3 to 39 storeys have been analyzed for static loads. (IV) Analysis for seismic loads has been carried out for 3-5, 7-15, and 19-27 storeys for representative low-, medium-, and high-rise buildings. The most appropriate mathematical models (Model-I for all staggered forms and Model-IV for vertical panel arrangement) have been used in the analysis. Results for the static and dynamic responses are presented and compared. Location of the shear walls/panels in a multistoryed framed building has a significant effect on the stiffness of the structural system and the distribution of forces within the structure, particularly under lateral loading. The dispositioning pattern of the wall panels has thus an important role in the behaviour of the building under lateral loads. The structural walls/panels may behave in two distinct ways- in flexure and/or in sheardepending upon their aspect ratio. These structural actions- the flexural and the shear- may be economically mobilized by the way of dispositioning of these walls/panels within the structure. Therefore the role of dispositioning is thus only to determine the principal mode of response to be fundamental mode. The concept of staggering and bounding of wall panels by boundary columns within the frame of building has not so far been investigated and is taken up in this study. The same has been critically assessed for its feasibility and advantages as compared to non-staggered (continuous) shearwall arrangement in building frames. Lastly, the effectiveness of various dispositionings of 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 drift of structures. At the same time it may be superior to continuous system of construction. The results of this study are expected to interest a wide section of the profession and the research community, since only a few studies (55, 56, 124, 125 chapter 2) on the staggered system of R.C. wall panels of building frames are available. (