TALL BUILDINGS 15.1 INTRODUCTION For the structural engineer the major difference between low and tall buildings is the influence of the horizontal loads due to wind and earthquake on the design of the structure. Lateral deflection of a tall concrete building is generally limited to H/1000 to H/200 of the total height H of the building. In the case of tall buildings, in addition to limiting this so called lateral drift, attention has to be focused on the comfort of the occupants because vibratory motion may induce mild discomfort to acute nausea. Another aspect that needs to be addressed in tall buildings is the vertical movement due to creep and shrinkage in addition to that due to elastic shortening. These movements can cause distress in non-structural elements and must be allowed for in detailing. This chapter is mainly concerned with the elastic static analysis of tall structures subject to lateral loads. An attempt is made to explain the complex behaviour of such structures and to suggest simplified methods of analysis of those types of structures which do not require full 3-D analysis. The behaviour of individual planar bents and the interaction between shear walls and rigid-jointed frames will be examined in detail as it highlights the complexity involved in the analysis of three-dimensional structures subjected to horizontal forces. 15.2 ASSUMPTIONS FOR ANALYSIS The structural form of a building is inherently three-dimensional. The development of efficient methods of analysis for tall structures is possible only if the usual complex combination of many different types of structural members can be reduced or simplified whilst still representing accurately the overall behaviour of the structure. A necessary first step is therefore the selection of an idealized structure that includes only the significant structural elements with their dominant modes of behaviour. It is often possible to ignore the asymmetry in a structural floor plan of a building, thereby making a three-dimensional analysis unnecessary. One common justifiable assumption is that floor slabs are fully rigid in their own planes but flexible out-of-plane. Consequently, all vertical members at any level This chapter is a modified version of the chapter on Tall Buildings by Hoenderkamp from Reinforced Concrete: Design, Theory and Examples by T.J MacGinley, 1990, Spon Press.