Modeling of Support Conditions at the Bases of Tall Buildings

Assumptions regarding the vertical and lateral support conditions at the base of a tall building can have a significant effect on the results of the analysis of the structure, and on the design based on that analysis. This is especially true when the building has a deep basement. There is no generally accepted "standard" way of modeling the base restraints or supports. Several alternative idealizations are in use. These different ways of modeling supports for tall buildings will be discussed and evaluated in this paper. Through an example, some of the simple support models that appear to be in use will be compared with a complete three dimensional analysis. A technique for using a basement as a "virtual"� outrigger to provide a wider base for resisting overturning effects, as proposed in a recent paper (Nair, 1998), will be investigated further in this work. This is shown to be a logical extension of the use of a complete three-dimensional model of the bottom of the building in the analysis of the structure.