Lateral Bracing of Buildings with a Basement - Influence on the Stress Resultants and Deformations of Reinforced Concrete Bracing Walls when including the Basement in the Mechanical Model

This paper presents an investigation of the influence on the shear force distribution and on the displacements of reinforced concrete bracing walls continuous through a basement, when modelling these walls. The shear forces and displacements are compared with those obtained when simplifying the wall by a cantilever with full fixity at the top of the basement, ignoring the transfer of shear force and bending moment into the basement. First, the influence of the basement is modelled by hinged connections between the wall and the base slab and the floor diaphragm above the basement, respectively. This will result in an unrealistically large wall shear force in the basement. Then, the flexural restraint of the wall by the base plate, and the lateral compliance of the floor diaphragm are modelled by a rotational spring and a lateral spring at those levels. Recommendations are given for reasonable values of the spring constants for practical design purposes. Special attention is given to the way in which including the basement in the mechanical model of such walls will influence aspects of their seismic design. Such aspects are the rigidity of the bracing system which influences the lateral earthquake loads, and the displacement ductility factor which influences the maximum admissible behaviour factor q.