Preliminary design of high-rise outrigger braced shear wall structures on flexible foundations

This paper presents a graphical method to optimise the position of outriggers on shear walls with flexible foundations. This location for the outriggers will cause a maximum reduction in lateral deflection at the top of the building. The method can be used for preliminary design of high-rise structures subjected to horizontal loading. The method requires the calculation of six structural parameters: bending stiffnesses for the shear wall and outrigger structure, an overall bending stiffness contribution from the exterior columns, rotational stiffnesses for the shear wall and column foundations in addition to a newly suggested bending stiffness parameter representing the structural behaviour of the flexible foundation beam connecting the foundation of the shear wall to the exterior column foundations. These parameters allow the derivation of two compatibility equations for rotations at the intersections of the neutral lines of the shear wall with the outrigger and foundation structures. They yield expressions for the restraining moments at outrigger and foundation levels that act in the opposing direction to the bending moment from the horizontal loading on the structure. Maximising the influence of the restraining moments on the horizontal deflections leads to the optimum location of the outrigger structure. Combining all stiffness parameters into two non-dimensional characteristic structural parameters allows the optimisation procedure for this type of structure to be represented by a single graph that directly gives the optimum level of the outrigger. It is concluded that all six stiffness parameters need to be included in the preliminary analysis of a proposed tall building structure as the optimum location of the outrigger as well as the reductions in horizontal deformations and internal forces in the structure can be significantly influenced by all the structural components.