Underground box structure as a foundation for shear walls in seismic resistant buildings

The present paper addresses a foundation problem in seismic resistant buildings braced by structural walls. In particular, the box-type foundation for an internal shear wall is investigated. This kind of foundation can be an alternative solution for direct foundations. It solves the problem where oversized ordinary raft foundations are needed. In fact, the box foundation gathers the gravity loads resting on the peripheral walls, so that the global soil reaction reduces its eccentricity. In most cases the elements constituting the box structure, such as peripheral walls, raft, and ground floors, are already part of any actual building; they only need a proper seismic design, which is herein proposed. A simplified theoretical approach is presented to allow a correct proportioning and design of each structural element, such as reinforcing ribs and panels constituting the box. Moreover, the solution given by this analytical procedure can be useful to check the reliability of any numerical analysis. Since the analytical approach is based on equilibrium conditions, it can be easily extended to the ultimate loading state. A practical application is finally presented for a seven floor building where both the shear walls and the box foundation are subjected to a severe seismic action.