Box basement Structures in Seismic Resistant Buildings

In reinforced concrete structures seismic design, structural wall system is often preferred with respect to frame system for different reasons, as it is possible to reduce the section dimensions of columns and beams, which in this case carry only gravity loads; and moreover because the damages, under serviceability earthquakes, are generally reduced, in fact the buildings braced by structural walls are invariably stiffer than framed structures. Uncertainties about shear walls ductile behaviour can arise, but a proper reinforcement design can assure good capacity to dissipate energy in a stable way. On the other hand problems arise in the foundation, because a large gravity load eccentricity usually occurs on the basement. As a consequence, too large sizes direct slab foundations are needed. In the present paper an alternative foundation solution is proposed, which consists in the structural organization of a box basement structure, made by the peripheral vertical underground walls, and by the raft floor and the ground floor as horizontal diaphragms. All these structural elements are already present in the buildings and they only need a proper design for the goal. Attention is focused on the different structural role of the box elements and on the evaluation of the shear stress flows which affect them. In the paper some numerical results are compared with the simplified theoretical approach presented. On these bases a correct design and proportioning of the box basement structures can be achieved.