Seismic rocking isolation of an asymmetric frame on spread footings

Rocking isolation is a relatively new design paradigm advocating the intense rocking response of the superstructure as a whole, instead of flexural column deformation. This is accomplished through intentionally underdesigning the foundation to guide plastic hinging below the ground surface rather than in the columns. A 2-story, 2-bay asymmetric frame is used to explore the effectiveness of this novel design approach. Finite-element dynamic analyses are performed using as seismic excitation idealized pulses and 20 real accelerograms, taking into account material (soil and superstructure) and geometric (uplifting and P-? effects) nonlinearities. A conventionally, Eurocode-designed frame and its foundation are compared to a design featuring the same frame but with substantially underdesigned (unconventional) footings. It is found that the performance of the unconventional system is advantageous, as not only does it escape collapse but it also suffers reparable damage. Despite their reduced width, the residual settlements of the underdesigned footings are comparable to those of the conventional ones. However, the analyses also reveal that residual rotation and differential settlement of the underdesigned footings may be unavoidable and must be critically evaluated-a need exaggerated by the asymmetry of the examined frame. Three possible ways of improvement at the foundation level are studied: (1) a single conventional tie beam, monolithically connected to the footings; (2) two separate tie beams hinged at each footing (allowing rotation, but resisting axial deformation); and (3) a hybrid system, comprising a single continuous tie beam connecting the three footings but externally hinged to each of them. The first solution hardly offers improvement, as it hinders rocking, and the second fails to reduce differential settlements. The hybrid solution provides encouraging results in terms of residual rotation and differential settlement, while it does not hinder the development of beneficial rocking isolation mechanisms and fully restrains horizontal differential movements.