Uplifting and overturning of equipment anchored to a base foundation

This paper concentrates on the rocking response of rigid equipment anchored on a foundation base with plan dimensions that are larger than the footprint of the equipment. With this configuration there are two distinct capacities to resist uplift and eventual overturning: (a) the equipment engages the base foundation into rocking motion, and (b) the restrainers fracture and the equipment subsequently rocks atop its foundation base. This study examines both moderate uplift (serviceability level) and overturning (ultimate state-failure). It is first shown that for specific equipment/base configurations, the high strength restrainers used in practice are sufficient to engage the foundation base into rocking motion for a wide range of recorded earthquake motions. Subsequently, the study uncovers that for earthquakes with long distinguishable pulses, the margin between exceeding the serviceability level of uplift and achieving overturning is minimal. None of the strong motions used in this study is capable of overturning the freestanding configurations examined. It is found that small values of the foundation protrusion from the footprint of the equipment reduces the uplift appreciably.