This study concentrates on the rocking response of rigid equipment supported on a foundation base. In most cases heavy electrical equipment is anchored on a concrete base with plan dimensions that are larger than the footprint of the equipment. In the event that the strength of the restrainers, Fu is sufficiently large and the ground acceleration is sufficiently strong, the equipment will engage its foundation in rocking motion. On the other hand, if the restrainers are too fragile they will fracture and eventually the equipment will rock atop its foundation base. Accordingly, equipment anchored to a base foundation exhibits two distinct rocking capacities: (a) the equipment engages the base foundation in rocking motion and (b) the restrainers fracture and the equipment subsequently rocks as a freestanding block atop its foundation base. The aim of this study is to compare these two capacities for practical values of the foundation footprint and the restrainer strength. The study examines intensity levels of ground shaking that will exceed serviceability levels (6 in. uplift at edge), and intensity levels that will result in overturning.
The study shows that for specific equipment/base configurations the high-strength restrainers used by PG&E are sufficient to engage the foundation base in rocking motion for a wide family of recorded earthquake motions. It was shown that the minimum strength capacity of the restrainer needed to avoid fracture is closely related to peak ground acceleration and that only the Cape Mendocino record is capable of fracturing the high-strength restrainers. It was found that the strength capacity of the restrainer should be as high as Fu =W to engage the base foundation in rocking motion. The study reveals that for earthquakes with long distinguishable pulses (Tp >2.5 sec ), the margin between exceeding the serviceability level of uplift and achieving over- turning is minimal.
Nevertheless, none of the strong motions used in this study is capable of overturning the free- standing configurations examined. Two records, the Rinaldi Receiving Station record (1994 Northridge earthquake) and the Takatori record (1995 Kobe earthquake), are capable of uplifting the two transformers of interest beyond the serviceability level; however, it is found that small fractions of the foundation protrusion, d, to the half-width of the equipment, b, reduce the uplift appreciably. Occasional exceptions to this conclusion have been identified and explained.
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