U.S. Geological Survey
Fact Sheet 068-03

EVALUATING EARTHQUAKE PROTECTION
Diagram showing structural cross section of USC University Hospital. Brown rectangles in foundation represent base isolators (see photos below).	Building response recorded in the 1994 Northridge earthquake.

Base isolators in laboratory tests—(left) undeformed isolator, (right) deformed isolator with sizeable horizontal displacement (see red arrow on right side of photo on right). Such displacement of isolators prevents large displacements of floors of the building above.

The USC University Hospital was built with base isolators to allow it to withstand strong earthquake shaking. The success of this design strategy was demonstrated in the 1994 Northridge quake, when the hospital and its contents suffered no damage, despite the servere ground shaking produced by the quake.

One engineering strategy for reducing earthquake damage is to partially decouple, or isolate, a building from ground shaking in an earthquake. This strategy, called base isolation, is increasingly used to safeguard important structures. Building response recorded by the California Geological Survey in the 1994 Northridge earthquake (magnitude 6.7) confirmed the promise of the base-isolation strategy. The 8-story steel superstructure of the University of Southern California (USC) University Hospital in Los Angeles is supported by 149 isolators (see photos above) sitting on continuous concrete footings. During the Northridge earthquake, motions recorded at the top of the isolators and at the roof were less than those recorded in the ground below the isolators and at a nearby site removed from the building. The isolators reduced the level of motion fed into the base of the building by about two-thirds. The peak shaking at roof level was only about 40% of that recorded on the ground about 200 feet from the building, whereas with a conventional foundation the roof-level shaking would have exceeded that measured on the ground.

| MAIN TEXT |
| MONITORING ONSET OF STRUCTURAL DAMAGE | EVALUATING EARTHQUAKE PROTECTION |
| SHAKING IN AN IRREGULAR STRUCTURE | ASSESSING SAFETY | CAPTURING BUILDING PERFORMANCE |