Shake Test

In January 1994 the earthquake nightmare that had long afflicted California’s healthcare system came true: A 6.7-magnitude quake left 11 Los Angeles-area hospitals partially unfit for occupation and caused $3 billion in damage.

The next year, California enacted legislation putting hospitals on a strict schedule for upgrading structures particularly vulnerable to earthquakes. Though modified in 2010 to more scientifically define the highest-risk Structural Performance Category-1 structures, in effect reducing their number from 40% to 20% of hospitals and extending compliance deadlines throughout the next decade, numerous structures are still reported to be in danger of collapse from a major earthquake—40 hospital buildings in San Bernardino County alone, according to a Center for Health Reporting article published in 2011.

Some 225 hospitals are under state mandate to report compliance progress for campuses having at least one building in danger of collapse, according to the state Office of Statewide Health Planning and Development (OSHPD). Yet hospitals, along with the rest of California’s economy, are struggling with financial shortfalls threatening completion of needed capital improvements.

With California facing a one in four chance of an earthquake of 6.7 or greater occurring during the next decade, according to a U.S. Geological Survey estimate, hospitals are in need of accurate information that can be used to adequately protect their buildings in a timely fashion despite fiscal austerity.

Enter a unique five-story "hospital" constructed atop a 7.6-meter-by-12.2-meter shake table designed to reproduce motions recorded or scaled from recent intense quakes ranging from 6.7 (Los Angeles) to 8.0 (Peru, 2007) to 8.8 (Chile, 2010). Under the auspices of the University of California at San Diego’s (UCSD) Englekirk Structural Engineering Center, this was the largest shake table-based structure ever to focus on nonstructural component function and survivability post-earthquake.

It featured two upper stories housing a fully equipped surgical suite and intensive care unit, as well as floors including furnishings and equipment both secured and unsecured, piping and conduits for HVAC, plumbing and electricity, a small network of computer servers and workstations, and a fully operational fire suppression system.

The building also featured standard egress via an elevator and stairway, 8-inch concrete slab flooring, and two types of cladding: precast concrete for the top two floors and lightweight metal studs overlaid with gypsum and stucco below.

The structure was put to the test in April and May 2012, first through a progressively vigorous shaking with and without base isolation, then with a controlled burn simulating a post-quake fire. The shaking encompassed a series of one-way thrusts eventually reaching as much as eight times horizontal gravity load.

With base isolators—flexible rubber mountings that can reduce horizontal motion in the building—the horizontal acceleration of the building was reduced to about a quarter of that experienced under the fixed condition. The shaking was resumed minus the isolators, leading to dramatically eye-opening results, and was followed by the controlled burn test.

The aftermath produced mixed feelings for the engineers, industry product suppliers, and state regulators involved in this multiyear collaboration. They were pleased by access to real-world experimental data they had never had before—data going far beyond standard post-quake assessments and expensive computer modeling.