Tilt-up designs upgraded to meet latest seismic standards
Among the buildings most susceptible to earthquake damage are the low-cost tilt-up buildings that dominate the industrial landscape of much of the Southwest. The seismic vulnerability of these buildings was dramatically demonstrated with the collapse or partial collapse of at least 500 tilt-ups during the January 17, 1994, Northridge Earthquake in southern California and a smaller number during the 1989 Loma Prieta Earthquake in northern California. As a result of this damage, structural engineers conducted detailed investigations leading to the development of new designs that have greatly strengthened these buildings and will reduce property damage and loss of life in future earthquakes.
These new designs are incorporated in the recent publications, “Guidelines for Seismic Evaluation and Rehabilitation of Tilt-Up Buildings…” by the Structural Engineers Association of Northern California and “Guidelines for the Seismic Retrofit of Existing Buildings” by the Structural Engineers Association of California. The most recent Uniform Building Code also includes these new engineering provisions.
Tilt-up buildings derive their name from the way in which they are constructed, with the concrete walls cast on floors or slabs and tilted into place by cranes. Then, roofs and floors are built and connected to the walls. It is the lack of adequate connections between the walls and the roofs that has led to the poor performance of these buildings during earthquakes.
The new designs developed by structural engineers and the new code provisions, accompanied by stricter detailing and inspection requirements, mean that today’s tilt-ups are as resistant to earthquakes as any other type of building. On the other hand, tilt-ups designed prior to the 1994 Northridge Earthquake remain vulnerable and could partially collapse in a major earthquake.
Throughout seismically active areas of California, some tilt-ups have been strengthened by conscientious building owners who realize that the relatively low costs of strengthening their buildings far outweigh the costs resulting from damage to the buildings and their contents, delays in business operations, and the potential loss of life. In addition, numerous local jurisdictions, such as the City of Los Angeles and other southern California cities, as well as the Bay Area cities of Fremont and Hayward have adopted mandatory retrofitting programs to upgrade tilt-ups and other earthquake-vulnerable buildings. But, elsewhere, the high risk associated with older tilt-up buildings remains largely unrecognized.
“Hopefully, it won’t take a major earthquake to realize the potential loss of life and heavy property damage that these older tilt-up buildings represent,” says David McCormick, structural engineer and lead author of the Structural Engineers Association of Northern California study. “Fortunately, previous quakes have occurred at hours when these buildings were largely unoccupied, but, if an earthquake were to occur during working hours, the loss of life could be substantial.”