Elevated sections of roadway are necessary in a terrain like Southern California's because they permit gentle grades and curves as the highways bridge hills and ravines and crisscross other roads. But constructing bridges that can survive a rumbling earth is an extraordinarily difficult engineering task, as Caltrans, the California transportation agency, is learning the hard way.
Caltrans began the massive task of retrofitting the region's infrastructure following a 1971 earthquake in the San Fernando Valley. On balance the engineering measures proved sound. But a combination of too little knowledge and too little speed prevented the retrofitting effort's success.
Structures are designed to withstand earthquakes, based on known faults; the fault that moved last week was not even known to exist. Moreover, most earthquake engineering is designed to cope with horizontal ground motion; last week, the ground moved up and down violently, and the severe vertical movements may have caused columns to punch through some roadways. Soil conditions are also not fully understood.
Structural analysts insist that they can handle a quake of last week's magnitude, whether building new bridges or refitting old ones -- but that they need time. Critics say they have had time but moved too slowly. Now, California faces a repair bill running into the billions for damaged Los Angeles freeways -- and uncounted losses because the arteries supporting the economy have been severed. With luck, Caltrans hopes to have the damage repaired by the end of 1994.
Highways are constructed from rigid concrete spans up to 500 feet long, but the joints between these decks are vulnerable spots. Following the 1971 San Fernando earthquake, California engineers "stitched" the decks together with steel cable on about 1,200 highway overpasses.
The columns are a second major weak point. They must be strong enough to withstand major force, yet not so brittle that they shatter. Engineers wrapped the columns in 3/8-inch steel jackets -- like a giant tin can -- and pumped the jackets full of grout.
Some overpasses are constructed on double columns. In those cases, concrete walls were constructed between them.
With both single- and double-column construction, the concrete foundations are enlarged and pilings are driven deep into the ground to channel the destructive force back to its source.
