No Easy Solutions on Spans : Bridges: For Caltrans, the continuing puzzle is how best to bring mammoth pre-’71 structures up to strength.

After the 1971 Sylmar earthquake, the California Department of Transportation revised its code requirements for all of the state’s bridges. New codes appeared in 1973 and have been progressively updated, the latest coming in 1985.

It is the columns of a bridge that are the most important elements for resistance to earthquakes. Thus, the integrity of these columns is essential to a bridge’s survival. Since 1971, the major changes in the codes have all concerned what is called a provision for “ductility” in bridge columns. By ductility, we mean the ability to deform or deflect the effects of an earthquake in a ductile, or pliant, manner, thus avoiding brittle fractures. Ductility allows for large side-sway deflections of a bridge’s superstructure, without danger of collapse. In an event such as Tuesday’s earthquake in the San Francisco Bay Area, bridges built since 1971 can be expected to behave well.

But what about those bridges that were built before 1971? The San Francisco-Oakland Bay Bridge and the elevated structure on the Nimitz Freeway in Oakland are both in this category, and large side-sway deflections appear to be the cause of their collapse.

Considering the potential for disaster, one could ask why something wasn’t done about these bridges. In fact, Caltrans engineers were well aware of the problem and had been actively pioneering various solutions for retrofitting bridges to bring them up to post-’71 codes. No other agency, state or federal, is as far advanced as Caltrans concerning this technology.

But when it comes to bridge retrofit, where do you start? There are more than 13,000 bridges in the state system. It is simply not possible to upgrade all of them simultaneously. So you begin with the easy ones--those that can be upgraded with the installation of cable restrainers that tie spans together, preventing their falling from bridge columns.

The difficult bridges, you leave for later. Structures like the Bay Bridge and the Nimitz double-deck are in this class. Their monumental size, plus their structural type, precluded the use of conventional methods of retrofitting.

Research is continuing in this field. But so far the results have been inconclusive concerning the most effective and satisfactory method of retrofit. What is needed is an intensified research and development program to explore innovative ways to upgrade these structures.

Now is not the time for knocking Caltrans. Rather, it’s time to push for more funds, both state and federal, to develop and implement strategies for the retrofitting of the remaining bridges in the Caltrans program.