Retrofitted Bridges Withstood Earthquake

SACRAMENTO —  Dozens of highway structures that were strengthened for earthquake safety rode out the Northridge temblor with only minor damage, proving that the state’s seismic retrofitting program can prevent major bridge collapses, a Caltrans report said Wednesday.

In their first detailed assessment of the damage sustained by highway overpasses and interchanges, Caltrans officials said the Northridge earthquake, unlike previous quakes, probably will not prompt major changes in retrofitting and design standards for bridges.

All the retrofitted bridges withstood major shaking, he said, with only minor damage.

“I don’t see any big surprises in this earthquake like we had in Loma Prieta. I think what it showed is that what we were doing is right,” said James Gates, chief of Caltrans’ office of earthquake engineering.

He said most of the damage to the bridges was predictable and would have been prevented if the structures had been retrofitted. Only one of the eight bridges that collapsed in the quake was not scheduled for seismic strengthening.

Gates said officials believe there was not the extensive movement at several sites that had been first reported and led engineers to speculate that retrofitting might not have saved some of the bridges.

His comments were based on a report issued by the Post Earthquake Investigation Team, a group of engineers who were dispatched to Los Angeles to comb through the wreckage of heavily damaged and collapsed bridges in an effort to determine some of the causes of their failure.

Their findings, written to serve as a guide for engineers who are designing new bridges or retrofitting old structures, pointed out several potential trouble spots that the team said probably contributed to some of the collapses in the Jan. 17 quake.

In particular, members urged Caltrans to conduct more studies on the flares at the tops of columns and certain kinds of abutments that caused the ends of some bridges to break away from the land.

Gates said that in bridges designed in the early 1970s, flares were added to the tops of columns as an aesthetic feature that would break away in an earthquake.

As it turned out, he said, the flares on the Simi Valley Freeway did not break away and forced weaker sections of the columns to fail, causing a bridge to collapse.

The department, he said, will search for any other bridges that might be like them and re-examine them for possible inclusion in the retrofitting program.

For an unknown reason, he said, the bridges had fewer columns than most bridges, meaning that each one would have to carry a heavier load than normal.

Although the team recommended further study of the impact of vertical forces on bridges during the Northridge quake, Gates said he believes those forces will not turn out to be a contributing factor to the collapses.

“Earthquakes are very complicated and there was a significant vertical component in this earthquake. It’s influence on structures we still don’t know but my gut feeling is (it did not have much impact),” he said.

His opinion differs from that of Caltrans chief bridge engineer James E. Roberts, who has said he believes the strong vertical forces may have been a major factor in the failure of bridges.