UCSD Engineer Blames Cracked Joints for Nimitz Freeway Collapse

SAN DIEGO —  A UC San Diego structural engineer on Thursday blamed the Nimitz Freeway collapse on cracks in the ill-designed joints that connected the double-decked roadway to its massive, reinforced support columns.

Speculation on what caused the bridge in Oakland to collapse in Tuesday’s Bay Area earthquake has focused largely on the bridge’s support columns. But Nigel Priestley, a UCSD structural engineering professor who studies how earthquakes affect bridges, said the temblor generated intense pressures that cracked the under-reinforced joints, allowing the upper deck to collapse on motorists traveling along the lower deck.

The joints provided a “natural place for weakness, and there was virtually no reinforcement” evident in the failed bridge or in original engineering designs, Priestley said.

Although substandard by today’s guidelines, the Nimitz Freeway’s relatively unique design was “state of the art” in the 1950s, said Priestley, who is helping the state Department of Transportation find ways to reinforce bridge columns that are susceptible to earthquake damage.

Few Bridges Share Problem

Priestley said other double-deck bridges should not be shut down because few bridges share the same fatal design problems.

Priestley, who traveled to Oakland on Wednesday to inspect the failed bridge and original engineering designs, described the columns as “under-designed by today’s standards.” But he maintained that the joints were “the real weak link.”

The Nimitz Freeway’s chances of surviving Tuesday’s earthquake would not have improved had its huge support columns been retrofitted during Caltrans’ continuing retrofitting program, Priestley said. The accident “could have happened anyway,” Priestley said.

During the 1970s, Caltrans installed improved expansion joints on the bridge’s roadway. But those improvements were not designed to lessen the strains that produced cracks in the joints between the roadways and the support columns.

“This is a complex and unusual bridge,” said Priestley, who studies bridges and other structures in UCSD’s unique earthquake engineering laboratory. “It’s not a standard . . . and there are some unusual aspects to it that make it prone to damage.”

Priestley said the steel bars that should have reinforced brittle concrete at the joints evidently proved unable to handle stresses generated when the roadways and columns lurched back and forth during Tuesday’s earthquake.

Although newer bridge designs take those stresses into account, engineers during the 1950s had a “very primitive” understanding of how bridges would transfer stress from one section to another. The joints, while up to standard in the 1950s, left the “potential for a very major crack to develop.”

That potential existed because reinforcing bars at the joints were “inadequately anchored” in the massive support columns, Priestley said.

Priestley and two other scientists at UCSD use a unique $2-million earthquake testing center to construct full-scale models of structures that are subsequently destroyed by earthquake-like forces. Those studies will be used to improve design and construction methods in the United States, reducing casualties and property damage after earthquakes.

Caltrans has awarded UCSD’s earthquake researchers a $717,000 contract to physically test a “steel blanket” that would be wrapped around suspect bridge columns across the state to improve their resistance to earthquake damage. Caltrans also has awarded the laboratory a $135,000 contract to assess damage caused to the I-5/I-605 interchange during the 1987 earthquake in Whittier.