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American bridge builders in the 1920s and 1930s figured they had made a big design improvement when they turned to metal pieces called eyebars.
Eyebars were simple, relatively strong and affordable ways to connect bridge parts by using pins inserted through the holes in the ends of slender metal beams.
But the improvement of yesteryear is an Achilles' heel of today, as Bay Area residents discovered this year when a crack was spotted on one of the 1,680 eyebars on the 73-year-old Bay Bridge. The discovery prolonged a Labor Day weekend closure of the span and later led to a second closure from Oct. 27 to Nov. 2 to repair a brace installed to reinforce the cracked beam.
Caltrans experts still are trying to design a longer-term fix for the eyebar, and anticipate another temporary Bay Bridge closure in early 2010 to make the modifications.
Meanwhile, Caltrans frequently inspects the eyebars on the 56-year-old Richmond-San Rafael Bridge, and cities and counties must watch out for another 50 smaller eyebar bridges they operate elsewhere in California.
Advances in welding metal and building wire cables made eyebars obsolete by providing another way of holding up bridges.
"Eyebars were considered quite safe," said Jack Moehle, a UC Berkeley civil engineering professor. "Our analytical tools were not as advanced back then. The problem is they have inherent flaws."
The problem: Microscopic cracks develop as the eyebar metal
Eyebar use became unnecessary in part because of changes in metallurgy and welding that made it practical to weld together large metal plates. Also, cable technology improved so that strands of hundreds or thousands of wires could be bound together to provide strength in numbers far superior to relying on any single piece of metal.
One of America's worst bridge disasters exposed the trouble with eyebars. At 5 p.m. Dec. 15, 1967, the Silver Bridge across the Ohio River between Ohio and West Virginia collapsed, killing 46 people and injuring nine.
"The bridge was bouncing up and down. It seemed like it just stopped, just hesitated for a minute, and that's when the cable started snapping," truck driver Frank Nunn told the Chicago Tribune in 1987. "Then zoom, it just gave way."
The accident shocked the nation, spurred a federal requirement for bridge inspections at least every other year.
In the Bay Area, the old Carquinez Bridge near Crockett, which has since been replaced by the Zampa Bridge, was shut down temporarily in the late 1960s until additional reinforcements were made on its eyebars, Caltrans spokesman Bart Ney said.
The Bay Bridge has redundancies lacking in the Silver Bridge, which collapsed when a single eyebar failed. The Bay Bridge's cracked eyebar is in a group of eight that work in unison to shoulder the load.
Still, the crack showed fortifications were warranted, Caltrans concluded.
"The inspection program on eyebars worked on the Bay Bridge. They found the crack," said Mark Ketchum, a San Francisco civil engineer and spokesman for the Structural Engineers Association of Northern California. "What didn't work was the temporary repair."
Three metal brace parts crashed down on the Bay Bridge's upper deck Oct. 27, forcing a six-day closure for repairs.
"I think when something like this happens, the anticipated response is to lose a little confidence," Ketchum said. "I don't think we're in a Chicken Little situation. We have a pretty safe transportation system. "
The important thing, Ketchum and Moehle said, is for operators of bridges to adequately inspect and maintain eyebars.
"The point here is that as funding goes up and down due to economic declines, we need to keep up our inspections," Ketchum said.
News researcher Camille Donaldson contributed to this story. Reach Denis Cuff at 925-943-8267. Read the Capricious Commuter blog at www.ibabuzz.com/transportation.
