For much of the past two weeks, the Bay Area's attention was focused on a previously obscure piece of structural steel on the east span of the Bay Bridge known as an eyebar.

The cracked eyebar forced the extension of a scheduled construction closure of the bridge over Labor Day weekend for emergency repairs. Then, when that repair failed on Oct. 27, flinging tons of steel onto the bridge during the evening commute, the bridge was barricaded for another 51/2 days while the repairs were fixed. And in the months to come, Caltrans officials have warned, another closure to install a more permanent repair is likely.

Despite all the attention it has received, and all the complaining it has caused, the eyebar isn't the biggest danger on the eastern span of the Bay Bridge. And it's not the reason the span is being replaced - at a cost of $6.3 billion - instead of retrofitted.

Like any old structure, the eastern span has many flaws, and because of its design, is considered more prone than other bridges to collapse during an earthquake. The flaws considered most serious by engineers and geologists include the span's foundations, its network of steel supports, and the supports for the road deck, which were what failed in the 1989 Loma Prieta earthquake.

Seismic experts consider the eastern span a likely candidate to collapse in another major temblor. Caltrans issued a 1996 report that concluded the bridge could not withstand a 6.5 magnitude earthquake, and that it would be cheaper - and safer - to replace the eastern span. But repeated delays slowed construction. The projected opening date is now 2013.

'Competing against time'

"I wish it would be done sooner," said Ahmad Itani, a professor of civil and environmental engineering at the University of Nevada, Reno, who has studied the span and reviewed the eyebar repairs. "We are competing against time."

Built in just three years, the Bay Bridge was considered an engineering wonder when it opened in 1936, with the western section being built as a suspension bridge and the eastern section as a cantilever span. But construction techniques and materials have changed, and much more is known about earthquakes and their effects on bridges.

"The foundations are the biggest part of problem," said Bart Ney, a Caltrans spokesman.

The existing eastern span is anchored in bay mud using a series of pilings that are actually treated Douglas fir trees - the biggest that could be found. Most are 70 to 80 feet long; the longest, Ney said, reaches 120 feet into the muck. In contrast, pilings for the new eastern span are 8-foot-diameter steel pipes, pounded at an angle about 360 feet deep into more stable soil formations.

Strong shaking

With little support from the foundations, the bridge would be subjected to strong shaking and movement during an earthquake, similar to what caused a 50-foot section - where the incline meets the cantilever section - to fall in the Loma Prieta quake after connecting bolts broke and the slab was pulled from its support.

The section that fell during the earthquake was not the only one damaged in the 1989 quake. At least one other section was near collapse, and five other connections snapped. Caltrans strengthened those areas during retrofitting work on the eastern span in the late 1990s "so the bridge could move more," said Andrew Fremier, deputy executive director of the Bay Area Toll Authority and a former Caltrans engineer. "But not enough for a major seismic event."

Engineers also say the eastern span's multitude of steel-to-steel connections - including the 1,860 eyebars, the narrow steel latticework supports and other steel members - that make up the cantilever structure are outdated and aging. Some engineers, including Itani, believe the structure is nearing the end of its life expectancy.

"With steel, if you want to break something, you bend it back and forth," he said. "This bridge has been back and forth by loads for more than 70 years. The steel has reached its limit."

A case for retrofitting

However, at least one Bay Area engineer believes the span could still withstand a major earthquake and should be retrofitted. Abolhassan Astaneh-Asl, a UC Berkeley civil engineering professor and a frequent critic of Caltrans, says the eastern span would be more resistant to earthquakes and terrorist attacks than the single-tower suspension span under construction.

Astaneh-Asl also criticized Caltrans for failing to properly maintain the existing span since it began construction of the new bridge in 2002, and said negligence may have contributed to the crack in the eyebar.

Bay Area transportation officials disagree, insisting they have continued to maintain the bridge as a vital transportation structure and will continue to do so as long as it remains open to traffic.

"The bridge is maintained well, and we are paying attention to it," Fremier said. "While the (eyebar trouble) is a sign the bridge has started to show its age, it is in good shape. It has to be. But it is seismically vulnerable."