San Francisco-Oakland Bay Bridge Eyebar Repair
By Bart Ney and Andrew Gordon
The San Francisco-Oakland Bay Bridge (SFOBB) is a towering testament to achieving the impossible. When the bridge was first conceived during the Gold Rush era, the public imagined such a monumental undertaking as an engineering and financial impossibility. In the end, the believers proved the naysayers wrong, and the Bay Bridge became a reality in 1936.
So 73 years later, as Caltrans was seismically retrofitting the bridge, including replacing the entire East Span, crews were unexpectedly faced with another “impossible” task during the 2009 Labor Day weekend closure of the bridge. The bridge was already closed for one herculean feat – cutting out and moving a 3,200-ton double-deck section of bridge in order to roll in a new 3,600-ton section to connect the East Span to the Yerba Buena Island (YBI) Detour.
During the closure, workers conducting a routine inspection of the bridge discovered a significant crack in a structural steel member, known as an eyebar, on the East Span. While the crack was unrelated to the weekend’s work, it was severe enough that it forced an emergency closure of the bridge for repairs.
An eyebar gets its name from its geometry – a long bar with two circular end pieces and holes in both of the end pieces, giving the appearance of two “eyes” connected by a long bar. Eyebars are connected to each other in chains by placing large cylindrical pins that pass through the holes at the ends, with two or more eyebars being connected with these pins.
Although eight eyebars constitute the overall structural member and create the appearance of a redundant system, each eyebar must carry its share of the load. The main concern was the apparent loss of about one-eighth of the load-carrying capacity of the structural member. A secondary concern stemmed from the fact that the crack caused a portion of the structural member to twist.
The crack was discovered Saturday night, and the bridge was originally scheduled to reopen for the Tuesday morning commute. Engineers quickly devised a temporary repair, as a long-term fix required more time to design and procure specialized materials. Fortunately, much of the material and manpower for the short-term repair was already on-site.
While engineers kept a long-term repair in mind, they developed an adjustable short-term repair design so that the bridge would be safer when it reopened than when it closed. The repair entailed creating a brace around the cracked eyebar that took the load off that eyebar and redistributed the weight of the load among neighboring eyebars. Engineers and construction crews worked for nearly 70 hours to reopen the bridge safely just one and a half hours later than originally scheduled.
Eyebars are not unusual in California; there are 1,680 eyebars throughout the entire Bay Bridge. Two other state spans have similar eyebars – the Richmond-San Rafael Bridge and the Highway 263 Bridge outside Yreka in Siskiyou County. California also has 52 local agency bridges with eyebars.
However, nearly two months after the initial repair, during the evening commute on October 27, one of the repair’s metal tie-rods cracked due to vibrations caused by high winds, which caused the tie-rod to rub against another portion of the repair. When the tie-rod cracked, two tie-rods and a crossbeam fell to the upper deck of the bridge. Fortunately, no one was injured. While wind was a consideration in the design, unique wind patterns created unusual behavior in the tie-rods, which vibrated at a high frequency and caused one of them to crack.
During a nearly weeklong emergency closure of the bridge, several enhancements were made to the short-term repair, including tensioning the tie-rods so they would not rub against other components and crack. The tie-rods were strapped together, and additional sections of the repair were welded together. If a tie-rod were to fail again, these enhancements would prevent pieces of the repair from falling.
While Caltrans kept an eagle eye on the repair using a combination of daily lane closures and video monitoring, engineers continued to devise the long-term repair, which was put in place in December. The long-term repair entailed cutting away the cracked section of the eyebar, placing a new U–shaped eyebar over the upper pin, and then bolting together the remainder of the original eyebar and the new eyebar head to create a single integrated unit that was stressed to redistribute the load evenly throughout all eight eyebars. This long-term repair would not have been possible without the short-term repair in place, and it preserves the safety and integrity of the bridge while requiring significantly less maintenance and inspection.
The long-term fix was done during the night, requiring only a partial lane closure, which did not adversely affect motorists. This fix will secure the bridge through the opening of the new East Span and the demolition of the original steel truss bridge.
Eyebars were once used prominently in bridge construction as a more affordable alternative to larger fabricated steel box beams or girders, and in some cases as an alternative to steel cables. Eyebars are no longer used in domestic bridge design.
Caltrans’ bridge inspection program identifies and documents cracks and other conditions that could compromise structural integrity, and also makes appropriate repair recommendations, which can range from minor repairs to closure of a bridge. Caltrans is now doing critical eyebar inspections every three months.
For more information regarding the repairs to the Bay Bridge eyebar please go to: http://baybridgeinfo.org/eyebar/mediabar