Failed connection shuts Oakland Bay Bridge
San Francisco’s Oakland Bay Bridge was shut for six days last week following the near-catastrophic failure of a recently repaired structural steel member.
A 2t tie bar arrangement supporting a cracked member on a steel truss forming part of the bridge’s eastern section plunged onto the main roadway last Tuesday.
The accident occurred at around 5.30pm on 27 October. Two vehicles were hit, but no-one was injured.
The repair was carried out in September, when a routine inspection discovered a crack on a steel truss member called an eye bar. To carry the load of the compromised eye bar, four high strength rods were installed above and below a saddle arrangement (see diagram).
However, last week one of the high strength rods failed, causing a second tie rod and part of the top saddle to also fail and fall onto the carriageway below.
Transport agency Caltrans blamed the failure on high winds, saying that fatigue stress caused the rods to fail. This was most likely caused by excessive movement of the rods during high winds which resulted in metal on metal contact.
BAY BRIDGE: REPAIRING THE CRACKED EYE BAR
Four high strength rods were installed alongside the cracked eye bar.
One of these then failed under wind loadings, falling to the road deck and causing another bar to fail and fall to the deck.
“Design engineers have established fatigue caused by vibration experienced by the rods as being contributory to the failure,” said a Caltrans spokesman.
“The vibration was something we were monitoring in our weekly inspections. In recent visits inspectors had noticed movement and we had already started designing how we would isolate those sections. However we didn’t get the modifications in place for the latest windstorm.”
The failed repair has already been replaced and the bridge reopened last Monday. The latest repair, which took 130 hours, is similar to the original although extra measures have been taken to minimise vibration.
“The vibration was something we were monitoring in our weekly inspections. However we didn’t get the modifications in place for the latest windstorm.”
Caltrans spokesman
The rods have been lashed back to the eye bar in three places to minimise vibration. Protective sleeves have also been installed over the tie rods to prevent them from rubbing against the eye bar and extra welds and straps have been used to prevent sections falling onto the carriageway, should there be another failure.
The bridge reopening was delayed by a day after tests last Saturday showed that the newly installed rods were rubbing against the eye bar.
This meant that the repair had to be reworked. This heightened public concern about the safety of the bridge.
Bay bridge upgrade
Work is ongoing on the Bay Bridge to strengthen it to withstand earthquakes.
The western suspension section, which runs between San Francisco and Yerba Buena island is being retrofitted with dampers while the steel truss eastern section, which runs between Yerba Buena island and Oakland, is being completely rebuilt.
The replacement bridge, which features a self anchored suspension span, is being built alongside the existing bridge. When it is complete in 2013, the existing eastern section (including the area in which the crack was found) will be demolished.
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Readers' comments (1)
M Seshagiri Rao | 5-Nov-2009 2:10 pm
It is sad that the present state of Technlogy could not foresee this possibility and the reair work was itself the cause for a failure. Lucky the pieces did not drop on a moving car and cause casualties.
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