OAKLAND — A Depression-era marvel, the 1936 eastern span of the Bay Bridge is once again pushing engineers and contractors beyond their limits.
Demolition is finally underway, but taking down the old gray lady over the next three years could prove as tricky as erecting the shiny new $6.4 billion span next door.
“The old bridge is the world’s largest armed bow and arrow,” Caltrans bridge engineer Brian Maroney said. “We have to de-string it very carefully or it will go boom!”
The bridge’s design is what engineers call “fracture critical.”
That means nearly every welded or riveted joint connecting the 77-year-old span’s 10,000 steel frames, called trusses, is a key link in the carefully balanced structural forces that hold the cantilevered truss bridge together.
Sever the wrong piece in the wrong order and the entire bridge or large segments of it could spring violently apart.
Workers could die or suffer injuries. Toxic lead paint could end up in the San Francisco Bay. And — horrors! — flying debris could damage the expensive new bridge that runs alongside.
The $281 million demolition project is so risky that Caltrans is requiring contractors at California Engineering Contractors/Silverado Joint Venture to submit plans signed and stamped by two licensed engineers rather than the customary one.
On a recent brilliant fall morning, Maroney and Robert Ikenberry, the contractor’s safety manager, led a small excursion up the old bridge’s steel grated catwalk to one of its two 385-foot peaks.
It was the last time journalists would stand on the summit. Within days, demolition work would make the climb too dangerous.
The bridge’s highest points overlook each side of a 1,400-foot span, the longest of its kind in the world when it was built. From a bird’s-eye view, the old workhorse is far more impressive than it looks from the grungy road decks where its oft-painted gray steel pales in comparison to the gleaming new white span.
Steel pieces of varying sizes and shapes are riveted and welded into alternately bizarre and delicate geometric patterns.
The pieces are arranged into massive steel frames called trusses, each carefully positioned for a particular structural purpose, Maroney explained.
The trusses, deck and piers on the Yerba Buena Island side closest to the new span will be the first segments to come down. Caltrans wants to clear the area so the contractor can finish the new span’s bike and pedestrian path all the way to the island and construct the permanent eastbound onramp.
At one particularly tricky junction, eight steel components — some big enough for an adult to stand in — merge into a single vertical column. Which member should be cut first?
“The contractor literally has to take this bridge apart one piece at a time,” Maroney said.
Then add worry about how the bridge will shift when contractors roll heavy equipment onto the bridge and start stripping away 71/2 million tons of asphalt, concrete, rebar and steel — the equivalent of 28 million loaded military Humvees.
As if these aren’t reasons enough to keep engineers and contractors up at night, the span is built out of fatigued World War I-era steel that may not handle unaccustomed strains.
“This is a beautiful and nightmarishly scary structure,” Maroney said.
If only original Bay Bridge chief engineer Charles H. Purcell were around to answer questions.
But Purcell died in 1951, so Caltrans and contractors must rely on original drawings and photos and apply modern technology that engineers of the 1930s couldn’t have imagined.
For example, the contractors will use strain gages to measure in real time whether a piece is stable before it is cut. They will use 500-ton jacks to balance and neutralize loads. Computer models will map the load paths to help avert disaster.
“We are essentially deconstructing the bridge using clues from 80 years ago along with the technology of today,” Ikenberry said.
Managing risk isn’t cheap or quick. The price tag for the demolition has grown from $46 million in 1997 to $281 million today, although it remains under the $318 million budget set in 2005.
The demolition work also started two months late. Caltrans planned to give the go-ahead to start taking the bridge down within days after the new one opened on Labor Day.
But after a broken bolt debacle in March put the new span’s opening schedule in serious jeopardy, demolition planning stalled. Then Caltrans and the contractor ran into delays obtaining permits for handling toxic lead paint debris.
Contact Lisa Vorderbrueggen at 925-945-4773 or lvorderbrueggen@bayareanewsgroup.com. Follow her at Twitter.com/lvorderbrueggen.
