The collapse that didn't have to happen
OAKLAND, Calif. (KTVU) - If the deadly collapse of that Florida pedestrian bridge seems bizarre, it really is. Failures of large scale construction projects are few and far between, provided you do it correctly or don't innovate too much or too fast. That's because speed can kill.
When you look at the rubble of the fast built Florida bridge collapse, something is missing. What's missing, as a rendering shows, is the huge tower, that would have held the bridge up. The similarly constructed San Francisco Oakland Bay Bridge eastern span was built tower first. The bus bridge into San Francisco's new Transbay Terminal; built tower first. Professor Robert Bea, emeritus of UC Berkeley's School of Engineering is world renowned civil engineer. We asked, "Have you every heard of anybody trying to build a bridge, or at least successfully build a bridge without first building the tower?" Bea's answer, "No."
Traditionally, when you build a long span bridge the way you do it is, you build the tower first. Then you can hang things off the tower including the roadways until you finish it off with those support cables.
That's because in long spans, concrete isn't strong enough to hold up it's own weight. To compensate, designers and builders string many stands of thick stretched steel cables inside the concrete to compress it, making it much, much stronger.
Last Saturday, the Florida contractor slid the bridge walkway across the highway, over a series of steel supports. It ended up resting on the pillars at each end. Professor Bea says those steel supports were critical.
"They actually were in place until Saturday," said Bea. But then, he says, the contractor made an ill fated decision. "(They) got it into place and then removed those supports." said Bea.
Removing the steel supports under the uncompleted bridge allowed traffic to flow; not a good idea beneath active construction. Bea says his expert sources say, workers were adjusting those strengthening steel cables during the week as the bridge hung in mid air. Had they left the steel supports in place would it have collapsed?
"No, I think it's very unlikely it would have happened," said Bea. In other words, without that support tower, did it fall of its own weight? "Yes," said Bea.
This alleged time and money saving process called ABC, Accelerated Bridge Construction, will now be seriously questioned and evaluated in the light of this tragic result.