Engineers say long-term water damage unlikely on new Bay Bridge span

With the predicted El Nino likely to bring heavy rains that might leak into and corrode the interior of the new $6.4 billion eastern span of the Bay Bridge, some have raised concerns about the damage water could cause to the very vulnerable chamber deep inside the new bridge.

Even the bridge's designer has expressed long-term concerns over the impact of possible leaks, but some wonder if these concerns overblown.

The guard rails on the tower span are bolted into and through the deck, so the guard rails won't fail if hit by a car. Though there has been some leaking through many of the bolt holes, it appears there's little or no damage to the structural metal.

That's because all that metal inside the bridge has been coated with zinc. The protective zinc coating reacts with water, forming a white powder. 

"What that is is zinc doing its job.  Water hits the zinc, it's oxidizing; it's protecting the base metal. That's exactly what it's supposed to do," said Bay Bridge Resident Engineer Bill Casey.

Casey says Caltrans knows where the leaks are coming from: where the guard rails are bolted into the roadway.

"We want to deal with it and make sure it doesn't continue, but at this point in time, I don't see any structural concern here," said Casey.

Where the bolts come straight through the deck, there's been no leaking because the metal pieces are held together under great pressure. But where some of the bolts connect to a flange, about an inch or away from the road deck, there has been some leaking. 

But Caltrans says it can put something in there that can take care of that. To plug the leaks, waterproof coated metal will be placed in the open areas where the bolts are not flush against the roadway deck metal which should end the leaks.

"It would probably take you 6 months easy to do all the work; maybe a little bit longer," said Casey. "We want to keep as much water out as possible, so if water does get in, if it goes into the bridge it finds its way out, so we designed all kinds of things and mechanisms to get it out."

For example, there are many open drain holes on the underside of the bridge, designed into the bridge from day one. Wherever water leaks in, there are channels for the water to go down to those drains. Result: water never touches the interior metal, only the protective zinc coating.

Some have also raised concerns that deep inside the bridge deck where the cable is anchored into the bridge, water could damage the massive cable that holds the bridge up.  It's in is the cable chamber where more than 17,000 individuals wires were made to form 137 bundles which created that one gigantic cable.

Some water has gotten in there, but it may not really be a long-term problem. Though the cables appear to be bare metal, they are also coated with zinc and will be highly protective when reacting with water. 

Casey says there is evidence water has leaked in because on some of the cables there is a telltale white powdery residue. Casey told KTVU this has had no structural impact and that the zinc residue will stop when the leaks are permanently plugged within the next few months.

"The real concern here is long term.  You don't want it year, after year, after year, for many, many years or decades having these cables get wet.  That's what you don't want," said Casey.

With no more water coming in, all corrosion stops.

"With no moisture in here, you don't really have corrosion. So with that kind of low humidity you won't have corrosion over the long term," explained Casey.

Nonetheless, from day one, it was assumed that some water might get into this water tight but not waterproof chamber from time to time.

"So that's why you've got all these layers.  You have the layers of the zinc primed steel. the zinc primed cables themselves and then the dehumidifier to really keep it dry in here. With proper maintenance, this bridge will last the test of time," said Casey.

Even the bridge designer, who expressed concerns about leaks, sees this is a long term problem only if the water continues to leak.
 

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