SAN RAFAEL (CBS SF) — Caltrans announced Friday evening that only one lane of the Richmond-San Rafael Bridge would be open in each direction overnight as crews made repairs in the area where loosened concrete fell from the upper deck the day before.

Caltrans said that the closure would run from 9 p.m. Friday night until 7 a.m. Saturday morning. There could potentially be another round of repairs during a similar window from Saturday night to Sunday morning.

The agency said all repairs were expected to be completed before the morning commute on Monday, Feb. 11.

Earlier Friday, a state transportation official said it was the failure of an expansion joint that caused several pieces of concrete to break loose and fall onto the lower deck, forcing Caltrans to shut down the span for hours on Thursday.

Caltrans District 4 Director Tony Tavares told reporters at a Friday news conference that engineers have determined that the bridge is safe for commuter use.

“The bridge is safe, it is open to traffic in both directions and the structural integrity of the bridge is sound,” he said. “Preliminary findings indicate that an expansion joint had an issue. It created some concrete to break off and fall on to the lower deck.”

Caltrans made a temporary fix Thursday by installing large metal plates over the expansion joint so the bridge could reopen. Tavares said work on a permanent fix would begin on Monday, weather permitting.

“The bridge was built in the mid-1950s,” he said. “The expansion joint — we think it’s wear and tear. It could have also been caused by a heavy truck load that went over that area that caused the damage to that expansion joint.”

Tavares said a team will inspect all the expansion joints on the 5 1/2 mile span over the weekend. All the expansion joints including the one that failed were inspected last August.

“At the time we inspected that expansion joint there was no indication that it would fail,” he said.

Crews will also monitor the bridge over the weekend to respond quickly if new issues arise.

“If we find any other issues we will have them repaired at the same time (as Thursday’s joint failure),” he said.

For a rainy Friday afternoon, traffic on the span was surprisingly light. Driving across the bridge, the steel plate bolted over the hole left by Thursday’s crumbling concrete was visible. The road bed is made of lightweight concrete, commonly used in bridges around the world.

“It’s not as strong as normal concrete, but it’s plenty strong,” said consulting engineer Dan Eilbeck.

Eilbeck explained that some structural tests conducted on the bridge are simple.

“Acoustic testing, where you go around and tap the concrete; listening for that hollow sound,” said Eilbeck.

He explained the hollow sound may indicate an internal problem.

For nearly 70 years, the bridge has endured some of the harshest weather in the Bay Area.

The falling concrete damaged a vehicle and created a traffic nightmare for Bay Area drivers for most of the day Thursday.

richmond-san rafael bridfge debris chp photo

Richmond-San Rafael bridge debris. (CHP Photo)

Traffic problems started after a Sig Alert was issued at 11:20 a.m. Thursday morning when authorities closed the bridge in the eastbound direction due to a report of falling concrete. A CHP spokesperson said that golf ball size or larger chunks of concrete were reported falling from the underside of the span’s upper deck.

CHP Officer Andrew Barclay said at least one car had been hit by the falling concrete, causing ‘major damage.’

“The initial report came from a caller who reported their car had been hit by concrete,” he said. “We asked them to stop so we could get a report, but they said they were on their way to the airport.”

At about 11:45 a.m., the CHP Marin Facebook page posted images of sizable chunks of concrete that had fallen from the upper deck as well as a cracked hole in the upper deck above.

Authorities shut down the westbound portion of the bridge that carries at least 80,000 vehicles a day at about 12:08 p.m. They were able to reopen a single lane in the eastbound direction shortly after 3 p.m., but the reopening of those lanes only lasted about an hour.

Finally, on Thursday evening all lanes were reopened.

The closure created traffic chaos on the span and on both sides of the bay as drivers tried to get around the closure. The approaches to the Golden Gate Bridge and the Bay Bridge getting out of San Francisco were congested by early afternoon, while eastbound traffic on the San Mateo Bridge was slow going across the entire span.

The traffic began back up from Highway 37 onto 101 north before 3 p.m. as hundreds of cars tried to get around the Richmond-San Rafael Bridge closure.

In San Francisco, traffic downtown had slowed to a crawl due to the heavy traffic headed to the Bay Bridge.

The SFMTA shut down the California Street cable car line due to heavy traffic conditions and later stopped service on all cable car lines for the evening.

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