Falling chunks of concrete from the upper deck of the Richmond-San Rafael Bridge struck a car on the lower deck and forced a complete closure of the busy span on Thursday, causing gridlock as far away as San Francisco as commuters jammed alternate routes.

Officials shut down both decks at around 11:20 a.m., and by 8:20 p.m., all the lanes were reopened.

It was the latest jolt for an aging 5.5-mile cantilever span that periodically suffers gashes and holes.

A protective layer of concrete that covers the steel of the upper deck of the bridge broke Thursday morning, plunging to the lower deck, where it bounced and caused major damage to a white Mercedes. California Highway Patrol officers received reports of football-size chunks of falling debris shortly after 10:30 a.m.

When officers arrived, they found concrete chunks scattered across all lanes of the lower deck about a quarter of a mile from the Richmond side. Cars traveling west across the bridge on the upper deck shook loose more bits, some as small as pebbles, others as large as basketballs, which showered down.

Inspectors found that the pieces had dropped from the upper deck’s center lane. On Thursday night, they were placing a steel plate over the area. Caltrans plans to put an emergency contract out to bid this weekend for a permanent fix.

Transportation officials stressed that eroding concrete, caused by rain and heavy trucks, has not affected the structural integrity of the bridge. In other words, it will not collapse.

“These issues have been around on this bridge for a long time,” said Randy Rentschler, legislative director of the Metropolitan Transportation Commission, which plans and funds road and transit projects throughout the Bay Area.

He described loose, deteriorating material as a feature of the span, which opened in 1956.

“If a vehicle hits the side of the bridge — even a glancing blow — that will loosen it up,” Rentschler said. “Rattling trucks can loosen it up, too.”

The shutdown snarled traffic on the region’s other bridges and on the roads leading up to them. In the North Bay, Highway 37 — which stretches between Novato and Vallejo and served as a detour — was jammed in both directions.

In San Francisco, city streets carried heavier-than-usual traffic as commuters who were shut out of their Golden Gate Bridge-to-Richmond bridge route headed to the Bay Bridge. The San Francisco Municipal Transportation Agency temporarily stopped running the California Street cable car, which clanks from the Financial District to Van Ness Avenue, due to the heavy traffic.

Golden Gate Transit suspended service for the Route 40 and 40 X buses, which run across the Richmond-San Rafael Bridge between San Rafael and the El Cerrito del Norte BART Station.

At one point at around 3 p.m., officials reopened one lane in each direction of the Richmond bridge — but after about 45 minutes, more chunks of concrete began to fall and officials ordered a second full closure.

They reopened one lane in the eastbound direction — on the lower deck — at around 4:30 p.m., but that did little to relieve the traffic jams of the evening commute.

Barbara Townsend of Novato said she had been headed east toward Richmond on the bridge’s lower deck around 11:40 a.m. when traffic came to a halt in the middle of the span.

“Cars were still going on the upper deck, which was disconcerting because it shakes the bridge,” she said. “I had a vague sense of panic because I couldn’t see the end of the bridge or the beginning of the bridge. I was just sandwiched between all these cars.”

After a few minutes, Townsend said she got out of her car and knocked on a few windows to ask if anyone else knew what had caused the jam. She eventually found out about the falling concrete from Twitter. Townsend made it off the bridge after about an hour and a half and then headed back to Marin County — through San Francisco by way of the Bay Bridge, then the Golden Gate Bridge.

A 25-year resident of Marin County, Townsend said she’s aware of the bridge’s problems. “The bridge has been needing some love for quite some time,” she said.

Some people who got stuck on the bridge panicked, according to CHP reports. Drivers on the upper deck made three-point turns and drove the wrong way to get off the span, while others abandoned their vehicles and started walking. One caller to the CHP asked that an officer be sent out to “calm down drivers at mid-span.”

No injuries were reported, and only one car, the white Mercedes, sustained damage, CHP spokesman Andrew Barclay said.

Though the bridge has been patched and resurfaced many times, its structure is old and prone to ruptures, some large enough to offer views of the water below. A series of potholes in the mid-2000s prompted bridge shutdowns — and traffic tie-ups — while temporary repairs were made.

A deck rehabilitation project designed to prevent pavement deterioration was completed in 2006, but new holes have opened since, including in 2008. In September, the bridge was briefly closed for emergency roadwork, but the nature of the work was not disclosed.

Chronicle staff writers Dominic Fracassa, Kimberly Veklerov and Lauren Hernandez contributed to this report.

Ashley McBride, Rachel Swan and Michael Cabanatuan are San Francisco Chronicle staff writers. Email: ashley.mcbride@sfchronicle.com, rswan@sfchronicle.com, mcabanatuan@sfchronicle.com Twitter: @ashleynmcb @ctuan @rachelswan