It’s one of the world’s engineering marvels.

But that distinction may be cold comfort to drivers scrambling for alternate commute during the four-day Labor Day weekend closure of the San Francisco-Oakland Bay Bridge.

The closure to move a new bridge section into place means rerouting about 280,000 motorists on Friday, Sept. 4, and a slightly smaller number — down around 220,000 daily — on the Saturday, Sunday and Monday of the holiday weekend.

Narrowly averted was a double whammy. The temporary closure of the Bay Bridge could have been combined with a staff strike at Bay Area Rapid Transit, the subway system that carries commuters in an under-bay tube.

That prospect was averted with an Aug. 16 settlement between BART and the Amalgamated Transit Union.

“We were really hoping we wouldn’t have that situation to deal with,” said Bart Ney, a public information officer for Caltrans on the Bay Bridge earthquake retrofit project.

Ney said Sept. 4 will be the first regular commute day without Bay Bridge service since the 1989 aftermath of the Loma Prieta Earthquake, in which the temblor knocked loose an upper, eastbound bridge section onto the lower, westbound lanes.

The studies emanating from that seismic failure led to this massive engineering project to retrofit the 8.4-mile bridge, due to be completed in 2013 at a cost of $6-plus billion. It includes a complete replacement of the eastern part (from Oakland to Yerba Buena Island) and a full seismic retrofitting of the western part (from the tunnel on Yerba Buena Island to San Francisco).

For a video tour, click here.

Area transit agencies, including cross-bay ferry services, will watch the commute patterns on Sept. 4.

For the whole temporary shutdown, BART will extend hours to have trains running in the wee hours of the morning as a “lifeline service,” with 14 stations (a fraction of the total) open during those times, said Luna Salaver, public information officer for BART.

Scheduled trains will be planned for the maximum number of cars (six to 10). BART expects a 20 to 25 percent increase in ridership on Friday, Sept. 4, pushing the system close to its maximum capacity of 400,000, Salaver said.

During a similar shutdown on the 2007 Labor Day weekend, when Caltrans and its contractors installed a new portal for the Yerba Buena tunnel, BART’s increase was 46 percent on Saturday, 49 percent Sunday and 5 percent on Labor Day Monday.

Regular traffic will be barred from the west span as well, but the 108 bus line connecting Yerba Buena/Treasure Island with downtown San Francisco will still run.

During the Bay Bridge shutdown, extra parking control officers will be on duty in San Francisco’s South of Market area advising motorists on alternate routes and mass transit, said Kristen Holland, a spokesperson for the San Francisco Metropolitan Transportation Agency. Roadway electronic message boards and broadcast service announcements have been alerting the public to prepare for the closure and avoid cross-bay travel.

“There has been massive outreach campaign to get to as many users and BART commuters as possible,” said Ney. “They get the message, but of course you’re never sure that they’re going to hear the message.”

Caltrans is also coordinating with public safety, emergency agencies and city engineers for San Francisco and Oakland, Ney said, as well as the Golden Gate Bridge Authority.

Transit agencies have notified large employers in San Francisco, Oakland and Emeryville, encouraging them to have employees consider commute alternatives and telecommuting on Sept. 4, Ney said.

There are many superlatives in this project, including:

• the eastern span will be the world’s longest self-anchored suspension bridge.

• the eastern and western sections are connected by the world’s largest tunnel bore on Yerba Buena Island (76 feet wide, 58 feet high, carrying two decks of five traffic lanes each).

• it is Caltrans’ largest lift ever with a custom-built crane (the “Left Coast Lifter”) setting gargantuan pieces in place to connect the temporary east span with the portal to the Yerba Buena tunnel.

During the closure, crews will cut out a 300-foot-long, 3,200-ton, double deck bridge section of the bridge, replacing it with a prebuilt, 3,600-ton prebuilt piece connecting the tunnel portal to the skyway. The skyway is two temporary bridges (eastbound and westbound) that will detour traffic after the reopening of the Bay Bridge (the schedule is for the temporary closure to start at 8 p.m. Thursday, Sept. 3, and end at 5 a.m. Tuesday, Sept. 8).

While motorists whiz over the temporary bridge admiring the views, workers will dismantle the old, double-deck eastern span of the Bay Bridge (which is on the National Registry of Historic Places), saving key pieces for museums. Then they will build the new suspension span in the footprint of the old bridge.

The temporary arrangement with a huge pre-built section lends itself to efficient bridge reconstruction. It should be a model for future Caltrans bridge projects, said Ney — depending on ownership of adjacent land and sufficient lead time to plan a temporary transit interruption.

For instance, he said, with the right circumstances and today’s technology, such a plan might have hastened the reconstruction of an I-10 bridge damaged in the 1994 Northridge earthquake in Los Angeles, another massive project with a highway bearing a similar volume of traffic.

Motorists will need to tap the brakes. The Bay Bridge’s temporary detour, just north of the current eastern span, introduces a gentle S-curve just east of the Yerba Buena tunnel, which will reduce the speed limit there from 50 to 40 miles per hour. (At peak commute hours, the speed of traffic is more likely to be 10 mph.)

Technically, the bridge project is well celebrated. By boat and bus, conventioneers representing civil engineers, architects, urban planners and the Army Corps of Engineers have received up-close tours.

Caltrans has had great success with a requirement since 2002 that all project companies with engineering expertise staff an office on a common project campus on Oakland’s Pier 7 — Ney said,  “It’s a fantastic principle that has cut down on the time for resolving problems.” 

During the Labor Day temporary closure, there will be 50 or 60 hardhats working at any time around the clock in the area of the replaced section, Ney said, with a couple of hundred workers for the whole project. There will be other workers on standby, round the clock, in case of emergency.

“There’s a fair amount of engineering, lifting and sliding things into place, 150 feet in the air,” Ney said.

The temporary closure will also give transit authorities a chance to make improvements to make it easier for drivers to access FasTrak lanes in the Oakland approach’s toll booth plaza.

The new eastern span will add a path for cyclists and pedestrians to pedal and stroll back and forth from Oakland to Yerba Buena Island.

The original Bay Bridge construction cost $77 million, completed in 1936.

Lance Howland can be reached at