Replacing the collapsed MacArthur Maze freeway ramp and replacing or repairing the ramp below could take several months or several weeks, but until the debris of Sunday's predawn gasoline tanker truck disaster is cleared, it's anybody's guess.

Only a few hours after the fire that buckled the steel holding up a section of the viaduct connecting eastbound Interstate 80 with eastbound Interstate 580, Caltrans had already picked a contractor for demolition as soon as accident investigators finished their work, Caltrans Director Will Kempton said.

"At this time, we have no estimate of the time it will take to complete that work," he said of the repair project, which he predicted would cost "tens of millions of dollars."

But he offered a ray of hope for commuters who regularly come off the Bay Bridge heading for the Oakland hills, Walnut Creek or other areas served by I-580 and state Highway 24.

Caltrans has done some rush repairs in the past, such as the 30-day repair of the Bay Bridge after the Loma Prieta earthquake in 1989, or the three-month reconstruction of multiple sections of the Santa Monica Freeway in the Los Angeles area after the 1994 Northridge earthquake.

Caltrans engineers, along with Kempton himself, were already onsite Sunday morning evaluating the extent of the damage and plotting a repair strategy.

Gov. Arnold Schwarzenegger has already signed an emergency order that would waive some of the state's contracting requirements to get the project started as quickly as possible.

In the next day or two, Federal Highway Administration inspectors will visit the site to determine the need for a federal emergency declaration, which will clear red tape for providing some federal funding and the necessary regulatory approvals for the reconstruction project.

"At the end of the day, I believe the public will be pleased with the work that's been done," Kempton told reporters at the Bay Bridge toll plaza Sunday afternoon.

The heat of the fire from the 3:40 a.m. gasoline tanker truck crash weakened steel girders supporting the viaduct section and melted bolts that held it onto the concrete pier keeping it aloft, Kempton said.

The viaduct section, estimated to be 80 feet long, slid off the pier and snapped cables intended to support it if an earthquake shifts the pier from underneath. The section of roadway that will need to be repaired, including the diagonally hanging deck, is about 250 yards, Kempton said.

The big unanswered question that could have a major effect on the speed of the repairs is what is hidden below the debris of the upper deck.

The lower deck, which connects westbound I-80 with southbound I-880, was where the tanker crashed, exploded into flames and super-heated the upper deck. Caltrans officials said the lower deck, too, might need to be replaced.v "Once we clear the debris off the lower ramp, we'll be able to do a better assessment on the damage to that structure and whether or not we can shore that structure up to allow traffic to use that as we complete permanent repairs," Kempton said. Initial evaluations Sunday showed that the lower ramp was "severely damaged and it will need some work," he added. Although it was too early Sunday for Caltrans officials to estimate the length of the project, Abolhassan Astaneh-Asl, a civil engineering professor at UC Berkeley, estimated that the job would cost as much as $20 million and could be done as quickly as the 1989 Bay Bridge earthquake patch-up -- if the lower deck is salvageable. "In one month, Caltrans should be able to put the steel girders in place and get this thing done," said Astaneh-Asl, who has studied the MacArthur Maze's collection of overpasses.

"The lower deck, because of the impact, might have a lot of damage," which, if it needs replacement, "might take two months," he said.

The key to doing the job quickly is building with steel girders, rather than concrete, similar to the old structure. Viaducts held up with reinforced concrete could take as long as 28 days to cure, and preliminary work would add two weeks to that.

"They need to modernize it and redesign it, but it can be done in a matter of days. They need to use new connections now because these are 1950s bridges," Astaneh-Asl said.

Another factor that would help speed the project is that Caltrans owns the right-of-way on all sides, easing access for construction equipment. In a few days, he predicted, demolition of the collapsed bridge decks should be completed.

"By next Sunday, steel is being fabricated, concrete foundations, if necessary, have been started and the piers are being constructed," if all goes well, Astaneh-Asl said. After that, the steel girders will be fabricated and in a few more weeks, motorists will be back in business.

Reach Erik N. Nelson of the Oakland Tribune at 510-208-6410 or enelson@angnewspapers.com.