Caltrans officials don't know how long it will take to replace the stretch of Interstate 580 that was destroyed early Sunday, but they started working on it soon after the steel-bending blaze was extinguished.

Engineers were at the scene of the collapsed I-580 connector before dawn to assess the damage, said Caltrans Director Will Kempton.

By noon, Caltrans had hired a demolition company to begin removing the melted and twisted freeway connection.

The agency has also started to design a replacement structure for the collapsed, 250-yard section.

Although one Bay Area transit official estimated that the I-580 connector would be closed for at least a month, Kempton refused to estimate how long it might take to rebuild the freeway or to reopen the connection between westbound Interstate 80 and southbound Interstate 880. Engineers are still assessing damage to I-880 and to the sections of I-580 adjoining the collapsed segments, he said.

Kempton vowed to move quickly to repair the gap that has been opened in the Bay Area's freeway network.

"We're moving very rapidly on this,'' he said. "We want it to be as smooth as possible for motorists.''

Caltrans has invoked a special emergency process that allows it to hire contractors without the usual competitive bidding. It also allows the agency to bypass environmental studies and to offer bonuses if contractors complete the work quickly.

Randy Rentschler, spokesman for the Metropolitan Transportation Commission, the regional transportation planning agency, said the I-580 connector would be shut for a least a month. The part of the freeway that connects westbound I-80 to southbound I-880 will be closed "until Caltrans determines that it's safe," Rentschler said.

Many motorists, tired of the lengthy Bay Bridge east span project or the slow replacement of the Cypress structure after the 1989 Loma Prieta earthquake, regard Caltrans as bureaucratic and slow.

But Kempton said the agency has a history of acting quickly in emergencies, citing the repair of the broken eastern span of the Bay Bridge after the Loma Prieta earthquake, and rebuilding collapsed sections of Interstate 10 in Los Angeles after the 1994 Northridge earthquake. Caltrans also used its emergency process to rebuild and reopen Highway 1 at Devil's Slide last spring.

"In past instances of similar circumstance, I think you have seen us act very expeditiously,'' he said.

Kempton said Caltrans has hired Cleveland Wrecking, the demolition company that has worked on the intensive weekend teardown jobs on the Bay Bridge's San Francisco approach. The contractor's employees visited the scene of the collapse late Sunday morning and were clearing the debris off I-880 and tearing down parts of I-580 starting Sunday evening.

Two segments of I-580 eastbound leading away from the Bay Bridge toll plaza were destroyed in the early morning fire when a tanker truck filled with gasoline crashed while traveling on southbound I-880 beneath the other highway.

That stretch of eastbound I-580 carries an average of 45,000 vehicles a day, while the south I-880 connection averages 35,000 a day.

The intense heat generated by 8,600 gallons of burning gasoline weakened the steel frame of the I-580 structure.

It bent, pulled away from support columns, began popping bolts and collapsed, Kempton said.

"Steel begins to melt at 2,750 degrees Fahrenheit,'' Kempton said.

John Osteraas, a civil and structural engineer for Exponent Failure Analysis Associates, a Menlo Park firm that explores the causes of structural collapses, likened what apparently happened on I-580 to "the principle of blacksmithing."

"The problem with structural steel,'' he said, "is that it loses strength and stiffness when it is heated. Think of a blacksmith with a red-hot horseshoe. It becomes very malleable.''

Steel loses half of its structural strength when heated to 1,000 degrees -- a temperature easily created by a burning gasoline tanker, Osteraas said.

The same phenomenon contributed to the collapse of the World Trade Center towers on Sept. 11, 2001, he said.

While the damage to I-580 is evident, it's tougher to tell the condition of I-880, said Bijan Sartipi, Caltrans director for the Bay Area.

Caltrans engineers are examining the area where the tanker burned, he said. Pavement on that section is not only charred but cracked, and there is some evidence of damage to the support columns.

Engineers took samples of the columns and roadbed to determine the extent of the damage, he said, but will wait until debris from the fire and collapse is removed to finish their analysis.

The goal is to open I-880 as soon as possible to reduce the impact on commuters and East Bay freeways, Sartipi said.

"The question is: What kind of load can it handle?'' Sartipi said. "Do we need to replace part of it or shore it up?''