But if everything goes according to plan, officials expect that construction crews working around the clock will be able to install four support towers around a severely cracked pillar holding up I-95 in time to reopen the heavily traveled section of highway tonight.

Last night, the crews were waiting for tower sections that were being rushed from Baltimore, Pittsburgh, Scranton and Coatesville, and the contractor was hopeful that all the pieces would arrive by 3 or 4 a.m. today.

Until the work is done, motorists are being advised to seek alternate routes or take mass transit, with SEPTA adding cars to the R7 line to handle more riders.

PennDot warned those who normally use I-95 to get into Center City to figure on an extra hour for their commute.

Yesterday, cars detouring around the closed two-mile section of I-95 between the Betsy Ross Bridge and Girard Avenue jammed secondary streets such as State Street, Richmond Street and Aramingo Avenue.

"It's a mess," said Debbie Blase, a carpenter, as she gassed up her Toyota Paseo at a Sunoco station on Aramingo. "I don't even know what streets I'm taking."

Truckers were being detoured at either I-676 or Woodhaven Road for a circular route involving the Schuylkill Expressway and Roosevelt Boulevard.

But many truckers and other motorists not making stops in the city simply headed to New Jersey, where state police reported volume far higher than usual on I-295.

As bad as it was, the blow may have been lessened because Philadelphia public schools are on spring break, keeping buses off side streets. And many commuters have taken the week off because their children's schools are closed.

This is the second time in 12 years that stretch of I-95 has been closed for an extended period. A tire fire under the elevated highway a mile north of the damaged pillar in Port Richmond severed the artery for a week in March 1996. The road was built in the 1960s and handles about 180,000 vehicles daily.

Peter Kim, an engineering consultant for the state Department of Transportation, set in motion the chain of events leading to the latest shutdown on Monday when he inspected a center support pillar near Cambria Street late in the afternoon.

There, he found that a crack that had been about a half-inch wide and four feet long in 2005, and two inches wide in October, had become an 8-foot-long gash ranging in width to 2.5 inches to 5.5 inches and exposing rusted steel rebar, officials said.

PennDot Commissioner Allen Biehler said that the pillar had been earmarked for repairs this summer after the October inspection but that "rapid erosion" opened it more. PennDot engineer Harold Windisch said rusting metal, which expands, created an "abrupt reaction."

Two hours after Kim's discovery, PennDot's regional executive, Lester Toaso, decided to close the highway after conferring with department engineers and officials in Harrisburg.

"We were looking at the safety factor," said Toaso, who stressed that the pillar was not in danger of imminent collapse.

During the next five hours, PennDot officials notified states and transportation agencies along the I-95 corridor of the planned shutdown, and worked with city and state police and the Fire Department on closing the highway at midnight.

The repair plan calls for installing four steel towers to relieve the damaged pillar, which then can be repaired or replaced, Biehler said.

The highway can reopen after the towers - which can carry a combined weight of two million pounds - are installed, he said.

The emergency project is expected to cost more than $250,000, officials said.

Gov. Rendell joined Biehler and Mayor Nutter at the work site under the highway yesterday afternoon and used the highway closure as an opportunity to press support for his $1.2 billion borrowing plan for the state's aging bridges, which include the elevated sections of I-95.

He said the shutdown also highlighted a need for a "massive, stepped-up federal commitment" to support infrastructure repair and renovation.

It will cost about $6 billion to modernize I-95 in Southeastern Pennsylvania, and Rendell said neither the city nor state could handle the tab.

"This corridor is crucial to the economic well-being and quality of life in the city of Philadelphia," he said.

The biggest economic impact of the shutdown is expected to be felt by truckers, especially those making pickups and deliveries in Philadelphia.

At the Tioga Marine Terminal, just off the closed portion of I-95, Robert W. Palaima, president of Delaware River Stevedores Inc., was concerned about two ships scheduled to arrive today, one carrying perishable fruit from Chile.

"I'm worried about getting people and cargo in and out," Palaima said.

Peter A. Robinson, an executive with Jevic Transportation Inc. in Delanco, said gridlocked secondary roads were making pickups and deliveries slow and difficult.

The added costs include diesel fuel at $4 a gallon and New Jersey Turnpike tolls paid to avoid the jams, he said.

Robinson also is worried that the delays may result in some trucks' not getting back in time to pick up their scheduled loads.

In a bid to avoid the congestion, UPS said it would dispatch 60 trucks earlier than usual this morning.

Contributing to this report were staff writers Robert Moran, Sam Wood and Henry J. Holcomb.