How the I-95 bridge will be rebuilt

Like highway bridges the world over, the Interstate 95 bridge that collapsed in a fire June 11 was supported by a stalwart technology that predates the invention of cars: massive steel beams in the shape of a capital letter I.

This profile is perfectly suited to withstand repeated forces from trucks and cars rumbling overhead, said Andrew Bechtel, chairman of the civil engineering department at the College of New Jersey. Railroad rails have the same shape. So do the steel beams that support skyscrapers, dating back to the first steel-framed skyscrapers of the late 1800s.

“It’s elegant in its simplicity,” he said.

Yes, the bridge collapsed when a tanker truck struck a supporting concrete wall and caught fire. But that wasn’t the fault of the design, Bechtel said. Any bridge would eventually succumb in a fire of that magnitude.

So contractors, overseen by Philadelphia-based Buckley & Co., will rebuild using the same design as when the bridge was built less than a decade ago.

Sixteen I-beams will be laid across the supporting concrete walls on either side of the Cottman Avenue exit ramp. Then an 8½-inch-thick bridge deck, made from sturdy, steel-reinforced concrete, will be placed on top, said Din Abazi, chief bridge engineer for PennDot District 6.

It’s not yet clear when the whole job will be finished, as engineers are testing the integrity of the supporting concrete walls, called abutments, to see if they need repair or replacement, he said. That’s why PennDot commissioned the building of a temporary roadway in the meantime, to restore a portion of traffic flow sooner on the busy corridor.

But in the coming weeks, construction of the permanent bridge will begin, and that means I-beams. Each one will be 104 feet long, measuring 46 inches tall, according to PennDot specifications.

To understand why the shape works, think of a diving board, which is oriented in a flat direction because it’s springy that way. But imagine if the board were turned on its side, so it looked like a long, low section of wall. Not so springy anymore.

The 16 I-beams that will be used to rebuild the I-95 bridge rely on the same concept. They are, in effect, long, steel diving boards, turned on their sides for maximum stiffness.

Welded to the top and bottom of each beam is a stout metal flange — the “serifs” of the capital I.

That’s because the greatest amount of compression and tension occurs at the farthest point from each beam’s center axis, said Bechtel, the engineering professor at TCNJ, in Ewing, Mercer County. The extra steel plates at the top and bottom of the I allow the beam to resist stress where it is needed most.

Most motorists drive over these beams their whole lives without giving thought to their letter-shaped profile, he said. But with the disruption of one of the nation’s busiest transportation corridors, suddenly they are front-page news.

“It’s the most publicity that I think civil engineering has gotten in a very long time,” he said.