One of the little-recognized wonders of Milwaukee's $810-million Marquette Interchange project are the eight ramps and bridges formed from double-box girders that Governor Doyle recently called "the country's safest bridges." That was a strong statement by the state's top public official. But the structures he's describing are remarkably strong, and, well, just plain remarkable.
One of the little-recognized wonders of Milwaukee's $810-million Marquette Interchange project are the eight ramps and bridges formed from double-box girders that Governor Doyle recently called "the country's safest bridges."
That was a strong statement by the state's top public official. But the structures he's describing are remarkably strong, and, well, just plain remarkable.
Many of the Marquette Interchange's overpasses and bridges are constructed in the conventional way, using straight I-beams of pre-cast concrete or welded steel. That's fine for sections that run straight.
But where the ramps and bridges curve, Wisconsin's Department of Transportation (WisDOT) and the system's designers, CH2M Hill and HNTB, both of Milwaukee, Wis., decided to use double-box girders of welded steel.
Says Robert Sisto, P.E., WisDOT structural engineer for the Marquette project, "Steel double-box girders lend themselves to curves and banking, and they resist the variety of forces in a curved shape very well."
Viewed from the end, the double-box girders used on the Marquette project look like two V's sitting side by side with their points flattened, each forming a trapezoid. The two trapezoidal boxes are strengthened internally by bulkheads and cross-bracing, and are joined to each other by full-height steel plates welded at points along their lengths. Each of the boxes stands about 7 feet high and 7 feet wide. Overall, the girder looks something like hull of a huge catamaran sailboat.
But double-box girders are not uncommon in the bridge industry, says Sisto, and the basic structure of those used in the Marquette project is standard for a double-box-girder design.
So what makes the Marquette Interchange's box girders so remarkable?
Finn K. Hubbard, P.E., Wisconsin state bridge engineer, says a combination of factors makes the Marquette's eight double-box girders so strong that they would not collapse even if one of the boxes were cracked completely through from bottom to top and three fully loaded semis (about 240,000 pounds) were stacked atop each other right over the crack.
"Extremely detailed finite-element analysis by computer," says Hubbard, "has proven that these girders are extraordinarily strong. The computer model is so powerful that these are the first twin-steel-box-type structures the Federal Highway Administration (FHWA) has ever approved as structurally redundant bridges.
"They are also the first ones," said Hubbard, "the FHWA has rated as 'non-fracture critical.' That means they won't collapse even if one of the two boxes has cracked all the way through. If something were to happen to the structure, we would have time to clear it and close it down before a catastrophe happened."
What makes these girders so spectacularly strong?
According to Hubbard, it's a combination of the design, the high-performance steel and the concrete deck that sits over the top. "The steel," says Hubbard, "is unusual because it combines 50,000-psi to 70,000-psi strength with about 10 times as much resistance to cracking as normal steel. In short, it's strong without being brittle."
In addition, says Sisto, the concrete slab atop the steel frame is special. Although it uses the epoxy-coated rebar common in cold climates, the concrete is poured to a depth 3-1/2 inches over the top of the rebar. That's 1 inch more than in a typical deck slab. The extra depth adds strength and helps keep salt and moisture from reaching the rebar.
But the concrete's real secret lies in its curing. Within 15 minutes of being poured, the concrete is covered in two layers of burlap and kept wet for a full 10-day wet cure. The result is concrete that is dense, strong and crack-resistant.
"These bridges, ramps and overpasses are being built to last 75 years or more," says Sisto.
The longest double-box girder on the Milwaukee project sits across 250 feet of open space. But longer spans could be could be engineered to meet a specific project's requirements, such as crossing a wider river, says Sisto.
Barbara Mikolajczak, P.E., is the Marquette project's financial management supervisor and public information officer. She says, "Representatives from other DOTs throughout the country are visiting this project to study these girders — as well as the Marquette project's overall success — to see how they can apply the lessons to their work. We studied other projects when we were planning this one. Now others are learning from us. The process helps improve things across the country."
With that kind of knowledge sharing, the Marquette Interchange's eight double-box-girder structures should soon be known as the first of the country's new safest bridges.