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Steel
Tuesday | 25 March, 2014 | 1:40 pm

Fixing the mix

Written by By Nick Wright

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Tampa Steel Erecting fabricates steel for I-30 bridge in downtown Dallas

March 2014 - When you think of Dallas, you don’t think of bridges. It’s usually New York, San Francisco or Pittsburgh, whose water-bound downtowns are bracketed by spans of diverse design.

Although Dallas sprawls across the north Texas prairie, a twin-arch bridge over the Trinity River, which runs through downtown, will soon become a signature of the city’s skyline.

The Margaret McDermott Bridge, named for a local philanthropist, will carry Interstate 30 into the impressive snarl of freeways that converge on the city’s west side, known as the “Dallas Mixmaster.” From two steel arches, the bridge will be suspended with steel cables and stand at 293 feet. It’s designed by Spanish architect Santiago Calatrava, famous for his modern works that use metal, glass and concrete. 

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Fabricating the steel superstructure is Tampa Steel Erecting Co., Tampa, Fla. It’s also fabricating the steel platforms for pedestrian and bicycle traffic, which will be separated from vehicles. The paths are 33 feet wide and along with the vehicle traffic span, are 1,125 feet long.

It’s been all hands on deck at Tampa Steel, whose 80 shop employees have been working exclusively on this project. The shop is no stranger to high-profile projects. Under president Bob Clark, Tampa Steel has done structural steel work frames at Disney’s Epcot Center and parts of Tampa International Airport.

For each arch, 39 octagonal-shaped sections of A709-50 fracture-critical steel are fabricated from 1/2-inch and 3/4-inch-thick plate. The grade has a yield point strength of 50,000 pounds per square inch. The platform is made of a similar grade, with two fabricated longitudinal girders and 105 transverse floor beams made of wide flange W24x104 and W24x146 beams.

The first sections of the bridge are expected to be delivered late this summer, according to Jeff Ames, Tampa Steel’s vice president for operations. The primary steel supplier is Infra-Metals, an arm of Reliance Steel & Aluminum Co., with locations along the East Coast.

The Margaret McDermott Bridge is just one part of the Dallas Horseshoe Project, a nod to the pattern created by the intersection of Interstate 30 and Interstate 35. According to Ana Bak, spokeswoman for the project, it’s an $798-million undertaking to alleviate traffic, as the area is among the 17 most congested roadways in Texas.

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Fabricating feats

As with any infrastructure project, there are challenges. The dimensional tolerance specs are tight. For example, at 48 locations across the bridge’s length, the suspension cables are connected to the arch with pins. Those pins must be within 1/4 inch in any lateral direction, says Ames. Because the cables will stretch 300 feet at the tallest point, the attachment point needs to be equally accurate.

To hit those targets, Tampa Steel loads its CNC files into its Peddinghaus FDB-2500 plate processor, which cuts, drills and scribes within tight tolerances. Skilled fitters and welders take it from there, assembling components. John Clark, vice president at Tampa Steel, then supervises the fabrication and final assembly, surveying each piece into place; and monitors cutting, bolting and finishing the full penetration welds.

Welding heat distortion is also under the microscope. The arch’s welds are full-penetration welds on relatively thin plate. To mitigate warpage, Tampa Steel minimizes the heat input by using flux core arc welding instead of submerged arc. Also, the fabricators are back-stepping welds, varying the direction of welding on longitudinal joints to limit the effects on section camber and balancing distortion with opposite and parallel welding techniques.

With a bridge like this, appearances matter. The final finish on the bridge’s steel is a high-gloss, brilliant white polysiloxane coating, says Ames.

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“While valued for its appearance and extended gloss retention, the polysiloxane is particularly challenging in its application, to achieve a flawless visual appearance and the high gloss finish tends to highlight the slightest irregularity in the surface of the underlying steel,” he explains. Working with Texas Department of Transportation, Tampa Steel tested four manufacturers’ paints and made a selection based on criteria that includes  gloss and efficiency of application.

Tampa Steel will assemble the arches for a trial fit before disassembling and trucking them to Dallas.

“We will have the entire bridge fabricated and stored at our shop prior to beginning shipment for erection,” Ames says, at which point American Bridge Co., based near Pittsburgh, will erect the bridge.

It’s expected to be complete in mid-2017. MM

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