Steel in the News
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Posted by Tasha Weiss on April 7, 2012 at 1:51 PM.
Steel tub girders easily handled the long spans on the interchange at Interstate 30 near Dallas. Completed tub girder sections were assembled in the fabrication shop of Hirschfeld Industries, San Angelo, Texas (NSBA/AISC member and AISC Certified Fabricator), with internal diaphragms and top lateral bracing installed. Photo: Austin Bridge & Road
With the recent completion of the Eastern Extension of the President George Bush Turnpike (PGBT), the North Texas Tollway Authority (NTTA) has completed a vital link and increased mobility between outlying cities to the west and north of Dallas.
Critical to accessing this new extension is the interchange at Interstate 30, constructed by the Texas Department of Transportation (TxDOT). The interchange consists of four direct connector ramps linking the PGBT main lanes to the reconstructed I-30 main lanes. The direct connectors consist of a single-lane ramp supported primarily on pre-stressed concrete U-beams with spans varying from 80 ft to 100 ft. As the bridges cross the I-30 frontage roads and main lanes, steel superstructures are required to accommodate the longer spans and the tighter horizontal curvature they involve.
Trapezoidal steel box girders (or tub girders), with spans up to 255 ft, were chosen to match the appearance of the concrete U-beams and for their desirable aesthetics. The project has eight trapezoidal steel box girder (or tub girder) units; each unit has a unique span arrangement and horizontal curvature with a minimum radius of 890 ft.
Tub girders on horizontal curves behave fundamentally differently than curved I-girders. I-girders have very little torsional stiffness and require cross frames between the girders to redistribute torsion in the system into shears between the girders in the cross section. Tub girders, on the other hand, have significant torsional stiffness and typically are stable without the need for additional cross frames or diaphragms between the girders.
Assembling the girder sections in the fabrication shop created a very stable section that was much easier to handle than a traditional I-girder. In the field, the additional torsional stiffness of the tub girders allows them to be erected without the need for external diaphragms, which greatly reduced the time needed for erection.
You can read more about tub girders and the new Eastern Extension of the PGBT in the April issue of MSC, available now. The article provides a preview of some of what the author, Greg Kochersperger, P.E., professional associate with HDR Engineering, Dallas, will present in Session B13 at the World Steel Bridge Symposium, April 18-20 in Dallas. The final program for the 2012 WSBS and NASCC: The Steel Conference is now available at www.aisc.org/nascc.