ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.
Issue link: http://www.aspiremagazinebyengineers.com/i/297033
Dallas Area Rapid Transit's (DART's) Irving 1 and Irving 2 segments of the 14-mile Orange Line will extend from Bachman Station in northwest Dallas (on the Green Line) to Belt Line Road at the southern portion of Dallas- Fort Worth International Airport. This will complete 90 miles of DART's rail network by 2014. The 9.3-mile segment includes six stations and eight bridges; one of them is a 7000-ft-long structure over the Trinity River. T h e p r o j e c t h a d i t s c h a l l e n g e s , specifically the design and construction of the Trinity River levee crossing, a three-span, post-tensioned, spliced, precast concrete girder structure. The alignment is vertically constrained by adjacent overhead power lines and a U.S. Army Corps of Engineers (USACE) levee below. These limitations required a s t r u c t u re c a p a b l e o f s p a n n i n g 2 6 0 f t w i t h o u t a n y t e m p o r a r y s u p p o r t s o r p l a c e m e n t o f h e a v y equipment on the levee. Through an integrated approach, the design-build team developed a constructable and economical solution. Alternatives Considered The completed dual-track structure is a three-span, continuous unit with span lengths of 145, 260, and 145 ft. Each of the six girder lines comprises five girder segments. Girder segments B and D are balanced over the central piers and are stabilized with a temporary support tower beneath the end spans, as shown in the girder layout. The remaining girder segments—A, C, and E—are supported using overhead steel, strong-back beams. The construction sequence, developed in concert with the contractor, during design, ensured that the levee remained undisturbed during construction. Critical Design Aspects Because of the tight project schedule and the need to begin construction activities as soon as possible, it was critical for the design of the foundations to be completed before the superstructure design. The foundation for all piers consisted of a single, 108-in.-diameter drilled shaft below each column. This foundation design was rapidly constructed and cost effective. The foundations and substructure were designed, approved by the owner and the USACE, and then constructed well in advance of the girder erection. Although precast concrete girders are used extensively in Texas, the deepest of the Texas Department of Transportation standard girders was 70 in., which would not be sufficient for the selected structural configuration. The 70-in.-deep standard girder was modified by increasing the web depth by 12 in. and the web thickness from 7 to 8 in. This newly created girder is designated the TX82. The increased web thickness was used to help with shear capacity and, more importantly, to accommodate the longitudinal post-tensioning tendons located within the web. This girder section is used for segments A, C, and E. The middle segment of the levy span is 160 ft long, and at the time of construction, was the longest precast concrete girder ever erected in Texas. For girder segments B and D, located over the main piers, the standard girders were once more modified by utilizing a variable depth that increased the maximum depth by an additional 4 ft, resulting in a 10 ft 10 in. total girder section depth at the pier. The transportation, construction, and final configuration of the segments posed a challenge for the prestressing design, requiring a combination of pretensioning and post-tensioning, as well as temporary prestressing. A number of different support methods w e re u s e d d u r i n g t r a n s p o r t a t i o n and erection, requiring a delicate balance of stresses for each of the c o n f i g u r a t i o n s . S t a n d a rd ( 7 - w i re ) precast concrete girder pretensioning profile DART TRINITy RIVER BRIDGE / DALLAS, TExAS BRIDGE DESIGN ENGINEER: Parsons, Denver, Colo. PRIME CONTRACTOR: Kiewit, Stacy and Witbeck, Reyes, and Parsons, a Joint Venture (KSWRP); Dallas, Tex. CAST-IN-PLACE CONCRETE SUPPLIER: Lone Star Ready Mix, Leander, Tex. (batch plant set up on site) PRECASTER: Bexar Concrete, San Antonio, Tex. POST-TENSIONING CONTRACTOR: DYWIDAG-Systems International USA Inc. Bolingbrook, Ill. Trinity River Bridge girder layout and erection sequence showing temporary supports. All drawings and photos: Kiewit, Stacy and Witbeck, Reyes, and Parsons, a Joint Venture. SEGMENT A SEGMENT B SEGMENT C SEGMENT D SEGMENT E E F F E C L LEVEE C L PIER 61 C L PIER 62 C L PIER 59 C L PIER 60 PHASE 3 DART Trinity River Bridge A Design-Build, Precast Concrete, Spliced-Girder Bridge Solution by Thomas W. Stelmack, Parsons, and Jonathan Kempfer, Kiewit Infrastructure South Co. 14 | ASPIRE , Fall 2012 P R O J E C T AspireBook_Fall12.indb 14 9/18/12 8:58 AM