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Roads & Bridges November 2012

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Roads & Bridges November 2012 BRIDGE DESIGN By Jamie Farris, P.E., Dan Carlson, P.E., and Eric Nelson, P.E. Contributing Authors Proper frame work Waco, TxDOT come up with right solution for IH-35 ruce Jackson, a noted professor of inspirational, in many ways bridges define their surroundings. Needing to expand I-35 American culture, said, “Bridges (IH-35), representatives from the city of Waco B are perhaps the most invisible form wanted signature bridges with an “art deco” look to complement other recent downtown of public architecture; they become frames economic revitalization projects. Because for looking at the world around us.” the frontage road bridges were going to be constructed first—the mainline bridges will be Taking that philosophy out of academia and replaced later—TxDOT and Waco representa- putting it into practice, the Waco District of the tives decided to create signature twin frontage Texas Department of Transportation (TxDOT) road bridges on each side of the mainline has initiated a unique bridge project spanning bridges. Though aesthetics was certainly a the Brazos River. Though only two frontage driving force behind the new bridge design, the road bridges are involved, the $17.3 million initial defining characteristic of the project was IH-35 Extradosed Bridge Project will pioneer a functional need. bridge technique in the U.S. As part of the American infrastructure, Brazos bravado bridges often become treasured landmarks IH-35 runs north-south through the heart in their communities. Majestic, grand and of Texas. A major transportation artery for the 42 November 2012 • ROADS&BRIDGES Debate rages over the boundary some 60 ft from the corresponding between extradosed and traditional mainline bridges. The roadway for each cable-stayed bridges, but extradosed frontage road bridge will carry three traf- bridges have a distinguishing feature— fic lanes with shoulders, as well as a 10-ft lower tower height (than traditional 6-in.-wide sidewalk for pedestrians and cable-stayed bridges) in proportion to cyclists. In addition, scenic overlooks the main span. Typically, cable-stayed providing unobstructed river views were bridges have tower heights around incorporated into the pylons to enhance one-fourth to one-fifth of the main-span the bridge experience for pedestrians. length. Extradosed bridges, however, Each of the new twin structures will have shorter tower heights equal to be a three-span bridge with a 250-ft cen- approximately one-10th of the main- ter span and 185-ft side spans. Matching span length. The shorter tower height the span configuration of the proposed results in shallower cable angles that new mainline bridges, this configuration in turn lead to an increase in the axial will align the piers within the river for compression in the superstructure and all the bridges in their final condition. a decrease in the vertical stay forces The resulting side span–to–main-span that act as supports in a conventional ratio is 0.74, common for ordinary cable-stayed bridge. So, cables on an girder bridges with the side-span length extradosed bridge serve a prestress- generally varying from 0.6 to 0.8 of the ing function. But that is not the only main span. For cable-stayed bridges, this notable feature of an extradosed bridge. ratio is typically between 0.35 and 0.50. Due to the additional support of the For extradosed bridges, the recom- cables, an extradosed bridge may have a mended side span–to–main-span ratio shallower superstructure depth relative is essentially the same as for ordinary to a traditional girder bridge. The span- girder bridges, but tends to average on to-depth ratio of extradosed bridges is the lower end—around 0.6—for many typically on the order of 35-to-1, versus existing extradosed bridges. However, approximately 20-to-1 to 25-to-1 for this ratio often varies for extradosed girder bridges. However, an extradosed bridges, depending on site conditions bridge still acts as a girder bridge, so rather than structural optimization. This state, the highway carries thousands the superstructure depth is greater than is exactly the case for the IH-35 bridges, of business travelers and local com- a conventional cable-stayed bridge, where the span layout has been set to muters on a daily basis. That demand with a typical span-to-depth ratio of match the proposed mainline bridges. has caused significant congestion approximately 100-to-1. In addition, due Each new bridge’s superstructure will along some areas of the route. Given to an extradosed bridge’s relatively stiff consist of a 10.5-in. cast-in-place concrete Texas’ ever-increasing population, this superstructure, which resists a majority deck with 6-ft 6-in.-deep steel trapezoidal congestion is only expected to worsen. of live-load forces (rather than having box edge girders and 3-ft 6-in.-deep To remedy this, the Waco District of the stays carry the load), these bridges steel-plate I-girder floor beams. Trans- TxDOT has committed to expanding also are often characterized by low verse floor-beam spacing will vary from a 94-mile stretch of IH-35 from the live-load stress ranges in the stay cables. 13 ft 3 in. in the end region of the side Williamson County line (north of Taking this all into account, what does spans to 15 ft in the regions near the Austin) to the city of Hillsboro (south this mean for the IH-35 bridges? pylons. The trapezoidal box edge girders 3 of Dallas). With its ongoing $1.9 billion Spanning the Brazos River and paral- are composed of ⁄4-in. web plates, with expansion program, TxDOT looks to lel to the existing mainline interstate 2-ft-wide top flange plates and a 5-ft-wide create continuous frontage roads along bridges, the new IH-35 frontage road bottom flange plate. Top flange plate the mainline interstate in areas where it extradosed bridges will each be 620 ft thickness varies from a typical 1 in. up is needed most. In Waco, the mainline long. Traffic on each bridge will be one to 3 in. for the regions over the bearings bridges over the Brazos River provided way, with the new bridges placed to the at the pylons, and the bottom flange 1 the perfect profile. After discerning the outside of the mainline bridges. Though plate varies from a typical 1 ⁄4 in. to 3 in. need, though, project designers turned the existing mainline bridges are steel- over the pylons. The box edge girders are their thoughts once again to aesthetics. plate I-girders, they will be replaced with continuous for the entire length of the Fielding input from the design team new steel box-girder bridges as part of bridge, supported on single disc bear- of TxDOT staff, AECOM staff, Waco the IH-35 corridor improvements. In the ings at the abutments and pylons. The city officials and the Waco-area public, final configuration, both the north- transverse I-girder floor beams consist of 1 TxDOT selected an extradosed style for bound and southbound frontage road ⁄2-in. web plates with 1-ft 6-in.-wide by the set of two 620-ft-long bridges. bridges will be separated horizontally 1-in.-thick top and bottom flange plates. ROADSBRIDGES.com 43 The superstructure of the I-35 frontage road bridges will have a steel-girder composite cross section. As for the edge girders themselves, bridge. In addition, the project team load-carrying systems, cable support can once again this is where extradosed used a steel trapezoidal box section for be provided for only a portion of the features helped the project depart from the edge girders, rather than a steel-plate span. Consistent with the geometry of the norm. The superstructure for almost I-girder section more commonly used on many extradosed bridges, the first stay all extradosed bridges to date has been composite cable-stayed bridges. This was for the IH-35 frontage road bridges will either precast or cast-in-place concrete done to provide greater superstructure be offset from the pylon by approxi- box girders or cast-in-place concrete stiffness and less reliance on the cable mately 20% of the main span. From slab cross sections. However, with the stays, common on extradosed bridges. this first stay, the cable support points IH-35 frontage road bridges, the project The project team also wanted to visually are spaced 14 ft 9 in. along the edge team chose a steel-girder composite match the new mainline bridges (the girder, for a total of 59 ft. This results cross section. Though a concrete cross current bridges are scheduled to be in approximately 50% of the main section is more the norm for extradosed replaced with steel box-girder bridges). span being cable supported, which is bridges, steel girders with concrete decks For aesthetic reasons, the design team consistent with most existing extradosed have been considered. But so far, the chose to use five cable stays. The cable bridges that have cables distributed only known bridge to be constructed system consists of a single plane of five across approximately 60% of the span. with extradosed features and boasting cable stays at each pylon supporting the But the extradosed features created yet a steel-girder composite cross section is edge girders (a total of 20 stays for each another unusual circumstance. the Golden Ears Bridge in Vancouver, bridge). The cable stays will be anchored Due to the use of the relatively more Canada. So, the IH-35 bridges will be the at the deck level to the web of the steel flexible steel superstructure (supported first use of this combination in the U.S.
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