Clearfork Main Street Bridge
Precast concrete was the ideal material for an innovative bridge design that meets both the floodway requirements of the U.S. Corps of Engineers and the lifestyle needs of Fort Worth citizens. Architects Freese and Nichols chose precast for the $11 million-$13 million, four-lane, split bridge with suspended pedestrian bridge beneath.
The 550-foot vehicular bridge carries four lanes of traffic, with a center span of 220 feet. It is built using a system of precast concrete beam segments that are spliced together using post-tensioned steel strands to form a streamlined structure. Walkers, cyclists, wheelchair users, and others have a separate level of access underneath, with scenic overlook spots and ramps that connect with trails on either side of the Trinity River.
Precast girders offered designers a solution to both aesthetic and practical concerns, creating a sleek look for less than the cost of steel. Likewise, innovative formwork helped meet the precast producer’s needs for producing the unique bridge elements.
Texas Concrete Partners produced the precast haunch girders and Type 6 modified beams for the bridge in two locations. At its Victoria headquarters, the precaster produced specialty haunch beams, including a massive 96-foot-long, 10-foot-deep haunch girder that tapers to 6 feet at the ends.
Hamilton Form fabricated formwork for the girders. The haunch girder formwork included an arched soffit and 10-foot, 4-inch sideforms. The soffit was formed with a 200-foot radius on one side and a 185-foot radius on the other.
The producer did not want to tie up one of its main beds to cast the limited number of specialty beams, so Hamilton Form provided a solution. The abutment at the bed they used was generally used for smaller beams, and did not have the capacity to resist the required amount of prestress force (480 kips) 10 feet in the air. To accommodate this setup, the formwork supplier designed sideforms with stressing bars to resist the prestress force in their upper flanges.
“The ends of the beams were congested with steel,” says Jeff Sexton, production manager for Texas Concrete Partners. Post-tensioned haunch beams have three 3 1/2-inch ducts to accommodate steel strand that is added onsite, post-tensioned, and filled with grout.
In Waco, Texas Concrete Partners produced Type 6 modified beams, which are similar to a Type 4 beam at the bottom, but with the wider flange of a Texas Bulb-T beam at the top. They cast three beams at a time, every third day. Like the haunch girders, these beams required self-consolidating concrete, which slightly reduced the amount of manpower needed during each pour.
The producer began casting bridge elements began in August 2011, and finished in Spring 2012. TCP
Longer, Leaner Beams Give Precast an Advantage
Many state DOTs are collaborating with industry associations and experts to develop bridge beams that are more efficient for both bridge designers and precast producers. The widespread adoption of these new designs has given precast producers a competitive edge.
The Florida Department of Transportation worked in collaboration with the Florida Prestressed Concrete Association and Dr. Maher Tadros of the University of Nebraska to develop the Florida I-Beam (FIB). First produced by Standard Concrete Products in Tampa in 2009, the beams feature a constant top and bottom flange with a variable web to facilitate the use of fillers in casting various beam sizes.
The FIB shape accommodates more strand than the old Florida Bulb-T beam design. The increased strand allows for longer beams, which means bridges can be designed with fewer beams and spans.
New FIBs have a wider bottom flange and lower center of gravity than the AASHTO beams, making them more stable in shipping, handling, and placement. The FIBs also have a fully predesigned reinforcement option that significantly reduces fabrication time.
In 2009, the Texas DOT (TxDOT) also replaced AASHTO Type 4 I-beams a new Texas I-Girder. The new beam features a thinner vertical section, thinner flange, and more bulbous bottom than the AASHTO design. Although it uses less material, the I-Girder is stronger so it can be designed in longer lengths, and bridges can be built with fewer beams.
The Precast Concrete Manufacturers’ Association of Texas (PCMA) consulted on the I-Girder design, with suggestions to make it easier for precasters to pour. Although its adoption meant precasters had to purchase new forms, the PCMA reports they have ultimately received more orders with the I-Girder,even with fewer beams being used per bridge.