Laying The Groundwork for Fast Bridge Construction
Prefabricated elements and systems accelerate construction of bridges to hours or days instead of months or years.
In February 2003, more than 200 bridge professionals viewed successful projects at the National Prefabricated Bridge Elements and Systems Conference cosponsored by the American Association of State Highway and Transportation Officials (AASHTO) and the Federal Highway Administration (FHWA). The presenters shared visions of bridges built in hours or days as opposed to months or years. Bridge engineers from 35 States and representatives from AASHTO and FHWA, professional associations, contractors, suppliers, and academia listened as the speakers described projects that met the need of State transportation agencies to “Get In, Get Out, and Stay Out.”
The Nation's bridges have a median age of 40 years, and today many structures need reconstruction. But increased traffic and urban congestion demand outside-the-box thinking to accelerate construction. In 2001 the AASHTO Technology Implementation Group, known as the TIG, chose prefabricated bridge elements and systems as one of the innovative technologies that promises the highest payoff. (Others include accelerated construction and intelligent traffic systems in work zones.) To encourage implementation of bridge prefabrication, the AASHTO group sponsors workshops, provides speakers for related conferences and other meetings, and publishes a Web site (www.aashtotig.org) that includes information on a number of prefabricated bridge projects that have been constructed to date.
In addition, FHWA, through its Innovative Bridge Research and Construction program and the Resource Center, champions prefabrication for accelerated construction. “Our vision is to get out in front of the bridge deterioration curve with accelerated construction such as prefabrication and stay there,” says Tom Saad, structural design engineer, FHWA Resource Center, Chicago. “FHWA bridge engineers will partner with States, industry, and academia to develop and implement technologies that produce more durable highway structures that can be constructed in a fraction of the time of conventional structures.”
The AASHTO group and FHWA are encouraging this technology because of the many advantages for bridge owners, engineers, builders, and the traveling public. First, use of prefabricated elements or systems minimizes traffic impacts. For example, contractors can perform time-consuming formwork assembly, concrete casting, and curing offsite in a controlled environment away from traffic. Prefabricated bridge designs are more constructible because the offsite work reduces time onsite dealing with constraints such as heavy traffic, extreme elevations, long stretches over water, and tight urban work zones.
Safety improves because prefabrication reduces the exposure time for workers and the public who travel through construction zones. Prefabricated elements and systems work well to accelerate both reconstruction and new construction. Prefabrication and shipment of components to the job site also reduce impacts on the environment. Finally, prefabricating takes elements and systems out of the critical path of the project schedule. The fabricator can take as much time as needed to produce a quality component or system in a controlled environment. Improved quality translates to lower life-cycle costs and longer life.
With traffic control running anywhere from 20 to 40 percent of construction costs and user delays priced at thousands of dollars per day in heavy traffic areas, States and owners will realize cost savings from accelerated bridge construction. Then as the technology becomes standard practice, costs will decrease.
The conference showcased a wide range of bridges of all sizes. Five outstanding prefabricated bridges presented here are Lake Ray Hubbard in Dallas, TX; James River in Richmond, VA; Baldorioty de Castro Avenue in San Juan, Puerto Rico; Mitchell Gulch in Castle Rock, CO; and Reedy Creek Bridge in Orlando, FL.
Lake Ray Hubbard, Dallas, TX
With Texas containing one-twelfth (approximately 49,000) of the Nation's bridges, the Texas Department of Transportation (TXDOT) has experimented with prefabricated elements for decades. The agency now is expanding its use of prefabricated elements to include entire systems. On the eastbound two-lane Lake Ray Hubbard Bridge, the contractor took one look at the power lines just 14 meters (45 feet) from the work zone and decided that the combination of a rocking barge and a crane's mast arms posed an unacceptable risk. Because the bridge's 43 pier caps had repeating elements, prefabrication could be cost effective.
TXDOT engineers designed the caps that extend between the supporting columns, and the contractor built a portable trestle to construct the 1,312-meter (4,300-foot)-long bridge. The defined work area from the trestle placed every component in the critical path so prefabricating the caps was crucial to maintaining the schedule, according to Tracey Friggle, TXDOT assistant director of construction for the Dallas District and project manager. “Without the prefabricated pier caps, the work would have taken us an extra year,” Friggle explains. “Instead of 8 to 9 days to form, tie, pour, and cure each cap, we took 1 day to set each one.”
To anchor the precast caps to the existing columns, crews used vertical grout sleeves cast into the caps and then pumped a high-strength grout into the sleeves to complete the connection.
James River, Richmond, VA
“Today engineering is the easy part, while traffic is a big consideration,” says Malcolm Kerley, Virginia DOT's chief engineer for program development. “The I-95 bridge over the James River carries 110,000 vehicles a day, so we wanted to open it to the public as soon as possible.”
After considering public input, VDOT closed the lanes from 7 p.m. to 6 a.m. Sundays through Thursdays for construction. The agency also requested A plus B bidding, with “A” being the unit price and “B” the number of days valued at $30,000 per day. VDOT did not consider any bids over 220 days. The winning contractor bid 179 days and ultimately finished in 140 days. For each day under 179, the contractor received a $30,000 bonus, and for each day over was to have been charged $30,000. Because daytime opening was critical, VDOT established a schedule of disincentives for time beyond 6 a.m. in restoring all traffic lanes. Fines ranged from $5,000 for failure to open at 6 a.m., an additional $10,000 if not open by 6:15 a.m., $35,000 if not open by 6:30 a.m., and so on to a cumulative disincentive of $250,000 for remaining closed until 6 p.m.
For most of the 101 spans, the contractor erected preconstructed composite units consisting of a 222-millimeter (8.75-inch) deck over steel plate girders. A nearby casting yard precast the units. Overnight, the work crews removed the old bridge span, prepared the gap for the new preconstructed composite unit, set the unit in place, sealed slab joints, and post-tensioned slabs transversely.
Baldorioty de Castro Avenue Bridges, San Juan, PR
This Puerto Rico project, which was highlighted at the conference, demonstrated how to deliver urban bridge projects in weeks instead of months or years using prefabrication methods. The contractor, with exacting sequencing, pieced together the two totally prefabricated overpasses in just two weekends.
To ease congestion on a San Juan road that carries more than 100,000 vehicles per day, the engineering contractor designed the prefabricated overpasses at two intersections for the San Juan Department of Transportation and Public Works. The construction contractor erected two 275-meter (900-foot)-long and two 214-meter (700-foot)-long totally prefabricated bridges in two stages.
On the first weekend, the crews drove piles, cast the footings in place, and then installed asphalt over their work. The next weekend the crews uncovered the footings and erected and post-tensioned the prefabricated substructure components. After the crews completed the first two substructures, they set the 30.5-meter (100-foot)-long superstructure span in place, complete with seven box beams, wearing surface, and parapets.
Two work crews erected the remaining spans simultaneously from the center span toward each end, post-tensioned each transversely, and then placed an asphalt overlay. To complete the process, the crews constructed retaining walls with select fill on the approaches. The first 275-meter (900-foot) overpass was ready for traffic in 36 hours, and the second overpass in just 21 hours.
“Commuters traveled at grade on Friday evening, and by Monday morning they were traveling over the new overpasses,” says John Dick, conference presenter and structures director for the Precast/Prestressed Concrete Institute in Chicago.
Mitchell Gulch, Castle Rock, CO
Plans by the Colorado Department of Transportation (CDOT) specified a cast-in-place box culvert to replace a 49-year-old deteriorated timber structure rated as one of Colorado's 10 worst bridges. But when the Denver-based contractor examined the long grade leading down to the Mitchell Gulch Bridge and the resulting dangerous detour, he decided that he could replace this 12-meter (40-foot)-long bridge in a weekend instead of a couple of months.
On a previous project with the same conditions, the driver of an 18-wheeler coming down the hill had lost brakes on 14 of the wheels, crashed through the barricades, and killed two employees. Two contractors approached CDOT with a value design/construction engineering proposal to replace this bridge in a weekend within the same cost parameters as a conventional project. Plus this plan would minimize inconvenience to the 12,000 daily commuters, who had no reasonable alternative route.
The prefabrication manufacturer from Littleton precast 90 percent of the new bridge, including substructure units such as wing walls and abutment walls, along with the more common precast deck units to enable rapid assembly. The contractor prepared steel piles to support the precast substructure units ahead of time. The contractor and engineering manager orchestrated every minute of the weekend with contingency plans such as backup equipment servicing, leaving little to chance. The contractor made field adjustments on several prefabricated elements.
At 7 p.m. on a Friday, the contractor rerouted traffic and began dismantling the old structure. By Saturday at 1 a.m., crews had placed abutments and wing walls, and welded them to the steel piles and to each other. When a fiber-optic line was encountered, the construction team adjusted the angle of the wing walls to accommodate the line. At the same time, crews rehabilitated the streambed with riprap. On Saturday afternoon, after placing the flowable fill behind the abutment walls, the team lowered, grouted, and post-tensioned the precast girders. Work stopped at 11 p.m. so the crew could rest and then resumed Sunday at 7 a.m. The crew completed the earthwork, backfilling, and asphalt paving on the bridge and approach, opening the structure by 5 p.m. in a record 37 hours of actual construction.
Reedy Creek Bridge, Orlando, FL
To mitigate the environmental impacts of heavy equipment on the Reedy Creek wetlands at the Animal Kingdom Entrance to Disney World, Walt Disney Imagineering committed to a top-down construction process using precast pile caps, prefabricated deck planks, and steel pipe piles. The Reedy Creek Bridge has two parallel 305-meter (1,000-foot)-long bridges widening from 13-meters (43 feet) for the first 73 meters (240 feet) to 16 meters (53 feet) wide for the remainder. Utilities cross Reedy Creek in a 4.3-meter (14-foot) gap between the two bridges.
The selected contractor won the project with a design that resulted in net savings of $950,000 on the $8.3 million project. The design met Florida Department of Transportation standards, maintained the bridge span and roadway deck configuration, reduced the number of support piles, and simplified the precast pile caps. In addition, the design used 2-meter (6-foot)-wide deck panels that are 381 millimeters (15 inches) thick at the center and 610 millimeters (24 inches) thick at the ends instead of 457-millimeter (18-inch) constant depth panels.
The contractor drove the steel piles and erected precast components from a traveling erection platform. The 104 pile caps are identical except for the length and number of conical holes (two or three) for integration with the steel pipe piles. Shear keys between panels and the reinforced concrete overlay are the only cast-in-place concrete. Traffic barriers were slip-formed.
What Needs to Change?
These accelerated prefabricated bridge projects illustrate a change in thinking from the traditional approach to a systems approach that considers traffic impacts during the planning stage.
“Contracting procedures need to change to provide incentives for contractors to build bridges rapidly,” Dick observes. “Whether it's A plus B bidding or other incentive/disincentive programs, the system has to change to encourage innovation.”
“To be cost-effective,” Rick Lawrence, president of Lawrence Construction Co. from the Colorado project, adds, “I need to build at least 10 bridges at a time, but the State awards projects one at a time.” By grouping single-span bridges, both the contractor and the State would realize volume savings in materials and labor. Another option is the design/build approach enabling the contractor to contribute practical ideas for accelerating construction.
Although the States have reaped the rewards of accelerated construction and superior quality with components produced in a controlled environment, some details of this technology can still be improved. Kerley cautions, “My concern is not with the quality of the components but with the quality of the connections.”
Saad agrees, “We still need research on developing the best practices for delivering this technology.”
Ian Friedland, FHWA bridge technology engineer adds, “Some standards and specifications need to be evaluated for accelerated construction.”
Looking to the future, Kerley says, “Through the AASHTO TIG we're establishing a network in which one State shares its success, and another State takes the idea and tweaks it for its project.”
With the variety of prefabricated systems available, bridges need not be “cookie-cutter” designs, but FHWA and the States are developing standardized designs with modular systems to replace the typical 50-year-old “bread-and-butter” bridges that need replacing nationwide. FHWA's Bridges of the Future initiative envisions a bridge with accelerated construction time, adaptability to widening and other demands, and lower life-cycle costs. Prefabricated bridge elements and systems are a step toward this vision.
Mary Lou Ralls, P.E., chairs the AASHTO TIG (Technology Implementation Group) on Prefabricated Bridge Elements and Systems. She serves as director of the Bridge Division at TXDOT where she started as a bridge design engineer in 1984. She earned her bachelor's in civil engineering in 1981 and her master's in structural engineering from the University of Texas at Austin in 1984. Since 1996 she has chaired and worked on various committees for AASHTO, the National Cooperative Highway Research Program, and the Transportation Research Board, and published numerous articles on bridge topics.
Benjamin M. Tang, P.E., serves as the senior structural engineer in the Office of Bridge Technology for FHWA and has spent most of his 27-year FHWA career in bridge engineering. He earned his bachelor's in civil engineering from the University of Maryland and his master's in structural engineering from the University of Illinois at Urbana. He serves on various task forces and committees in bridge design and construction engineering. He has published numerous articles on the technology of fiber-reinforced polymer composites for bridges.
For more information, States and professional organizations may request speakers on prefabricated bridge technology for workshops and conferences by contacting Mary Lou Ralls, email@example.com, or Benjamin Tang, firstname.lastname@example.org. Upcoming workshops, conferences, research results, and other opportunities for bridge professionals are announced on the AASHTO TIG Web site www.aashtotig.org, along with links to specific FHWA Web sites on prefabrication and accelerated construction. For information on A plus B bidding, visit www.fhwa.dot.gov/programadmin/ contracts or www.ic.usu.edu. For precast prefabricators, check the geographical list of prequalified Precast/Prestressed Concrete Institute members at www.pci.org.