By using precast bent caps, designers may also improve worker safety. For over-water bridges, precast bent caps reduce the amount of time workers need to operate over water. For bridges over existing roads, they minimize formwork required, reducing traffic disruptions on the lower roadway. For bridges with jobsite constraints, such as power lines that affect work zone safety, they limit the amount of time that workers are at risk.
Engineers have found that bridge construction times can be greatly reduced by using precast columns on cast-in-footings or drilled shafts. Columns can be segmental, post-tensioned, and either hollow or concrete-filled.
But a new method is gaining acceptance in many areas. Bridge designers and builders are finding ways to prefabricate entire segments of the superstructure. According to the FHWA, a total substructure system may consist of individual pier(s) or prefabricated bent cap(s) supported by prefabricated column(s).
Preconstructed composite units may include steel or concrete girders prefabricated with a composite deck, cast off the project site, and then lifted into place in one operation. Truss spans can also be prefabricated. This method offers tremendous potential advantages in constructibility, onsite construction time, and the need to have equipment on the construction site.
In many ways, UDOT is the nation's leader in adopting accelerated bridge construction (ABC). The interest was spurred by the need to rebuild I-15 in time for the 2002 Winter Olympics in Salt Lake City. The 17-mile corridor was completed ahead of schedule, in four and a half years using a design-build approach.
Before 2008, most UDOT ABC projects involved deck replacement on existing structures. Each project used many prefabrication details. But following a significant public outreach that included workshops and public meetings, UDOT began to standardize ABC details to provide a more consistent product and improve efficiency for the local design and construction industry. Officials used the decision model designed by FHWA but added additional criteria with which to determine whether a project could use ABC procedures.
Following this effort, UDOT selected full-depth precast concrete deck panels as one of its first two standard products for many upcoming rehabilitation projects. It then selected designs of self-propelled modular transporters (SPMTs) to quickly remove and replace bridges as the second ABC standard product because of the short construction season and the need to avoid impacting rush hour traffic during bridge replacement projects.
Transportation officials recognized that adopting ABC techniques had greater out-of-pocket concerns. The traditional approach to public projects is one in which the project is awarded to the contractor with the lowest construction bid, often without considering impacts other than completion delays.CONTROLLING COSTS
They developed a business model that accounts for the cost of additional time users must spend traversing construction work zones, detours, or sitting in traffic. This business model combines the construction cost curve with the user cost curve to obtain the total project cost curve and lowest societal cost. UDOT uses this new business model to evaluate the total costs of its projects, resulting in political capital and public praise.
UDOT planners have been reviewing construction costs, in addition to user costs for its ABC projects. When they began their ABC efforts, they incorporated a maximum 30% additional cost over conventional construction. The amount is an estimated percentage of additional per bridge cost statewide that the department felt would allow a breaking point for costs not known in the conceptual phase of the project. This percentage includes initial startup costs for developing the process.
They watched this premium diminish over time as contractors integrated ABC into standard practice. For example, the bid price for Utah precast deck panel projects in June 2007 was about $70 per square foot. By April 2008, the bid cost per square foot was reduced to about $55. Accordingly, contractors have been able to streamline the movement of these superstuctures. On a series of projects that include site preparation, transportation time, and skidding and setting the bridge, contractors reduced their total time to one-third of the original move.
UDOT officials expect to save more time and costs as the number of ABC projects increase. To help continue these savings, the department has developed a standard bid document for projects that use either Full-Depth Precast Concrete Deck Panels or SPMTs to move entire structures. The standards enable all contractors to better understand the details of these innovative projects.
Source: “Utah's Move to Accelerated Bridge Construction as Standard Practice” by James McMinimee and Rukhsana Lindsey, Utah DOT, Salt Lake City; and Mary Lou Ralls, Ralls Newman LLC, Austin, Texas.
To watch a video of crews moving the I-15 bridge in American Fork into place, visit www.udot.utah.gov/pioneer.