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Marketing Plan for Prefabricated Bridge Elements and Systems (PBES)
Bridge projects are one of the most notorious culprits for traffic delay. In many cases, a bridge provides the only passage across a geographic barrier. Taking such facilities out of service for any length of time can mean alternatives that cause great delay and that take drivers many miles out of their way. Also, because of the limited construction work space on bridge projects, safety hazard levels for construction workers and motorists are much higher than those of roadway projects.
The Current Bridge Inventory
Today there are more than 595,000 bridge structures on the National Highway System. These include bridges made of steel, concrete, wood, aluminum, and masonry. Current thinking is that decks should be able to last at least 100 years, and while there still exist functioning bridges that were built more than 100 years ago, the average life of a bridge deck is between 20 and 25 years. For details on the number and types of bridges in each state, see the charts in Appendix A.
Product Line Analysis
As previously noted, there are all sorts of design approaches and technologies currently being used in bridge construction. These include the bridges built with steel girders, concrete girders, and even timber. There are cable-stayed bridges, bridges built on piers, and many other approaches. It’s important to note that PBES-type construction is not based on the materials used or the structural principles employed to keep the bridge in place. Rather, PBES bridges are merely bridges which can have all or part of their construction done offsite and then erected at the site in a much shortened time period from a bridge built in situ.
What Exactly is a Prefabricated Bridge System?
Prefabricated bridge systems include superstructure systems (composite units, truss spans), substructure systems (abutments, caps/columns, piers), and totally prefabricated bridges. Examples of prefabricated bridge elements include full-depth deck panels and substructure caps. Using prefabricated bridge elements and systems can facilitate meeting several key needs:
Traffic and environmental impacts are reduced; constructability and safety are improved because more of the work is moved away from the bridge site, minimizing the need for lane closures, detours, and use of narrowed lanes. Prefabrication of bridge elements and systems can be accomplished in a controlled environment without concern for job-site limitations, and that, in turn, can increase product quality and lower costs. Prefabricated bridge elements and systems especially tend to reduce costs where use of sophisticated techniques would be needed for cast-in-place techniques, such as in long water crossings or high-elevated structures, like multi-level interchanges.
As the December 2004 issue of the FHWA publication Focus notes,
For highway agencies, the use of pre-fabricated bridge elements and systems, ranging from substructures to entire bridges, is proving to be not only a best practice but good business. Prefabrication can also lower costs by eliminating the need to perform the construction in a restrictive sequence of operations. Instead, the work can be done ahead of time, reducing the risks posed by bad weather and other variables.1
A number of completed PBES projects, including contact names and some contract documents, can be found at www.fhwa.dot.gov/bridge/prefab/projects.htm. In addition, an Accelerated Bridge Construction List, which includes several examples where PBES was used, can be found at www.fhwa.dot.gov/bridge/accelerated/abclist.htm.
How Does PBES Compare with Other Methods?
Prefabricated bridge elements and systems offer bridge designers and contractors significant advantages in terms of onsite construction time, safety, environmental impact, constructability, and cost.
Minimize Traffic Impacts of Bridge Construction Projects
Using prefabricated bridge elements and systems means that time-consuming formwork, concrete curing, and other tasks associated with fabrication can be done offsite in a controlled environment without affecting traffic.
Improve Construction Zone Safety
Because prefabrication moves so much of the preparation work for bridge construction offsite, the amount of time that workers are required to operate onsite, frequently in or near traffic or at high elevations or over water, is greatly diminished. Job site hazards and constraints such as nearby power lines are minimized when workers can complete most of their construction offsite.
Make Construction Less Disruptive for the Environment
Bringing prefabricated superstructures and substructures to the site ready for installation reduces disturbance to the land surface at the site, and it reduces the amount of time required onsite for heavy equipment. Keeping equipment out of sensitive environments is less disruptive for those environments.
Many job sites impose difficult constraints on the constructability of bridge designs—heavy traffic on an Interstate highway that runs under the bridge being constructed, difficult elevations, long stretches over water, or restricted work areas due to adjacent properties, to name a few. Using prefabricated bridge elements and systems relieves such constructability pressures.
Increase Quality and Lower Life Cycle Costs
Prefabricating bridge elements and systems takes them out of the critical path of the project schedule: work can be done ahead of time, using as much time as necessary, in a controlled environment. This reduces dependence on weather and increases quality control of the resulting bridge elements and systems. All projects that use prefabricated bridge elements and systems increase the quality of their components; most also lower life cycle costs.
Elements and Systems
Rapidly expanding technologies associated with innovative materials and equipment have made it possible to prefabricate the components of bridges—and sometimes even entire bridges. Increasingly, bridge engineers are turning to prefabrication of the following bridge elements and systems to save money, to solve project-specific challenges, and to increase the quality of bridges by conducting fabrication in a controlled environment.
Prefabrication offers exceptional advantages for deck construction, particularly for removing deck placement from the critical path of bridge construction schedules, for cost to place the deck, and for quality of the deck. Partial-depth prefabricated deck panels act as stay-in-place forms to speed construction and allow more controlled construction for a more durable deck than fully cast-in-place decks. Full-depth prefabricated bridge decks facilitate and speed construction, and bridge designers are finding innovative ways to connect full-depth panels to ensure durable connection details.
Superstructure: Total Superstructure Systems
Increasingly, innovative bridge designers and builders are finding ways to prefabricate entire superstructures. 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 also can be prefabricated. Prefabrication on this scale offers tremendous potential advantages in terms of constructability, onsite construction time, and the need to have equipment on the construction site.
Substructure: Bent Caps
Cast-in-place bent caps require sequential construction processes, including extensive formwork erection and removal, as well as concrete curing time. If they are fabricated offsite, these sequential processes are not a factor. As a result, bridge owners and contractors are turning to prefabricated bent caps:
Substructure: Pier Columns
Bridge construction times can be greatly reduced by using prefabricated columns. Columns can be steel or concrete (segmental, post-tensioned, either hollow or concrete-filled).
Substructure: Total Substructure Systems
A total substructure system may consist of individual pier(s) or prefabricated bent cap supported by prefabricated column(s) or prefabricated footings.
Totally Prefabricated Bridges
Totally prefabricated bridge systems offer maximum advantages for rapid construction and depend on a range of prefabricated bridge elements and systems that are transported to the work site and assembled in a rapid-construction process.
Prefabricated bridge elements and systems can be the most cost-effective solution in terms of both initial and life cycle costs. This cost competitiveness results from the speed of onsite construction and the improved quality that can be obtained with prefabrication.
Prefabricated components typically have lower unit costs relative to conventional cast-in-place construction due to economy of scale (e.g., fabricators’ fixed costs such as steel forms are spread over a large number of bridges). In addition, shortening the construction time at the bridge site by quickly installing larger prefabricated systems can further reduce construction costs, as listed below:
Lower Life Cycle Costs
Prefabricated bridge components are built offsite or near-site in controlled environments. Improved quality of materials and construction is achieved due to reduced weather impacts and established materials suppliers and standardized plant operations for consistent quality of materials and production. In addition, the off-the-critical-path construction allows adequate time for curing to obtain more durable concrete. Provided the connections between the prefabricated components are properly designed and constructed, prefabricated systems can be expected to provide extended service life with reduced maintenance requirements.
Delay-related User Costs
The reduced duration of onsite construction time that is possible with prefabricated bridge construction also will result in a reduction in delay-related user costs. Delay-related user costs are real costs to the traveling public in terms of hours of lost productivity and increased gasoline and maintenance costs for their vehicles as they wait in traffic queues and travel additional miles on detours. Increasingly, user costs are being considered in determining contracting strategies, and often they are the basis for the magnitude of incentive/disincentive on a project.
Examples of Cost-effective Prefabricated Accelerated Bridge Construction
A number of examples are available to show the construction cost savings that can be achieved using prefabricated bridge elements and systems on accelerated construction projects. Three such projects are described below. For each project, cost savings are defined as awarded bid price minus engineer’s estimate. These three projects saved a total of $23.2M in construction costs and significant onsite construction time.
Lewis and Clark Bridge Deck Replacement in Washington State
In 2004, a total of 18,000 vehicles per day crossed the mile-long Lewis and Clark Bridge on State Route 433 over the Columbia River between Washington and Oregon. The shortest detour route is 40 miles, necessitating the use of full-depth prefabricated panels and an innovative accelerated installation procedure to replace its deteriorated deck. Using SPMTs and a specially designed frame, 3,900 feet of deck were replaced during 124 night closures plus 3 weekend closures. Conventional cast-in-place deck construction was not a viable option, as it would have required 4 years and significant impact to traffic. The full-width prefabricated deck system combined with innovative construction equipment allowed the bridge deck to be replaced with no impact to rush-hour traffic.
The Washington State Department of Transportation (DOT) used A+B+C bidding, where "A" was the bid items for contractor payment. "B" and "C" were only used to determine the lowest responsible bidder, where "B" was the total number of bridge closures established by the bidder to complete the work multiplied by a closure rental cost and "C" was the total number of single lane closures established by the bidder to complete the work multiplied by a single-lane rental cost. An incentive was included for early completion. Incentives and disincentives were included for reduced and increased closures, respectively.
The low bid of $18.0M was 38 percent ($10.8M) below the engineer’s estimate of $28.8M. The contractor also received the $100,000 early-completion incentive and additional incentives for reduced closures, for a total incentive payment of $185,000. The Washington State DOT obtained the new deck ahead of schedule with no impact to rush-hour traffic.
I-95 Bridge over James River Superstructure Replacement in Virginia
In 1997, a total of 110,000 vehicles per day crossed the twin 4,185-ft-long Interstate 95 bridges over the James River in Richmond, Virginia. With such a large traffic volume, conventional construction was not an option for replacement of the deteriorated superstructures. In 2002, after soliciting the preferences of the public, the existing spans were removed and new prefabricated segments of half the roadway width were installed using high-capacity cranes and conventional flatbed trailers during night operations. The 102 spans were replaced in 137 nights during 17 months, with no impact to rush-hour traffic. Conventional construction would have required 24 to 36 months and significant impact to traffic.
The Virginia DOT used A+B bidding, where "A" was the bid items and "B" was the number of calendar days with nighttime lane closures, with bids greater than 240 days considered to be non-responsive. An incentive and disincentive of $30,000 per day was included for early completion not to exceed $2.0M and late completion with no dollar limit, respectively. An additional disincentive that accumulated up to $250,000 per day was included for not having all lanes of the bridge open to traffic on time.
The low bid of $43.4M was 11 percent ($5.1M) less than the engineer’s estimate of $48.5M. The contractor bid 179 days to replace the spans and completed the work in 137 night closures, receiving $30,000 for each of 42 nights, for a $1.3M incentive. The Virginia DOT obtained a new superstructure ahead of schedule with no impact to rush-hour traffic.
State Highway 66 Bridge over Lake Ray Hubbard Precast Bent Caps in Texas
Unlike the above two projects, in which the owner agencies required the use of prefabricated bridge components because of traffic needs, the twin State Highway 66 bridges over Lake Ray Hubbard northeast of Dallas was bid with conventional cast-in-place substructures. After award of the project, the contractor proposed a field change for precast reinforced concrete bent caps on the 4,360-ft-long, 40-ft-wide eastbound bridge to reduce the handling of formwork and materials over water and to minimize the construction workers’ exposure to high-voltage transmission lines that ran adjacent to the bridge. The Texas DOT approved the contractor’s proposal to prefabricate the caps with no change in funding. The contract did not include incentives or disincentives. Precasting the 43 identical caps saved 5 to 7 days per cap, for a total of 215 days. Conventional bent caps would have required 7 days of critical path activity per cap for forming, concrete placement, and curing, totaling an additional 9 months of construction time. Prefabricating the caps off the critical path also allowed the use of normal-strength high performance concrete with its greater durability but slower strength gain due to the 35 percent replacement of cement with ground-granulated blast-furnace slag.
The low bid of $40.9M was 15 percent ($7.3M) less than the engineer’s estimate of $48.2M. The Texas DOT obtained a more durable bridge ahead of schedule.1 “Prefabricated Bridges Deliver Quality, Safety, and Savings,” Focus, December 2004, published by the Federal Highway Administration, publication number FHWA-HRT-05-022.
This page last modified on 08/17/12