Accelerated bridge construction
Bridges are vital to the transportation system, but in practically every state, a number of them have received failing grades and need to be replaced.
One way that federal, state and local officials are addressing the problem is by knowing their ABCs — accelerated bridge construction. Accelerated bridge construction uses prefabricated components instead of cast-in-place to repair or rebuild the structures.
Prefabricated bridges have been used for some time for temporary bridges, but John Simkulet, P.E., bridge department manager at Greeman-Pederson Inc., New York, noted they are being used more often in a permanent capacity. His firm has some experience with prefabricated bridges, mostly with components like three-sided boxes or trusses for pedestrian bridges.
“They definitely have their place and their application,” he said.
History
According to the Federal Highway Administration’s report, “Pre-fabricated Steel Bridge Systems — Final Report,” available at www.fhwa.dot.gov, the first truly modular prefabricated steel bridge systems were developed in the 1930s for British military serving in remote areas. The main members are trusses composed of panels that are bolted together. The floor then spans between the truss members with a combination transverse floor beams and steel decking or grating. These systems are referred to as panel/floor beam/deck type bridges, and are probably the most widely recognized.
The second-most prominent prefab system was developed in the 1950s as a replacement for deteriorating wooden bridges. They used prefab structural steel plate girders or a fulllength truss member with a steel deck on top of the main member, and are known as deck/ girder bridges.
The modern-day prefabricated panel/ floor beam/deck type system was patented by A.M. Hamilton in 1935 and is known as the Callender-Hamilton System. That system was modified by Sir Donald Bailey, a British civil engineer, and was patented in 1943 as the Bailey Bridge. Similar to the Callender-Hamilton System, modifications were made so that long spans would be built in multiples of panel lengths. Load-carrying capacity was increased by utilizing double trusses in vertical and horizontal planes.
The Bailey M2 Military Bridge is still being used by the United States military and is also sold to state departments of transportation for temporary bridges.
The third generation system is the Acrow Bridge, patented in 1973 and updated in 1990. It is lighter than the original designs, with trusses that are 50 percent deeper, 50 percent stronger in bending and 20 percent stronger in shear. The current Acrow Bridge and Bailey Bridge systems can span up to 450 feet, offering width accommodations of three lanes of traffic. Sidewalks can also be cantilevered on either side of the bridge. The Acrow Bridge is used in both temporary and permanent structures.
Other bridge systems, all based on the original design, include the Maby Johnson Bridge — with modifications mainly to design — and the Janson Bridge, which addressed fatigue performance the earlier systems did not, since they were built to be temporary. The Quadricon Module Bridge System is widely used in Asia and expected to last 75 years, but whether it adheres to U.S. standards still needs to be investigated.
PBES
A prefabricated bridge element and system has several components. Depending on need, it can involve just certain elements of the superstructure or substructure, the entire system to build a new bridge from the foundation, all the structural elements and the process to put the structure in place — whether by self-propelled mobile transport system or conventional lifting equipment.
In cases where the structural elements of the trusses or girders are still sound, and there’s only deterioration to the deck panel, just one length of concrete deck can be replaced. In a case like this, the prefabricated panel can be installed in just hours, during the night, to greatly reduce the traffic impact.
Benefits
The biggest benefits to a PBES are the amount of time saved, the controlled environment where the components are made and increased safety. According to the ABC system, considerable time is saved at every level — from fast-track contracting using the construction manager/ general contractor method, to the foundation and embankment by using elements such as geosynthetic, reinforced soil and expanded polystyrene geofoam, which is lightweight and can be cut to whatever shape needed.
With this method, bridges can be built in a matter of days, not months. In the traditional method, concrete decks have to be cured for 30 days.
Prefabricated concrete decks and steel trusses and girders are also made offsite in controlled environments, allowing for more uniform components.
“You also don’t have to worry about the weather as a restriction,” Simkulet said. “They can work year-round.” In some cases, prefabricated elements are made nearby but not directly on the bridge.
Another benefit is that there’s flexibility to the design, so it can be easily modified for unforeseen site conditions. The Federal Highway Administration also stated that with PBES there is less impact to the environment, since there is less heavy equipment on site.
Using a PBES method means increased safety for workers and the general public. A PBES reduces the amount of time in the work zone, and using a self-propelled mobile transport system eliminates having workers up in the air. According to Simkulet, this significantly reduces work zone time and workers being exposed to traffic.
The bridge system is reportedly easier to maintain, because there are fewer parts. As for cost savings, the FHWA said they could be in the range of 25–60 percent depending on the standard of construction. One graph listed a price range of $30-$60 per square foot for precast concrete panel deck, and the cost to move a bridge span at $200,000-$900,000.
Having more than one project, to gain economy of scale, is recommended when initially moving to this method. As Simkulet pointed out, “What you don’t have to pay for labor in the field, you’re paying to the pre-caster — and for transporting to the site.”
The Tappan Zee Bridge, which crosses the Hudson River in New York, is currently under construction using the ABC method.
“Every span is being built and assembled in Albany, barged down the river and then set in place. So it’s moving very quickly.”
Disadvantages
Simkulet said the disadvantage he sees of building with a prefabricated system is the increased amount of plant inspections.
“Now we have to cover where everything is being assembled. We have to follow the different pieces and inspect in each facility as well as on site,” he said. Still, the controlled environment is definitely a positive.
In New York, the goal for the accelerated bridge program is to limit the amount of time the bridges are shut down. That goal is currently being met with PBES.
“The impact to society is the reason the industry is moving in this direction,” Simkulet noted.
Did You Know?
According to the Federal Highway Administration, there are 40 states with one or more prefabricated bridge element and system projects, seven states with 20 or more projects and 11 states actively pursuing this method as standard practice.
20+ PBES BRIDGES: New York, South Carolina, Utah, Texas, Maryland, Illinois and Alaska
16–20 PBES BRIDGES: Ohio
11–15 PBES BRIDGES: Minnesota and Oregon
6–10 PBES BRIDGES: New Jersey, North Carolina, Michigan, California and Colorado
1–5 PBES BRIDGES: Washington, Idaho, Wyoming, Arizona, New Mexico, Nebraska, Iowa, Wisconsin, Louisiana, Kentucky, Georgia, Florida, West Virginia, Virginia, Pennsylvania, Maine, Massachusetts, Rhode Island, Connecticut, Hawaii, Puerto Rico and Washington, D.C.
STATES WITH NO PBES: Nevada, Montana, North Dakota, South Dakota, Kansas, Oklahoma, Missouri, Arkansas, Delaware, Indiana, Mississippi and Alabama
Source: www.fhwa.dot.gov/everydaycounts
