Bridge Construction on the Fly?
Back in early July I posted a short commentary on the state of highways and bridges in the U.S. The American Society of Civil Engineer’s (ASCE) comprehensive 2013 Report Card for America’s Infrastructure offers an in-depth analysis as well. But wouldn’t you know? There may be hope. The classic entrepreneurial American spirit has hatched an idea that aims to tackle at least a part of the problem. Every Day Counts (EDC) is the Federal Highway Administration’s two-part initiative to spear-head a solution to the dilemma of our compromised U.S. infrastructure. This post is devoted to some of the technological innovations that are coming out of that effort.
Early US History: Open Spaces to Roam and Build
Most of our bridge and highway system was built up and completed in the 1950s. Construction took place on a landscape that was mostly undeveloped or lightly settled and made up of farm fields and open land.
The story is pretty different now of course. Where we once had open spaces, we now have dense urban development. And we’ve all experienced the mess and headache of summer highway projects. Highway and bridge reconstruction slows and redirects traffic, aggravates drivers, puts workers and travelers at risk, incurs costs and forces public officials to scramble for alternatives.
ABC (Accelerated Bridge Construction) Offers a Solution
ABC (accelerated bridge construction) is a set of technologies designed to tackle the problems of building and rebuilding bridges in the 21st century. Here is a key issue: Many of the bridges that are considered structurally unsound today are located in heavily traveled urban areas. Obviously these bridges are challenging to renovate for the reasons mentioned above. The difficulties presented by many urban sites makes ABC often the only practical alternative. Many states are adopting ABC methods in order to get these projects done.
ABC Construction is a set of tools and practices that not only speeds up the time it takes to complete a project, but improves the quality of the final built product as well. Often there are cost-savings as well (more detail on the topic can be found in the manual published by the Federal Highway Administration). Some of the components of ABC methodology include:
- Geosynthetic Reinforced Soil-Integrated Bridge System (GRS-IBS) is a methodology that combines closely spaced geosynthetic reinforcement and granular soils into a new building material. This composite material can be used to build abutments and embankments that are less prone to settle and create irregularities and raised areas at the end points of the bridge. Use of this technology is 25 to 60 percent more cost effective than standard methods.
- Prefabricated Bridge Elements and Systems (PBES) involves building the main components of a bridge offsite or parallel to the site in designated staging areas.
- Slide in Bridge Construction (SIBC) is the technique of using PBES, or pre-built structures, to efficiently replace a standing bridge. This methodology involves building the new bridge adjacent to the existing bridge on temporary supports. Once the new bridge is built and tied into the approaches as well as paved, the existing bridge is demolished, all within 24 to 72 hours.
Advantages of ABC Methods
There are several advantages to the use of ABC methods:
- Less time is spent at the construction site which makes for a safer environment for workers and motorists.
- Less time is spent rerouting traffic and making other accommodations for on-site construction, all of which reduces project costs.
- Bridge components that are built off site in temperature controlled environments are of higher quality, resulting in a bridge with a longer service life. Major components of bridges, from deck panels to piers, abutments and wall elements, as well as entire bridge “systems” (the substructure or superstructure) can be built off-site.
- New construction projects as well as reconstruction projects can both use ABC techniques.
- Use of ABC technologies results in less construction time spent in environmentally sensitive areas.
Limitations on Use of ABC
Implementation of ABC technologies at different sites requires some out-of-the-box thinking and solid planning. Through a process called Accelerated Construction Technology Transfer (ACTT) agencies are given resources to evaluate the potential to apply ABC at specific sites. Some of the site constraints that are frequently considered include:
- Whether a water crossing will allow the delivery of prefabricated elements. Is the waterway navigable and does it have sufficient depth to accommodate a barge hauling bridge components?
- Is there a staging area large enough in the vicinity of the proposed bridge reconstruction to allow for the fabrication of the superstructure or other bridge system?
- Are there any geotechnical constraints? Cranes moving prefabricated elements can produce large concentrated loads in small areas. Poor soils can compromise safety in these situations.
Potential for Long-Lasting Solutions
ABC technologies represent one small, but effective remedy to a national crisis. The Transportation Research Board is undertaking a number of other research projects to figure out how to make highway reconstruction more efficient and durable through the Strategic Highway Research Program (SHRP2). All of these efforts are needed. That ABC projects are underway in multiple states across the country shows that Americans have a way of rallying to the task.
Also Read: The Unhappy State of Bridges and Highways in the U.S.