When a newly installed pedestrian bridge collapsed and killed six people at Florida International University in Miami in March, investigators including the National Safety Transportation Board and the Florida Department of Transportation began scrutinizing the project's construction process.

The investigation is ongoing, and so far there's been no conclusion yet that the method used — known as accelerated bridge construction, or ABC — is inherently flawed or even fit to blame in this particular instance.

So, while there are still many unknowns surrounding the FIU bridge failure, one question can be answered, and that is why the ABC method is being chosen in increasing numbers by bridge design teams, state transportation departments and infrastructure contractors.

What is ABC?

But first, what is ABC? The system of bridge building is still being studied and perfected and is still unfamiliar to many of those outside the infrastructure arena. FIU is actually home to its own Center for Accelerated Bridge Construction, which researches issues and technology related to ABC and serves as a basis of consultation to the transportation industry on the method.

Using language from the FIU's website, ABC is a "delivery solution method of building and repairing bridges with the capability to reduce the interruption to traffic and increase safety." The method, also known as rapid renewal, essentially boils down to being a process that allows teams to build a bridge in some location other than its future footprint and then install it on-site in as little as a day.

The method actually encompasses a range of individualized strategies. For example, Bala Sivakumar, professional engineer and resident ABC expert at engineering, construction and infrastructure planning firm HNTB, said that for span replacements of approximately 50 feet to 200 feet, a contractor might build a new bridge next to the one slated for replacement and then, using temporary towers of support, roll the old bridge out and the new one in. For longer spans, the bridge can be divided up into shorter segments and replaced the same way, using expansion joints to link the new sections together.

If there isn’t a time crunch, Sivakumar said, or if the terrain around the bridge does not allow for a lateral slide of spans, the bridge can be prefabricated in pieces, which are transported to the site, if necessary, and then lifted into place and connected after the old bridge is removed. “It’s almost like a Lego set,” he said.

In January, Oklahoma City residents were able to see the process unfold in person when the Oklahoma Department of Transportation trucked in prefabricated railroad bridge pieces to replace an old section of Interstate 235.

Advantages to ABC

“The reason to use ABC,” Sivakumar said, “is to reduce the impact of traffic delays during major bridge replacement projects.” HNTB has performed ABC bridge replacements in a week, two weeks, during a single weekend, and all of these scenarios drastically decreased the chaos and number of safety issues that frequently come with the extended road closures and detours necessary when bridge construction takes place on site.

Another advantage to using ABC, said Amanda Bao, associate professor in in the civil engineering technology, environmental management and safety department at Rochester Institute of Technology, is that the offsite location, which can be miles from the jobsite or as close as 100 feet, can be indoors or set up to shield workers and materials from the elements, eliminating weather-related delays and harm to the finished product. ABC also has the potential to reduce construction costs by eliminating the need for the temporary bridge structures sometimes required during traditional construction.

ABC can be less risky for workers as well. “Now we’re also seeing safety benefits,” Sivakumar said. “It’s a lot safer to do an ABC project because [workers] are not next to traffic or 50 to 100 feet above ground tying rebar or trying to pour concrete.”

Schedules can also be compressed by eliminating the sequential process inherent in traditional bridge construction and splitting tasks into phases that can be executed in parallel to one another, he said. For example, in conventional bridge building, the order of construction and installation is typically foundations, piers, beams and then the deck. With ABC, all of those phases can be completed simultaneously.

Weighing pros and cons

However, with the speed, safety and convenience of ABC comes a price: Projects typically cost 20% to 30% more than traditional bridge construction jobs, according to Sivakumar. The agency in charge of the project then has to weigh the costs of traffic delays and congestion as well as other potential economic losses, such as nearby businesses being cut off from customers.

“What we find,” Sivakumar said, "is that ABC is becoming more and more accepted … because bridge owners are starting to recognize the user impacts of bridge and other transportation projects, and they are willing to pay more to get it done faster.” It also helps owners reduce their administrative costs due to the reduced schedule time.

Where the ABC method cannot be used, however, is on a bridge project that has been designed for conventional construction, Bao said. “It cannot be converted automatically,” she said. “You have to redesign it.”

Contractor training comes into play as well since, Sivakumar said, approximately 50% of the contractors taking on ABC projects currently are doing it for the first time. There are Federal Highway Administration seminars and other training opportunities available to contractors, but, typically, construction companies are willing to invest in the necessary training to ensure that project managers, supervisors and workers are prepared.

Bridging the learning curve

In a one- to two-week installation process, he said, there is a learning curve for contractors for the first few days, but usually by day three or four, production goes up and they’re able to make up for any lost time. “Contractors get a lot faster and better the more they do it,” Sivakumar said.

Of course, Bao said, the same rules that govern traditional bridge projects also apply to those built according to ABC techniques — careful, daily inspections during construction and installation in order to identify cracks, disconnections or any other potential defects at the earliest stages.

Paul Diederich, president of Industrial Builders in North Dakota, said the company’s first and only ABC project involved it building a railroad bridge above 14 railroad tracks in 22 hours. The active railroad yard only had to be shut down for one day, which made it a good fit for ABC, he said, and safer as well. The bottom chord of the steel structure would have put workers 25 feet above grade, but using ABC and building the structure offsite put the bottom chord near ground level.

Diederich said there’s not much demand for ABC projects in North Dakota but noted that it’s another tool in the toolbox — a way for the traveling public to avoid the inconveniences that come with onsite bridge construction. 

And the more experienced contractors get with ABC, Sivakumar added, the narrower the price gap between it and traditional bridge building methods.

Commuters could end up having the most influence as to whether or not state transportation departments start using ABC regularly, though. “We have found once the public has seen ABC in action, states find it difficult to tell taxpayers they’re going back to old methods,” Sivakumar said.