Bridges: The ABCs of ABC
Accelerated bridge construction cuts long term costs
It almost seems like magic: you go out of town for a week or two and find yourself driving home over a new bridge – one that wasn’t even under construction when you left.
The technique of replacing bridges rapidly has been gaining ground in Canada for just over a decade now. Known as Accelerated Bridge Construction (ABC) or Rapid Bridge Replacement, the process has become a mature and effective way of minimizing disruptions to traffic flows caused by bridge construction – besides offering other benefits.
The key element is minimizing – to an unbelievable degree – traffic disruptions caused by bridge construction. In the bestknown examples, bridge components have been prefabricated offsite and installed in a couple of days.
“People tend to focus on the big SPMTs (self-propelled modular transporters), where they build a bridge off-line, bring it around and set it in place. And people say, ‘Okay, that’s accelerated bridge construction,’” says Patrick Malone, corporate business development manager, PCL Civil Constructors, Inc. But that is not the only approach.
The issue is those long lines of vehicles snaking through a conventional bridge construction site. Conventionally, a staged approach is used to leave some lanes open for traffic during construction. Depending on the bridge, traffic mobility may be impacted for months or even years. In some cases, contractors agree to work only at off-hours. That means some improvement to traffic flow and an often considerable increase to the total time needed to deliver the project, especially when weather delays are involved.
Traffic congestion represents a real cost to the community that may be hard to define. For example, the traffic delays related to construction on Toronto’s Gardiner Expressway are costing an estimated million dollars a day – spurring a hurry-up approach to the project in December.
“That reduction in wait time is why the government and the ministries of transportation are so willing to pay a premium to have this done,” says John Almeida, vice president of Construction Operations, Aecon Construction Materials Ltd. The market is significant. Roughly 25 per cent of Canada’s bridges need repair or replacement, according to the Canadian Precast/Prestressed Concrete Institute (CPCI). Many older bridges were constructed without air-entrained concrete or coated rebar, leading to corrosion, according to the Canadian Society for Civil Engineering. In 2011, it estimated the maintenance, rehabilitation and replacement backlog for highway bridges at $10 billion.
Besides minimizing driver frustration and economic loss, there are other benefits to ABC.
Safety: there are risks for both construction workers and the public as long as vehicles drive through a jobsite. Traffic control on a jobsite can reach anywhere from 20 to 40 per cent of construction costs, according to data presented by the CPCI.
Environmental: emissions from all those idling cars represent a major environmental problem. Emissions from construction equipment and vehicles will also mount up during a multi-year project. “The labour is less (at an ABC site). You don’t need parking for 100 people; you need parking for five or 10 people,” says Malone. The ability to build a bridge quickly is also a plus in environmentally sensitive areas – fish-spawning waterways, for example.
Societal impacts: You don’t need to look much further than the residents living near a bridge replacement site to see some of these.
Quality: Using precast components confers certain benefits, all by itself. “If you do a cast-in-place structure, you can get a lot of salt on top of it. You have a higher probability of corrosion,” says Malone. With ABC, the longevity of the structure is dramatically increased because you are able to prestress all the top plates. “You’re producing the product efficiently in a facility that supports a higher level of quality,” adds Malone. The workplace conditions are better controlled and staff may have a higher level of skill.“You get something that is built faster, built with less mobility impact and you get have better durability,” says Malone.
There are two core concepts in ABC, according to many authorities. In the first, prefabricated bridge elements and systems (PBES) are built offsite and assembled on-site, with the minimum of on-site work.
In the second, the entire superstructure is built off-site and lifted into place in the final position. This is sometimes called Slide-in Bridge Construction. PBES may consist of a superstructure, a superstructure and substructure or a complete bridge. Typical elements include deck, beam, pier, abutment and wall elements, as well as such miscellaneous elements as precast approach slabs, prefabricated parapets, deck closure joints and overlays.
The components are rolled, launched, slid, lifted, or otherwise transported. When the deck and parapets are part of the installation, no subsequent construction phase is required. The installations sometimes require the use of the poster child of ABC, the SPMT. These massive transporters can carry enormous bridge components to the site for installation. The slide technique is the other common approach. “In the slide, one bridge is skidded out and the other bridge is skidded in, typically utilizing some type of Teflon pad.” says Almeida.
The new bridge is built on temporary supports beside the bridge to be replaced. Once complete, the existing bridge is demolished or slid out of the way. The new bridge is slid into place, tied in to the approaches and then paved. Traffic is closed for only a few days.
PCL has also worked with a float-in approach in the USA. “You can either construct a new substructure or you utilize an existing substructure. We build the main span offline, and then we float those pieces and using barges and fit the new pieces on top of the substructure. This can be done in days, instead of months,”says Malone.
There are many ways to accelerate bridge construction and reduce traffic mobility issues. Common practices like design/build can help minimize the duration of a project, according to Malone.
He cites a large bridge project in the U.S. where traffic flow was largely unimpeded thanks to putting piers beside the roadway, not in the middle as is the conventional practice. Precast segments were installed at night, enabling them to do about a lane at a time. Another related concept is the GIGO (Get In, Get Out) bridge technique. While not directly comparable to the PBES or Slide-In approaches, it does involve offsite fabrication to reduce construction times and doesn’t require specialized equipment like SPMTs.
The bridge is designed in such a way that you can do the fabrication during winter, move the components with cranes and install them onto the substructure. “That was done in London (on the Westminster Rd. Bridge) on the 401 last year,” says Bala Tharmabala, head of the Bridge Engineering Office, McIntosh Perry.
At present, Aecon is doing a demonstration project for a complete rigid frame movement. That’s picking up not only the superstructure but the deck, walls and footings together – the whole thing. “You take the old bridge out and place the new bridge on top of the old footings, and dowel them together,” says Almeida.
The components being moved are massive… “Some of these pieces might weigh 1,500 tons,” says Almeida. “The timing for rigid frames might be a little longer than two days, it might be a week. But it will not be six months.” Testing is being carried out of the Cornwall Centre Road Bridge, according to Almeida.
Ontario has been the bellwether for ABC, with government endorsement for the concept and about a dozen bridges already complete. Development started with lab testing scale models in 2001, according to Tharmabala and Quazi Islam, managing engineer, MTO Bridge Design Team, McIntosh Perry.
Along with another McIntosh Perry colleague, Clifford Lam, they worked for the Ontario Ministry of Transport for many years and were part of a team of ministry engineers that developed the ABC concept for use in Ontario. They emphasize that their work at MTO is the foundation of their expertise.
In 2004, the first bridge of this type was constructed near Timmins. In the early days, “We were quite nervous about this type of approach,” says Islam. It was used on the Ottawa Queensway, a very heavily travelled route. “There was no secondary route the traffic could take,” he adds. If something had gone wrong and the bridge had been closed for an extended length of time, it would have been a major traffic problem.
In fact, the old bridge was kept in one piece, with the idea that if there were problems with the new one, the old one could be reinstalled, says Islam. Fortunately, the project was a complete success. “We closed the bridge in the evening on Saturday and opened by noon on Sunday,” he says.
COSTS AND CONCERNS
“It’s a simple enough concept that people get it quickly. I don’t think there’s any misunderstanding,” says Almeida. While to some it seems like a slam-dunk, ABC does face some friction. Costs are the prime one. ABC bridges are significantly more expensive, in most cases. But as with anything else in construction, there’s the price, and then there’s the cost.
“With owners, we talk about costs a lot,” says Tim Davis, district construction engineer, PCL Civil Constructors, Inc. “The price depends on how you look at it. The initial, up-front price might be higher, but when you factor in the user costs of impact on the traveling public, ABC can actually turn out to be cheaper. You have to consider the soft costs.”
This may require the owner to revise his methods in planning and costing a bridge project. “Price is an issue. There is a lot of demand for infrastructure right now, and people need to stretch their budgets,” says Malone.
As the technology evolves and standardized approaches evolve, costs and risks should decline. “Rather than custom engineering every solution, pre-engineered modular systems configured for traditional construction equipment could promote more widespread use of ABC through reduced costs and increased familiarity with these systems among owners, contractors, and designers,” according to a presentation by CPCI.
Public awareness is another concern. Tharmabala cites a residual level of public concern about this process and says more education is needed.
“People want guarantees that the bridge will open at exactly the time promised,” he says. He emphasizes that ABC is now a proven process and that, with planning, only minor issues should emerge. Not every installation may have gone as smoothly as planned, but that’s an engineering reality in all construction, he notes. The public tends to object when a bridge is going to be completely shut down, even if only for a matter of a few days. Some prefer the idea of intense traffic congestion for six months to closing it for a few days. “They don’t understand that they will get long-term gain if they go through short-term pain,” says Tharmabala.
Malone has a simple analogy: “It’s better to rip the Band-aid off,” with two days of complete closure instead of months or years of traffic snarls, he says.
There is a positive at play, though. “People are intrigued by it. They’re fascinated when the SPMTs show up to do the job,” says Almeida.
Outreach to the public can help. Pre-construction town halls, a dedicated website with detailed information, jobsite webcams, enclosed viewing areas during installation and tweeting on construction progress are just a few ways the public can be brought on board.
For the contractor, construction risk can be a concern. When the owner has paid to have a closure of two or three days and it turns out to be a week or two, that is a big issue. Some jobs have gotten more than their share of negative press. “The timelines that are being stipulated are incredibly demanding. When there are delays, the penalties are punitive. It’s disproportionate,” says Almeida. Even though four days might be an amazing turnaround, if the owner contracted for a two-day closure, it’s a problem.
Logistics is another important factor.
“That’s where a lot of these bridges are a challenge,” says Almeida. “You need a fairly substantial temporary yard. You have to be sure that your founding material is suitable, because you are building a superstructure and you have to build it on something solid. You can’t have deflection or differential settlement. There are a lot of issues to contend with, in respect to building a bridge in a field.”
Even when satisfactory space is available, transportation can cause unexpected costs. “Make sure the logistics of the move are planned so that you don’t have to do so much secondary work. If it’s just a matter of taking down a couple of street lights, that’s not a problem… You want to make sure you don’t have to move a 44 KV transmission line,” says Almeida.
Ontario has taken the lead in Canada, but the interest in ABC in the USA is much more intense.
“In the US, the federal government offers five per cent additional funding to any project that utilizes an approved ABC technology,” says Malone. The Federal Highway Administration has taken a lead role. Comprehensive information has been posted to the web, a national conference has been launched and successful projects have been completed in many states. ABC fits its niche application well… replacing bridges on heavily travelled highways where no reasonable detour is available.
The technique is slowly gaining recognition. Tharmabala expects steady growth in Ontario, due to increasing traffic congestion in urban areas. “At the moment, the ministry does three or four a year. But that could become 10 or 15 a year, five years down the road,” he says.
Precast has been around for a long time and ABC is just bringing some of its advantages into focus. ABC concepts are influencing many other fast-tracked projects. “You don’t just apply ABC, carte blanche. You have to look at all the factors in that project,” says Malone.
Jim Barnes is a contributing editor to On-Site. Send comments to firstname.lastname@example.org.