April 9, 2018

Following Florida collapse, accelerated bridge construction under the spotlight

Kevin Nursick Spokesman for the state Department of Transportation

Q&A talks with Kevin Nursick, a spokesman for the state Department of Transportation, about accelerated bridge construction.

Q. The bridge that collapsed at Florida International University used a construction method called accelerated bridge construction, or ABC, which is something the state Department of Transportation has used more readily in recent years. What is ABC and why is DOT using it?

A. ABC is a bridge construction technique involving the use of pre-fabricated bridge elements. The prefabricated elements range from as small as precast concrete bridge deck panels to as large as fully constructed bridge spans.

Typically, in ABC projects, construction of bridge components takes place in a different location from where the structures are actually installed. Bridge components are usually built off-site, in areas away from vehicle traffic, and then installed in a short time period.

ABC methodology is gaining wider use because it improves work zone safety for construction workers and the traveling public, reduces overall construction-related traffic delays, and reduces the duration of bridge construction, among other benefits.

Q. Can you provide a few examples of Connecticut bridge projects that used ABC?

A. A recent example was the replacement of the I-84 bridge spans over Marion Avenue in Southington in 2014 , which used self-propelled modular transporters (SPMTs). SPMTs are multi-axle, heavy-lift transport devices that can be linked together to lift and transport heavy loads such as pre-constructed bridge spans.

The new bridge spans were constructed near the Marion Avenue bridge site but away from traffic. During one weekend in June, and with much advance notice and coordination with multiple parties, all traffic was diverted off I-84 onto on-and-off ramps, while the existing spans were demolished and new pre-constructed spans moved into place.

While there was significant cost using the SPMTs to transport pre-constructed spans into place, DOT saved money in other areas. For example, DOT didn't need to widen bridges or the highway to accommodate traffic, since that section of I-84 was closed for a few days during construction.

Another recent ABC example in 2016 involved the replacement of two bridge spans on I-95 over Oil Mill Road in Waterford using a lateral slide method.

In this approach, the new spans were constructed in two halves. Then, during a 30-hour construction period, the existing spans were demolished and the new spans were slid into place.

During the installation period, one lane of traffic was left open. While traffic backup was significant, it was short in overall time duration.

Q. Is accelerated bridge construction safe?

A. Accelerated construction has been shown to be as safe, or safer, than conventional construction. While ABC is not as common as conventional construction, its safety track record has been excellent.

Improved safety derives from greater separation between construction activities and highway traffic. Typically the construction of bridge components in ABC projects takes place off-site or in adjacent construction staging areas well separated from motor-vehicle traffic.

Q. How prominently will ABC be used in Connecticut going forward?

A. ABC construction methodology will continue to be prominent in the future for DOT projects. There are approximately 10 ABC projects currently in construction and 10 more in the design and pre-construction award phases.

Significant ABC projects now in construction include the replacement of the Metro-North Railroad bridge over Atlantic Street in Stamford where SPMTs will transport pre-constructed bridge spans into place. The method was selected to minimize the temporary closure time of Atlantic Street, a highly important transportation corridor between the Stamford train station and downtown Stamford.

Also in construction is the rehabilitation of the Route 8 southbound bridge in the Waterbury Mixmaster interchange where five of 21 bridge spans will be replaced during weekend traffic closures, with traffic diverted to a local street bypass.

These ABC methods are being selected during preliminary design phases where analysis demonstrates a significant advantage in speed of construction and reduction in overall traffic.

While the construction bid price of an ABC-designed project may be slightly higher than conventional construction as a result of using specialized heavy-lift equipment, it saves money as well. For example, shortened construction duration reduces DOT's overhead costs.

Q. It's still early in the investigation, but were any lessons learned about ABC from the Florida bridge collapse?

A. Regarding the Miami Florida International University pedestrian bridge collapse, it is premature to speculate on the cause or attempt to draw conclusions from the tragedy. Connecticut bridge engineers, however, are attuned to the investigation undertaken by the National Transportation Safety Board and await the findings and any lessons to be learned.

But, to be clear, our conventionally built bridges and bridges built using ABC techniques are verifiably safe.

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