Better Bridges: Building blocks for bridges
Constructing a bridge via Accelerated Bridge Construction (ABC) that uses lightweight fill material can involve what appears to be assembling giant building blocks. But these are made of polystyrene so contractors are working with huge plastic foam blocks.
“They are literally like assembling giant Lego blocks,” explains Fred Doehring, deputy bridge engineer for the Utah Department of Transportation (UDOT). It’s the use of innovative material such as this in construction that is enabling state agencies to speed up the repair of bridges.
UDOT believes that by using some of these innovative construction technologies, including Self-Propelled Modular Transporter (SPMTs) and Accelerated Bridge Construction (ABC), the agency was able to dramatically reduce the impact on roadway users of replacing Salt Lake City’s I-215 bridge at 3300 South. The use of SPMTs — a series of hydraulic trailers — in conjunction with the lightweight expanded polysytrene Geofoam blocks rapidly sped up the replacement of the bridge, Doehring says.
Traditional bridge replacement requires lane closures and traffic impediments for many times a month a more, says Rick Chestnut, a principal at Terracon Engineering. However, with the I-215 at 3300 South bridge project, UDOT did a benefit-cost analysis of the project, known as the “delta” cost. It determined that using ABC technology would make a significant difference to the motoring public.
When the UDOT put the structural repair of the bridge up for bid a requirement was to minimize delays. Utah’s I-15 is the major corridor and I-215 wraps around it, making both highways major arteries. Shutting down the area or even rerouting traffic temporarily would be a major undertaking and severely interrupt traffic. That’s why Ralph Wadsworth Construction, the contractor that UDOT ultimately awarded the job, proposed the project using ABC technology incorporating Geofoam (manufactured by ACH Foam) to minimize traffic impact. Just a year before, the I-215 bridge at 4500 South project also was completed through ABC by the same contractor.
“By using rapid replacement techniques, we only shut down I-215 for 19-1/2 hours,” Bryan Jensen, field engineer with Draper, Utah-based Ralph Wadsworth Construction, points out. “After we reopened I-215, we finished the tie-ins and turned the bridge over to UDOT for use within eight days. According to UDOT, this was the fastest single-span bridge replacement. The rapid techniques allowed us to finish the bridge all within a short time frame instead of months of lane shifts and road restrictions.”
Jensen notes that UDOT would only allow a window of 54 hours during with the bridge could be shutdown. “We accepted the challenge to do it,” he says. Jensen is quick to point out, “With this project, it wasn’t just about cost. UDOT was really good about considering the public’s own time to make sure they aren’t sitting in cars stuck behind orange barrels.”
Although the substructure and superstructure took several months to complete, the impact on users was minimal until the bridge was ready to be removed and replaced. The success of this project and the other ABC projects completed by UDOT has led to a statewide adoption of this technology. In fact, UDOT is in the process of awarding a $1.2 billion ABC project — a single design-build contract — to reconstruct 1-15 through Utah County.
UDOT has built/replaced 17 bridges using SPMTs, where sections of the bridge are built offsite and moved onsite by the special trailer transporters. The agency also has used several other methods of rapid replacement: It has completed five bridges where they were built adjacent to the existing bridge and then slid in place; two “drop in-place” bridges, where a superstructure is built and then cranes are used to put it into place and UDOT is currently working on its fourth and fifth bridge where all the components, including abutments, columns, bent caps, and decks, are pre-cast offsite and then hauled to the site.
“We’ve done a lot of tearing off of the old deck, where the existing girders are left in place and the deck is replaced using pre-cast deck panels,” Doehring says.