Jigsaw bridge puzzle

News February 26, 2002
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When the Virginia Department of Transportation needed to replace the mainline structure of the I-95 James River Bridge, they kn

When the Virginia Department of Transportation needed to replace the mainline structure of the I-95 James River Bridge, they knew it had to happen without affecting rush-hour traffic through the middle of downtown Richmond. The initial contract called for a traditional replacement of each span with precast units consisting of individual girders and a deck, with transverse and longitudinal closure joints for each piece. But this system was not a good fit for the project’s tight 7 p.m. to 6 a.m. nightly schedule.

Under the original plan, the project team would come in at night, cut out one piece, replace it, construct a temporary transition and open the bridge in the morning. It would take two or three nights to complete each section.

Joining forces with the Virginia DOT and Archer-Western Contractors Ltd., Parsons Bridge and Tunnel Division developed constructibility modifications for the project. For Parsons, the key to making the process work better was match-casting.

In match-casting, pieces are cast together and then separated. The idea is that the pieces will fit back together at the jobsite better than pieces that were cast separately. Parsons likes to compare match-casting to creating a jigsaw puzzle: you make the picture first, then cut it up, instead of making each piece separately.

Using a match-cast process, the James River Bridge team was able to increase the size of the replacement pieces. This reduced the total number of pieces in the project by approximately one-third. The precast composite units (PCUs) are typically three girders across with a concrete deck; they vary in length from 45 to 95 ft with a deck width of approximately 22 ft. Reducing the total number of replacement pieces allowed the team to cut their work in half.

The team built each span completely and used a transverse match-cast joint to pour one against the other. The bridge structure was cast in three 22-ft pieces for a total width of 66 ft. This was the first time a long thin slab had been successfully match-cast. Implementing a match-cast system, in which adjacent PCUs are match-cast longitudinally and erected side by side to make up the full width of a given span, increased efficiency and saved time. With the modifications and the match-cast approach, the I-95 crew could replace three units–and complete an entire span–in just one night.

For more on the story, read the March issue of ROADS&BRIDGES.

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