PROVIDENCE — The state Department of Transportation intends to build a new bridge across the Pawtucket River in addition to replacing the pair of bridges that now carry Route 95, to avoid disturbing interstate highway traffic during construction.

With an estimated 162,000 vehicles crossing the river each day, “We can’t really afford to shut down any lanes,” the DOT’s chief engineer, Kazem Farhoumand, said. He said the agency hopes to complete the first of three new structures during the summer of 2010.

The DOT is also planning to spend $3.5 million on repairs beginning early next year to keep the seriously deteriorated bridge safe until it can be replaced.

Although it’s referred to as one bridge, the Pawtucket River Bridge is really two separate structures built side by side, very close together. Both are deteriorated, to the point that large trucks are banned from them, one entrance ramp has been closed and crumbled concrete has been removed from the bottom of the bridge deck so it wouldn’t fall. Traffic has been shifted away from weak spots and parts of it are held up by temporary supports.

The $3.5 million in repairs aren’t intended to allow the weight limit to be raised above the present 18 tons, meaning that heavy trucks must continue to detour around it. Instead, the repairs are intended “just to keep it where it is” until it can be replaced, Farhoumand said.

If the repairs aren’t done, he said, it’s likely that the weight limit would have to be lowered.

The repairs will strengthen several beams in the bridge’s superstructure, including the massive steel girders that run lengthwise across its concrete piers and hold up the rest of the steel structure and deck above.

Farhoumand said the DOT also plans to repair the bridge’s deck joints and repave it, not just to improve the ride, but also to help seal the deck against water, which is speeding the bridge’s deterioration.

The DOT hopes to put the repair project out for bids in January, he said.

Farhoumand said that the first new structure will be built across the river south of the existing ones, between them and the Division Street Bridge. Once that is built, traffic will be shuffled between it and the two existing bridges while both are replaced.

He said the DOT hopes to start construction of that new bridge late next year and to have it completed “sometime in the summer of 2010.” That bridge should take less time to build than the others because there’s no existing bridge to demolish.

Opening that first new structure will eliminate half of the immediate problem by ending the need for a weight limit for northbound traffic. He said the whole project will take two-and-a-half to three years to finish.

To replace the existing bridges, he said, “We have to demolish two bridges and build three new bridges.” Two will carry north- and south-bound traffic on Route 95, with the third serving as a service road for them.

First, he said, northbound traffic will be shifted to the new bridge, and southbound traffic will be shifted from the existing southbound bridge to the bridge that now carries northbound traffic. That will allow the existing southbound bridge to be demolished and replaced.

Southbound traffic will then be shifted back to the new southbound bridge. That will leave all highway traffic on new bridges, and the existing northbound bridge free to be demolished and replaced, too.

Finally, northbound traffic will be shifted from the first new bridge to be built to the new northbound bridge. That will leave all of the interstate traffic on new bridges, and leave the first bridge to be used as a service road. Farhoumand said that will eliminate a traffic hazard, the short distance between the northbound entrance and exit ramps before and after the bridge.

The current estimate for the bridge replacement project is $75 million, but the design is only 10-percent complete and the figure is likely to change, Farhoumand said.

blandis@projo.com