PROVIDENCE — The state Department of Transportation said yesterday that it will for the second time lower the weight limit on a key state bridge, over the Sakonnet River, because its steel beams have continued to corrode.

The reduction, to 18 tons per vehicle from 22, together with a new state law also aimed at excluding heavy vehicles, will force many trucks to make a long detour to Bristol and the Mount Hope Bridge, the next closest way to get from the mainland to Aquidneck Island.

DOT Director Michael P. Lewis said he understands that the new limits “will continue to adversely affect the Aquidneck Island community.” However, “our goal all along has been to preserve the safety and integrity of the bridge until we are able to replace it.”

The parts of the bridge — which connects Tiverton and Portsmouth — most affected are not the relatively complicated arch span in the center or the truss spans on either end of the arch, but rather the less complex sections closer to shore on both ends of the bridge. There, layers of transverse and longitudinal beams hold up the concrete deck.

Kazem Farhoumand, the DOT’s acting chief engineer, said that many parts of the bridge are deteriorating. But he said that the immediate cause of the reduced weight limit is corrosion of 40 beams running crosswise immediately under the deck. The affected beams are near open joints that allow salty water from the deck to reach the steel and corrode it, he said.

The DOT said signs announcing the new restrictions will go up within two weeks. As it has said in the past when it announced weight limits and detours at the Sakonnet and at the Pawtucket River Bridge, which carries Route 95, agency officials said the affected bridge is safe.

In particular, Farhoumand repeated yesterday, having a vehicle a few tons over the weight limit crossing them would not cause a collapse. Instead, he said, the DOT is trying to protect the bridge and slow its deterioration to prolong its life until they can be replaced.

The DOT plans to replace both bridges, but that will take years. The agency said yesterday that it expects to advertise in late summer for bids to build the replacement Sakonnet River Bridge and to open the new bridge in 2011.

Along with the weight limit reduction, a new state law will force an additional segment of truck traffic to take detours around both the Sakonnet River Bridge and the Pawtucket River Bridge, which also had a 22-ton limit reduced to 18 tons.

That law, which went into effect when Governor Carcieri signed it Thursday, bans most vehicles with more than two axles from both bridges. The legislation was introduced because officials said that overweight trucks continued to risk fines by crossing the bridges, and the police could not tell for sure whether many trucks were overweight without stopping and weighing them.

Farhoumand also said that although the law includes no such exception, the state police won’t fine the drivers of trucks that are obviously under the weight limit even if they have too many axles. He gave as examples car carriers and the flatbed trailer trucks used to haul bulldozers and similar construction equipment, both of which can clearly be seen to be empty and are below the limit then.

The new law carries a fine of $3,000 for the first offense and up to $5,000 for additional violations.

The new restrictions still mean more trouble for the trucking industry, because rising fuel costs multiply the cost of detours.

The weight limit on the Sakonnet River Bridge could have a bigger effect than the one on the Pawtucket bridge. John Atwood, president if the R.I. Trucking Association, said that the detours the DOT set up around the Pawtucket bridge are working well and “hardly add anything” to a trip, in contrast with the long detour around the Sakonnet River Bridge.

At 15 to 17 tons, school buses remain unaffected by the weight limit, Farhoumand said. Fire trucks and other emergency equipment, along with other state and municipal vehicles, are exempt from the new two-axle limit.

blandis@projo.com