A construction team with local bridge experience has filed the lowest bid to build a new Highway 520 floating bridge.

The team of Kiewit-General and Manson — one of three bidders — offered to do the job for $587 million, the state announced Wednesday. That figure is less than a state engineer's estimate of $640 million and well below a budget limit of $750 million.

Officials expect to sign a contract with the company by Sept. 1, pending final review of documents and insurance, said Mike Cotten, the state Department of Transportation's design-build director for the 520 bridge.

This contract will include manufacturing 44 of the 77 pontoons, assembling the entire floating segment of the six-lane bridge and building a connection at the eastern shoreline. The new highway is to include a bike trail as well as two general lanes and a carpool lane in each direction.

Kiewit-General's bid was based on a mid-2015 opening. But if the team finished in 2014, it could win $2.5 million in incentive pay, said Cotten. Even assuming success on this and earlier contracts, the state remains $2 billion short of funding the Seattle portion through Foster Island, Montlake and Portage Bay, to complete the $4.65 billion highway.

Future tolls on I-90 are an option to fund the 520 project, but lawmakers have yet to propose that in a bill. Current law requires tolls to be used on the highway where they're collected. Tim Eyman's Initiative 1125 would affirm a ban on shifting I-90 funds to 520.

Until the entire crossing to I-5 is built, there could be a period of years when the new six-lane deck switches to the current four lanes, toward the west side of Lake Washington.

But for now, state officials proclaim good news.

Kiewit-General built the new Hood Canal floating bridge and partnered on the new Tacoma Narrows suspension bridge. Manson specializes in marine construction and dredging.

Kiewit spokesman Tom Janssen said one reason for its low bid is "we've probably got the most experience in floating-bridge construction anywhere in the country," including the advantage of already having the initial pontoon-fabrication contract, for 33 sections, in Grays Harbor County.

Officials expect to sign a contract by Sept. 1, Cotten said.

"The bids we opened today show that we can deliver a great transportation project at a great value," Transportation Secretary Paula Hammond said in a statement. "When complete, the new bridge will be safer, better able to withstand winter storms and provide improved commuting options for Puget Sound drivers."

The existing bridge is at risk of sinking in a severe windstorm. Its fixed columns near Seattle have eroded from within and could crack in an earthquake.

The other bid teams are Flatiron, Skanska and Traylor, who bid $643 million, and declined to comment Wednesday; and "520 Corridor Constructors," led by Walsh Group and Weeks Marine, who bid $690 million.

Mike Lindblom: 206-515-5631 or lindblom@seattletimes.com