The new St. Croix River bridge, one of the largest construction projects in Minnesota, won't open in 2016 as promised because building it has become more complicated than expected, state transportation agencies in Minnesota and Wisconsin said Friday.

"This is a very complex engineering project," said Commissioner Charlie Zelle of the Minnesota Department of Transportation (MnDOT). "This is only the second major bridge of its type built in the United States and the methods of construction are taking longer than anticipated."

The four-lane bridge will link Hwy. 36 at Oak Park Heights with Hwy. 64 in St. Croix County, Wis., replacing the Stillwater Lift Bridge as an interstate connection. Its delayed opening almost certainly will increase labor costs and could hurt Stillwater's plan to build a pedestrian plaza in its historic downtown district.

Problems became apparent in recent weeks when contractors told MnDOT "they're just to the point now where there's no way to recover that schedule anymore," Michael Beer, the project manager, said Friday. "As the summer progressed, certain items haven't gone as quickly as we hoped."

The delay wasn't caused by one problem, Beer said, but stems from a combination of challenges related to the unusual style of the bridge. It relies on tensioned cables to hold segments together, and fusing the metal known as rebar is taking longer than expected, he said.

"It's such a dense network of interweaving rebar," he said. "It takes longer for the contractor's workers to assemble that than on a typical bridge. There are a lot of areas like that on this project, so those portions just haven't been able to be produced as quickly as what the contractor had originally anticipated."

The transportation agencies have said the new bridge and its approach highways could cost up to $676 million. That price tag also includes money for historic preservation and for environmental work to protect the St. Croix River.

"Disappointed — but given the scope of the project, it's understandable," Gary Kriesel, chairman of the Washington County Board, said of the delay. "You've got to do it right the first time."

A later bridge opening, he said, could delay a traffic study the county had planned, but he said it would be worse if workers were hurt because they were trying to hurry to meet a deadline.

In recent years, bridge project managers told audiences at public gatherings that the bridge would open to traffic in November 2016. Instead, Zelle said Friday, the work will continue into 2017 and a new completion date won't be announced until later this year.

MnDOT, the Wisconsin Department of Transportation and the chief bridge contractor, Lunda/Ames Joint Venture, are working on a new schedule, he said.

"While we know that residents of this region want the bridge to be open for traffic as soon as possible, our highest priority is to complete the project safely and to the high standards we've set for it," Zelle said.

Beer, who works from a temporary bridge project office in Stillwater, said the November 2016 deadline meant construction would have concluded just before winter.

"That pushes us right up against cold weather," he said of the delay, and now work can't resume until the spring of 2017. "It's just an unknown. What we want to do is put out a date that [government] departments and the contractor both have a high level of confidence we can achieve, reasonable and achievable barring anything unforeseen happening."

The city of Stillwater, which lobbied hard for the new bridge, has established a revitalization plan for its downtown that includes a pedestrian plaza on a portion of the street leading to the Lift Bridge. Once the new bridge opens, the Lift Bridge will close to vehicle traffic and engineers will convert it to a pedestrian trail.

Mayor Ted Kozlowski said the delay didn't surprise him given the scope of construction, but he doesn't want it continuing into the summer of 2017.

"We want to get on with putting our downtown together with the Lift Bridge closed," he said.

Mayor Mary McComber of neighboring Oak Park Heights — where most of the highway work has been completed — said the delay won't have much of an effect on her city of 4,800 residents.

"I'd rather see them be safe, and not try to rush this and put somebody's life at risk," she said of bridge workers.

Beer, at MnDOT, said 400 workers are building the bridge portion of the project, including working in two casting yards.

"There's an increased level of complexity for the vast majority of the project. It takes experienced skilled workers everywhere out there," he said. "The workers we have out here are doing a great job, we could just use more of them."

Simpler bridges, he said, are either built from one direction or from both directions and joined in the middle. The St. Croix River bridge, he said, is different: All portions of it are being built at once.

Possible cost increases could relate to workers having to stay on the job longer, but material costs were set before the work began and won't change, he said.

"We want quality and safety, and safety is No. 1," he said of the delay.