VERONA ISLAND — The removal of the cable suspension bridge that spans the Penobscot River between Verona Island and Prospect is nearly as much of an engineering feat as its construction 81 years ago.

Six years after the sleekly designed Penobscot Narrows Bridge and Observatory opened in 2006, the demolition of the historic Waldo-Hancock Bridge has begun.

The state Department of Transportation hired the S&R Corporation of Lowell, Mass., to demolish the Waldo-Hancock Bridge at a cost of $5.3 million.

According to DOT, the total budget of $7.6 million also includes engineering, site stabilization, landscaping and other improvements. All demolition, except for the bridge towers, is expected to be complete by late April and the total project is scheduled to wrap by June.

The project is complicated by shifting tides and currents, and by a prohibition on explosives demolition for this particular site, said DOT’s project manager, Douglas Coombs. Depending on tide levels, the deck of the bridge can range from 125 feet to 145 feet from the surface of the water.

S&R crews are in the process of disassembling the bridge in segments. The bridge is basically coming down in the opposite sequence of its construction, he said. The biggest challenge was to stabilize the structure’s massive steel towers, which stand in excess of 237 feet above the surface of the water. To that end, crews cored into the tower piers and grouted in 22-foot rods, 16 rods per pier.

According to the DOT, the bridge totals more than 2,040 feet in length. Its main span is more than 800 feet long. The bridge’s approach girders span an additional 520 feet over land, before connecting to each of the 360-foot-long truss approach spans.

The S&R crews have both land-based and barge-mounted cranes on site.

“The deck, suspended off the cable that’s there now, will come down in small sections, alternating from shore to shore and then to the middle of the structure,” said Coombs. “There’s a very elaborate plan of how to do this, and the sequence of how it’s done, so that we don’t overload the existing towers in any way.”

Bridge segments are loaded onto trailers and taken to a disassembly yard that’s been set up on nearby DOT property. All steel materials will be recycled. Concrete will either be disposed of or recycled.

The Deer Isle bridge is now the only functioning suspension bridge left in Maine.

Early in the last decade, DOT looked into rehabilitating the Waldo-Hancock Bridge, but found hidden corrosion of the suspension cable system that was too extensive. Supplemental cables were put in place above the original cables, and construction of the new bridge began.

For its time, the Waldo-Hancock Bridge represented a number of technological firsts. According to the structure’s listing with the Historic American Engineering Record, in the U.S. Library of Congress, it was one of the first two bridges in the United States, along with the St. Johns Bridge in Portland, Oregon (completed in 1931) to employ prestressed twisted wire strand cables, which were first used on a bridge built in 1929 in Quebec.

“The prefabrication and prestressing of the cables decreased the number of field adjustments required, saving considerable time, effort, and money,” the listing says. “As an additional experiment in efficiency, the Waldo-Hancock cables were marked prior to construction, ensuring proper setting. This method had never been used before and proved successful in this instance. These innovations, invented and pioneered by [the designer, David] Steinman, were a significant step forward for all builders of suspension bridges.”

The Waldo-Hancock’s main span was twice as long as any other bridge in Maine. And it was the first bridge to make use, in its two towers, of a type of open-web truss with vertical members but no diagonals. The truss design was subsequently used in other notable structures, such as the Golden Gate Bridge.

The bridge opened in November 1931. In 1932, the design earned a Most Beautiful Bridge award from the American Institute of Steel Construction.

Several months ago, with the start of demolition, stabilization of the towers took longer than expected, putting the project about 30 days behind, Coombs said. However, that time might be made up thanks to an innovative rigging system, using multiple rigging points, to remove the deck segments on the suspension portion of the bridge.

In the meantime, said Coombs, the Penobscot Narrows Bridge — home to the only bridge observatory in the United States, and only one of four bridge observatories in the world — is expected to have a lifespan about as long as its elderly neighbor, 75 to 100 years. Maintenance on the cable-stayed bridge includes constant checks for moisture and corrosion, and the modern design accommodates access within the structure to make sure all is well.

“Certainly our technology has come a long way since 1931,” he said.