July 13, 2020

Towering Achievement Penobscot Narrows span bridges a transportation gap with form and function

The Penobscot Narrows Bridge and Observatory is arguably the most ambitious public construction project the state has seen in recent years.

That is appropriate, since it will replace the 75-year-old Waldo-Hancock Bridge which itself was an engineering marvel when it opened in 1931.

Today, hundreds of people are expected to descend on Bucksport, Verona and Prospect to walk over the new structure and listen to its builders describe its features.

In 2003, the state’s transportation department was already well into a $25 million rehabilitation of the old bridge when engineers discovered extensive deterioration in the bridge’s main cables. That discovery forced the department to place weight limits on the bridge until officials could install supplemental cables to bear some of the load on the bridge.

The discovery also put the construction of a new bridge on the fast track. Before the end of the year, officials had broken ground for the new span.

Its builders note with some pride that the new $85 million span will be the tallest publicly accessible structure in the state. It will be the only cable-stayed bridge in the Western Hemisphere with an observatory at its top.

The bridge was built in less than half the time it usually takes to build a project of this magnitude. It was done by two Maine companies – Cianbro Corp. of Pittsfield and Reed & Reed of Woolwich – working in a joint venture collaboration. More than 700 workers participated in the construction over the course of the three-year project. A core group of about 150 workers, almost all of them from Maine, have been on the project from the beginning.

The workers have made the project move ahead as smoothly as it has, according to Chris Burgess, lead engineer for the Figg Engineering Group of Tallahassee, Fla., which designed the bridge.

“I’ve worked throughout the country,” Burgess said during a recent media tour of the bridge. “These guys are the best.”

The bridge will be the first cable-stayed span in Maine, and there are some elements in its design and construction that make it unique, Burgess said.

The cable-stayed bridge is compared most often to the suspension-type bridge, the design used for the Waldo-Hancock Bridge which Penobscot Narrows will replace. Although both designs use cables, they differ in the way they handle the weight of the bridge deck, according to Burgess.

The suspension bridge has a main cable draped over the towers and uses suspender cables to support the deck. The main cables are anchored to the ground at each end of the bridge.

“With the cable-stayed bridge, the cables that support the deck are anchored in the deck and run directly up to the tower,” Burgess said. “They transfer the load from the cable into the tower and down into the foundation.”

The observation deck is a special feature of the bridge, Burgess said. Penobscot Narrows is just one of three bridges in the world that have the observatory built into the design. The other two are in Slovakia and Thailand.

In order to accommodate the elevator that will carry visitors to the observatory atop the western tower, the normal cable-stayed design had to be modified, according to Burgess. Normally, the cable stays are attached vertically, passing through the center of the tower, one directly above the other.

Because the elevator shaft is located in the center of the western tower, the cable stays in the Penobscot Narrows Bridge had to be offset so that they run from the center of the bridge deck through the tower in pairs, one cable stay on each side of center. Viewed from above, the cable stays form a V-shaped pattern which is unique among cable-stayed bridges.

Each cable stay holds individual, much smaller cables, the number varying between 41 and 73 cables based on the stay’s location on the bridge. There are fewer individual cables in the stays nearest the towers and increasingly more cables the farther away the stay is from the towers.

Another special feature in the bridge is the use of a cradle (similar to a culvert or pipe) that carries the cable stays through the tower. Each cable stay travels through a sleeve in the cradle which is embedded in the tower, so that the cables run from one deck anchor up through the tower and down to a corresponding anchor on the other side of the tower.

“With the individual cables there, you can replace one at a time, while maintaining traffic on the bridge,” Burgess said. “And the crews can work from under the bridge deck where they are protected inside the bridge.”

Only one other bridge uses the cradle design, he said. That bridge is under construction in Toledo, Ohio.

Other unique features of the bridge include:

. A gas protection system for the cables. The cable will be protected by a pressurized gas system that includes monitors that will alert DOT to any leaks that develop in the cable stays.

. A built-in force-monitoring system. This allows the state to monitor the force loads on individual cable stays easily and inexpensively to ensure that they are performing as designed.

. A curved, banked deck on the Prospect side of the bridge. That allowed the state to save money on land acquisition and blasting of ledge on the western approach to the bridge.

. Speed of design and construction. The work was completed in about half the time it normally takes to design and build a project such as this one.

. Construction method. An owner-facilitated, design-build format allowed the DOT, the designer and the contractor to work together as a team to complete the bridge. The method allowed construction to begin on the bridge while other sections were still being designed.

Join the Bridgewalk

The bridge will open to the public from 9 a.m. to 4:30 p.m. Saturday, although most events are scheduled to end by 3 p.m. Shuttle buses from parking areas in Bucksport, Verona Island and Prospect also will stop running at 3 p.m.

On the cover

An aerial view of Fort Knox in Prospect, the Waldo-Hancock Bridge

and the new Penobscot Narrows Bridge connecting Prospect and Verona Island right after sunrise on Sept. 30.

To purchase reprints of BDN photos, call 990-8100.

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