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Arch or not? Possible designs unveiled to replace Bourne and Sagamore Bridges

Arch span.jpg
Eve Zuckoff / CAI
Screenshot from Tuesday's virtual public meeting. Another meeting will be held on Thursday.

Officials at a virtual public meeting last night unveiled three designs that they’re considering to replace the aging Cape Cod Canal bridges.

Only one of the designs features an arch that resembles the existing Sagamore and Bourne bridges.

Design engineers say they’re leaning toward the arch structure option as a preference, as they begin seeking public input.

Nearly 500 people virtually attended the meeting. Many singled out the arch design as their preferred choice.

Questions raised by the public about the construction ranged from possible land taking and impacts on homeowners, to bike access and lane configurations.

Project managers from the Massachusetts Department of Transportation (MassDOT) said possible lane configurations and bridge widths would be discussed at a future meeting, and all three designs could accommodate a variety of lane configurations or alignments that provide ways for bikes, pedestrians, and vehicles.

The Arch design

The arch design, officials said, would be consistent with historical context. Also in its favor, among the three options, it would best be able to accommodate ramp connections with the smallest project footprint. And the arch itself could be built offsite, then floated in, simplifying construction.

This type of construction could easily adapt to wind conditions, said lead structural engineer John Smith. He also said he expects it to be cheapest to maintain over its lifespan.

"I think the arch type is the only one to go with," one commenter said in a chat box.

“I would agree that the arch type bridge is best suited for entry to Cape Cod,” added another. “I’ve owned a home in Sagamore Beach since 1965 and [I’m] excited to see this next step in upleveling living on the Cape.”

Box girder.jpg
Eve Zuckoff / CAI
Engineers are also considering a "concrete box girder" design, which would function and feel essentially like an upraised roadway, with no overhead support structures.

The Concrete Box Girder design

Smith and his team also considered the “concrete box girder” design, which would effectively look like a long road over the canal with little additional design features. Drivers would look up at only the sky above, rather than an arch of any kind.

It would be a “departure from the graceful existing steel bridges,” Smith said. It would also require an extended construction schedule, and longer approaches would need to be built, which would mean more impacts to the surrounding area.

But in the pros category, Smith said, it would perform best of the three under wind loading and construction wouldn’t affect canal operations.

Cable stayed.jpg
Screenshot by Eve Zuckoff / CAI
Also being considered is the "Cable Stayed" design option, resembling the Zakim Bridge in Boston.

The Cable Stayed design

The third option, “cable stayed,” would resemble the Zakim Bridge in Boston, and feature two large triangles connected by cable wiring overhead. Like the concrete box girder, it would be a significant change visually, and construction wouldn’t affect canal operations. But officials pointed out that it’s vulnerable to wind throughout construction, and it would require long cable-supported back spans, adding to cost and requiring steeper ramps.

It was not particularly favored by meeting participants.

“Cables,” one commenter said, “that’s more Boston, Tampa Bay. That’s not really the Cape feeling.”

Completion date? How about 2034

During the meeting, several locals raised questions about whether the government would be able to take their homes to accommodate the construction in a process called eminent domain. One participant, Hilary Anderson, said a realtor told her sister, who lives near the bridges, that she wouldn’t be able to sell her house until it’s clear whether eminent domain would affect the property.

“At this time we do not have an eminent domain because this is just the beginning stages, but when the plans are completed and if anyone is going to be affected you will be notified by a right-of-way agent," one official explained.

The timeline for construction depends largely on funding, but whichever design is chosen, officials said, the hope is that construction will begin around 2027 and last for an estimated six to seven years. Neither the existing Bourne or Sagamore bridge will be demolished until new bridges are built, they added.

During construction, officials said, they would provide a sidewalk for walkers, maintain two lanes in each direction at each crossing, maintain all existing roadway connections during construction, and get traffic off existing bridges as soon as possible to avoid the cost and disruption of having to perform extensive repairs on the aging infrastructure.

The existing bridges, which were built in the 1930s to span the Cape Cod Canal, were found to be “functionally obsolete” and needing replacement by the U.S. Army Corps of Engineers in 2019.

No matter what bridge design is chosen, the construction process will involve building “twin bridges” at each location to avoid major disruption to the Cape and Islands region. During phase one, a new bridge would be built “fairly close” to one of the two existing bridges. It’s not clear yet whether the first twin bridge would be built by the Sagamore or Bourne. But either way, the existing bridge wouldn’t be demolished until the twin bridge is finalized. Then the process would be repeated for the remaining bridge.

At the outset, officials considered seven feasible bridge types, narrowing them down to three based on screening criteria like initial cost, snow, ice and wind response, highway geometrics, and durability.

The project is still in the early stages of planning, funding and permitting, but it will be paid for by federal money and managed by the Army Corps of Engineers. Once work is complete, responsibility for the bridges will shift to MassDOT. Officials initially estimated the cost of the replacement would be more than $1 billion, but inflation has since caused that number to rise.

MassDOT will host a second virtual meeting on Thursday at 6 p.m. to present project updates once again, and hear public comment. A link to register for the meetings is here.

Eve Zuckoff covers the environment and human impacts of climate change for CAI.