Why New Bridges Are Needed to Cape Cod

The Bourne and Sagamore bridges are “functionally obsolete” and well past their lifespan, mirroring infrastructure problems nationwide, says a Tufts engineering professor
A large continuous truss bridge. The Bourne and Sagamore bridges are “functionally obsolete” and well past their lifespan, mirroring infrastructure problems nationwide, says a Tufts engineering professor
The Sagamore bridge, built in 1933. “The existing bridges have exceeded their design life,” said Brian Brenner. Photo: Matt H. Wade/@thatmattwade/via Wikipedia
November 6, 2019

Share

For New England beach lovers, going to the Cape is a ritual of summer. Another less pleasant ritual is sitting in traffic at the Bourne and Sagamore bridges. Their poor condition has increasingly required major maintenance work, causing significant delays.

In response, the U.S. Army Corps of Engineers released a draft recommendation last month calling for the complete replacement of the bridges, both of which were built eighty-four years ago—and originally intended to last fifty. The report called the $1 billion project the most practical solution for the bridges, which were described as “functionally obsolete” because of their inability to handle traffic needs.

The new bridges would each have four travel lanes, two added lanes for merging traffic, and, unlike the current crossings, a median separating the on-Cape and off-Cape-bound traffic. The current bridges would remain in service until the new bridges open to traffic.

The need for bridge infrastructure improvement is a common refrain in the United States.  According to the American Society of Civil Engineers 2017 report card, the average age of U.S. bridges was forty-three years, which means many are close to the traditional lifespan of fifty years old. In the report, the ASCE also found that 13.6 percent of U.S. bridges were functionally obsolete.

Tufts Now spoke with Brian Brenner, a professor of the practice in the Department of Civil and Environmental Engineering who specializes in bridge analysis, design, and structural engineering, to gain some insight on the project.

Tufts Now: What can you tell us about the bridges?

Brian Brenner: The existing two bridges were designed by Fay Spofford & Thorndike, my former company. The type of bridge is a continuous truss, in which the structure rises above deck. This design was innovative at the time, and widely used for longer span bridges from the 1940s to 1960s.

The physical shapes of the bridges are iconic. Millions of drivers have started their Cape Cod vacations driving across the Bourne and Sagamore bridges. That being the case, the aesthetic design of the new bridges will probably attract a lot of public input and debate.   

What are some of the issues that led the Army Corps to recommend complete replacement of the bridges?

The existing bridges have substandard highway geometry and are classified as functionally obsolete. This condition seems pretty clear to anyone who has ever driven across them in the narrow lanes—and especially if you are driving next to a truck. Both bridges have one narrow sidewalk each, which is tough and unpleasant for pedestrians and cyclists. I bike to the Cape from Boston every summer, and I have to walk my bike across the Sagamore bridge—it is not practical or safe to ride. The replacement bridge designs are planned to have wide separate pedestrian paths and bikeways. 

The existing bridges have exceeded their design life. The Bourne and Sagamore bridges were built in 1933. The salty canal environment has taken its toll on the steel framing, leading to the need for more frequent structural repair. At a certain point, it is more cost effective and less disruptive to replace the bridges instead of ongoing repair and rehabilitation.

What are some considerations for replacing the structures?

Building two new bridges greatly improves plans for construction staging, because it is possible to build the new bridges alongside the old ones with limited traffic impact. If the plan only was to repair the existing bridges in place, the traffic impacts probably would be more significant. Today there are a total of eight traffic lanes connecting the Cape to the mainland, four on each bridge. Taking only one lane out for any period of time, even off-season, has led to large traffic backups.  

Projects to replace the Cape Cod bridges will likely also include improvements to connecting roads. In addition to the announcement by Army Corps of Engineers, which owns the bridges, the Massachusetts Department of Transportation has prepared studies to evaluate concepts for improvements to the connecting roads. For example, there are studies to improve—and possibly eliminate—the Bourne Bridge rotary.  

Kalimah Redd Knight can be reached at kalimah.knight@tufts.edu.