Big Bridge, Little Bridge: the big dig soars across the Charles River - includes related article on the Central Artery/Tunnel project - two new bridges across the Charles River in Boston, Massachusetts - Cover Story

Public Roads, Sept-Oct, 1999 by Sybil Hatch

It's the Friday before Memorial Day 1998, and Boston motorists are swarming onto the freeways for their annual weekend getaways. Commuters are anxious to get home for the holiday weekend. Truckers are making their final deliveries for the week. And someone notices that the upper deck of the existing Interstate 93 bridge crossing the Charles River has settled several inches near an expansion joint.

The bridge is closed. Urgent phone calls are made. Central Artery/Tunnel engineers and inspectors rush to the site. The project team has spent upwards of three billion dollars worth of hours and effort to keep Boston open to traffic during construction. "Did we cause this?" is the question foremost on their minds.

Keeping I-93 and its Charles River bridge open while building under and around it is a monumental effort. The plan calls for underpinning and transferring the load from the existing I-93 footings to the walls of the new underground tunnel that leads up to the bridge. After traffic is flowing through the tunnel and onto the new Charles River bridges, I-93 will be demolished.

But the underpinning work in that area was not scheduled to begin until later in the summer of 1998. It turned out that several stringer beam webs had buckled, causing the bridge span to settle. Central Artery/Tunnel managers breathed a sign of relief, and the Massachusetts Highway Department directed emergency repairs, opening the bridge again in time for traffic returning to the city after the long weekend.

Past Its Prime

The I-93 closure confirmed what the Massachusetts Turnpike Authority (MTA) had known since first conceptualizing the Central Artery/Tunnel project in the mid-1980s: the bridge and freeway system was sorely in need of replacement.

When the I-93 double-decked steel-truss bridge over the Charles River was built in 1959, 75,000 vehicles traveled over it each day. On any given day in 1998, the bridge carried more than 190,000 vehicles.

The existing bridge is completely overwhelmed. Because of local-access traffic patterns, the existing northbound I-93 mainline narrows from three lanes to two, allowing traffic from Boston's Government Center via Storrow Drive to access the freeway.

"Very congested and very confused," says Terry Brown, an MTA spokesperson. "We have disparate pieces of highway that don't work together."

An overarching goal of the Central Artery/Tunnel project is to eliminate confusion. (See the sidebar, "Revolutionizing Boston.") Mainline, interchanges, on- and off-ramps, and bridges are being completely rebuilt so that the entire highway system through Boston works.

Bridges in the Making

The new Charles River bridges - the mainline bridge and the Storrow Drive Connector bridge - are an integral part of the new system. The people designing and constructing these two bridges call them "Big Bridge" and "Little Bridge," respectively. These bridges will replace the existing I-93 bridge.

The Big Bridge holds four lanes in each direction plus two dedicated lanes for direct access to a nearby neighborhood. The Little Bridge has two lanes in each direction. The total of 14 lanes more than doubles existing capacity. But perhaps more importantly, the new bridges eliminate the confusion and the safety problems caused by the Byzantine web of weaving, merges, and bottlenecks.

The Charles River bridges were designed by HNTB under the supervision of a joint venture of the Bechtel and Parsons Brinckerhoff companies. Daniel O'Connell Sons is constructing the Little Bridge, and an Atkinson-Keiwit joint venture team is building the Big Bridge, with the Bechtel/Parsons Brinckerhoff team providing program management, inspection, and oversight during construction. The Federal Highway Administration (FHWA) is providing technical and administrative expertise throughout the entire project.

History and Elegance

The mainline bridge is the gateway to downtown Boston. From many proposed designs, MTA ultimately chose a cable-stayed bridge by Swiss bridge designer Christian Menn. Its stately 82-meter-tall inverted-Y towers mirror the shape of Boston's Bunker Hill Revolutionary War Monument in the neighboring Charlestown section of Boston.

The 429-meter-long bridge, with an $93 million price tag, has five spans: two back spans of 34 and 40 meters on the downtown (south) end, two back spans of 76 and 52 meters on the Charlestown (north) end, and a 227-meter main span. The bridge, with steel box edge girders and steel floor beams in the main span and cast-in-place post-tensioned concrete on the back spans, is the first "hybrid" cable-stayed bridge in the United States.

Girders and two planes of cables will support the main span. The back spans are supported by a single plane of cables. Maintaining the traffic flow from the existing I-93 to Storrow Drive effected the layout of the cable stays. It was necessary to move the back-span stay-cable connections to the superstructure from the edge (from the outside similar to the main span) to the center. But this is only one of many other features built into the bridge to optimize traffic flow and accommodate obstructions around and below the bridge.