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.

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.

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.

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.

A composite steel box girders with a cast-in-place concrete deck will form the $26 million Storrow Drive bridge, which is about 30 meters west of the cable-stayed bridge. It has three continuous spans, 69-, 116-, and 69-meters-long, respectively. The 9.4-meter-wide steel box girder, with cantilever floor beams, make up a finished roadway width of 22 meters. They are thought to be the largest of their type in North America.

A Busy Corridor

There are major obstructions all around the project. Some were known before construction and some were not.

The Massachusetts Bay Transit Authority's (MBTA) Orange Line tunnel crosses the river at the same location as the north tower of the mainline bridge. The bridge foundations straddle the Orange Line tunnel, a concrete box about 10.7 meters wide by 9.1 meters deep.

Its roof is about 12 meters below the top of the bridge foundation. Prior to construction, Atkinson-Keiwit probed widely and deeply to verify the Orange Line tunnel's exact location. However, during installation of steel sheeting for cofferdams at the north tower, they found more than they bargained for - the coffer and a 0.91-meter waler, used during original Orange Line installation, at a depth of 12 meters.

The north tower foundation plans originally called for a 6-meter-thick reinforced concrete footing with a 2-meter-thick concrete tremie seal. This would be eight meters deep.

The south tower foundation straddles a 0.91-meter-diameter water main and is two meters away from the Orange Line tunnel. The water main, parallel to the slope down to the water, was not located at the elevations originally thought, causing an extra cofferdam to be installed for protection. Portions of the pipe were encased in concrete for added protection.

The ultimate solution was two cantilevered lanes on the east side of the mainline bridge. The cantilevered portion will accommodate northbound traffic from the Sumner Tunnel and the North End, but it creates an asymmetric cross-section.

The asymmetric layout posed significant design challenges. The cantilever loads along the main span are carried by the east side cables at stresses up to 50 percent higher than on the west side. Also, with 10 lanes of traffic, this bridge is the widest cable-stayed bridge ever built. The entire bridge design has undergone intense scrutiny from a panel of experts convened by FHWA to evaluate its viability and integrity.

But construction access space is not the only item that requires precision coordination and planning. The entire traffic management sequence is a carefully choreographed effort of building structures, moving traffic onto them, demolishing existing structures, and building replacements.

For example, the Storrow Drive bridge will be open for traffic in autumn 1999. A portion of the I-93 traffic, both northbound and southbound, will be routed onto the new bridge, freeing up existing off-ramps that cut across the new mainline. These off-ramps will be removed just prior to opening the new mainline northbound lanes in 2002.

Building It Right

The Big Bridge's main span is being built piece by piece, said Al MacPhail, Bechtel/Parsons Brinckerhoff's resident engineer.

"First you build the uprights, then install the steel edge girders. You place three floor beams on each girder and put spacers between them. You tie up the whole thing with cables, then lay precast concrete decking onto the top," MacPhail said.

"Easy," he said with a smile.

The original design and specifications for the north back spans called for construction by incremental launch to avoid disturbing three active ramps to and from I-93 on the north side of the bridge. However, the Atkinson-Kiewit team proposed using conventional falsework supported by 0.91-meter-diameter pipe columns throughout the complex interchange, saving the project almost a million dollars and shaving nearly a year off the schedule.

Many pieces of steel in the bridges are more than 22.5 metric tons, and there is a lot of steel in these bridges. Steel fabricators throughout the United States are working at their highest capacity, and it was difficult to obtain the amount of steel needed for the project in a timely manner.

In traditional cable-stayed bridge construction, a multistrand jack weighing about 1.8 to 2.7 metric tons is used to tension the entire bundle of strands. However, a French isotensioning technique will be used, wherein each strand within the bundle is tensioned independently using a comparatively tiny jack weighing only 45 to 90 kilograms. This isotensioning method is new in the United States, but not in Europe.

Elastomeric dampers will be installed within the cable bundle at bridge deck level to reduce vibrations in the cables. Cross-ties have been routinely used to mitigate wind-induced vibrations, and although bridge designers are still analyzing vibrations, four main-span planes will still need cross-ties.

Other Central Artery/Tunnel contracts also incorporate material from various states. Most notably, structural steel is being fabricated or fit up in Alabama, Arkansas, California, Colorado, Connecticut, Florida, Georgia, Kentucky, Maine, Maryland, New Hampshire, New Jersey, North Carolina, Ohio, Oklahoma, Pennsylvania, Rhode Island, South Carolina, Texas, and Virginia. In addition, other project elements have varied points of origin: ventilation fans from Ohio, ceiling panels from Oklahoma and Vermont, tiles from Indiana and North Carolina, and temporary bridge elements from New York.

Furthermore, a precast concrete plant has been established in Sanford, Maine. This plant, which was converted from a vacant, pre-World War II airport hanger, was established specifically for a Central Artery/Tunnel contract, but it will continue to cast segments for other construction contracts around the country.

And the project's solid foundation, upon which all decisions and actions are based, is the team relationship between FHWA, MTA, Bechtel/Parsons Brinckerhoff, and Atkinson-Keiwit.

"Good communications is the key to teamwork and a successful job," noted MacPhail. "Everyone who is working on this project, from the laborers all the way through the management team, has an immense sense of pride at building such a challenging and significant project."

Sybil Hatch is a technical and marketing communications consultant, specializing in the engineering and construction industry. She has a bachelor's degree in civil engineering and a master's degree in geotechnical engineering, both from Virginia Tech. She is a registered professional engineer in California and has been practicing civil and geoenvironmental engineering for 13 years.