A new feature of the Boston skyline is actually an old, familiar sight. Poking their heads above the forest of cranes that now mark downtown Boston are a pair of obelisk-shaped spires designed to echo the Bunker Hill Monument, a historic landmark of the Charlestown neighborhood to the north. Splayed out from these towers will be 116 cables that will support the new Charles River Bridge, which is the signature piece of the Central Artery/Tunnel Project, a.k.a. "The Big Dig."
The $86.4 million Charles River Bridge was conceived by Swiss bridge designer Christian Menn. It represents several firsts. It is the first asymmetric cable-stayed bridge in the U.S. With a 183-ft-wide main span, it is the widest cable-stayed bridge in the world. It also is the first cable-stayed bridge in the U.S. to use both steel and concrete in its frame: The back spans are constructed of post-tensioned concrete; the main span connecting them is made of steel box girders and steel floor beams.
The 1,457-ft-long bridge will take I-93 as it emerges from the new Central Artery tunnel under downtown Boston and carry four northbound and four southbound lanes of traffic across the Charles River to Charlestown. In addition and separately, the bridge will carry two northbound lanes, cantilevered on the east side of the bridge outside the inverted Y-shaped towers, from the Sumner Tunnel and the North End. The new bridge replaces an existing six-lane, double-deck span.
At the time of a visit by ROADS & BRIDGES in late October, the south tower was basically completed, along with the south back span. On the north end, everything was completed except for the top 50 ft of the tower. Construction had begun on the main span that will join the two ends over the Charles River.
The towers reach about 320 ft above the water level on the north end and about 290 ft high on the south. When completed, each tower will have 58 cables attached to it, with the other end of each cable anchored in the bridge deck. The deck will be about 50 ft above the river.
Meet me in the middle
The 745-ft-long main span is being constructed in 60-ft segments, according to Richard Raine, project manager for Kiewit Construction Co., Boston, which is the prime contractor for the Charles River Bridge. "We basically build the structure in 60-ft pieces, and there are several cables that are installed at various steps throughout the construction of that 60-ft segment."
The structural steel girders and beams come from Grand Junction, Colo., according to Raine. "The steel pieces come by rail to a local yard," he explained. "They?re transferred to trucks, and they?re brought to the site. On site, we load them onto barges and we dress them out, and then we install all those parts and pieces one by one."
There also are precast concrete panels?shipped by barge from Virginia?that sit on top of the steel framework and closure concrete that ties the panels together and to the structural steel. Raine said that Kiewit expected to have the structural work on the bridge completed next summer. "Then we?ll finish up with barrier rail and overlay," he commented. "The actual opening of the bridge will be dependent upon the completion of the rest of the artery."
One of the challenges of constructing the bridge has been the close quarters imposed by the other transportation structures crammed into the north end of Boston. For instance, one of the bridge cables passes only 3 ft from an existing structure, which the south back span ducks under. The leg of the existing structure actually passes through the bridge. "In the future, we?ll demolish that structure and build that piece back," Raine said. "That?s an indicator of how tight things are."
On the north end of the bridge, the north tower straddles the Massachusetts Bay Transit Authority?s Orange Line subway. "We had to drill shafts on either side of it and build a giant footing right over the top of the active Orange Line," said Raine. "That makes a lot of the logistics really tricky."
The trickiest part of building the bridge has been the logistics of receiving materials. "We don?t have a big yard to stockpile materials," said Raine, "so we try to schedule them on an as-needed basis. It?s been a juggling act for all of us, including our suppliers."
Framing a fearless asymmetry
What makes the bridge asymmetric is the two-lane roadway tacked onto the east side of the main roadway. The weight of these two northbound lanes tends to "want to push the bridge to the west while you?re constructing it," said Raine, "so we compensate to the east, and when we?re done theoretically we match in the middle."
T.Y. Lin International, San Francisco, developed a detailed computer model that lets Kiewit know where the bridge has to be positioned, at each stage of construction, in order for the pieces to fit together at the end. "Where we place the bridge during construction is not where it ends up," said Raine, "because we have to account for what?s going to happen to the bridge downstream in our construction process, and that?s going to cause the bridge to move in different places."
So, for the addition of each element, whether a cable, a piece of the main span, barrier rail or overlay concrete, Kiewit follows a prescribed sequence of actions. "When we bring a piece in, we place it," said Raine. "We have to push and pull it to make sure the tip of the piece is where it?s supposed to be. We lock off the bolts, and we go to the next step."
Each cable has to be added in a specific sequence and loaded to a certain force, which is not the final force it will feel. The engineering computer model predicts the final load each cable will carry. The cables on the east side of the bridge will feel more load than the ones on the west side because of the extra traffic lanes.
The stay cables that carry the Charles River Bridge have a few innovations of their own. The basic constituent of one of the cables is a high-grade steel wire. Seven of these wires, in a hexagonal arrangement, are put together to form a strand, with grease in the spaces between wires to provide corrosion protection. Each of these strands has a strength of 270 kips/sq in. (1 kip = 1,000 lb). Then each strand is coated with high-density polyethylene plastic to provide a second layer of protection. The biggest cables on the bridge combine 72 strands; the smallest have 17.
Each cable is then encased in a protective sheath, which is continuously welded closed, with an expansion slip joint at the top and a gasket at the bottom. That sheath is what is visible from the outside.
One of the innovations of the Charles River Bridge is that the space inside the sheath is not grouted. "This will be the first bridge in the U.S. that utilizes ungrouted cables," said Raine. "I think that some folks believe that the grout provides a higher degree of long-term protection for the cables," he explained, "but there?s another school of thought that says it?s unnecessary and it makes future cable replacement or repair much more difficult and more costly."
Raine said he thought most cable-stayed bridges outside the U.S. used ungrouted cables, "but the U.S. has been favoring grouted cables to date, so it?ll be interesting to see what happens from here on out."
The bridge is engineered to support itself with one cable missing, according to Raine, so that, if one of the cables is ever damaged, for instance by a vehicle crash, the cable can be removed and replaced.
Each cable, as it is installed, picks up its share of the deadload.
The bridge designers have implemented a couple of innovative ideas for damping out cable vibrations that could otherwise resonate through the structure, become amplified and cause damage. "I?ve seen videos of cables moving pretty violently," noted Raine. "Not these. So far, we haven?t noticed significant cable vibrations."
Raine said the biggest threat for setting up resonant cable vibrations was a combination of light wind and rain. The designers with the computer models tried to predict the resonant vibrations in the bridge but were not entirely successful, commented Raine, because "it turns out it?s more of an art than a science." The designers also conducted real-world wind testing on the structure.
One simple feature that provides a big benefit, according to Raine, is the bead that traces a double-spiral pattern down the outside of the sheath of each cable. "I?m no wind expert," Raine remarked, "but they say that that interrupts the wind-rain phenomenon, and the cable can?t get started to vibrate."
Each cable also will have a visco-elastic damping device developed by the cable supplier, the Freyssinet Group. Raine described the device: "Basically, it?s an oil bladder in a steel collar that surrounds the steel strands. If vibration begins, it has a damping effect." Such a visco-elastic damping device will be installed in the anchorage of each cable.
The damping devices are just one element in keeping the new Charles River Bridge strong and stable. As functional elements, the towers and stay cables will support the I-93 roadway over the Charles River. As an aesthetic element of the Boston skyline, the bridge will serve as a symbol of the monumental feat that is "The Big Dig" and give the people of the city a reason?and possibly the spare time?to look up from the bumper in front of them.