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CABLE-STAYED Bridge Forms Boston Landmark.

Contractors building Boston's new signature bridge, the $100-million highway structure over the Charles River, are charging full speed toward completion of the bridge next summer.

"It's right on schedule," said Terry Brown, a spokesman for Bechtel/Parsons Brinckerhoff (B/PB), the joint venture managing consultants for Boston's $12.2-billion Central Artery/Tunnel (CA/T) project. In July the southern half of the bridge was completed and crews will proceed to the northern half and work south.

With its graceful cable stays and two 266-ft-high towers, the new bridge will be a symbol linking the city's future to its historic past. As conceived by Swiss bridge designer Christian Menn, the bridge's inverted Y-shaped towers will reflect the shape of the Bunker Hill monument in nearby Charlestown. Built to carry Interstate 93 across the Charles, the big bridge will be 1,457 ft long with a 745-ft long, 183-ft wide main span.

WHO'S DOING WHAT

The owner of the bridge, the Massachusetts Turnpike Authority (MTA), has the construction contract with Atkinson/Kiewit, which is a joint venture of Guy F. Atkinson Company (San Bruno, California), and Kiewit Construction Company (Omaha, Nebraska).

As managing consultants for the entire CA/T project, Bechtel/Parsons Brinckerhoff did or will do project planning, environmental studies, right-of-way work, conceptual development, preliminary design, contract negotiation, management and review of final designs, construction assistance, and management and close-out. By law B/PB could not do final design.

The Boston office of Kansas City-based HNTB, in association with Figg Bridge Engineers, was selected to do the final design. The final design effort required that the preliminary plans be extended to contract documents. During construction, HNTB/Figg assisted by answering contractors' questions and reviewing shop drawings.

ASYMMEIRICAL DESIGN

The bridge will carry 10 lanes of traffic on the main span. Eight lanes will pass through the legs of the twin towers, and two lanes will be cantilevered from the east side. Both back spans will have eight lanes. In the back span areas, the two lane ramp consists of bridges separate from the cable-stayed structure.

By having two "extra" lanes cantilevered from its east side, the bridge will become the nation's first asymmetrical cable-stayed bridge. It is also the widest cable-stayed bridge in the world (1183 ft). And by using both steel and concrete in its frame and deck construction, the structure will be the first "hybrid" cable-stayed bridge in the United States.

The main span consists of precast concrete deck panels acting compositely with longitudinal steel box edge girders and transverse steel floor beams. Cast-in-place closure strips allow for the composite action. Girders and two inclined planes of cables support the bridge's main span. Steel floor beams underpin the main span and extend out transversely to support the cantilevered lanes. The back spans consist of cast-in-place, post-tensioned concrete.

FOUNDATIONS IN BEDROCK

The foundation for the bridge consists of 15-ft deep footings supported by 8-ft diameter drilled shafts filled with reinforced concrete. The South Tower has 14 drilled shafts, and the North Tower has 116 drilled shafts. Where the Orange Line subway passes close by, the shafts are doubly encased and the gap between the casings is left unfilled to isolate the subway Wan earthquake should shock the bridge.

A sheet pile cofferdam is constructed around the perimeter of each footing before constructing the drilled shafts. The shafts, each 84 ft long including a 40-ft deep rock socket, were drilled using permanent steel casings that were vibrated down through soil layers to weathered rock. When drilling is complete, the reinforcing steel cage is placed in the casing. Next, 5,000-psi concrete is tremied into the casing, filling it from bottom to top as it displaces river water.

Six two-in. diameter PVC pipes are placed within the full length of the shaft to allow for cross-hole sonic logging -- a test that is run after the concrete has hardened to make sure the shaft has no voids or imperfections.

After completion of the shafts, a concrete seal is tremied into the cofferdam while water is still inside. The seal is 5 ft thick at the South Tower and 6.5 ft thick at the North Tower. After the tremie seal gains strength, the cofferdam is dewatered and footing construction begins. Each footing is placed in two separate layers using concrete pumps. The South Tower footing is 154 ft by 40 ft in plan and requires 3,400 cu yd of concrete, and the North Tower footing is 1138 ft by 55 ft and contains 4,200 cuyd of concrete.

Construction started on the bridge's south side, so the south tower pier is ahead of the north tower pier. But the north tower pier is catching up fast. The tower cross section varies From 22 ft by 24 ft at the base of one leg to 10.5 ft square at the top. The tower legs are hollow with wall thicknesses ranging from four ft to one ft.

The tower legs above the roadway are built using a huge temporary steel A-frame that forms the insides of the tower legs. Jump forms are used to form the outside and sides of the inclined tower legs. A typical lift of concrete is 20 ft deep with a concrete volume of 70 cu yd. Up to 130 ft above ground, concrete was placed using a pump truck. Above 130 ft, concrete was placed by a tower crane with a bucket.

The spire portion of the tower pier is formed around a prefabricated steel stay cable anchorage unit, and concrete, bucketed into place, encases the anchorage. The very top of the tower is extended by about 50 ft to give shape to the tower piers, which are capped by a pyramid to resemble the nearby Bunker Hill monument.

Below the roadway deck level, the tower legs are inclined inward because of the close proximity of the existing Orange Line subway ventilation building and because the inclines help minimize footing sizes.

VALUE-ENGINEERED CHANGE

Both the back spans are built of heavily post-tensioned cast-in-place concrete. They join the steel main span at the tension struts that connect the two inclined legs at the "knees" of each tower. The concrete for the back spans was pumped to them. The big back span girders were built starting with the bottom flange, moving to the webs and finishing with the top flange or deck. The total area of forms for the back spans is 260,000 sq ft.

Originally the bid documents showed an incremental launching scheme -- using precast concrete segments--for the north back span. But because of Atkinson-Kiewit's experience with building cast-in-place bridges over active roadways in the West, the contractor proposed a value engineered change to build the north back span on falsework, similar to what B/PB had proposed in the preliminary design stage. After a careful review by the agencies involved, cast-in-place construction was approved for the north back span.

The south back span was always going to be cast-in-place construction on falsework. Cast-in-place construction on falsework is considered a tried-and-true method, whereas the incremental launching of a nearly 126-ft wide precast structure was considered to be more risky.

The stay cable anchorages in the back spans are cast into the 10-ft by 10-ft spline beam located on the center line of the concrete box girder. Workers gain access for stressing the cables by the use of temporary scaffolding under the bridge. Each back span has a total of 24 stay cable anchorages. The 14 cables nearest to the tower are placed in pairs with cables radiating upward and outward to anchorages located in the inclined legs of the towers. The 10 cables furthest from the tower are single cables that radiate upward to anchorages located in the vertical portion or spire of the tower. The number of spans in each stay cable vary from 18 nearest the towers to 72 furthest from the towers.

The Charles River bridge marks the nation's first use of iso-tensioning to stress the stay cables. Simply put, the method allows the stay cables to be stressed one strand at a time. Based on the calculated installation force for the stay cable, the force required for the first or reference strand is calculated. After the reference strand is installed, each subsequent strand is installed to match the force in the reference strand.

Forces are measured by load cells on the reference strand and on the strand being stressed -- and an electronic control stops stressing when the forces are equal. When the stressing operation is complete, the force in each strand multiplied by the number of strands equals the calculated installation force. The final dead-load force in the stays ranges from 300 to 1,600 kips (kip = one thousand pounds) and the live load force in the stays ranges from 14 to 300 kips.

COUNTERWEIGHTS FOR BALANCE

Because the south back span is only about 126 ft wide and 275 ft long, there was not enough counter-acting force to resist the weight of the main span--which is 745 ft long and about 183 ft wide.

So the south back span had to be counter-weighted. The contractor came in with ship ballast with cement mixed into it to act as the counterweight. The material was somewhat less dense than the designed concrete, but the material was pumpable and more of it was used to create enough dead weight.

Both the back spans are heavily post-tensioned in both directions. Temporary piers are installed at critical locations. Then after some of the post-tensioning is installed in the back spans, most of the falsework can be removed. And the temporary piers are also removed as the stay cables are installed and stressed.

In the main span, steel edge girders reach the span's full 745-ft length. To construct the steel structure, the steel edge girders are first cantilevered out in 60-ft increments. The edge girders are fabricated into the 60-ft segments and field bolted to the previous segment. The 130-ft long steel floor beams are spaced at 20-ft centers and fit between the edge girders. The floor beams cantilever out on the east side to support the two-lane ramp called SA-CN. A separate cantilever floor beam is bolted to the edge girder to support the cantilever on the east side. All the steel in the main span is erected by a barge-mounted crane of 310-ton capacity.

To complete the steel construction on the main span, stringers connecting the floor beams are installed. Next the precast concrete deck slabs closest to the previously erected section are installed and the first stage stressing of the main span and back span stay cables takes place. Then the next set of precast panels is erected and corresponding stay cables are stressed to their first stage limit.

That process is then repeated for the third set of precast panels. Then longitudinal post-tensioning is installed followed by the placement of transverse concrete closure strips over the floor beams. After the closure strip has attained 300 psi concrete strength, the slabs are post-tensioned longitudinally. The final steps are to complete the longitudinal closure pours over the edge girders and stringers and do the second-stage stay cable adjustment of all cables in the unit.

TECHNICAL FIRSTS

Another first for the bridge is the use of visco-elastic dampers. To prevent the stay cables from vibrating due to aerodynamic and structural disturbances, the visco-elastic dampers are installed in the stay guide tubes just above deck level. The visco-elastic damper comprises a neoprene collar with a viscous fluid-filled bladder. Wind passing the cables at a certain speed and direction, coupled with light rain, can cause the cables to oscillate. To control this movement, spiral beads on coextruded high-density polyethylene sheathing as well as visco-elastic dampers are used.

The structure also marks the first use of grade 70 high-performance steel in a cable-stayed bridge. The top nine cables of both the back and main spans are anchored into a composite steel and concrete box in the spires. Both the stay cable tubes inside the concrete walls and the steel anchorage assembly are made of the high performance grade 70 ksi (kips per square inch) steel. The steel box core is 50 ft long and has shear studs to connect it to the cast-in-place concrete walls. The steel assemblies were fabricated by Grand Junction Steel (Grand Junction, Colorado), and brought to Boston by rail and truck.

Forces from the cables come down the inclined legs from above the roadway. At the roadway level these forces can be resolved into horizontal and vertical components. The horizontal component is an outward force that tends to push the legs outward at the "knees" of the inclined legs. To counteract this outward force from each leg of the towers, heavily post-tensioned struts -- one per tower -- are attached between the legs. Normally the strut takes the tension force. Rather than make this a regular reinforced member with a lot of reinforcement, it is customary to post-tension the strut and maintain a residual compression all the time. That enhances the durability of the strut concrete.

At the Charles River cable-stayed bridge, in the steel main span the cables are attached to the longitudinal edge girders. However, in the concrete back spans the cables are attached to a spline beam in the middle. In addition, both the top and bottom slabs of the back span carry the load from the cables toward the towers. That all creates an imbalance in the force distribution at the strut; torsion is developed at the location of the strut, which is the transition between the main and the back spans. That torsion has to be controlled, otherwise permanent deformations will occur. Post-tensioning was used to counteract this torsion and keep the strut in stable equilibrium. The strut is about 138 ft long, and the total post-tensioning force in the strut is 42,000 kips at final completion.

Two centuries after the American Revolution, in which Boston played a leading role, the city is now in the forefront of another revolution -- in the field of cable-stayed bridge technology. New technologies and innovations are hallmarks of the Charles River Crossing.

Vijay Chandra is a senior vice president, Parsons Brinckerhoff, New York City, and a consultant to Bechtel/Parsons Brinckerhoff on the CA/T project. Dan Brown is principal, TechniComm, Des Plaines, Illinois. Assisting in this article were Paul Towell and Keith Donnington from the CA/T Bridge Group in Bechtel/Parsons Brinckerhoff.
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Author:Chandra, Vijay; Brown, Dan (American writer)
Publication:Public Works
Article Type:Statistical Data Included
Geographic Code:1U1MA
Date:Aug 1, 2000
Words:2407
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