Design & Construction Team
Project name: Tacoma Narrows
Bridge owner: Washington State Department of Transportation
Design-build contractor: Tacoma Narrows Constructors (a joint venture of Bechtel Infrastructure Corporation of San Francisco, and Kiewit Pacific Co. of Omaha, Neb.)Designer: Parsons, New York and San Francisco, and HNTB, Bellevue, Washington
Ten years after the infamous "Galloping Gertie’s" spectacular collapse in 1950, the second Tacoma Narrows Bridge opened to traffic. Built directly on Gertie’s caissons and over Gertie’s remains, it has since provided a reliable route across the Narrows for thousands of motorists. However, in recent years, increasing traffic demand along State Route 16 has exceeded the highway’s and the bridge’s four-lane capacity. Commuters living in Kitsap County experience extreme directional peak traffic flow as they travel to Tacoma, Seattle, and other points east of Pudget Sound. Traffic backups are commonplace and were the impetus for the Washington State Department of Transportation’s (WSDOT) decision to build a parallel suspension bridge to provide needed highway capacity. The state partnered with a design-build team to achieve its design and construction goals for the project. The new bridge (the left bridge in the rendering above), which is currently under construction, is scheduled to open to traffic in early 2007.
This rendering shows the two bridges side by side.
The scope of the project
On Sept. 25, 2002, WSDOT issued Tacoma Narrows Constructors (TNC)—a 50/50 joint venture of San Francisco-based Bechtel Corporation and Omaha, Neb.-based Peter Kiewit and Sons—a notice to proceed. This agreement allowed TNC to design and construct, under a design-build contracting arrangement, 3.4 miles of improvements to State Route 16. Within that 3.4-mile segment was the design and construction of a new, 1-mile long parallel suspension bridge. Other elements of TNC’s scope include upgrades to the existing bridge, relocating a memorial park, and building bridge maintenance and toll operating facilities.
When completed, the new bridge will be the world’s first major suspension bridge to be constructed under a design-build contracting arrangement. The bridge, designed by Parsons’ New York and San Francisco offices and the Bellevue, Wash., office of HNTB under a contract from TNC, will consist of a 2,800-foot main span supported by reinforced concrete towers. The towers will be founded on massive gravity caissons of open-dredge construction. Gravity anchorages on the hillsides of the Narrows will secure the main suspension cables. A unique orthotropic deck system will be integral with the superstructure stiffening trusses, and the design will include provisions for a future, lower roadway or light rail system. The project schedule includes a fast track design, which is now complete, to meet the 55-month, overall project delivery schedule.
The new bridge
The existing bridge is located at the narrowest point of the sound where tides run through at speeds up to 7.5 knots. The existing bridge piers generate significant turbulent eddies as these tides ebb and flow each day. Furthermore, the debris field resulting from the partial collapse of the original bridge cannot be disturbed, as it is now designated as a National Historic Monument. These factors combine to make construction in the water highly challenging. Additionally, the banks are steep, which creates geotechnical slope-stability challenges. No approach spans will be used on the new bridge.
The rectangular bridge caissons are 130-feet-wide x 80-feet-long reinforced concrete boxes with 15 internal, 22-foot-square dredge wells. The west caisson is 192 feet 6 inches tall and the east caisson is 218 feet 6 inches tall. Following a complex sinking operation, the west and east caissons touched down to the sea floor in December 2003 and January 2004, respectively. In all, each caisson includes 31,000 cubic yards (cy) of 4,000 pounds per square inch (psi) concrete—more than three times the amount of concrete as the towers that they will support. Each tower will be constructed with approximately 8,500 cy of concrete. The photo later in the story provides a bird’s eye view of one of the two caissons in the Narrows.
The east and west towers will stand 505 feet above the Sound and consist of inclined legs and three cross struts. The tower legs are hollow, rectangular reinforced concrete boxes that measure 14 feet in width and taper in length from 29 feet at the base to 24 feet at the tower top. Above the roadway, the walls of the tower legs will be 2 feet thick. Below the roadway, the transverse wall thickness is 4 feet to accommodate the reaction of the bridge superstructure under lateral loads. The lower, middle, and upper post-tensioned, cross struts all will be 15 feet wide and will be 25 feet, 20 feet, and 15 feet deep, respectively. Using jumpforming techniques, the two towers will be constructed with a total of 17,122 cy of 7,000 psi microsilica concrete. Tower construction is scheduled to begin in June and could take up to 10 months to complete.
The pull of the main cable tension will be resisted on both sides of the bridge by keyed-in, gravity anchorages. Each anchorage is being constructed of cast-in-place reinforced concrete and contains approximately 20,600 cy of concrete. The main features of the anchorage includes the splay chambers (where the main cable strands flare out), the anchor block, the splay saddle support plinth, the shear key, and an open center chamber.
The roof of the center chamber also serves as the roadway for vehicles traveling onto and off the bridge; precast, prestressed girders support the roadway slab for most of the anchorage length. At the expansion joint, precast, prestressed box girders are required for increased rigidity and torsional resistance. Should the lower level of the bridge be constructed in the future, traffic could pass though the center chamber once planned modifications to the anchorages were completed.
Both anchorages currently are under construction. As of February 2004, one anchorage was nearly ready to accept the anchor frame and grout tubes, and the shear key of the other anchorage was nearing completion.
The main cables will be constructed of galvanized, high-strength No. 6 gage (0.196-inch diameter) steel wire. The wire will be air spun by the modified controlled tension method into 19 strands per cable in a hexagonal pattern. Each strand is to be composed of 464 wires, resulting in a total of 8,816 wires per cable. The wire is provided from the manufacturer in coils weighing approximately 2,200 pounds, and it is spliced and reeled onto 11,000-pound capacity reels for spinning operations. Once spun, each strand will be formed into a circular shape and bound with seizing straps and individually adjusted to the correct sag profile in each span. This adjustment will be carried out at night to ensure consistent temperature conditions. The hexagonal configuration of the strands of the finished cable will be compacted hydraulically into a single, circular bundle with a diameter of 20-1/2 inches. It also will receive cast steel cable bands and will be wire-wrapped and painted. The total weight of the cables will be approximately 5,200 tons.
In the anchorage splay chambers, the individual wires will be looped around semicircular cast steel strand shoes. The strand shoes are anchored to threaded steel rods that will be embedded in the anchor block through grout tubes and bear against the anchor frames embedded in the anchor block concrete.
The cable wires will be protected from corrosion at several layers. First, the wires themselves are hot-dip galvanized as part of the manufacturing process. Following main cable spinning and compaction, a layer of protective zinc paste will be hand-applied to the exterior cable wires. The cable then is wrapped in galvanized steel wire placed in the bed of zinc paste. The wrapping wires are then cleaned of excess paste and painted with a Noxyde paint system—a semi-paste acrylic paint with elastic polymers capable of sustaining 200-percent elongation. The need for a climate shelter may be necessary at times during the application of the paint system to meet the atmospheric requirements called for in the special provisions of the project located in the Pacific Northwest—where cool, wet winter conditions prevail.
Each rectangular bridge caisson is a 130-feet-wide x 80-feet-long reinforced concrete box with 15 internal, 22-foot-square dredge wells. These critical structures were constructed above the water and positioned into place through a complex sinking process.
The suspended superstructure will consist of two, 23-foot, 6-inch deep, continuous trusses with an integral orthotropic deck. The west, main, and east suspended spans measure 1,400, 2,800, and 1,200 feet respectively, for a total of 5,400 feet of suspended superstructure. The cross section provides a 56-foot-wide roadway between reinforced concrete traffic barriers. This will provide two travel lanes, a high-occupancy-vehicle lane, and two, 10-foot-wide shoulders. The top and bottom chord members are sealed box sections, while the verticals and diagonals are fabricated I-sections. The truss is fully welded with the exception of shop-bolted splices at 40-foot intervals in the truss top chord, 60-foot intervals in the truss bottom chord, and bolted field splices at 120-foot intervals in chords and diagonals.
Ultimately, the orthotropic deck plate will be welded into a single plate measuring about 1 mile long. Typically, structural members are to be fabricated from American Association of State Highway and Transportation Official’s (AASHTO) M270 Grade 50 (Grade 345) steel, though extensive use of stronger Grade HPS 70W (HPS 485W) steel is used in regions of high member demand, particularly in the areas near the towers. For economy, the traveler rail for the bottom maintenance traveler is designed and detailed to be integral with the bottom flange of the bottom chord. This design eliminated the need for separate traveler rail beams and, therefore, reduced the overall superstructure depth and the profile exposed to the wind, while providing easy access for fieldwork and future maintenance.
At the bottom chord level, the lateral bracing configuration consists of a diamond-braced system with perpendicular cross struts at the even panel points. Also framing into the bottom lateral system, at the centerline of the bridge at the even panel points, are the "K-type" tubular cross frames. These tubular members are also fabricated of AASHTO M270 steel to comply with the AASHTO/American Welding Society’s D1.5-2002 welding code.
The center ties are comprised of additional diagonals erected at midspan above the stiffening trusses after full dead load is in place to connect to the main cables at articulated cable bands—which allow for movement without damaging the cables. This unique center tie transmits longitudinal loads from the superstructure to the cables and maintains their positions relative to one another to help keep the superstructure centered to improve serviceability. Vertical loads primarily are transmitted to the cables at these locations through suspenders.
The orthotropic deck will consist of a 5/8-inch deck plate and 29 sealed trapezoidal ribs. The deck plate also will extend over the top chord and acts integrally as the top flange of the top chord. This provides for a rigid system and prevents the need for a separate, upper lateral system—which provides significant savings in material, fabrication costs, and fieldwork. In regions where the top chord is specified to be of HPS 70W (HPS 485W) steel, the deck plate and ribs also are being fabricated from the same high-grade material. The orthotropic deck will be coated with a durable, acrylic resin-based waterproofing membrane prior to placing a two-course, 2-inch thick Trinidad Lake Asphalt modified overlay.
The bridge sections are being fabricated in Korea by Samsung Heavy Industries, under subcontract to a joint venture of Nippon Steel and Kawada Bridge. The bridge will be fabricated in 46 segments, typically 120 feet long. They are scheduled to be shipped across the Pacific Ocean stacked four segments high on three ships. Each segment, weighing approximately 400 tons, will be hoisted into place from a pair of gantries mounted atop the main cables. Sections near the anchorages will be swung into place and/or slid into position on skids. The segments will be painted and outfitted completely with utility lines and access platforms in the fabrication shop to expedite construction.
Designing for future double deck
Requirements were included in the project design criteria to accommodate a future lower level. A secondary cable in new anchorages could be added to support the additional load placed on the suspension system, with little affect on the current system.
However, to accommodate the heavier reactions at the tower, the towers and caissons were designed for the loads to be imposed upon them from the future lower level, and the anchorage design had to allow for the passage of the lower level between the splay chambers of the new bridge.
In many cases, the superstructure design was governed by the geometric demands and traffic loads anticipated for the future lower level. For example, the depth of the truss was set to accommodate future truck and/or light rail clearances. Also, the top and bottom chords and truss diagonals were designed to accommodate the future lower level without additional reinforcement. Furthermore, the bottom lateral bracing and cross frames are envisioned to be removed and replaced with an orthotropic deck integral with the bottom chord—similar to the upper deck—with knee braces to provide rigidity. How this second level is constructed ultimately will be determined when and if WSDOT decides to move forward with future expansion.
The new Tacoma Narrows Bridge is only the second major suspension bridge to be built in the United States during the past 40 years; and it’s the longest to be constructed since the Verrazano Narrows Bridge was completed in New York. With its integral orthotropic deck, specialized foundations, and unique center tie, the new bridge is setting the standard for the next generation of suspension bridges.
At this point in the project, the design-build process has been successful in keeping the fast track design on schedule and under budget. In addition, the constant interaction between the owner (WSDOT), the design-build contractor (TNC), and the designer (Parsons/HNTB) throughout design process has ensured that potential problems are resolved before construction begins.
David J. Climie, C. Eng, FICE, is the superstructure design and construction manager at Tacoma Narrows Constructors. Seth Condell, P.E., is the suspended superstructure work package lead at Parsons. Augusto Molina, P.E., is the suspension system work package lead at Parsons. Joseph M. Viola, P.E., is the superstructure design manager at Parsons. Viola is the contact for this article and he can be reached at firstname.lastname@example.org.
Sidebar: The Infamous Galloping Gertie
The Tacoma Narrows in Washington State is the narrowest waterway in Puget Sound and separates the Olympic Peninsula from Washington’s mainland. In 1940, the first suspension bridge spanning the waterway opened to traffic. Dubbed "Galloping Gertie," it collapsed in just four months and still endures as the textbook example of a bridge engineering failure. But the bridge has become a great teaching tool for bridge engineers to perfect their knowledge on how to design and build suspension bridges.