Big Bridge, Big Equipment: Contractors Rely on Putzmeister America Equipment to Construct the Tacoma Narrows Bridge
Big Bridge, Big Equipment: Contractors Rely on Putzmeister America Equipment to Construct the Tacoma Narrows BridgeSTURTEVANT, Wis. (October 20, 2004) – The new Tacoma Narrows Bridge – one of the longest suspension bridge to be built in the United States in almost 40 years – is taking shape on Puget Sound, south of Seattle, Wash.
At a point where the blue waters of the Puget Sound become a narrow channel almost a mile wide, the Tacoma Narrows Bridge connects a gap from the Washington mainland to the Olympic Peninsula. The location presents significant challenges, as the Tacoma Narrows is located in a seismically active area complicated by high winds, 15 ft. tidal swings and seven-knot currents.
The first cable-suspended bridge was built there prior to WWII. However, it was an architectural failure, nicknamed "Galloping Gertie" for the way it swayed in the wind. The
ill-fated bridge collapsed four months and seven days after it opened in 1940.
As a result, its replacement was a more aerodynamically sound steel bridge; and today it is the fifth largest suspension bridge in the United States. Since the bridge’s opening in 1950, traffic volume has soared to over 90,000 vehicles a day – 30,000 more than the maximum for which it was intended. It became paramount to construct another bridge that would reduce traffic congestion.
The new 2007 Tacoma Narrows Bridge – termed for its date of completion – will be 5,400 ft. in total length and 2,800 ft. in the main span. Being constructed directly parallel to the current Narrows Bridge, it will provide three eastbound traffic lanes, shoulders on both sides and a separate bicycle/pedestrian path. Meanwhile, the current bridge will be reconfigured to carry three westbound traffic lanes.
Although the center lanes of the two bridges will only be 200 ft. apart, the two bridges will not look alike. Unlike the existing green, steel towers, the new bridge towers will be coated gray with pigmented sealer and built of reinforced concrete. Today’s technology makes concrete much more practical than steel as a more cost-effective medium and with significantly lower maintenance.
After concluding the design, the first major task was to construct two of the largest caissons ever built, forming the foundation for the 510 ft. tall towers or piers. Each caisson, equivalent to a 20-story underwater building, contains about 38,000 cu. yd. of concrete and can accommodate the weight of a second deck (either road or light rail) in the future.
Proposals of how to efficiently concrete the caissons were presented to Tacoma Narrows Constructors, a joint venture of Bechtel and Kiewit Pacific. Product specialists Bill Carbeau of Putzmeister America and Rolf Dose of Putzmeister-AG Germany masterminded a strategic plan. They relied upon owner Skip Gribble of Northwest Concrete Pumps and Systems, Inc. in Seattle to successfully handle the plan’s implementation and mobilization of equipment. As a result of these combined efforts, the Putzmeister proposal was selected.
First, the technical plan allowed for pumping concrete from land versus hauling concrete on a barge. This allowed the ready-mix trucks to conveniently discharge directly into a concrete pump’s hopper from shore. Concrete would travel via a 5 in. delivery line along the current bridge until it became necessary to cross down 165 ft. and then over to a pumping barge. At that point, a ramp would hold the delivery system, which featured two 5 in. steel braided Putzmeister hoses to accommodate the tide.
Second, the use of Putzmeister’s highly advanced ZX pipeline alleviated the tremendous environmental concern about concrete spillage in the water. The ZX delivery line and couplings are leak-proof and rated for extremely high pressures.
Carbeau, Sales and Product Manager of Telebelt® and Specialty Equipment for Putzmeister America says, "The ZX pipeline and couplings were originally designed by Putzmeister for use in sewage treatment plants where it is critical to prevent leaks. The technology was successfully applied in this application and proved to be absolutely leak-free and trouble-free."
A third factor in selecting the plan was the pumping equipment’s design. The proposed Putzmeister BSA 14000 concrete trailer pumps were the only units on the market to offer both the high volume and high pressure combination needed for pumping concrete on this job. The units had electric twin motor drives and were capable of outputs up to 130 yd3/hr. Their maximum pressure was 2176 psi, which is crucial for effectively pumping concrete over the long distances that this job required.
Finally, no counterweight on the separate placing booms was required, even with the larger sized models selected for extended reach. The lack of counterweight was a significant factor in keeping the center of gravity as low as possible on the barge. With a 125 ft. horizontal reach, the Putzmeister MXKD 38/42 placing booms were the obvious choice. They were both long enough for efficient concrete placement and high enough to clear the rebar above the pour.
Once the Putzmeister strategy was selected, the next step was developing the two needed caissons. At a nearby Port of Tacoma site, 62 ft. high steel forms were built atop a cutting edge. Then the BSA 14000 trailer pumps pumped concrete into its walls, whereby a caisson was started. Because of the weight, the structure dropped 30 ft. beneath the water’s surface as planned. Both caissons were initially prepared in this manner.
In July 2004, three tugboats towed the first caisson to the construction site 11 miles away. The theory was for the in-coming tide to bring the caisson in from the port and the out-going tide to handle the final maneuvering. Nothing is simple when a 15,240 ton structure is being towed. At this point, it was essentially a seven-story high box that was 130 ft. long, 80 ft. wide and 80 ft. tall – and most of it was submerged.
The days following, the structure was precisely maneuvered into position. With only a 36 in. margin of error allowed, divers used global positioning systems to help secure the caisson with cables to pre-positioned anchors embedded in the Tacoma Narrows floor. The second caisson traveled to its final destination in August.
Over the next few months, the two structures, mostly hollow and each floating on 15 steel air domes, gradually sunk as a series of 10 ft. reinforced concrete walls were further added atop each caisson. Two MXKD 38/42 placing booms were responsible for simultaneously placing the concrete. Each placing boom had the ability to cover the other’s area as a backup measure, although this proved unnecessary as no technical problems were encountered. The average output was 60 yd3/hr with line pressures as high as 2000 psi.
Obviously, the bigger placing booms required larger versions of the Putzmeister’s standard lattice towers. Carbeau notes, "Two freestanding 40 ft. towers were bolted to a single floating barge where it was necessary to flood the chambers to achieve a zero-degree elevation with no more than a plus or minus three-degree list. This was to offset the loads imposed by the two booms that were mounted next to each other on the same side of the barge."
From land, concrete was pumped by two BSA concrete trailer pumps and 1500 ft. of ZX delivery line to one caisson while the placing booms placed an average of 1200 cu. yd. of concrete in a one day pour. Then, the barge moved to the caisson on the other side of the Puget Sound to pump the following week. To pump the other caisson, two BSA trailer pumps were set up on the other side of the bridge using 2200 ft. of ZX delivery line.
This flip-flopping approach meant the 210 ft. tall caissons could be built in a concurrent manner. When completed, the cutting edge was sealed with concrete and eventually went through about 150 ft. of water and 60 ft. of seabed to reach solid footing.
Northwest Concrete Pumps and Systems assisted with operators, training and service during concrete placement of the caissons over a six-month period. Gribble comments, "We never encountered even one minor problem with the high pressure trailer pumps or separate placing booms on this highly technical job."
Pier construction started in September 2004, and will continue over the next year. Pumping with the Putzmeister equipment was so successful that it is also being used to construct the piers.
Therefore, before being disassembled, the existing placing boom setup was used to place concrete as high as the units could reach – another 70 ft. above the caissons. Then, the same Putzmeister BSA 14000 trailer pumps were set up underneath both ends of the new bridge, again using a total of 3,700 ft. of ZX pipe to deliver the concrete to a bucket. The bucket places the concrete in the tower’s forms, which can only withstand a maximum of 24 yd3/hr.
When reaching the struts, a BSA 14000 trailer pump will pump the concrete all the way to the top of the strut’s 140 ft. height while a tower crane assists by holding the pipeline.
A system of concrete forms called "bird cages" is being used to build each leg of the hollow towers in 17.5 ft. increments. With each caisson and pier nearly as tall as the Space Needle, they will consume a total of 16,000 cu. yd. of concrete when completed next April.
Of special note is the concrete mix for the piers. It is both strong and dense to keep salt spray and acid rain from penetrating the piers’ steel skeleton; and it incorporates both plasticizers and fly ash. Typically, bridge tower specs call for a 7,000 psi; however, this mix is closer to 11,000 psi. Nearby concrete production takes place at a temporary batch plant set up by Glacier Sand and Gravel of Tacoma, Wash.
Anchoring the Cables
Presently, Telebelt conveyors are placing concrete to anchor the cables for suspension. Carbeau says, "The truck-mounted Putzmeister belt conveyors are an ideal solution because they can place the larger size aggregate extremely fast and without a possibility of plug-ups. When finished, the two cables will each consume 26,000 cu. yd. of concrete."
Gribble says, "We’re involved with about 95 percent of the concrete work on this high profile project. This includes providing the operators, training and service during the caisson and pier construction along with supplying the concrete placing equipment for the cable anchors, three bridge overpasses and several retaining walls. When finished, the project will have used over 255,000 cu. yd. of concrete."
Project construction started in 2002, and the new Tacoma Narrows Bridge is expected to open in 2007. Upgrades to the existing Tacoma Narrows Bridge will finish in 2008. Overall, the project cost is presently estimated at $849 million.
General contractor: Tacoma Narrows Constructors – a joint venture of Bechtel and Kiewit Pacific
Concrete supplier: Glacier Sand and Gravel – Tacoma, WA
Authorized dealer: Northwest Concrete Pumps and Systems, Inc. – Seattle, WA
Pumping contractor: Ralph’s Concrete Pumping – Seattle, WA
Concrete placing equipment for caissons and piers: Putzmeister MXKD 38/42 separate placing booms (2); freestanding 40-ft PM towers (2); and BSA 14000 trailer pumps (3)
Concrete placing equipment for anchors: TB 105 and TB 130 Telebelt conveyors
Concrete placing equipment for retaining walls and bridge overpasses: Putzmeister concrete boom pumps – various sizes