Placing Concrete In Suspension

The new Tacoma Narrows Bridge — one of the longest suspension bridges to be built in the United States in almost 40 years — is taking shape on Puget Sound, south of Seattle, WA. The bridge will connect a gap from the Washington mainland to the Olympic Peninsula. Its 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 new Tacoma Narrows Bridge will be 5,400 ft. in total length and 2,800 ft. in the main span. Constructed directly parallel to the current bridge, it will provide three eastbound traffic lanes, shoulders on both sides and a separate bicycle/pedestrian path. The existing bridge will also be reconfigured to carry three westbound traffic lanes.

Although the center lanes of the two bridges will be only 200 ft. apart, the appearance of the two bridges will be quite different. 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 more practical and cost effective than steel with significantly lower maintenance.

Developing a plan

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 is equivalent to a 20-story underwater building, contains about 38,000 cu. yds. of concrete and can accommodate the weight of a second deck (either road or light rail) in the future.

Proposals for 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.

The technical plan called for pumping concrete from land vs. hauling it 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., 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.

The use of Putzmeister’s ZX pipeline alleviated the environmental concern about concrete spillage in the water. The ZX delivery line and couplings are leak-proof and rated for extremely high pressures.

Putzmeister BSA 14000 concrete trailer pumps offered both the high volume and high pressure combination needed for pumping concrete on this project. The units had electric twin motor drives and were capable of outputs up to 130 cu. yds./hour. Their maximum pressure is 2,176 psi, which is crucial for effectively pumping concrete over the long distances involved on this job.

No counterweight was required on the separate Putzmeister MXKD 38/42 placing booms. This proved a significant factor in keeping the center of gravity as low as possible on the barge. With a 125-ft. horizontal reach, the placing booms were long enough for efficient concrete placement and high enough to clear the rebar above the pour.

Developing the caissons

Once the strategy was selected, the next step was developing the two caissons. At a nearby Port of Tacoma site, 62-ft.-high steel forms were built on top of a cutting edge. Then the trailer pumps transferred concrete into its walls to start the caisson. Because of the weight, the structure dropped 30 ft. beneath the water’s surface as planned. Both caissons were prepared in this manner.