Final Pontoons Nearly in Place for Floating Bridge in Seattle

520re-posted from enr.construction.com

The world’s longest floating bridge will soon be eclipsed by a newer, longer span a few feet away on Lake Washington in Seattle.

As crews from Kiewit-General-Manson (KGM) move toward swinging the final 1,000 ft worth of concrete pontoons into place this July, the owner—Washington State Dept. of Transportation—looks toward opening this project in April 2016, a key component of a project worth $4.47 billion overall.

Moving the final “raft” of pontoons into position “will be a shock,” says Adam Geyer, KGM superintendent. “There is still a lot of work, but that is a huge milestone. This thing is close to being done.”

Gun lap for route upgrade

After 19 years of planning and five years of construction, less than a year remains until the ribbon-cutting for the 7,710-ft-long bridge, a completely updated version of the span drivers currently traverse on State Route 520 connecting Seattle to points east.

The floating bridge represents one part of the larger State Route 520 Bridge Replacement and HOV Program project that includes new land-based highway construction in Medina, the project’s home base—work that wrapped in fall 2014—and improvements on the west side of the new bridge—some that is planned and some that is unfunded.

The floating bridge, though, remains the project’s focal point. The new structure is slightly north of the existing bridge, which will be removed, utilizing historic right-of-way and avoiding concerns over a bay and protected wetlands. The distance between spans varies from 60 ft to 28 ft.

The old bridge, which opened in 1963, is overdue for replacement. In addition to seismic concerns, the four-lane structure bottlenecks traffic, and with a roadway sitting directly on pontoons, high-wind and high-wave weather events close the bridge. Any bridge or pontoon maintenance on a bridge with no shoulders also requires a closing of lanes.

“It has taken its pounding,” says Dave Becher, WSDOT floating bridge and landings project director. “We have done a lot of retrofits, and at a certain point, you can keep putting money in, but not getting value out.”

The new bridge is a significant upgrade. The size has changed, with six lanes and the ability to add light rail in the middle. To handle the increased load, the pontoons were designed to the largest dimensions possible to float from the casting site 578 nautical miles away in western Washington via the only possible channel—Seattle’s Ballard Locks, with a total of 5 ft of leeway.

To overcome weather and maintenance issues, the design team raised the new bridge 20 ft off the pontoons, allowing complete access to all bridge systems from below.

“We will be able to do 95% to 99% of the work on this structure from underneath,” says Becher.

To help with maintenance, crews also built a complete facility, housed under the east approach of the bridge, into the hillside with direct access to both Lake Washington via a dock and the pontoons. The facility contains the command station for all bridge systems.

Concrete pontoon and anchors

The new bridge includes 77 total pontoons, 23 that make up the spine—21 longitudinal pontoons and two cross pontoons—and 54 flanker pontoons, smaller supplemental pontoons designed for stability and attached to the sides of the spine for extra buoyancy.

Each of the larger pontoons for a bridge 116 ft wide is 360 ft wide and weighs 11,100 tons. The design called for specialized high-strength marine concrete mixes with 7,500 psi.

Initial pontoons proved faulty, requiring on-the-lake fixes for the first wave, a plan that required more steel reinforcement.

Once on the lake, though, the pontoons needed a bridge deck. KGM brought in a 24-barge derrick crane, the largest that would fit through the locks, to transition the cast-in-place concrete deck and steel girders, with pieces weighing 100 tons, all the way across the width of the bridge. Access to the new bridge comes from the north only.

“The pontoons were designed to maximize what could get through the locks; the barge was limited by the size of the locks,” says Becher. “You always have that limiting factor. It limits what you can build and the equipment you can use.”

To hold the pontoons in place, KGM used a concrete-mixing site it created on the north end of the lake in Kenmore, Wash., to build anchors. The gravity anchors, the size of a two-story house, are basically giant hollow concrete boxes measuring 40 ft by 40 ft by 23 ft. The derrick picked them off a barge and helped sink them into the lake.

The 58 cables for the new bridge feature 3-1/8-in.-dia strands that run to three different types of anchors. Along with the gravity anchors, crews use drilled shaft anchors where the ground is hard, but not deep, and fluke anchors in the really soft soil buried in the lake. All anchors are intended to remain in the lake forever, designed to give maintenance crews the ability to change cables via a pin-connection release point.

Lake Washington’s topography requires cable anchors to secure a floating bridge. The water is 200 ft deep atop another soup-like 200 ft of soft silt and lake sediment. To build a non-floating bridge would require massive caissons, a series of costly towers and huge approaches that would extend inland for miles, a prohibitively expensive scenario that would be out of character with the largely residential area.

Bridging the bridge

The last big push for the pontoons that sit 6 ft above the water level comes with the joining of pontoons B, C and D, the westernmost section, near the current bridge and then swinging the assemblage into place. By joining the three pontoons at once instead of each after the other, WSDOT was able to keep boating lanes open longer. Geyer says building B, C and D—1,080 ft long altogether—was like doing a bridge in itself.

Work on the bridge started with the W cross-pontoon on the east side. Then the contractor assembled staging areas and worked “right down the line, letting all activities line up linearly.”

“It allowed everything to flow in progress across the lake,” Geyer says. “To avoid an island scenario, we rafted pontoons on the north side of the bridge.”

The vast majority of the bridge contains 776 precast deck panels match cast at 6,500 psi in Kenmore and floated to the bridge. The B, C, D raft, as it is called, rises toward the west approach, requiring the use of cross beams, columns, precast concrete girders and bridge decking. In all, the bridge contains 772 columns and 331 girders.

Working on water created some difficult logistics. For example, longitudinal joints require 80 high-strength bolts each 20 ft long and 3 and ½ in. in diameter to fit together. “Nobody thought we would be able to line those things up,” Becher says. “We are lining them up at different angle points. Then we take those bolts and tension up to 750 kips, 80 times.” At 1,760 bolts, there is a lot of “opportunity for success or failure at each joint,” especially with the bridge bobbing up like a cork. Crews had trouble with only four, even getting those to work in the end.

Greg Meadows, floating bridge and landings deputy construction manager, says wind loading adds a new on-lake element for building a bridge. “You are a creating a big sail and how do you deal with that during construction?” he asks. Based on sea state and winds, new criteria added to the construction phase. The added complexities of surveying a moving structure when everything is built to a 16th of an inch requires new thinking—vertical over plumb—and specialized surveying tools.

Geyer says the surveying system was a key logistical hurdle to overcome, as were the scheduling challenges associated with moving equipment around a lake. “Every day you are getting curveballs,” he says. “You just have to stay on your toes.”