A conceptual image of the proposed pontoon casting basin in Aberdeen, Wash., on Grays Harbor, shows facilities that would produce 33 concrete pontoons for emergency replacement of the SR 520 floating bridge.
As it nears the end of its useful life, the Governor Albert D. Rosellini Bridge that carries State Route 520 across Lake Washington from Seattle to Medina is the focus of two projects, according to the Washington State Department of Transportation.
The first involves building a casting facility to construct 33 pontoons for emergency relief in the case of catastrophic failure. The second, a related project, will use those 33 pontoons and 44 more to replace the aging bridge.
Known to locals as the Evergreen Point Floating Bridge, it is the state’s oldest and world’s longest floating bridge. Opened as a four-lane toll bridge in 1963, the 60-ft. (18.2 m)-wide roadway features a 1.42 mi. (2.2 km) long floating section supported by 33 pontoons held in place by 62 anchors attached with high-strength cables ranging from 2-3/16 to 2-3/4 inches thick.
Despite a seismic retrofit in 1999, the bridge and its approaches are vulnerable during severe windstorms and earthquakes. When sustained wind gusts reach 50 mph, the bridge is closed to traffic in order to protect its drawspan, anchor cables and pontoons from cracking or breaking, which would cause the bridge to sink.
Bridge crews closely monitor the bridge, the effects of wind and water on it and road conditions, especially now that the weight of various reinforcements over the years has lowered the bridge about a foot. When storms require a full bridge closure, it causes a bottleneck for commuters, who have few options for alternate routes. Designed to carry 65,000 vehicles per day, the Evergreen Point Bridge actually handles twice that number.
The potential for congestion is the reason for this two-part project, said Dave Ziegler, principal engineer on the Pontoon Construction Project for WSDOT. SR 520 is one of only two east-west roadways across Lake Washington.
“Due to the geographic challenge of the area, there are few detours available,” he said. “If the bridge was closed, commuters would either have to drive around the lake or take the I-90 bridge, which already has a lot of traffic.
“Closure of the bridge would overwhelm the I-90 with re-routed traffic.”
In the event of catastrophic failure, Ziegler estimates it could take at least five years to construct pontoons and restore the bridge. Pontoons, which are the foundation of a floating bridge, are large, hollow concrete structures designed to support the weight of the road and the daily traffic. They can take years to construct. Another floating bridge in Seattle, the Hood Canal Bridge, sank in 1979 and was closed for several years waiting on a replacement.
Washington State has more floating bridges than any other single location. The reason a conventional suspension wouldn’t work in this application is because SR 520 is a curved corridor; suspension bridges need to travel in a fairly straight line. Also, conventional fixed bridges are expensive to build in deeper waters with soft beds, such as Lake Washington, which is 214 ft. (65.2 m) deep at its deepest point. To support the bridge, the support towers would have to be 630 ft. (192 m) tall — nearly the height of the Space Needle.
First Contract Awarded
In January 2010, WSDOT awarded a contract to Kiewit-General Joint Venture to design and build 23 large pontoons and 10 smaller, 60 by 100 ft. (18.2 by 30.4 m) stability pontoons to replace the existing four-lane bridge. The pontoons will be stored in Grays Harbor until they are needed either for the rapid replacement of the bridge as a result of a catastrophic failure or for the planned SR 520 bridge replacement.
Ziegler called the project a design/build hybrid because, while Kiewit-General won the contract to build the casting facility, the second contract to replace the bridge has yet to be awarded. All 77 of the pontoons are designed by the DOT.
“We need a single designer for the pontoons so they fit together,” said Ziegler.
Plans call for the pontoons to be approximately 28 ft. tall, 75 ft. wide and 360 ft. long (8.5 by 22.8 by 109.7 m) — about the length of a football field.
The contract does not include towing the pontoons 120 to150 mi. (193.1 to 241.4 km) to Lake Washington or construction of the new roadway.
“They would need to build columns and a bridge deck to use it as a completed structure,” Ziegler explained. “This contract is just for bare pontoons.”
What it does include is selecting a site for construction of the casting facility. The long process of site selection involved evaluation and environmental process analysis of 20 to 40 sites. Two candidates remain: Aberdeen and Hoquiam, both in Gray’s Harbor, the open-water temporary mooring site for completed pontoons.
Construction of the casting facility poses a challenge because it has to be built 30 ft. (9.1 m) below ground and will require a dewatering system to draw down the water table.
Ziegler explained that as each batch of pontoons is completed, the water level will be allowed to rise, the gate will be opened and the pontoons will be floated through the launch channel, to be moved to Gray’s Harbor by tugboat for inspection and storage until needed.
The facility will then have to be drained to begin the next cycle. In the first five cycles, four big pontoons and two small (SSP) pontoons will be cast. The final cycle will pour three big pontoons.
Ziegler called it “simple box construction” using concrete, standard reinforcing steel and post tensioning. First, on a 920 ft. long by 200 ft. (280.4 by 60.9 m) wide slab inside the facility, wood forms are assembled around a steel framework. The forming system is three stories tall and 360 ft. long. Concrete is poured into the forms to create the pontoon floors, walls and top slabs.
But it’s not as simple as it sounds.
WSDOT conducted preliminary tests in advanced construction methods in engineering, using a full-scale model.
“We recognized that our pour-in form and curing led to cracks,” Ziegler said. “So we analyzed our method.”
To combat thermal cracking and cracking from stress or shrinking, they installed tubing as a means of thermal control and hydration of the concrete.
“We pour the slab first,” Ziegler explained. “When we pour the walls, we run hot water in the slab to heat it to wall temperature. We lower the temperature of the slab as the wall cools. By controlling the temperature during pouring, we reduce the incidence of cracking.”
Each pontoon is inspected for cracks, with divers checking under the hull at the mooring site.
Ziegler said that some of the large cracks can be sealed with an epoxy injection, while small cracks can be treated with a “paint-on” seal. Constructed of dense concrete to limit water migration, the pontoons have a 75-year life cycle.
Construction of the casting facility is scheduled to begin mid-March 2011. Some precast elements, such as the interior walls, will begin production in mid-2011, but the first cycle of pontoons isn’t expected to commence until Spring 2012.
The last cycle should be completed in 2014. It is the largest pontoon bridge ever built and it is being built faster than ever before.
The pontoons won’t be moored in Gray’s Harbor for long, Ziegler said. An RFP for the bridge replacement project was issued in December 2010 to three pre-qualified teams, who were given until spring 2011 to submit bids and proposals. The prequalified teams are Flatiron-Skanska-Traylor Joint Venture, SR 520 Corridor Constructors (Walsh Construction Company, PCL Construction Services and Weeks Marine) and Kiewit-General-Manson Joint Venture.
The second contract includes construction of 44 supplemental stability pontoons and 58 anchors, which will be combined with the 33 from the first contract for bridge replacement. It’s a modular solution, Ziegler said, with potential for six lanes: four general-purpose, two high occupancy vehicle (HOV) and two light rail.
Because the remaining 44 are smaller, they can be cast at another place — even an above-ground facility.
Wherever they’re cast, they will be towed to Washington Lake and secured in place by steel cables anchored to the lakebed. The road and concrete columns will be built on top of the pontoons. The new bridge will be built adjacent to the old, with a slight shift in alignment. Unlike the five-week closure for the Hood Canal Bridge, this connection will be brief.
The replacement pontoons, bridge deck and anchor cables will be designed to withstand winds up to 92 mph. Other improvements are intended to enhance mobility, protect the environment, and improve safety.
New noise reduction techniques will reduce highway noise for nearby neighborhoods and parks. The new structure will remove fish migration barriers and facilities will be built to treat stormwater from the roadway before it reaches Lake Washington.
This is the third design-build project for the SR 520 corridor program in the past year. WSDOT began a thorough environmental analysis of the project in 2000.
The draft and supplemental environmental analyses published in 2006 and 2010 show that, when completed from I-5 to Medina, the floating bridge project will improve travel times and reliability in the corridor. Construction will begin in 2012, and the new bridge will open by the end of 2014.
Crunching the Numbers
The budget for construction of the casting facility and 33 pontoons is $367 million.
“We thought it would cost $180 million more,” Ziegler said. “It came in significantly under estimate.”
The bridge replacement budget is expected to cost $600-750 million.
To fund these improvements, in May 2009, Gov. Gregoire signed ESHB 2211, which authorized tolling on the SR 520 bridge beginning in 2011.
It’s an unusual move, Ziegler said. Typically, tolling commences with the opening of a new roadway. However, by initiating the toll on the old bridge, funds will be generated to help begin construction of the new pontoons.
The legislature set the SR 520 program budget at $4.65 billion for improvements from I-5 in Seattle to SR 202 in Redmond. Toll revenue and state and federal funds provide about $2.37 billion in funding for the SR 520 improvements. Additional funding comes from bonds, a fuel surcharge and federal funds. CEG
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