Omega Morgan moves Sellwood Bridge
By Laura Hatton27 March 2013
In one of the longest bridge moves ever made, heavy haulage specialist Omega Morgan moved the Sellwood Bridge in Oregon, USA, from its permanent concrete supports to temporary steel piers. The move was to make way for a new bridge to be constructed across the Willamette River. IC reports.
The Sellwood Bridge, at 1,972 feet long, 75 feet high and, 28 feet wide (600 x 23 x 8.5 metres), is one of Oregon, USA’s busiest bridges with 30,000 vehicle crossings each day. The spectacular project to move Portland’s Sellwood Bridge was a joint venture between Omega Morgan and Slayden and Sundt. The move lasted 12 hours and was watched by hundreds of onlookers. It was the result of months of precise planning.
The old bridge was put into its new location in late January. It is now a temporary route for motorists while the new US$ 307.5 million bridge is built in the old bridge’s original location. The new bridge will open in 2016. “We are really pleased to be involved in this highly important, complex and exciting project,” said John McCalla, Omega Morgan CEO and president.
The east end of the bridge needed to be moved only 33 feet (10 m) while the west end had to be moved 66 feet (20 m). This made the job more complicated because it was not a straight-across move. According to the company, however, both Omega Morgan and Slayden and Sundt have successfully used this detour bridge method on other projects. “Omega Morgan has moved bridges weighing upwards of 8,000,000 pounds [3,629 tonnes] but this one offered some additional challenges,” McCalla said.
Omega Morgan won the contract after it devised a strategy to move the bridge in one piece. The plan showed that it would save time, money and duplication of effort. Other proposals had suggested expensive and redundant structural features and extensive staging.
The plan involved sliding the aging bridge on skid gear to the north of the existing bridge and then mounting it on new piers built in the river. The bridge would then become the “shoofly,” or detour, while construction began on the new bridge.
Engineers used 10 sliding jacks, 40 lifting jacks and a central control system to make sure the move progressed as planned. The truss span was designed as a continuous structure rather than a series of connected spans, which is unusual. This design required the contractor to move the entire span in one piece.
In preparation for the move, crews removed short spans at the east and west ends of the truss span that will not be part of the new detour bridge. Hydraulic jacks lifted the truss span several inches off the old concrete piers, and then horizontal jacks pushed it on rails along steel translation beams linking the old piers with the detour bridge piers.
The Multnomah County Board of Commissioners approved the rigging method proposed by Omega Morgan in June 2012. According to Larry Gescher, construction manager at general contractor Slayden/Sundt Joint Venture, advantages of this method included savings of $5 to 10 million over other methods proposed; it allowed for the new bridge to be built in one phase; it saved a year in construction time; the temporary detour structure would be as strong if not stronger than the existing Sellwood Bridge (including seismically); and, finally, there would be no need for redundant structural features on the new bridge. In addition, because drivers will use the temporary bridge, they will be separated from construction workers, ensuring a safer work environment.
Prior to the bridge move, temporary approach spans were installed at the west and east ends of the relocated bridge to link Highway 43 in southwest Portland to South East Tacoma Street. These will remain in place throughout construction of the new bridge.
The bridge move started early on Saturday 19 January 2013, moving at a snail’s pace of about six feet per hour. About 35 crew members remained on the bridge throughout the move, operating the network of 50 hydraulic jacks and that lifted and pushed the bridge on ramps to its new location and monitoring the truss.
According to The Oregonian newspaper, engineers had calculated that the structure could tolerate about four inches of bend. The routine was to push the bridge a couple of inches and then stop to assess progress. The company used survey laser targets, 10 GPS sensors, 30 stress-strain gauges, 10 smart levels and the review of 30 staff members to monitor the process.
One of the complications of the move was that the new interchange at the west end is larger than the older one. To provide for the extra space, the bridge had to move in a skewing motion to compensate.
Omega Morgan’s equipment was used to lift the truss off the concrete piers and then slide it along the translation beams to the steel temporary bents. Hydraulic jacks pushed the truss on its journey. Omega Morgan uses this same equipment regularly for operations such as moving newly-built barges around the fabrication yard at Port of Portland and loading and unloading container cranes on and off barges at ports around the United States. Some of the same equipment was used to load the arch span for the Sauvie Island Bridge onto a barge in 2007 in preparation for setting the span in place at the bridge site.
To prepare for the truss-sliding operation, Omega Morgan installed U-shaped track beams on top of the translation beams from the concrete piers to the steel bents. Teflon pads are glued to the track beams to provide slick sliding surfaces.
To actually lift and slide the bridge truss, Omega Morgan used its standard skid beams, which are 14 foot (4.3 m), ski-shaped steel units that slide on the Teflon pads in the track beams. Four skid beams were used at each of the concrete piers, with two of the skid beams located at the north side bridge bearing and two at the south side bearing. At each bridge bearing, the two skid beams sat on the track beams on the east and west sides of the bearing.
For the Sellwood operation, each skid beam had two vertically-orientated 150 ton (136 tonne) capacity hydraulic jacks for lifting the truss off the concrete piers and lowering it onto the temporary steel bents. With two skid beams at each bearing, this meant that four jacks lifted the truss at each bearing. Since there were 10 bearings in total (two per pier), Omega Morgan used 40 jacks to lift the truss. At each of the three river piers, the weight of the truss, including concrete roadway deck, was about 900 tons (816 tonnes). At each of the end piers, the bridge weight was about 340 tons (308 tonnes). The total weight of the truss span was estimated to be about 3,400 tons (3,084 tonnes).
In preparation for the lifting and translation operations, the SSJV installed custom-designed steel cradles at each truss bearing (10 cradles total). The purpose of the cradle at each bearing was to carry the weight of the truss from the bearing to the four lifting jacks.
To move the skid beams and truss along the track beams, Omega Morgan used 10 horizontally-orientated 75 ton (68 tonne) capacity hydraulic jacks to push on the south side skid beams. The north skid beams and south skid beams were tied together to assure that they moved together. In Omega Morgan’s system, the pushing jacks were pinned to the rear ends of the skid beams and pushed against clips on the sides on the track beams. Due to the slick surface provided by the Teflon pads in the track beams, only a small part of the pushing jack capacity was needed to move the truss. The pushing jacks could also be used to pull back in case a skid beam moved too far.
History of the Sellwood Bridge
The 87-year-old bridge replaced a ferry called the John F. Caples. The steam-powered ferry took an average of 582 vehicles and 482 pedestrians across the Willamette River every day, compared to the 30,000 vehicles that crossed the bridge in 2012.
The Sellwood was Portland’s first fixed-span bridge, which means it was the first bridge in Portland that did not have a draw or swing span. In 1960 the Sellwood Bridge underwent extensive repair to prevent it from collapsing after settling issues caused cracked concrete.
Through the years since then, however, the aging bridge deteriorated. Among the bridge deficiencies that hastened the decision to replace it were that it could not handle the volume of traffic; had a rating of two out of 100 on the bridge efficiency scale; buses and heavy trucks are restricted from using the bridge; it has narrow lanes and sidewalk and no shoulders; it has no bike facilities and poor connections to a trail system; and it was not designed to withstand earthquakes even though the area is in one of the most dangerous earthquake zones in the USA.