Bridge Design & Engineering
The need to maintain transportation links during bridge replacement and rebuilding works is particularly critical in locations where travellers have no practical alternative. Sometimes this is done by building a new temporary bridge before the existing structure is demolished, but in Portland, Oregon last month, a steel truss bridge was moved sideways and is now being used as the ‘detour’ bridge while a new crossing is built.
The 335m-long Sellwood Bridge over the Willamette River in Multnomah County carries more than 30,500 vehicles per day, and alternative routes require long detours. Hence the prospect of closing it for an estimated 2-year period while a new crossing was built was not considered an option for the bridge owner or the local community, and a solution was sought that would enable the bridge to stay in use during this time.
The proposal was to build a set of new piers directly in line with the existing ones and shift the steel truss sideways during a short closure. The operation was carried out successfully last month (January) and the bridge is now in use on a new alignment and will remain there until the new US$308 million structure is ready to open in 2016.
Main contractor Slayden/Sundt employed Omega Morgan as a specialist subcontractor to plan and carry out the translation operation, with the latter also providing its own specialist equipment for the procedure.
Multnomah County is the most populous area in Oregon, and the county includes the city of Portland. The replacement of the 87-year-old Sellwood Bridge is one of the largest the county has undertaken in decades.
According to Omega Morgan vice president of engineering Ralph Di Caprio, the move went exactly to plan, and he believes that the bridge itself is possible in better shape now than it was before the move.
“The bridge had tweaked out of alignment over the years since it was built,” he says, “and as we lifted it out onto the skid units it straightened itself out.”
Naturally the behaviour of the bridge during the move was heavily monitored, with the translation operation being halted at regular intervals for progress to be checked and measurements taken. The greatest risk during the operation was the the steel truss would be overstressed and would incur damage, but careful planning and monitoring meant that this fear proved unrealised.
The existing steel truss is a continuous structure about 335m long with four spans; two end spans each 75m long and two central spans of 91.5m. It was designed by Gustav Lindenthal who chose this type of structure in order to minimise the amount of steel required, since the budget for the bridge, which was built in 1924, was very limited. At each of the five piers, the truss was supported on two large steel roller bearings, one bearing under each side of the truss.
The operation involved the crew raising the truss span vertically by around 100mm and then pushing it north using hydraulic jack-and-slide beams. It was then slowly moved twice as far on the west end – some 20m in total – compared to the east end. This is to accommodate the new, permanent bridge, which is wider at the west end where the highway expands from two to four lanes to add traffic capacity.
At the new location, the truss is supported on five temporary steel piers at the same spacing as the existing concrete piers, to ensure that the support points remain under the same positions on the truss.
The Sellwood truss is a long, slender structure with a finite amount of inherent strength and there was risk that truss members could be damaged if there was any excessive bending or twisting during translation. An engineering firm on the county’s team analysed the truss to see how much movement it could bear without damage. Based on this, tolerance limits were established for the permissible amount of deformation from vertical bending, horizontal bending and twist.
“We were particularly concerned with the top and bottom chords,” says Di Caprio. The intention was to support the truss at the same places and in the same way as it was supported in its original position and to move it without overstressing any of the members.
In order to support the truss at all ten bearing points during translation, the joint venture installed steel translation beams between the five old concrete piers and the five new temporary steel bents. Two translation beams were used at each pier to accommodate Omega Morgan’s skidding equipment; one at each side of the bearings. This skidding equipment was used to raise the truss of the concrete piers, then slide it along the translation beans to the steel temporary piers using hydraulic jacks. Preparation for the truss-sliding operation involved the installation of U-shaped track beams on top of the translation beams, with Teflon pads glued to them to minimise friction.
To lift and slide the bridge truss, the contractor used skid beams; these are 4.3m-long 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, two at the north side bridge bearing and two at the south side bearing.
Each skid beam had two 150t-capacity hydraulic jacks installed to operate vertically, and these were used 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 were needed to lift the truss at each bearing; eight per pier and 40 in total across the full length of the bridge. At each of the three river piers, the weight of the truss and its concrete deck amounts to about 900t; added to the 340t weight at each of the end pier, this totals an estimated 3,400t for the whole bridge.
To support the truss itself, once it was raised from its bearings, the contractor installed a total of ten custom-designed steel cradles, one at each truss bearing. Ten horizontally-oriented 75t capacity hydraulic jacks were then used to move the skid beams and truss along the track beams, by pushing on the skid beams on the south side. The skid beams on the north and south side at each pier were fixed together to ensure that they moved simultaneously. With the Teflon pads reducing the friction to a minimum, only a small part of the capacity of the pushing jacks was needed to move the truss. These jacks were also able to pull the skid beams back if necessary.
Once on the move, the truss had to travel along a curved path to reach the final skewed alignment of the detour bridge, and the translation beams were designed to allow for this. The jacks were set up so that those on the west end of the bridge pushed twice as fast as the east end jacks, and those at each of the three intermediate piers pushed at proportional rates. A special digitally-controlled power pack was used to control the amount of hydraulic fluid going to each jack in order to achieve this movement.
Monitoring the differential movement of such a large structure was particularly important, and the contractors used two different ways of checking its progress. Before the operation began, Omega Morgan marked each beam to show how far the truss should have moved at any given point in time; the operation was halted periodically so that the progress of the truss could be checked. These actual locations were compared to predicted movements which had been calculated during the planning of the translation operation.
The second check was carried out by a surveying subcontract on land, who monitored the locations of targets at the bearing points on the truss.
Once the truss was in its final location over the detour bridge piers, the span was lowered about 50mm onto temporary bearings by the 150t vertical jacks on the skid beams. The temporary bearings are steel plates installed at the correct heights to support the truss span. At the new pier in the center of the river, these bearings are fixed in place, while at the other four temporary piers, the truss will be able to move longitudinally to accommodate temperature changes. On the old bridge, this movement was accommodated by the use of steel rocker bearings.
The operation went smoothly, Di Caprio relates. “We monitored progress throughout to check that the truss was moving as planned, and we had incorporated ways in which to make adjustments as we went along, if necessary. We also monitored the pressure in the jacks so that we could see quickly if the pressure was changing which would have indicated something unexpected.”
The movement of the truss was stopped regularly to carry out checks, in all about 66 times Di Caprio says, so the actual translation procedure took about 14 hours in total. With the preparatory work and the need to tie the bridge into the road connections, the operation required a five day closure in total.