Slip Slidin’ Away
Better Roads Staff
• Each skid beam will have two vertically oriented 150-ton 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 means that four jacks will lift the truss at each bearing. Since there are 10 bearings in total (two per pier), OM will be using 40 jacks to lift the truss. At each of the three river piers, the weight of the truss (including concrete roadway deck) is about 900 tons. At each of the end piers, the bridge weight is about 340 tons. The total weight of the truss span is estimated to be about 3,400 tons.
• In preparation … the SSJV will install custom-designed steel “cradles” at each truss bearing (10 cradles total) … 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, OM will use 10 horizontally oriented 75-ton capacity hydraulic jacks to push on the south side skid beams. The north skid beams and south skid beams will be tied together to assure that they move together. In OM’s system, the pushing jacks are pinned to the rear ends of the skid beams and push 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 will be needed to move the truss. The pushing jacks can also be used to pull back in case a skid beam moves too far.
• As noted, the truss must translate along a curved path … The steel translation beams are designed to account for the curve … To move the truss along the curved path, OM will control the pushing jacks such that the jacks at the west end will push twice as fast as the east end jacks, with the jacks at the other three points pushing at proportional rates. To accomplish this, OM will use a special “digitally controlled power pack” that can control the amount of hydraulic fluid going to each jack.
Attaching the Truss to the Detour Bridge Piers
Once the truss is in its final location over the detour bridge piers, the span will be lowered about 2 inches onto temporary bearings. The 150-ton vertical jacks on the skid beams will be used to lower the truss span. The temporary bearings are steel plates installed at the correct heights to support the truss span.
At Detour Bridge Pier 19 (in the center of the truss and river), steel guide bars attached to the pier top bearings will surround the truss bearings to keep them from moving in any direction.
At the other four temporary piers, the truss bearings will be held in place with guide bars on the north and south sides but will be allowed to slide back and forth in the east/west direction as the truss expands and contracts due to temperature changes. To allow the truss bearings to slide freely on the pier top bearings, Teflon pads will ride on stainless steel sheets between the bearing surfaces.
The old Sellwood Bridge was also designed to allow for this natural expansion and contraction of several inches. For the old bridge, steel rockers were used to allow east/west movement rather than Teflon pads.
Monitoring Truss Deformations During Translation
The Sellwood truss is a long, slender structure with a finite amount of inherent strength. Truss members could be damaged if the truss is bent or twisted excessively during translation. To assure that no damage occurs, the translation program includes the following measures.
• An engineering firm on the county’s team has analyzed the truss to see how much it can be bent or twisted without causing damage. Based on results of the analysis, “tolerance limits” have been established for the permissible amount of deformation from vertical bending, horizontal bending and twist.
• During translation, the contractor team will use three methods to check the amount of deformation in the truss:
(1) Marks on the translation beams or track beams as described above. The contractor team will periodically check the actual location of the truss along the beams against numbers on OM’s “Offset Table” to see if the truss is within tolerance for sideways bending.
(2) Laser instruments attached to the truss itself with laser beams aligned along the truss. SSJV personnel will monitor the laser beams at each truss support point to make sure the truss stays in a straight line up and down as well as sideways.
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