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.

(3) Visual surveying by an SSJV subcontractor with an instrument set up on shore. This survey program mentioned above will provide data on truss shape as well as on truss location.

In addition to deformation checks by the contractor team, the county team will also do the following:

(1) GPS surveying by a county engineering team.

(2) Strain gauge measurements on truss members. A local engineering firm specializing in monitoring stresses in bridges and other structures has installed electrical strain gauges on critical truss members. These gauges will measure deformations in the members during translation. Engineers on the county team have set limits on the amount of allowable strain.