Maintenance crews from the Multnomah County Bridge Section recently completed upgrades to two bent caps under the Hawthorne Street Ramp of the Hawthorne Bridge and one bent cap under the Morrison Avenue Ramp of the Morrison Bridge. The bents are the structures beneath the bridges and ramps that support the deck. A load rating analysis of the ramps conducted by the Oregon Department of Transportation indicated that some of the bents were under-strength for current traffic loadings. In-depth analysis by the engineering firm of CH2M Hill identified three bents which required reinforcement. The engineers recommended a system of externally mounted post-tensioned bars to increase the compressive strength of the concrete caps, thereby increasing the load carrying capability of the bents. The project cost approximately $52,000.
A steel I-beam was used as an anchoring block on each end of the bent cap. Through this beam were inserted two or four bars which ran the length of the bent cap. Hangers were used to support the weight of the rods.
County Maintenance personnel, working from mobile lifts, positioned a pair of hydraulically powered jacks over one end of the tensioning rods. The jacks are supported from a truck crane located on the structure above, or to the side of the work site.
Once the jacks were in place, the hydraulic system was activated, tensioning the rods to a final pressure of 584 kilo-newtons (131,289 pound-force). The pressure was monitored from the support vehicle. Tension was increased in stages to prevent placing too much stress on the rods. Engineers from Ch2M Hill supervised the process.
After the desired tension was reached, the amount of movement in the rods was measured and large holding nuts were tightened. This allowed for the removal and repositioning of the jacks. The Morrison Avenue bent required a set of four rods. The Hawthorne Avenue bents only required two rods per bent.
As a last step, protective caps were installed over the ends of the rods to prevent corrosion to the rods and tightening nuts.