Replies

DWR Board Expresses Concern of Breakage of 50 year old Anchor Tendons in Spillway 3 years ago

For 3 years the Oroville DWR Board has been expressing concern of "breakage" within the array of 37' 6" steel post-tensioned Anchor Tendons (rods). Ultrasonic wave testing has been used to inspect the rods, however this technique is limited in quality of defect (corrosion, cracking) detection short of the full length of the rod. It has limitations to a quality analysis for the full length of the rod due to attenuation (signal reflection losses & other artifacts). Ultrasonic wave testing does not provide another important structural result; i.e. the remaining post-tension load "stress" that is left in these 50 year old rods. Knowing the remaining "stress" in these rods will reveal how much structural capacity the Radial Trunnion Gates may withstand in water flow operation (hydrostatic & hydrodynamic forces). Thus, the DWR Department of Engineering (DOE) has been seeking to implement a new technology to determine the stress left in the rods using "dispersal wave technique". More on this later.

DWR Board is concerned regarding tendon anchor breakage due to age and other breakages at dams of similar construction: 2014 Division of Safety of Dams DSOD Inspection report.

View of Trunnion Anchors (tendon ends, rows of dots) extending out of the Trunnion anchorage blocks. 2014 Division of Safety of Dams DSOD Inspection report.

You note that each anchor tendon is a 6 inch thick steel rod. That is a fairly hefty chunk of steel. Even if the outer 16th inch got corroded, which would be a lot, the rod should still maintain better than 95% of its tensile strength.

Steel in the Golden Gate Bridge, and the Brooklyn Bridge, and many other applications has been in continuous tension for quite a bit longer than the Oroville dam has existed, and that steel is still quite serviceable. Are these anchor tendons so under designed that they commonly fail?

In my experience as a civil engineer, most structures are overdesigned A minimum of 300 to 500%. Maybe with dams, it’s a whole different ballpark though. Considering the consequences, it seems like they’d want to do way better than that.