Wow, you bring up a good point about structures like the Golden Gate Bridge.
Don’t want to get too far off the Orville Dam topic here, but do you guys have any idea of the assessment of how long the Golden Gate Bridge is built to withstand the continual tension since it was built in the 30’s?
The working life or longevity of Internal Pre-stressed or Post-tensioned steel tendons within concrete is a different animal than "suspension bridges" (excluding the bridge Cable anchoring). The Golden Gate Bridge suspension cables will remain tensioned by design (i.e. "suspension"). Bridge cables are accessible for inspection and repair & not buried in concrete. Original designers estimated the life of the bridge to be 200 years, assuming proper maintenance. The current engineer in charge of the bridge recently stated that the life could be much much longer. Keep in mind that this bridge did have a major replacement of the 500 wire bundle suspension "ropes". They found a design flaw where water pooled at a joint of the deck and the rope - causing severe corrosion (see article below - has interesting history).
With the Oroville Spillway anchor tendons, these are imbedded in the concrete. Physical inspection requires chipping a viewing hole to the steel rod. Not exactly a wise choice for 340 cables (or even a statistical sampling). That is why they are developing methods to stimulate the end of the rod and analyze the reflections - in seeking corrosion reflections and determining how much physical tensioning remains. One of the factors that reduces tensioning is called "creep". It essentially is a slow change in the pressure axis of the concrete where the concrete effectively shortens - thus relaxing the stressed steel rod or tendon. Microcracks may also form in the steel where these defects may lead to reduced stress, or worse, rupture. Corrosion is the ultimate enemy.
Golden Gate Bridge Corrects History (thousands of mathematical calculations - forgotten engineer)