"The painted red crack shown in post 3671 is from a Feb 2015 DSOD report, isn’t it?"
Yes, the crack image is from Feb 3, 2015 DSOD inspection report. A key point in this image is the dashed red arrow the inspector placed in the photo. This dashed red arrow was where the crack "was" before it "extended" downward 2+ feet. The date is not revealed, in this report, when the crack was at the tip of the dashed red arrow.
Your question is insightful regarding bridge loading, foundation stability (seasonal rains & water table flow to the "lowest point"), and operational loading from recent spills.
You are a bit ahead of me in questions as I've been doing more research to get further documentation to possibly get better data. One aspect that should be noted is that the higher column section is more massive in its relationship to the lower column section. In addition, the 5 foot thick column abuts the massive 40 foot thick monolith #27 in the headworks.
The crack may be a result of the construction of anchorage (or not) between monolith #27, Gate 8 pier, and the spillway bridge.
A 1998 DSOD inspection report noted that there are "shear cracks" at each end of the spillway bridge junctions in the gate columns. Quote: "Shear cracking was noted in the first and last bay of the headworks structure. The cracking occurred through two lifts at the reentrant corner below Elevation 868.5 (the spillway road bridge elevation). The cracking is cited in past inspections and evaluations and is noted here, for completeness."
To me, this indicates that the Monoliths at the ends of the FCO structure are not secured to the FCO piers with reinforced anchorage (rebar). (the Finite Element Analysis simulation of the FCO affirms this too). This creates a fulcrum point at the extension of concrete that supports the ends of the spillway bridge. This extension of concrete is noted in blueprints to be part of the monolith profile, but not noted whether there were isolated sections to "float" with the bridge & FCO pier columns. The bridge is structurally part of the FCO pier columns.
Because of this, the bridge abutment concrete supports may be torsioning the end 5 ft thick piers by settlement (and/or in addition to heavy load crossings), causing the shear cracks. This is a serious design flaw if so.
"Question: if 2 of the 384 anchor tendons “failed” in 2010, how did they determine that?"
The 2 failed tendons were first noted in a 1999 FERC/DWR communications. At Pyramid dam, finding the failed tendon was easy, the whole end assembly fell off the bearing plate. Stood out like a missing tooth. At Oroville, it was not stated how the failed units were identified, but my guess is that they were found to be loose since the tendon failure shear was within inches of the bearing plate. Post failure analysis of the face of the shear would reveal the corrosion progression to "critical crack size failure" via the distinction line to uncorroded or less corroded surface indicators.
"..but none of the other 382 have structurally failed since then."
DWR doesn't know how many have failed as they cannot accurately test beyond 10 inches. Thus, DWR is only testing 2.1% of the total tendon length. When a tendon "fails" it removes the tensile stress that assists in corrosion crack growth. Even if they don't detect a growth in a crack, the tendon may already be useless. P.S. The only reason why so much information is available is due to the CEII secrecy enactment process was enabled in 2001. The detailed Ultrasonic test report was filed with FERC in 2000 (before the CEII lockup of information). So, DWR can keep "failures" bottled up in CEII secrecy.
DWR may be operating with less functional tendons today. Even they admit they don't know as they have turned to a new method tryout of Dispersal Wave Testing. They will not have the results of this testing (accurate results) until Sept 1, 2017. Sort of late to find out if your FCO is going to "work" for the new repair spec of 100,000cfs....