I did a tensile test on a total of 28 3-inch diameter anchor bolts (set 30 inches into concrete with a nut at the end, an projecting up 30 inches into a steel hold down seat) holding down two water reservoirs 15 feet in diameter and 96 feet tall. They were only tightened with a wrench with a 6 foot cheater bar when new in 1996. After the Nisqually earthquake in February 2001 in the Seattle area I wondered for some time about the bolts,(many of the nuts looked loose), well I decided to prestress them all to 100-kips (.25 fy and 3 of them were broken off 4" below the concrete surface before I started), and we prestressed the remaining 25. It was very difficult to do this because the nuts only clear the reservoir by 3/8 inch. It looks like recent fatigue failure to me, no necking down and all broke the exact same way. I sent them into a lab for a chemical, tensil, brittleness test, and an opinion of failure mode. I had thought the concrete anchorage seat was pulling out slightly or the bolts deformed somehow during the earthquake - wrong. I am now quite concerned about the remaining bolts and how I could repair/replace the broken ones, or if I should replace them all (huge job).