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A675 GR90 Anchor Bolt Fatigue Failure 1

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BrandonC

Civil/Environmental
Joined
Dec 15, 2005
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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).
 
Good idea to send the failed anchor bolts for metallurgical analysis. The results from the analysis will dictate options. For the remaining anchor bolts, the diameter is large enough (3") and the end should be accessible to consider ultrasonic testing (UT) - in place - to verify if the anchors contain transverse cracks (in other words cracks that are 90 degree to the length of the stud) along a vertical length before the end of the anchor. Unfortunately, you will not be able to UT the entire length of the anchor because of the geometry. However, if you state these are embedded 30” in length, you should be able to NDT a good portion of that length.

Ultrasonic examination is a nondestructive test (NDT) method that utilizes high frequency sound waves to detect internal flaws in material. This is a highly specialized test and would require an experienced Level II examiner. Call an NDT company in your area and have them evaluate the option of UT. I have seen this performed in the field numerous times and it might be feasible for your application.

Regarding the failed anchor bolts - you should have a licensed structural engineer review the situation to determine if there is enough redundancy to avoid having to replace the failed anchors. I would not recommend any butt weld repairs to failed anchors.

 
Great UT idea, going to do that next, after the lab tests are returned for the broken bolts, and then look at either ignore, repair, or replace the bolts.

The bolts broke off 4 inches below the concrete surface in a break that looks like a classical fatigue failure, roughly strait across and you can see how it propagated.

For repairs we have considered chipping down and attach long coupler nuts to the remaining imbedded section with a 40 inch shaft extension, or try to unscrew the bolt from the imbedded nut with an ultra high torque impact wrench, but not sure if this is possible. Welding on a piece is not an option
 
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