Tower mast fatigue failure of anchor bolts
Tower mast fatigue failure of anchor bolts
(OP)
Hi,we have a case where concerns have been raised about some anchor bolt installations (tower mast). It is believed that there may be a fatigue problem with the bolts under high tension loads from wind. What are the best ways to prove/disprove this. Also what are others experience regarding this type of failure? Likely? As far as am aware they were not pretensioned, although that has yet to be confirmed.






RE: Tower mast fatigue failure of anchor bolts
Most fatigue issues happen over many millions of cycles. Their locations are predictable and usually at stress concentrators, like corners or welds.
RE: Tower mast fatigue failure of anchor bolts
RE: Tower mast fatigue failure of anchor bolts
RE: Tower mast fatigue failure of anchor bolts
RE: Tower mast fatigue failure of anchor bolts
contrary to other posts, fatigue cycles as measured by AISC falls in 4 categories
loading condition 1 - 20,000 to 100,000 cycles
loading condition 2 - 100,000 to 500,000 cycles
loading condition 3 - 500,000 to 2,000,000 cycles
loading condition 4 - over 2,000,000 cycles
your tower probably fits loading condition 1 or 2
for instance, you would fit into condition 1 if your tower freqency was 9 seconds.
without additional information, there is no way to know the stress levels in your tower
RE: Tower mast fatigue failure of anchor bolts
RE: Tower mast fatigue failure of anchor bolts
if you have exceeded the values established by AISC, there is an argument that fatigue is a "potential" problem, regardless of which country you are in.
RE: Tower mast fatigue failure of anchor bolts
RE: Tower mast fatigue failure of anchor bolts
RE: Tower mast fatigue failure of anchor bolts
I seriously doubt that you'll find any in-situ forensic testing that will be adequate (I don't think X-ray will cut it). You'd probably need to cut out one of the bolts and take it to a lab. A compenent forensic metallurgist can do the investigation for you (I can recommend one in the US if you can't find one in the UK). That's probably not a realistic solution though.
RE: Tower mast fatigue failure of anchor bolts
If possible, you could take an fatigue prone, exposed weld or other fatigue prone exposed detail on the baseplate area and analyze that detail in fatigue as compared to the anchor bolts. If you prove that the exposed detail would fail before the anchor bolts (for the same loads, of course), the check is as simple as inspecting the visible detail. Visible detail OK, hence, anchor bolts should be OK too.
Now, you also talk about other issue, violent movements and the possibility of the bolts being overstressed and not torqued. For that, I would calculate what stresses the bolts should take in service, and then torque them about that value. That will have several effects, first it will load test them, second, it will eliminate the issue of fatigue. Third, it will reduce any possible movements on the mast.
RE: Tower mast fatigue failure of anchor bolts
Low frequency vibrations require high stress to fail in fatigue. Correspondingly, high frequency vibration loadings require much lower stress to result in the same failure.
RE: Tower mast fatigue failure of anchor bolts
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Tower mast fatigue failure of anchor bolts
RE: Tower mast fatigue failure of anchor bolts
you can get the same fatigue from:
smaller number of cycles coupled with higher stress
larger number of cycles coupled with lower stress
RE: Tower mast fatigue failure of anchor bolts
Tata