Tower mast fatigue failure of anchor bolts

[londonkid]

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.

 

[JedClampett]

IMHO, this is unlikely. The stress levels in the steel of the anchors is probably low. The design loads (90 mph wind or whatever) are only seldom approached and don't always stress the same bolts.
Most fatigue issues happen over many millions of cycles. Their locations are predictable and usually at stress concentrators, like corners or welds.

 

[nutte]

Yes, the full design wind force does not occur frequently enough for fatigue to be a concern.

 

[cvg]

what is the natural frequency of the tower as it sways in the wind? how many stress fluctuations has the bolt experienced? Have any of these exceeded allowable stress per AISC Appendix K? The bolts should have been pre-stressed. It is also possible that other members in the tower and especially at the anchor point could be subject to fatigue and would be fracture critical members.

 

[londonkid]

cvg, we do not have any data that you mentioned. Anything we come up with will be based on a lot of assumptions. We are acting as a consultant to a third party. There were apparently some violent movements of the mast during and just after constuction (due to sections since removed). These movements are impossible to quantify now. The concern is that the fatigue life of the bolts may be reduced due to this period, (it lasted about 2-3 months).

 

[cvg]

you will have to calculate the natural frequency, determine the number of stress fluctuations per day based on that frequency, multiply by 3 months and you will get a number of cycles. plug that into the table in Appendix K, see below

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

 

[londonkid]

sorry should have said this is UK based. so how would you prove the bolts have fatiged have fatigue cracks forming or have ever been stressed over the design stress (i realise this is slightly different to fatigue).

 

[cvg]

xray or microscopic, forensic evaluation of the bolts in a lab...

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.

 

[londonkid]

agreed fatigue in US is fatigue anywhere. the word 'potential' is key here. we could end up doing expensive remedials that are not needed. at the moment no one seems able to prove or disprove either way.

 

[hokie66]

Is this a mast supported on the ground or on the top of a tall building? If the latter, the condition is much more severe, and there have been failures due to the difference in dynamics of the building and mast.

 

[Gumpmaster]

Like cvg said, only forensic evaluation will tell if you actually have fatigue damage.

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.

 

[kelowna]

You do not seem to have a lot of information. I would suggest a comparative analysis.

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.

 

[Ron]

Keep in mind that fatigue of structural section exposed to wind (aolean vibration), can occur without the design winds occurring. For fatigue consideration, you have to look at the whole vibration spectrum, not just the wind loads anticipated for failure.

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.

 

[paddingtongreen]

Learning from the past, is this a free standing tower or does it have guys?

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[hokie66]

Not going to get taken down that path again, eh, Michael? Silly me, I should have asked that as well as whether the mast was on top of a building.

 

[cvg]

it really doesn't have anything to do with the frequency of the vibrations, just the number of cycles and the stress level.

you can get the same fatigue from:

smaller number of cycles coupled with higher stress
larger number of cycles coupled with lower stress

 

[corus]

I would have thought that the bolts would have been pretensioned or else the thing would just rattle about. By pretensioning the bolts the fatigue loading on the bolts is reduced considerably as the joint stiffness is taking much of the load variation. You'd have to know what the pretension was before looking at tower frequency, wind load spectrum etc.

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