In other words, the bolts failed because the tower leaned over, the tower didn’t lean over because the bolts gave out. The bolts were stretched beyond their yield point.
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It’s fun to speculate when dealing with ‘crash dynamics’ and your third picture at post 18 probably wasn’t the best example for the point I was trying to make….or at least it’s more complicated than I was suggesting. With other IWT crashes I’ve seen where it appears that the bolts at the flanges have just departed (like your 3rd picture), the surfaces where the bolt separation has occurred have the classic striation markings which unmistakenly identify them as bolt fatigue failures…or at least fatigue is present even if the fatigue fracture happened secondarily to something else but failed anyway because of being in a severely weakened state. Thus when a ‘clean flange’ is seen like in the picture, fatigue is automatically assumed. Having said that, you raise a good point which is essentially… what came first and what explains the fold in the tower.
Let’s say the fold happened first for some reason…. Towers are design for some serious wind speeds as worst case scenarios and that is likely to be in the 100 to 150 mph range depending on where this one was located. I don’t know where this failure occurred and whether there was some unusual ‘500 year event’ that occurred…. If you know the details of location and date, a survey of some archived weather data would give you the wind speed data you need. Regardless, if high wind speed played a role in the sense that it had increased to the point of the tower failure, all the flange bolts on the windward side would have seen an increase in tensile stress. However, at the instant that the tower gives way and folds, that additional tensile force in the bolts drops instantly… essentially the restraining force they need to provide is dramatically reduced. Thus the folding of the tower didn’t happen first… the only thing that could have happened is that the fold in the tower and the popping of the bolts essentially happened at the same time but that would mean that a scenario was encountered where the stresses at the flange bolts are increased. That of course would be very difficult to believe unless the bolts were already at the point of failure anyway. It needs to be remembered that the flange joint is designed to be at least as strong (or stronger) than the rest of the tower… thus if there were still forces going on after the fold that could cause a secondary failure in the tower, it wouldn’t happen at the flange joint but at some point between the flanges. Here’s another scenario… let’s say a blade flew off for some reason and put the entire tower into a wild state of imbalance. There are two outcomes from that… one is that the onboard vibration instrumentation will ‘put the brakes on’ and this has been known to create a reaction that is so severe that it can bring a tower down. Or there is the scenario where the out of balance forces were serious enough to cause the fold… if either of those occurred, then it would just fold over and everything from that point on up would just crash to the ground. What would cause the bolts to fail since the tensile forces would be reduced as soon as the fold occurred.
Now, you didn’t say that it failed due to high winds…. It could have buckled for some other reason but it starts to get very difficult to envision other causes for the fold. If for example, the foundation let loose and failed, the whole tower will just fall over without a folding of the tower…. and I can show you many pictures of those. There is one cause I’ve seen a few times that caused a tower to fold and that is if it was struck by a blade…. and I think that there is a good chance of it happening in this case. I think it’s possible that the first part of the sequence was that the flange bolts failed and it started to fall around what I referred to as the ‘hinge point’. At some point, inertia and the rotor gyrsoscopic effects play an important role… and after the failure at the flange had already happened, a blade may have clipped the tower and caused it to fold. In that sense, it’s like a pop can that is reletively stiff when weight is applied from above but very weak if a force is applied to the side of it.
The picture looks vaguely familiar. Is this one that failed in Oklahoma a year or two ago… north of Oklahoma City an hour or two as I recall?
Those pics were just random pics from a "windmill collapsed" Google image search. However, doing a Google Lens search on the image turned up this story (among others):
HUNTER, Okla. — Enel Green Power North America is investigating why a wind turbine belonging to the company collapsed Tuesday, May 21, 2019, near this rural community."Enel Green Power North America is working with the turbine manufacturer GE to investigate an incident that occurred yesterday involving a turbine failure at our Chisholm View II wind project in Grant County, Oklahoma," the company said Wednesday in a statement. The wind farm's footprint extends into both Grant and Garfield counties.
According to EGP, there were no injuries as a result of the incident.
The turbine was part of Enel Green Power's 6,000 acre, 64.8 megawatt Chisholm View II wind farm, located immediately east of Hunter.
This wind farm was built adjacent to the original, much larger, Chisholm View project, which sits on 45,000 acres. Construction of Chisholm View II began on the project in summer 2012, and the wind farm began operating in 2016, adding another 64.8 megawatts to the first project's 235 megawatts.
Access to the turbine was blocked Wednesday morning for safety, an Enel Green representative near the site said. Nobody had been within 300 feet of the GE 2.4 megawatt turbine since it collapsed, he added, though one employee had piloted a drone up to the wreckage earlier in the day, attempting to assess structural damage.
Workers were waiting on an Enel Green investigation team to arrive at the site to determine cause, he said.
"Our first concern, as always, is ensuring the safety of our workers, contractors and the surrounding communities, and given the ongoing nature of the investigation, we ask that the community avoid the area of the incident," the statement said.
This story will be updated as more details become available.
So there you go. Hunter, OK, May 21, 2019. I used to work in commercial broadcasting, so I'm familiar with tower wind loading. (I'm not a structural engineer by any stretch, just a broadcast engineer.) But we did have to obtain certified stress analysis of our 1,000' broadcast tower before we added a new antenna, additional transmission line, or different lighting, not only for wind but also wind plus icing.
As I speculated in another post on this thread, I think the common element to these tower collapses and also tower fires is failure of the blade feathering system, causing wind loads that exceed tower design limits. Yes, they should be able to withstand 80-100 MPH winds, but that also assumes a properly feathered blade system offering minimal resistance. If the blades get stuck in a power producing pitch, or the azimuth motor that rotates the doghouse to face directly into the prevailing winds fails, that can 1) cause more wind resistance than the tower was designed for (collapse,) and/or 2) overspeed the gearbox and generator (fire, if only the blade pitch mechanism fails but the doghouse is still facing the wind.)