Looking at that third picture, it appears to me that the steel failed first above the joint, causing the mast with the doghouse to fold and lean over. It was this off-center load that then sheared the bolts as you described, arresting any further folding of the steel at the failure point and instead finished toppling over at the failed joint.
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.
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?