Lifting and hoisting
I was reading an interesting backstory on a TV antenna collapse after seeing a video on Youtube which included a clip of it in a longer "Engineering marvels" video.
The document explains the sheer forces extremely well, and it's one we all have to deal with when using hoisting devices etc.
The job was installing a 1700 or so foot tall TV antenna tower, the last section that had to go up was according to the video 6 tons, but according to the document the riggers thought it weighed 1,000# I don't know where the discrepancy in that is but it's not extremely important for this post.
The section contained the microwave baskets which were sealed at the factory, the lifting ears worked fine to get it off the flatbed truck but not to hoist it vertically without the hoisting cable damaging the baskets.
The antenna co refused to allow the baskets to be removed for the hoisting, and refused to suggest how to hoist it, so the riggers came up with an on site jury rigged extension to clear the baskets.
The extension was a steel channel about 6' long and bolted to the ears with two 1/2" dia U bolts.
Figure 1 shows the rigger's assumption for the sheer forces on the two U
They selected bolts they thought were strong enough, rated at 1,200# each and they thought the 2 would be plenty for the 1,000# section (it MAY have actually been 6 tons or 12,000#)
Figure 2 shows what they should have used for their calculations:
Note that the 6' long extension acts on this exactly like a crow bar! That changes the whole dynamics and puts a LOT more stress on the bolt on the right (they were spaced about one foot apart)
The riggers did not have an engineer on staff, and the antenna mfr refused to even look at their hoisting device despite being requested to do so.
They managed to get this up to 1,000 feet before the U bolts broke, there were 3 riggers riding INSIDE the section being raised and 2 on the tower to keep the section from hitting the tower etc.
Between 5 and 7 (again conflicting numbers) were killed when the whole tower collapsed- the falling section sheared one of the guide cables on the way down and the the whole thing came crashing down.
It was found the bolt mfr's specs on the U bolts was wrong, and that the bolts were half the rated strength, they settled out of court, but even with bolts properly rated for the 1,200# the riggers thought they were, their amateur design for the lifting extension unknown to them was putting 7 times the sheer force on the right hand bolt than the left hand bolt due to the lever action of the 6' long channel they used.
They also had 3 men riding up inside the section adding their weight too, figure maybe another 500-600#
Some of the back story:
They thought two bolts rated at 1,200# each were enough to lift what they thought was 1,000# load, and if both bolts had equal loading they would have, maybe, because they were "rated" to hold 1,200# but upon tests were found to be half that: 600# which means the two bolts together would only have supported 1,200# total before breaking. They might still have gotten away with it with a straight up lift and even forces on both bolts, but the 6' extension caused 7 times the sheer force on one bolt (which then was using the other as a pivot point as can be seen in the diagram) plus the weight of the 3 men added in. So the 1,000# section they thought the thing weighed added to the 500-600# of workers they apparantly forgot to factor in! meant they were lifting 1,600# with 2 bolts they were led to believe would hold 1,200# each (2,400# total) leaving only an 800# margin of safety.
You add in the force of movement, stopping, starting, men moving around and that 800# is used up.
In actuality they were lifting 1,600# with bolts that ultimately were only going to hold 1,200# and that's what happened, the bolts failed, the right hand bolt would have failed even if they were actually at the rated capacity, and with the loss of that one, the other one would have snapped as well.
Make sense RWolff.
What part is not clear to you?
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