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RWolff 06-16-2013 01:15 PM

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:


The riggers initially had approved the antenna plans, which provided for placement of the antenna hoisting lugs. These lugs provided attachment points for lifting cables that would be used for removing the antenna sections from the delivery truck, and for hoisting the antenna into the air for final assembly on a 1000- foot tower. A crew of riggers who had constructed such towers for many years was on site The crew used a vertically-climbing crane mounted on the already constructed portion of the tower to lift each new section of the tower, and finally, the two-section antenna onto the top of the tower. The design called for a three-legged tower, and as each new section was lifted, it was positioned and bolted onto the previous tower sections, one piece at a time. The tower legs were solid steel bars with 8-inch diameters. The tower sections weighed approximately 10,000 pounds and were each 40 feet long. They were raised without incident to a height of about 1000 feet.

The two final antenna sections arrived at the site and assembly proceeded as planned, until the last antenna section was ready to be hoisted into position. This section was different from the other sections of the antenna because it had microwave baskets attached to the sides of the antenna. The placement of the hoisting lugs allowed the antenna to be lifted horizontally off the delivery truck, but the baskets interfered with the lifting cables when the antenna was rotated to a vertical position. A makeshift extension to the lifting lug had to be fashioned by the riggers to permit the last section's vertical hoisting. Unfortunately, on the day of videotaping during the hoisting of this last section, something went wrong, and while the antenna was being hoisted, the bolts on the makeshift lifting lug extension failed. The result was a tragedy. Several riggers fell a thousand feet to their death.
The calculations used:


Calculation of stress in bolts = Total Antenna Weight / (2 * Total Bolt Cross-sectional Area)

In Figure 1, the shear stress in each of the bolts is Antenna Weight / (2 * A bolt). Figure 2 shows the proper analysis, involving summation of moments around the center of the left hand bolt. The moment equations are then used to determine the force applied to each bolt. Dividing the force applied to each bolt by the bolt's cross-sectional area will give the shear stress acting on the bolt.

Model riggers should have used (Figure 2)

Free-body diagram of lifting bar and analysis of riggers' solution

Assuming that the angle that the tower hangs is relatively small, which it was, and that the bolts were about 1 foot apart, and that the supporting channel was about 6 feet long, the load in the bolts is actually a combination of direct shear (as found previously), plus the load due to the moment caused by the length of the channel. Thus:

Sum of Moments about the bolt on the left end of the channel =

0 = Weight of antenna * 6 feet - Added force in bolt * 1 foot, such that

Additional force in bolt = 6 * weight of antenna, which when added to the direct force of 1 * Weight of Antenna gives a total load on the right hand antenna bolt of 7 * Antenna Weight.

The corresponding shear stress on each bolt is thus: Stress = (7 * Antenna Weight)/Area of bolt
or, in other words, the stress (for these assumed numbers) in the bolts is seven times what the Riggers thought it would be.
This is a good example of how easy it is to make assumptions not only about the bolts, equipment, and misc items we take for granted are truly rated to what the label CLAIMS it is, but also about designing things such as that hoisting extension to lift heavy objects, or whatever else we need to get moved or done.
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.

roofingquotes 06-22-2013 11:15 PM

Make sense RWolff.

RWolff 06-23-2013 01:43 AM

What part is not clear to you?

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