TV Antenna Collapse
Staff posted on October 24, 2006 |
TV Antenna Collapse

Riggers attached a long channel to the antenna section with several U-bolts. By attaching the lifting cables to the end of the added steel section, the antenna section could be raised vertically without interference from the microwave baskets. They used bolts that they thought were of the proper strength, and figured in a factor of safety, but they did not realize that the piece of steel they attached also added a large moment which was transmitted through the U-bolts. Figure 1 shows what Riggers, Inc. thought was the correct model and Figure 2 shows the model that should have been used.

Figure 1: Model riggers thought was correct

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

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.

Only hours before the accident, the following conversation took place on site between Kevin Chapp, Rigger's cable operator, and Bob Peters, Riggers' lead lift.

Chapp: Well, what's the verdict?

Peters: Looks like the wind will be below the limit, so we'll go up. Sure is foggy though. These radios are a lifesaver when you can't see the ground.

Chapp: I never ran tag lines before radios, so I wouldn't know. Before my time. How did you get your instructions down before that?

Peters: Did a lot of yelling, but they weren't nearly as tall then either.

Chapp: Dang that thing is really up there, huh. I don't envy you up there. I'd be scared to death. Have you ever had one of these things fall?

Peters: I've had close friends on one that came down, and was in the vicinity of one that blew down in a storm while it was going up, but that's about all. It's not really the extra height that gets you. You're just as dead if you fall 100 feet as you are from 1000 feet.

Chapp: Some of the guys were kidding around last night about if they were on a tower and it fell should you unbuckle your belt and jump, or stay strapped on.

Peters: Got me. OSHA would probably fine you if you didn't stay belted.

Chapp: Ready for the next one?

Peters: Yep. Hey — I've got my dog in the car, checked out of the motel, got the car packed and everything. Would you let him out and watch out for him? I'd hate to have him locked in there all morning.

Chapp: Sure, but when we get really busy back he goes. There's our TV camera man. He has been out here almost every day now, taking pictures. What's that for?

Peters: I think they are keeping a record of it on the news as it goes up. Like a documentary.

Chapp: Who is going up today?

Peters: It will take four of us — two on the antenna to keep it lined up, and two on the tower to receive it. We'll be through about 2 or 3 this afternoon, and be at the next job tonight...

But they never made it to the next job. The antenna collapsed catastrophically when the bolts failed while the antenna was being hoisted. All four people lost their lives in the accident, in addition to another rigger working on the construction site. When they found the bolts (by viewing the videotape) and ran tests, the shear strength of the bolt material was half of what it should have been. Thus, the bolt manufacturer was sued, while Antenna Engineering suffered no monetary loss for the accident. A year before the accident, the bolt manufacturer received a lot of bad publicity due to another substandard bolt. They did not need another messy case, so they settled out of court and the case was closed.

The public has become increasingly aware that benefits of industrial progress must be measured against the growing concern that someone must protect the public from damages caused by the products and byproducts of technology. Naturally, the spirited, and swelling public debate puts engineers at the center of the product safety and social responsibility versus legal liability controversy. As such, this case raises questions about the engineering firm's social responsibility versus its legal liability. What alternatives could have been employed so that seven riggers would not have lost their lives?

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