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Total collapse during installation of a 6-ton FM antenna being placed on a new 1800 ft. tower. 5 technicians killed, 3 on the hoist and 2 on the tower. It was determined that insufficient sized bolts on a makeshift lifting lug extension failed. The falling debris severed one of the tower’s guy wires, causing the tower to collapse.

The following case is based on an actual incident. However, individual actors and company names have been changed due to possible pending litigation. In addition, although conversations and memorandums used are based on evidence surrounding the case, they are hypothetical in nature, and are used to illustrate important issues rather than to attempt an actual reenactment of what “really” happened.

Introduction To The Case

In 1982, a television station video crew was filming the raising of their new television tower. The antenna was designed and manufactured by Antenna Engineering, Inc., a moderately-sized local firm. Riggers, Inc., a small local firm, was contracted to raise and assemble the antenna. During the initial design, Antenna Engineering submitted antenna plans to Riggers for their approval. Riggers approved the plans which provided for placement of the antenna hoisting lugs. These lugs provided attachment points for lifting cables which 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..

Introduction To The Case

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 of the delivery truck, but the baskets interfered with the lifting cables when the antenna was rotated to a vertical position. A make-shift 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 make-shift lifting lug extension failed. The result was a tragedy. Several riggers fell 1000 feet to their death.

The video camera caught this catastrophe on film, and through its footage, investigators were able to discover where the failure initiated, and why the accident occurred. The case of the antenna tower collapse raises serious questions about the design engineer’s social responsibility to ensure safety on the construction site, and poses additional questions about product liability issues in engineering and ethics.

Photos

The Cast of Characters

Antenna Engineering, Inc. Designed and Built the Antenna

William (Bill) Harris — President. Harris recommended to Jordan that Antenna Engineering, Inc. not get involved with Riggers problems regarding lifting the antenna tower, due to legal liability issues.

Harry Jordan — Head of the Engineering Division. Jordan told Riggers that they could not authorize removing the microwave baskets, yet he also told Riggers that the engineering firm signed off responsibility once Riggers accepted their design plans.

Riggers, Inc.: Contracted to Assemble the Antenna

Frank Catch — President.

Randall Porter — Vice President. Made initial call to Antenna Engineering, Inc., detailing the problems Riggers was having lifting the top antenna section with the microwave baskets on it.

Bob Peters — Lead Lift. One of the workers killed in the collapse.

Kevin Chapp — Cable Operator. Talked to Peters before the catastrophe, asking about the safety of the operation.

The Problem:

Riggers, Inc. could not hoist the last antenna section using the lifting lugs Antenna Engineering provided due to interference with microwave baskets. Antenna Engineering refused permission for Riggers to remove the basket and reassemble after hoisting.

Analysis of the Antenna Lift

MODEL RIGGERS THOUGHT WAS CORRECT

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

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.

Ethical Issues Of The Case – Points for Discussion

1) Where does the responsibility of Antenna Engineering end and Riggers begin? Should Antenna Engineering have provided adequate hoisting lugs in their original design?

2) Should Riggers have looked at the original design more carefully?

3) Should Antenna Engineering have allowed Riggers to remove the microwave baskets?

4) Should Riggers have devised their own hoisting solution without consulting an engineer? What is their responsibility for contracting a consulting engineer?

5) Should Antenna Engineering have recommended another consulting engineer to assist Riggers? Should they have notified their professional society? Was it ethical for the engineers at Antenna to wash their hands of the project without attempting to find a resolution for Riggers? What other measures could they have taken to assist Riggers without becoming legally entangled?

6) Should Antenna Engineering have refused to review the new hoisting design?

7) If social responsibility comes before legal liability, surely there were other things Antenna Engineering could have done. What would you do in a similar situation?

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