TV Antenna
I. Case Statement
A. Date and Time: 1985
B. Place: Missouri City, TX (a southwestern suburb of Houston)
C. Characters: (names have been fictionalized)
1. Antenna Engineering, Inc.- designed and built the antenna
William 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 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.
2. 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.
D. The Situation
A Houston television station decided it needed to expand its antenna coverage area by erecting a new, taller (1,000 foot) transmission antenna in Missouri City, TX. They hired Antenna Engineering, Inc. to design the antenna. The design called for twenty 50-ft sections to be stacked onto one another, with the last two sections having microwave antenna baskets on them.
Riggers, Inc. was hired by the television station to assemble the tower. They would use a crawling jib crane to lift the sections into place and then they would manually bolt them together. The crane was capable of crawling up the tower and thus would be able to place section after section in place.
Each 50-ft segment of the tower had a lifting lug in the middle of the section. This was used to lift the section of of the truck it was on. Riggers' Inc. decided to use this lug to lift the sections of the tower into place. They would lift it by the center and rotate it using additional wires so that it would be vertically oriented. This method worked for 18 of the twenty sections. The last two sections had microwave baskets along their length. The wire would hit these baskets if the riggers tried to rotate the section around the lifting lug.
Riggers, Inc. called Antenna Engineering and asked if they could take the baskets off during the lifting phase and then reattach them once the section was in place. Antenna Engineering, Inc. had let one set of riggers take the baskets off once, and they completely destroyed them in the process. They were not going to let that happen again. They told Riggers, Inc. that the baskets must stay on the sections. Riggers' Inc. asked how they were supposed to lift the section and were told that that was their problem.
The Riggers devised a solution for their problem. they decided to take some channel steel they had and attach it to the section at a right angle. They would attach the cable to the end of the channel steel and rotate about that point. The cable now would not hit the baskets. They called Antenna Engineering, Inc. and asked if they could come look at the solution they had devised since Riggers, Inc. had no engineers on staff. Antenna Engineering, Inc. said that they could not look at the solution since then they would be liable if anything went wrong. In fact, the president of Antenna Engineering, Inc. told his engineers to stay as far away from the site as possible, so they would not be linked to anything the riggers were doing.
Their solution had some problems that even a freshman engineering student could identify. But, they had no engineers, so they were unable to perform an analysis like the one below.
Model used by Riggers
Model Riggers should have used
Free body diagram of the lifting bar solution
Analysis of lifting bar solution
Sum(MA) = TL - FBd = 0
Sum(MB) = T(L - d) - FAd = 0Solving the above equations for FA and FB,
FA = (T(L - d))/d and FB = (TL)/d
and the corresponding shear stress on each bolt is:
sigA = FA/Abolt = (T(L - d))/(dAbolt)
sigB = FB/Abolt = (TL)/(dAbolt)where:
FA = Force on bolt A
FB = Force on bolt B
Abolt = Cross-sectional area of bolt
d = distance between the boltsR = (Shear stress with moment arm)/(Shear stress from Riggers) = Error Factor
R = ((TL/d)Abolt)/((T/2)Abolt) = (2L)/d
Assuming one set of bolts were used, placed one foot apart, and the steel channel was six feet long:
R = 2(6[ft])/1[ft] = 12,
or, in other words, the stress (for these assumed numbers) in the new lug bolts is twelve times what the Riggers thought it would be, based on their erroneous analysis.
Riggers, Inc. proceded to lift the second to last section up to the top of the tower. Everything went smoothly. This was not to be the case with the last section. Its ascent was captured on video by the television station. As the last section rose, physics caught up with the riggers. Near the top of the tower, their solution failed. The bolts holding their channel steel to the section failed and the section fell. The falling section hit one of the tower's guy wires and the entire tower collapsed. All of the riggers on the tower and on the section were killed in the collapse (seven men total).
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