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.
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
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 = 0
Solving 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)
FA = Force on bolt A
FB = Force on bolt B
Abolt = Cross-sectional area of bolt
d = distance between the bolts
R = (Shear stress with moment arm)/(Shear stress from
Riggers) = Error Factor
R = ((TL/d)Abolt)/((T/2)Abolt) =
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