Practice Question: Maximum Tension on a Rigging Cable

NCEES Civil PE Specification II.A.: Construction loads

Problem

A mobile crane is being used to organize structural steel beams in a laydown yard. The crane utilizes rigging cables produced my Very Good Rigging Company (VGRC) Inc. VGRC has specified the maximum tension load allowed for each cable is 5 kip. If length of the beam L equals 20 feet, what is the maximum beam weight allowed in pounds per linear foot?

Assume each cable supports the same amount of load.

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Solution

Answer: C) 354 plf

Consider weight of the steel beam, W.
$$ W=w\ast\ L $$
where:
$$ w=weight\ in\ pounds\ per\ linear\ foot $$
$$ L=length\ of\ beam $$
Weight supported by each cable:
$$ W_{cable}=\frac{w\ast L}{2} $$
Tension in each cable based on angle:
$$ T=\frac{W_{cable}}{cos(45°)} $$
Solve for weight per foot based on tension capacity:
$$ 5\ kip\ast1000\frac{lb}{kip}= \frac{w \ast 20 ft}{2 \ast cos(45°)} $$
$$ w=354\ plf $$