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  • Date: Oct 21, 2013
  • Posted by John Klein
  • Tags: Staging and Rigging, Engineering

"Stronger is not always better"

Why over-building every component does not result in a better design

"Let's just make it so strong that it won't fail": Fail safety is an important topic in structural design. Any structure will fail if it is subjected to a large enough load. The term "Fail Safe" does not mean that a structure is too strong to fail; if done properly it means that failure does not result in collapse without any warning. If a structure is loaded beyond its capacity, then at some point one component or part will fail first, which could be thought of as the weak link, or the fuse. The fashion in which it fails is of the utmost importance and should be determined and designed.

"That come-along isn't cheap. It was designed correctly": Without considering fail safety, one would probably think that a come-along that failed by the handle bending over was cheaply made. If a 1-Ton come-along's handle bent under a load just over 1-Ton then it may have been well designed. Why? What could result from a come-along's handle bending over while under high-effort ratcheting? Maybe a busted knuckle, or a pinched fingernail? Conversely, if the handle was so strong that it could keep ratcheting away until the wire rope (cable) snapped, then serious injury could occur.

"Can't we just limit the load?": Maybe. But how much do 20 people weigh? Well, are they adults or children? Are they slim gymnasts or average people that may need to lose a few pounds? Would they be standing still, or would they be jumping up and down together in rhythm while exercising? The point is that specific loads can be difficult to predict. Loads can be even tougher to control when that responsibility is passed on to a third-party. If a box truss/decked platform has been designed to raise a four-piece band off the stage at the end of a concert, what happens if five eager fans rush the stage and jump on with them?

"So we should make things weak on purpose?": No. Design to put the fuse where you want it. Let's take the example of a platform that is raised up to hoist a band into the fly loft after a concert. Let's say that four cables attach to the corners of the platform, and that four motors are intended to lift the platform and band. If the system were to be overloaded unexpectedly by fans jumping onboard, what component should be the first to give way? Certainly not the cables, cable clips, or shackles. If they were to snap the whole group could come crashing down. What about the platform? If it bent in the middle and dished out as the cables were hoisted people might notice that something was awry and may jump off. What if we use the motor limits? If the motors limit out and could not lift the platform then there would be no height to fall from. So design the platform, connections, and rigging to handle at least the rated load of the motors. Don't spend money making all components five times stronger.

"What are you saying!?!?" : Just make it "stronger" leads to extra cost, too much material in the air and insufficient design. Design the overall system to give warning before catastrophic failure. Put thought into the components that make up your system, and put material where needed. Make it "fail safe" not a "money pit"!

Let me know if you have some thoughts, or would like to talk through.

jklein@phoenixengineering.com

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