[SML] Question about Safe Working Loads

[Ford Sellers]

Thanks Alex!

Next question... is anyone interested in (and capable of) doing destructive load testing on our Rig Bar?
The Lab we have been using can only go as high as 2000kg.
We think we need to go to between 4000, and 6000kg.

It could be really fun.... I'll bring the beer.

-Ford

-----Original Message-----
From: Stagecraft [mailto:stagecraft-bounces at theatrical.net] On Behalf Of Alex French via Stagecraft
Sent: Thursday, April 23, 2015 8:29 PM
To: Stagecraft Mailing List
Subject: Re: [SML] Question about Safe Working Loads

On Wed, Apr 22, 2015 at 6:16 PM, Bill Conner via Stagecraft <stagecraft at theatrical.net> wrote:
> varies based on component.  WIre rope is 8:1; fibre rope 10:1, a 
> number of hardware components 5:1 or the manufacturer's WWL.  It 
> really depends on what this is and what kind of wear in normal use, as 
> well as other factors.

To put it a little more philosophically- any standards or conventions come down, either through data or assumptions, to variability.

Structural design standards use much lower safety factors (and the correct term is "safety factor" in that context, "design factor" is not a common term in structural engineering), because an I-Beam is a big hunk of steel that doesn't vary a lot- in general nothing's going to happen to your I-Beam that unexpectedly weakens it without being obvious.

A shackle has a lot more ways it can get over-stressed or damaged, but it's still a good sized hunk of steel.  So 5:1.

Wire-rope can be much more easily damaged (8:1), but fiber (10:1) rope can be damaged even more easily, and without it being obvious.  All sorts of hardware and rope used in technical rescue is likely to be considered using even higher safety factors, in part because the environment and use conditions has huge variability.

So you might get a little guidance from thinking about your hardware in that light.  Is there the possibility for variability in manufacturing that wouldn't be obvious? Is there a possibility of damage that wouldn't be obvious or could be ignored?

Alex French

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