[SML] Unistrut safety factor specs?

Alex French flaggday at gmail.com
Thu Feb 26 21:01:31 UTC 2015


On Thu, Feb 26, 2015 at 3:35 PM, PATRICK MCCREARY via Stagecraft
<stagecraft at theatrical.net> wrote:
> Beams and column members are rarely given safety factors - rather they are
> rated for deflection. Failure in a connector or hardware item is just that -
> failure being torn apart, crushed, etc. - failure in a beam or column is a
> function of deflection. If you're planning something, best to use 1/360 of
> span, which, for all practical purposes, is a safety factor over 1/180 of
> span of about 2. Deflection ratios are based somewhat on usage - 1/360 being
> ideal for flat office/living room/occupied floors, 1/180 being tolerable for
> such things as warehouse racks, etc., where a bit of sag doesn't bother
> what's riding on them.

You're missing a big chunk of structural design here.

An engineer will always do two calculations for a beam:

1) Deflection- where the ratios chosen are about what else is
affected- a stringent standard like L/360 or L/480 may be appropriate
for a plaster or drywall ceiling, while a loose standard like L/180 is
acceptable for shelving.  But deflection isn't how something "breaks".

2) Bending stress- when something "breaks". The threshold used isn't
about "everything comes crashing down", but about "it will never be
the same again".

In some cases the allowable loads may be similar based on both, in
some cases they may be very different.  The bending stress
calculations are based on the steel's yield strength and a coefficient
applied to it.  The coefficients used are consistent, they wouldn't
generally be varied based on usage (though you might sitll want to
apply a *further* safety factor based on usage).  I gave a little more
explanation in another email a moment ago, and am happy to ramble on
at length if asked.

Alex French




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