[SML] Unistrut safety factor specs?

[Alex French]

On Thu, Feb 26, 2015 at 2:54 PM, Stephen Lee via Stagecraft
<stagecraft at theatrical.net> wrote:
> Page 80 for general fittings says "Design load data, where shown, is based
> on the ultimate strength of the connection with a safety factor of 2.5,
> unless otherwise noted".
>
> The pipe hangers on page 128 list a safety factor of 5, etc...
>
> However, the max loads for the strut itself don't have a safety factor
> listed. I kind of doubt that the loads they have listed are the ultimate
> failure strength, but it doesn't say one way or the other.
>
> There's gotta be something I'm missing...

I believe... when they're looking at a specific piece of hardware or
connection method, they've selected and explicitly list a safety
factor that's appropriate.  The number the safety factor is applied to
is probably from real testing (because these aren't things that are
reasonable to calculate no matter how good your math is, you can test
and/or you can do finite element modeling).  I'm not sure if the
safety factor is their judgement or based on codes at all, but it
varies sensible based on the application- on page 86 they're talking
about a crushing strength where the channel or hardware will deform,
but that doesn't mean that everything comes crashing down.  On page
128 they're talking about hangers, where a failure is more likely to
result in things actually crashing down, so a higher safety factor is
used.

For beam and column loading, behavior can be calculated based on
geometric properties + material properties (assuming failing in pure
bending for a beam... if the failure mode involves the C deforming or
other types of local failure, that's much more complicated.  Hence the
note " Long span beams should be supported in such a manner as to
prevent rotation and twist" on page 64).  An example of material
properties is at the bottom of page 65.

Standard steel structural design calculates a load where the maximum
stress in a beam is 0.6Fy (uh... that 0.6 may vary depending on grade
of steel, and be a little different for column design too).  Fy is the
"yield strength" of the steel- the stress at which the steel stops
being "elastic".  Push past the yield strength, and when you remove
the load, you still have some permanent deformation.

So, looking at the beam loading data, one fair answer is 1.67 (1/0.6).
But that's in relation to the yield strength, which means "everything
is permanently affected" but not "everything comes crashing down".
The safety factors that we think about in theatre rigging are not
going to apply to structural steel (or things like truss for that
matter).

Sammler's "Structural Design For The Stage" is a great place to learn
more about wood and steel structural design for anyone interested.
While I was getting a civil engineering degree, it did more than any
other resource to help me really get my head wrapped around some of
the intuition and math in basic beam analysis.

Alex French