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RE: Sheet Metal Screw Properties

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I admit to previously using the bending stress of screws to justify some
connections in the past but I agree with Conrad and more recently have
based designs on a document OSHPD provided over a year ago in draft form
that gives ASD values for SMS in the typical condition where they are
installed with gyp between the base and side steel members.  Also
included in this document are values where 1 5/8" unistut is attached to
gyp with a single line and loaded such that prying controls.
Coincidently OSHPD has just released the final version of this document
yesterday.  The values are low for shear but as I understand they are
based on tested values in combination with AISI, ICC and SSMA documents.
These may or may not be overly conservative but these are the proper
values to use for all OSHPD jobs (and arguably DSA) in my opinion.  It's
use outside of the OSHPD/DSA environment may not be required but it may
be used as a tool among other criteria to select the proper fastener.

The document is supposed to be posted on the OSHPD website in the near
future but if you don't want to wait for the California Government's
"near future" email me and I will forward a pdf copy.

Donny Harris, SE
Los Angeles


From: "Conrad Harrison" <sch.tectonic(--nospam--at)bigpond.com>
To: <seaint(--nospam--at)seaint.org>
Subject: RE: Sheet Metal Screw Properties

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Checking Australian code for screws it only specifies minimum forces,
and states materials are not specified. I hazard a guess that the screws
are smaller than any tensile test token, and would not give a reliable
indicator of the strength of material in the screw. The stress
distribution on the threads and between mating part also complex, so
have standard tests for the screw.
 
Also self-cutting-tapping screws also have hardened surfaces to
facilitate cutting, so the screws may be brittle. So not a good idea to
place the screws in bending.
 
 
regards
 
Conrad


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