Have you tried the cold formed steel framing manual (not sure of the
title) available from AISI. It is supposed to have information on
connections, including screws. I know the Canadian version which came
out prior to the AISI one has a lot on screws and bolts in light gauge
steel.
Gary Hodgson
>
>
>
> */Brad Cameron <bsc@keymark.com>/* wrote:
>
> Hello from a long time Seaint "digest mode" lurker! I have benefited
> many times from the comments I see on this forum.
>
> I often design connections using screws in cold formed steel
> connections
> to resist eccentric shear. Little bitty #10 screws into 33 mil (20
> gage) sheet steel. Sometimes the steel is as thick as 54 mil (16
> gage).
>
> The easy way out is to design this connection using the elastic
> method.
> But how conservative is this for screws?
>
> The ultimate strength analysis method for bolts assumes a
> nonlinear load
> deformation relationship. The equation that governs this relationship
> in the AISC Manual is:
>
> R = Rult ( 1 - e ^ (-10 DEL) ) ^ 0.55
>
> Sorry if the formatting is hard to read. I learned on Salmon and
> Johnston's "Steel Structures", 2nd edition. Oops, now you might know
> how much of an old timer I am. In this book they state that the
> coefficients "10" and "0.55" in this equation are experimentally
> determined, resulting in a maximum delta at failure of about 0.35
> inches, using 3/4" A325 bolts. On page 135. I think this means that
> the bolt is assumed to drag through the steel for almost 3/8 inch,
> allowing the forces to redistribute amongst the fasteners.
>
> Given this, I wonder whether the equations as stated in the AISC
> manual
> for ultimate strength analysis are applicable to the dinky little
> screws
> I sometimes use. No way will I get 0.35 inches of movement out of
> a #10
> screw before it breaks or otherwise just gives up.
>
> Back on page 134, they say "Actually the concept of instantaneous
> center
> is identical to the elastic (vector) method if the resistance Ri is
> proportional to the deformation (i.e. stress is proportional to
> strain)." Makes me wonder what the load deformation curve of a #10
> screw
> looks like in cold formed steel.
>
> Anybody got any insights or suggestions - perhaps a way to derive
> coefficients appropriate for screws?
>
> Thank you in advance for your help!
> Brad Cameron
> Keymark Engineering
>
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