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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