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Re: Eccentric Shear on Screws

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Brad,
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(--nospam--at)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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