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Re: steel deck diaphragms

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While it is true that metal diaphragms do not suffer from cross-grain
tension problems, they still have to be able to transfer inertia forces
from the walls into the diaphragms.  Metal diaphragms have their own unique
behavior in this regard, in that the deck behavior to tension/compression
forces is different in each axis.

In the axis perpendicular to the flutes (corrugations) of the deck, the
deck offers essentially no resistance to tension/compression forces.
Fortunately, the support joists tend to be spaced at 8 ft or less in this
direction.  In this direction, the joist anchorage to the wall should be
designed to resist the entire wall inertial force as point load, with the
deck providing no resistance.

In the axis parallel to the flutes, the continuous metal sheet which is
attached to the ledger with puddle welds or screws is very rigid in
resistance to tension/compression forces.  Against tension, the deck alone
should provide sufficient resistance.  However, against compression, deck
sections which are relatively shallow (i.e., 1.5 inches) and which have a
relatively long deck span, may have a relatively high L/r ratio.  Thus,
they may be susceptible to compression buckling due to the wall inertia
forces.  Thus, in this axis, in some instances some small compression
struts may be necessary to transfer the compression forces into the
diaphragm, in a manner essentially equal to the subdiaphragm concept.  I
generally check for this by treating the deck as a column to determine the
permitted axial compression stress based on kL/r.

regards, Martin.