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RE: steel deck diaphragms

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	-----Original Message-----
	From:	Martin W. Johnson [SMTP:MWJ(--nospam--at)eqe.com]
	Sent:	Monday, September 20, 1999 9:29 AM
	To:	seaint(--nospam--at)seaint.org
	Subject:	Re: steel deck diaphragms



	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.