Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

Re: Steel deck diaphragms

[Subject Prev][Subject Next][Thread Prev][Thread Next]
I have been struggling with the same issue.  I have posted a few questions
on the list server and have received a variety of answers.  Some engineers
have great confidence in the performance of the metal deck to provide wall
anchorage.  Others feel that the deck should be detailed in the same
manner as a wood diaphragm.  Unfortunately, as far as I know,  the metal
deck industry has not performed any tests to prove the adequacy of the
deck puddle welded to the ledge angle with regard to wall anchorage.  In
LA county and in the City of LA, I have been required to provide wall
anchorage details similar to that required of wood diaphragms.  At the
side walls (joists parallel to the wall), I provided steel angle struts at
4' o.c., bolted to the wall; the length of the struts was equal to the
calculated subdiaphrgm depth based on an allowable subdiaphragm shear of
250 plf (I think this is too conservative for metal deck.  A more
reasonable value would probably be 1/2 the allowable diaphragm shear for
that deck).  The metal deck was puddle welded to the struts at 6" o.c.  I
provided continuous crossties at 24' o.c. by using double angle stuts.  I
used the steel joist girder top chords as continuous crossties where
possible, with a note to the manufacturer to design the top chord of the
joist girder for the required axial force.  Also be sure to note whether
the force is being applied through the joist girder seat or not.  At the
walls perpendicular to the joists, the wall is designed to span the 6 feet
or so to the joists which are used as continuous crossties.  

After the Northridge earthquake, the SEAOC task force issued a report
regarding corrective action to improve wall anchorage for tilt-up
buildings with flexible diaphragms.  The report was later adopted as part
of the City of LA Div. 91 ordinance.  The task force report had one line
regarding the anchorage of walls to metal deck diaphragms.  The report
stated that wall anchorage of metal decks should not be accomplished by
puddle welding the deck to the continuous steel ledger.  This language was
not adopted as part of the Div 91 ordinace.

Our home office in the midwest has made the same argument that you are
making regarding the strength of the puddle welds to resist wall anchorage
forces and the capacity of the deck in compression to act as a continuous
crosstie for projects they do in CA.  They have made this argument
successfully for the buildings that they have submitted for permit so far.

I understand that the State of CA requires wall anchorage with the struts
and development of subdiaphragms that I described above for schools and
hospitals using metal roof decks.

I have also been witness to the same poor installation of deck fasteners,
especially at the perimeter, as that noted by another respondent to this
posting.

I am anxious for more guidance on this issue as well.  Let me know what
you hear.

Christopher M. Harris, P.E.