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Re: Steel deck diaphragms[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaint(--nospam--at)seaint.org
- Subject: Re: Steel deck diaphragms
- From: cmh(--nospam--at)wallacesc.com (Chris Harris)
- Date: Sun, 19 Sep 1999 15:56:58 -0500
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.
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