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Re: Wall anchorage to steel deck[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaint(--nospam--at)seaint.org
- Subject: Re: Wall anchorage to steel deck
- From: CarlS95(--nospam--at)aol.com
- Date: Sat, 3 Jul 1999 02:33:24 EDT
In a message dated 99-06-24, Chris wrote: << Some time ago I designed a CMU wall building with steel joists and a metal deck diaphragm in LA County. The plan check required that I detail the side wall anchorage (the walls parallel to the direction of the steel joists) in the same manner as is required for wood roof diaphragms. She limited my subdiaphragm shear to 250plf also. I created subdiaphragms using steel angles at 4'-0" o.c. as substitutes for subpurlins. The angles were located directly under the metal deck, were bolted into the CMU wall, and extended into the roof diaphragm a distance equal to the calculated subdiaphragm depth. The steel deck was puddle welded to the steel angle struts at 6"o.c. Steel angle-to-angle ties were provided where the struts intersected with the joists. Cross-wall continuity ties were provided at 24' o.c. and were either the joist girder top chords or double angle struts similar to the subdiaphragm struts. This approach seems reasonable to me since steel deck diaphragms are flexible diaphragms similar to the wood deck diaphragms. However, I am puzzled because I do not see this kind of detailing being performed on the single story, steel deck flexible diaphragms currently being constructed in my community, south Orange Co. I was wondering what other engineers were doing regarding the anchorage of masonry or concrete walls to steel deck diaphragms. Why is this type of subdiaphragm detailing required in some locations but not in others? Chris: I believe the plan checker was wrong but maybe was right for a different reason. If the steel joists referred to are open-web joists, then the purpose of adding steel struts, or bridging, is to cut down on the l/r of the joist flanges. It is not required for the purpose of providing a sub-diaphragm. The fact that steel deck and plywood are both flexible diaphragms has nothing to do with it. I'll take a crack at explaining: When out-of-plane forces try to pull the wall away from the diaphragm, the diaphragm panel segment adjacent to the wall is in tension. If you are using plywood, the diaphragm is "discontinuous" at the first edge in from the wall because the edge nails cannot transfer forces across the wood joists. Therefore, you need to create subdiaphragms by adding blocking and continuity ties to develop the forces into the full diaphragm. However, in the case of steel deck, the diaphragm is continuous at the first edge in from the wall because the steel deck is plug-welded to the joist and lapped with the next section of deck. This provides a complete diaphragm system totally different from the situation created by using a wood deck. Steel deck is a continuous diaphragm throughout the whole building and there is no need to provide sub-diaphragms. Did that explanation work? Thanks. Carl Sramek
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