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Re: Wall anchorage to steel deck

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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?

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?


Carl Sramek