Need a book? Engineering books recommendations...

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

Re: Wall anchorage to steel deck

[Subject Prev][Subject Next][Thread Prev][Thread Next]
In a message dated 7/3/99 1:35:20 AM EST, CarlS95(--nospam--at) writes:

<< < Some time ago I designed a CMU wall building with steel joists and a 
  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 
 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 
 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

If I am not mistaken, both the City of Los Angeles and Los Angeles County 
Building Departments require that cross ties be spaced not more than 24' on 
center for both directions of the building and that subdiaphragm shears not 
exceed 250 pound per foot.  This applies to steel deck diaphragms without a 
structural concrete fill which is considered to be flexible. 

Since many of the bay sizes are now 40 to 50 feet in one direction, the steel 
joist act as a tie in this direction, while the girder acts as the cross tie 
in the orthogonal direction at every 40 to 50 feet.  Eventhough the metal 
deck is perpendicular to the wall, cross ties are still required at the 24 to 
25 foot spacing.   The metal deck is not considered to always act as a 
CrossTies due to interruptions for openings (skylights, mechanical, etc).  
Cross ties help prevent diagonal tension buckling from occurring in the 
diaphragm.  Remember on long narrow diaphragms, as the wall moves 
out-of-plane you have a horizontal shear in the diaphragm (like a wood beam 
bending).  This shear is ultimately resisted at the cross or end walls of the 
building, which creates a diagonal tension in the diaphragm  as the shear 
changes from parallel to the long walls to parallel with the cross walls.  
The cross ties provide a means to resolve the diagonal tension forces into 
components (force parallel to the steel joist and parallel to the cross ties) 
without distorting the diaphragm due to the diagonal tension.  This diagonal 
tension would be pulling perpendicular to the metal deck flutes.

This diagonal tension buckling was observed during the Northridge Earthquake 
in would diaphragms at about the quarter points of the diaphragm between the 
cross walls.  I don't know if it was observed in metal deck roof, but appears 
reasonable to assume the same could happen.

Also consider that there is a big push to use screws instead of welds in new 
construction for attaching the metal deck to cut down on construction costs.  
Side laps also may only be button punched instead of welded. Of course these 
are things which would have to be considered during design.

I don't believe the plan checker was thinking about the unbraced length of 
the steel joist.

LA City and LA County are the only two local Building Department 
jurisdictions that I am of that aware require this.  Orange County typically 
does things differently and has not adopted similar types of LA City 
Ordinances, since the Northridge Earthquake, for seismic retrofit of 
buildings and the implications for new construction.  Orange County typically 
has just stuck with the UBC.  Things may be different in the future if the 
different Orange County Building Departments are participating in the LA 
Basin Building Code (or similar name) which I believe is near completion.

Mike Cochran