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Re: Plwd: Rigid Diaphragm Analysis - Opinions Wanted

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Dennis Wish wrote:
> 3. The joint where two rectangular diaprhagms meet can not be made entirely 
> rigid so as to eliminate rotation at the joint - by nature of the imperfect 
> framing.
> 
> 4. In the common direction (parallel to the long legs of a "U" shaped 
> structure) - especially if the legs are symetrical the distance between the 
> CM and CR will be governed by the 5% requirment of the code. 

In response to 3. - True, but by placing straps at each point of
intersection you create a situation similar to that of a shearwall.  You
place a holdown at the end of a shearwall to prevent uplift (with only
minor displacement).  By placing straps at both sides of the
intersection of you "U" shape, you will get tension and compression at
each of you chord locations.  This can give a close model to a fixed
connection, again think about it like a cantilevered wood shearwall. 
But this comes back to how you want it to perform.  If you don't place
the straps, then the building will separate there (needs an expansion
joint).  If designed to accommodate the separation, then either method
should be acceptable. I guess it's kind of like a WF baseplate
connection, you can detail it as either fixed or pinned.

In response to 4. - I have seen people increase the shear by
approximately 10% at shear walls to accommodate the 5% CM offset.  If
you solve a simple span beam, and place the center of mass off 5%, you
get about 60/40 ratio for loading.  If you read the SEAOC blue book in
regards to this, it suggests taking into account the 5% CM displacement
regardless of diaphragm rigidity. This statement is only found in the
commentary, and not in the code.  Take that as you will.

Jake Watson, E.I.T.
Salt Lake City, UT