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[SEAOC] RIGID DIAPHRAGMS

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Solosimon writes:

> Assuming a square building with full length cmu walls along the north, 
south
> and west sides and only half of the east walls has a CMU wall;
>
> 1) Does a rigid diaphragm have a chord force? like flexible diaphragms?, 
or
> is it similar to boundary members for concrete and cmu shearwalls?
>
> 2) On the east wall, is a drag strut (collector) required (where the is no
> wall,)  to transmit the lateral forces to the cmu wall?
>
> 3) Are there any publications related to rigid diaphrams, that address 
design
> requirements other than distribution of lateral forces to shear walls and
> determination of center of rigidity.
>
> 4) Does the UBC address Rigid diaphrams.


In response to your questions:

1. Yes, a rigid diaphragm does have a chord force, and it may be required to 
be similar to a boundary member of a shear wall.  Check Section 1921.6.2.3 
of the 1994 UBC for confinement requirements for struts, ties and collector 
elements (This also applies to chords).  I had always designed and detailed 
chord members, but had the feeling that in your slab, there was enough 
flexural steel to be added up to satisfy the chord force requirement, but 
Northridge taught us all that that isn't the case.  I have seen pictures of 
a parking structure where the chord steel had yielded in tension in the 
middle of the diaphragm, and then popped out of the topping slab after 
yielding (there was approximately 6-#6 bars sticking up a foot or so). 
 Since seeing this, I have been a firm believer in the proper detailing and 
confinement of your chord elements.

2.  Yes, a drag strut is required here , likely your chord element can be 
used as your drag as well, just be sure to check the critical loading 
condition.

3.  I know of no publication which specifically addresses the design of 
rigid diaphragms, surely there are some out there and hopefully someone else 
will recommend a good one.

4.  The UBC offers no design procedures, but does have design requirements 
as mentioned above in section 1921.6.

Hope this helps.

Jeff Crosier, S.E.

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