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Re: Precast Double Tees Diaphragm

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Jake:

I believe that Sections 21.7.2 and 21.7.3 of ACI 318-99 (and the 
associated commentary) more clearly present the intent of the 1997  
UBC.  This belief is supported by Section C401.1 of the 1999 SEAOC 
Blue Book.  Concerning the changes made to Chapter 21 of ACI 318-99, 
the Blue Book says, "In general, SEAOC supports these changes. ... 
Other ACI changes, notably those regarding foundations and 
diaphragms, are endorsed by SEAOC, but are not presented here because 
their inclusion would involve a complicated modifying and 
re-numbering of UBC sections."  This interpretation is further 
supported by the last paragraph of Section 6.12.1 of FEMA 273 which 
says, "Untopped precast concrete diaphragms have been limited to 
lower seismic zones by recent versions of the Uniform Building Code.  
This limitation has been imposed because of the brittleness of 
connections and lack of test data concerning the various precast 
systems."   I interpret 97 UBC Section 1921.6.11 to apply to 
non-composite topping slabs (similar to ACI 318-99 Section 21.7.3). 

All of the preceding applies to design of new structures.  Your 
original message said "I am evaluating an existing school with such a 
diaphragm."  Because detailing requirements have continued to evolve 
(among other reasons), I believe that the UBC is not the most 
appropriate tool for the evaluation of existing structures.  As you 
appear to be aware, FEMA 273 does address existing structures.  
Section 6.12.2 says, "Welded connection strength shall be determined 
using the latest version of the Precast Concrete Institute (PCI) 
Handbook, assuming that the connections have little ductility unless 
test data are available to document the assumed ductility."

You might want to keep in mind a point that some designers miss: the 
demands on brittle components may well be limited by the ductile 
response of other components.  If (as you indicated in your original 
post) "the weld plates embeded in the top flanges won't work if I use 
FEMA provisions" and your demands are consistent with what can really 
be delivered by the yielding system, the problem is real and 
rehabilitation is required to meet your design objective.

Regards,

Mike

**********
> Date:          Mon, 13 Dec 1999 18:36:55 -0700
> From:          Jake Watson <jwatson(--nospam--at)inconnect.com>
> Subject:       Re: Precast Double Tees Diaphragm

> Michael Valley wrote:
>  
> > Section 1921.6.12 of the 1997 UBC (item 1): "Thickness shall not be
> > less than 2 inches (51 mm)."
> 
> This is true, however the previous section 1921.6.11 reads:
> 
> "A cast-in-place topping on a precast floor system MAY serve as the
> diaphragm, provided the cast-in-place topping acting alone is
> proportioned and detailed to resist the design forces"
> 
> This section seams to hint that the top flange can be designed to carry
> the diaphragm forces.  I agree that that the thickness is zero at the
> joints.  This is the discrepancy I have been trying to work out. 
> Secondly as much as I would love to use ACI 318-99 and FEMA, they have
> not been adopted by the UBC and my client is requesting UBC guidelines.
> I am only using FEMA to point me the right direction.  Are there any
> other place that may prohibit the use of the flanges from double tee's?
> 
> Thank you Mr. Valley, I sincerely appreciate your help.
> 
> Respectfully,
> Jake Watson, E.I.T.
> Salt Lake City, UT
> 
> P.S. Is California letting these styles of structures be built without a
> topping slab?

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Michael Valley                                   E-mail: mtv(--nospam--at)skilling.com
Skilling Ward Magnusson Barkshire Inc.                  Tel:(206)292-1200
1301 Fifth Ave, #3200,  Seattle  WA 98101-2699          Fax:        -1201