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Re: Precast Double Tees Diaphragm

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Roger Turk wrote:
> Michael Valley wrote:
> >>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." <<
> In addition to this, I would like to add that I believe that the 1964 Alaska
> earthquake (Prince William Sound EQ) exposed some dramatic failures of
> precast T's that were constructed without topping.  IIRC (I will have to look
> in the book tomorrow), the transverse ties (flange-to-flange weldments)
> simply pulled out of the flanges.  (IOW, they were no damned good as
> diaphragms!)
> A. Roger Turk, P.E.(Structural)
> Tucson, Arizona

One last question, do you have any recommendations on an economical
upgrade.  The diaphragm shears are to high to add a metal roof (upwards
of 4klf) and the walls won't support a topping slab.  We have talked
about adding plates along the flange joints.  Simply place (2) or (3)
bolts vertically through the top flange of each double tee and a common
plate, then place each of these at say 4ft on center.  Unfortunately, in
my mind this is the same style of connection and gets me nowhere.  A
second thought, the PCI book I referred to earlier has a design
methodology for doing a diaphragm without a topping slab.  I just can't
find a reference in the UBC to let me use it.

Again, I sincerely thank you for your time and help.

Happy Holidays all.
Jake Watson, E.I.T.
Salt Lake City, UT