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Re: pt-tensioned slab system

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In a personal response to Jo Jo See, I wrote:

. > If you use section 1606 (1994 UBC), the requirement is that the "member" 
. > supports more than 150 square feet.  In designing a slab, I design a one 
. > foot strip, therefore, it is virtually impossible for that one foot (or 
. > two foot, or even five feet) to be considered as supporting 150 square 
. > feet.  Each one foot width supports only one foot of width.
. > 
. > If you look at section 1607, which is supposed to be calibrated to 
. > section 1606, you need an influence area of *400* square feet before you 
. > can apply a live load reduction.  Check the "Commentary" to ASCE 7-95, 
. > pages 143-145 for additional considerations.

With respect to two-way post-tensioned slabs with banded tendons in one 
direction, it is my contention that the slab is only a one-way slab, spanning 
in the direction of the distributed tendons.  The ACI provisions that P/T 
slabs with banded tendons can be designed as two-way slabs is based on papers 
by Burns, et. al., in ASCE Structural Journals (June, 1977; September, 1985 
[2 papers]), however, the crack patterns shown in tests performed indicate 
that the failure modes of the slabs with banded tendons were as one-way 
slabs, not as two way slabs.

In a paper by Paul Roschke and Masamichi Inoue, "Effects of Banded 
Post-Tensioning in Prestressed Concrete Flat Slab," (ASCE Structural Journal, 
February, 1991) strain measurements indicate that the effect of the banded 
tendons is restricted to only a few feet either side of the banded tendons, 
which means that a post-tensioned slab with distributed tendons in one 
direction and banded tendons in the other direction will behave as a one-way 
slab.

Repeating what I said in my original post, the only reason that banded 
tendons in one direction "works" is because of the method of analysis that is 
used, the equivalent frame method, in which *all* of the load is assumed 
supported by an equivalent frame in one direction, **and** *all* of the load 
is *also* supported by an equivalent frame in the other direction.  This is a 
very conservative technique developed in the 1940's to permit hand (and slide 
rule) calculations in lieu of impractical hand calculations for two-way slabs.

A. Roger Turk, P.E.(Structural)
Tucson, Arizona

Nemencio Macario wrote:

. > Roger,
. > 
. > We've been following responses/discussions on re-slab live load reductions
. > and your input have been of great help. 
. > Would you care to site more references on this regard.
. > 
. > Thanks a lot,
. > 
. > N.C.Macario P.E.(Sructural)
. >