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Re: A structural engineering journal

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Paul Feather wrote:

>>Roger Turk wrote:

>
> Concrete:
>
> Expand on Raucher's(sp?) (Texas Transportation Institute) research on
banded
> tendons in post-tensioned concrete and debunk the myth that distribution
of
> tendons is not relevant in P/T slab construction.
>

Hello Roger,

Could you expand on this?  I design a lot of P/T flat plates, and all the
research I have read indicates that testing has shown banded and distributed
cable systems to be highly efficient and effective, particularly with
adequate bonded reinforcement.  (T.Y. Lin, R. Hemakon, and N.H. Burns)  This
is the recommended procedure according to the PTI.  Seismic evaluation of
existing structures after Loma Prieta indicated excellent performance of
flat plate systems (I cannot quote the study right now, but there is a
published report available from the PTI).

Paul Feather<<

The research on which banding of tendons is permitted is based on the 
research by Burns, et al.  ("Tests of Post-Tensioned Flat Plate with Banded 
Tendons," by Burns & Hemakom, ASCE Structural Journal, Sept. 1985, [1985a]  
and "Test of Four-Panel Post Tensioned Flat Plate," by Kosut, Burns, & 
Winter, ASCE Structural Journal, Sept. 1985 [1985b]) and is cited in ACI 
Committee 423 report, "Concrete Members Prestressed with Unbonded Tendons," 
ACI Structural Journal, May-June, 1989.

The flat plates were that were tested were designed using the equivalent 
frame method, a method developed in the late 1940's to simplify hand 
calculations of two-way flat plates and slabs, with the simplification being 
that *all* of the applied forces are carried by an equivalent frame in one 
direction *and* all of the applied forces are *also* carried by an equivalent 
frame in the other direction.  This is a very conservative method that lends 
itself to hand calculations of two-way flat plate and slabs.  If an analysis 
was done in only *one* direction, and beams were used to support the slab in 
the other direction, we would have a one-way slab and beam system which would 
be safe because of the original premise, all of the load is carried by the 
equivalent frame in *each* direction.

By banding the tendons in a P/T flat plate in one direction, we have that 
situation; a one-way slab in one direction with very shallow wide beams in 
the other direction.  This is shown by the crack patterns of Burns tests 
(Figure 8 & 9 [1985a] and Figure 5 & 7b [1985b]) where the concrete cracks in 
the top of the slab are parallel to and directly above the banded tendons, 
and the concrete cracks in the bottom of the slab are near mid-span and 
parallel to the banded tendons.  The failure pattern is the same as one would 
get with a beam and one-way slab system.  This is not the failure pattern 
that would be obtained with a two-way slab system.

"Effects of Banded Post-Tensioning in Prestressed Concrete Flat Slab," by 
Roschke & Inoue (ASCE Structural Journal, February, 1991) provides 
information on the how the stress from banded tendons is distributed out 
into the flat plate.  The research was done for the Texas Highway Department 
who considers the stress from the banded tendons is distributed at a 2:1 
ratio, not the 1:1 ratio that PTI endorses.  Figure 14 shows that even this 
assumption is excessively broad.

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