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Re: Tension Stress Graphs

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Tension stress graphs ---> Confusing terminology

1. If *you* are being asked to do the prestress design using your system(s), 
then I could assume that these graphs are the theoretical *tension stresses* 
in the concrete due to loading, DL and LL and possibly other loads, that the 
prestressing has to resist, and does not include the prestressing force.  
Does it appear that the graph represents an envelope?

2. Tendon tension stress, for all intents and purposes (except for friction 
in P/T), is uniform.  If the "Tension stress" graph is not uniform, or nearly 
so, then it would not be the tendon tension stress.

3. As Jason said, ask whomever sent you the graph what it means.  That way 
any misinterpretation is eliminated.

In P/S design, I prefer to specify the final prestress force and its c.g., 
and let the prestress fabricator select the strand pattern that will satisfy 
that profile and temporary stress conditions.  If the prestresser cannot 
satisfy the c.g. and final prestress force with his/her system, then he/she 
can submit their design.

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

Jason Emoto wrote:

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<<<frp 2000 wrote:

I received "flexural" demands for some prestressed beams.

What I received were : Tension Stress Graphs (length on x-axis, psi on
y-axis)

Are these equivalent to the moment curve divided by the Section Modulus ??

i.e. stress = M/S = Mc/I ,    where c = distance to furthest tension fiber>>>


Any response you get from the list will be an educated guess.  You should ask
the person who sent you the submittal for clarification.  That said, here's my
guess.

ACI code limits tensile and compressive concrete stresses in prestressed
members at service loads (at transfer of prestress and after losses).  The
intent of the graphs may be to show compliance with these stress limits.  The
concrete stresses are computed by:

f = P/A + M/S

where

P = axial compression due to prestress force
M = bending moment due to profile of prestressing, self-weight and applied
loads

The prestressing steel stresses are also limited by code, but should not vary
drastically along the length of most members.  Some variation will occur due
to friction, but it should be obvious by looking at the graphs whether they
show tendon stresses.

Jason Emoto
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