Pre-stressing Strand as Mild Reinforcement 1

[Lion06]

I have a strong recollection that there are limitations on using pre-stressing strands as mild reinforcement (not prestressed), but I can't find it anywhere in ACI 318. Am I way off base, or just looking in the wrong location?

 

[yakpol]

Lion6,
Assuming that you check against fracture strain, the cracking of concrete in tensile zone still remains a issue. Typically steel stress associated with allowble crack width is below 36 ksi.
Also, search the papers and make sure the bond between strand and concrete at high stress level is still there. The codes do not cover this stress range.

 

[Lion06]

Yakpol- I'm still not seeing the need to check the fracture strain. The very instant the steel reaches fpu (provided the concrete doesn't crush before then) I'm done. That's my nominal moment strength.

As far as the bond strength at this stress range; this is done every day with bonded, prestressed construction - double tees, hollow core plank, etc. Granted, these shallower members aren't getting up to 270 ksi in the steel, but they're pretty close.

 

[yakpol]

Lion06,
Note:
1. Not fpu, but fpy=0.9fpu
2. The concrete stress will be of triangular shape if strain is less than 0.003. You will need to run strain compatibility analysis to determine flexural capacity rather than simplified approach based on rectangular compression block.
3. Release stress 0.8(270)ksi is taken by concrete in compression. In the case of non-prestressed strand, concrete around the strand will be in tension and badly cracked. You really need a test proof. You may end up with unbonded, non-prestressed steel.
4. Serviceability?

 

[Lion06]

1. I don't have my PCI Design manual in front of me, but I think you can take pretty close to fpu if the strain compatibility analysis shows that you develop that strain in the cable.

2. This technically correct, but at the level of strain I'm talking about with two cables in a 20' deep member, whether rectangular or triangular, the steel strain will be well above where it needs to be. This is a non-issue in my mind.

3. Point taken.

4. Point taken here, too, but again, I started this thread with the sole purpose of finding out if you can develop the breaking strength of a prestressing cable that isn't prestressed. I definitely appreciate and welcome the other comments and thoughts, but they're outside the direct scope of the question.

 

[rapt]

Lion06,

All of this still relies on being able to develop the bond between the concrete and the strand, and the tests I have been involved with show that you cannot develop sufficient bond to develop the yield stress in the strand. The amount you can develop depends on the strain/stress profile along the member and varies from about 900MPa to 1200-1400MPa depending on the situation.

Anchorage at the end would also be important in a tie situation!

 

[TXStructural]

ACI 318 does not recognize the use of PT strand as non-PT reinforcement. ACI 318 section 9.4 specifically prohibits the design of members using deformed reinforcement over 80 ksi, with the commentary clearly indicating that the reference to "prestressing steel" means "prestressed reinforcement."

Using strand in place of mild steel will not allow the use of ACI 318 load and resistance factors, development lengths (obviously), formulas or computational methods. These all assume certain behavior of materials which are not approximate by kludging the code for use of strand. The behavior of a member with properly bonded reinforcement will be different from one which does not bond in a similar way. Doing as you suggest does not meet the standard of care required unless you fully evaluate not just the tension and strain compatibility, but also the assumptions underlying the code used for design. Although strand would not meet the definition of deformed reinforcement, that is the use you propose.

 

[Lion06]

TX-
That's kind of what I'm looking for - if it's explicitly not allowed or if it's just buried somewhere.

This is not my design. This is being suggested by a PC supplier and he claims they do this all the time. I've asked for some literature on the subject from him, but he has yet to provide any.

 

[slickdeals]

@Lion,
Also, see below in the Structural Engineer Magazine on strength of reinforcing

http://www.gostructural.com/magazin...ngth_of_reinforcement_in_aci_318_08-8639.html