Pre-stressing Strand as Mild Reinforcement
Pre-stressing Strand as Mild Reinforcement
(OP)
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?






RE: Pre-stressing Strand as Mild Reinforcement
thread507-173910: Unstressed Strands as Reinforcing
RE: Pre-stressing Strand as Mild Reinforcement
It looks as though the limiting factor for this is the ability to develop the breaking strength before the concrete crushes.
I can see how that's a problem in typical members, but if you have a very deep member, you can easily develop the breaking strain in the strand before the concrete reaches a strain of 0.003.
rapt, are you around? Does that sound about right to you or am I missing something?
RE: Pre-stressing Strand as Mild Reinforcement
I do not see how the depth of the member affects this. It is related more to the amount of tension force being developed comapred to the depth.
But this all comes out in the calculations if you do not make the assumption of yield, instead do the calculations by strain compatability and you will know what stress is developed in the strand. To ensure a balanced design you will have to limit the neutral axis depth much more severley than the default limits in codes.
The biggest problem still is how much stress can be developed based on bond and developemnt lengths. In the tests I mentioned in the previous discussion on this, they show, as expected, that it is not possible to develop full yield stress if the strand is not prestressed.
In areas where stress/strain changes gradually along the strand (positive moment areas with UDL loading), you might be able to develop up to 1200-1400MPa.
In areas where the stress/strain rate of change is more severe (negative moment areas, changes in section, point loads, etc) only 800-900MPa might be able to be developed.
So, no you cannot simply use strand without stressing it and assume full yield strength.
RE: Pre-stressing Strand as Mild Reinforcement
RE: Pre-stressing Strand as Mild Reinforcement
The correct solution is to limit the concrete strain to less than .003/.0035 to reduce the steel strain to less than the peak strain. This will result in a deeper neutral axis depth which is the only thing codes really limit, for ductility (this does not mean codes are correct, just lazy).
RAPT gives the designer the option to do this in a design if desired.
RE: Pre-stressing Strand as Mild Reinforcement
Just humor me here for a minute. Looking at this from only a strain and subsequent force standpoint for unstressed prestressing strand - say you have a 20' deep member (let's say it's 100' long so we're not concerned about bond length) with two 1/2" diameter, 270ksi strands. The member is 10" wide with 5ksi concrete. The neutral axis depth is assumed at ((2*0.153*270)/(0.85*5*10))/0.8 = 2.43". With the depth of the strands at say 19' = 228", the strain in the strands when the concrete reaches 0.003 is 0.278. This is much higher than the strain needed to reach 270ksi, which would be 270/28500 = 0.0095.
Is that all there is to it, other than the bond length, of course?
Do you happen to have any literature on the subject? My searches have turned up little.
RE: Pre-stressing Strand as Mild Reinforcement
RE: Pre-stressing Strand as Mild Reinforcement
RE: Pre-stressing Strand as Mild Reinforcement
In your example, the strand will yield at strain 0.9(270)/28500 = 0.0085 and will rupture at strain = 0.05 well before concrete compressive concrete strain reaches 0.003. As rapt said, prestressing strands perform poor if not prestressed.
RE: Pre-stressing Strand as Mild Reinforcement
How do you figure that the concrete is reaching a strain of 0.003 before the cable yields or ruptures? As I noted above, the strain in the strand when the concrete reaches a strain of 0.003 is 0.278. This is well above the two strains you just listed, which means that teh steel is straining before teh concrete crushes.
RW002- Point well taken.
RE: Pre-stressing Strand as Mild Reinforcement
The strand will rupture first. According to your calcs the strand tensile strain 0.278 at the time concrete compressive strain is 0.003.
The ultimate strain is near 0.05 for prestressing steel and 0.12 for A706 mild steel (both less than 0.278). The 20-foot deep section is grossly underreinforced, so steel yields and ruptures before strain in concrete reaches 0.003.
RE: Pre-stressing Strand as Mild Reinforcement
That's exactly what you want to have happen to develop the strength of the cable. If the concrete crushed before the breaking strain was reached, THEN you wouldn't get the full strength of the cable. I'm not following the point you're making.
RE: Pre-stressing Strand as Mild Reinforcement
For a typical RC member, you simply check the strain of the steel when the concrete is at 0.003 and if the steel strain is above yield, then you use the yield strength of the bars in the calcs. Why would (from an purely analytical standpoint) this condition be the exact opposite?
RE: Pre-stressing Strand as Mild Reinforcement
But what if the strain is above fracture?
RE: Pre-stressing Strand as Mild Reinforcement
RE: Pre-stressing Strand as Mild Reinforcement
RE: Pre-stressing Strand as Mild Reinforcement
BEFORE anything fractures. The only thing the strain diagram is telling you is that the concrete doesn't crush first. I've never seen any requirement to check steel strain against fracture.
The point I'm trying to make is that the steel will not see the strain associated with fracture, because the moment capacity (moment associated with steel strain reaching fpu) is achieved long before that.
Your point is well taken, but I'm really only concerned with understanding if it's possible to develop the full breaking strength of a non-prestressed prestressing cable. The use is chord reinforcement in a diaphragm. It's common in precast construction to use very little steel (far below code minimums for flexural members) for chord reinforcement. I've always seen mild steel, but I have a guy who wants to use non-prestressed cable. I just want to make sure he's using the right cstrength for his calculation - i.e. 100% of fpu, not like 25% of fpu.
RE: Pre-stressing Strand as Mild Reinforcement
I don't understand the counter-argument. Is it that the steel is not ductile and will fracture instead of yielding therefore making this a 'brittle' failure state similar to the crushing of the concrete?
EIT
RE: Pre-stressing Strand as Mild Reinforcement
RE: Pre-stressing Strand as Mild Reinforcement
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.
RE: Pre-stressing Strand as Mild Reinforcement
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.
RE: Pre-stressing Strand as Mild Reinforcement
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?
RE: Pre-stressing Strand as Mild Reinforcement
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.
RE: Pre-stressing Strand as Mild Reinforcement
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!
RE: Pre-stressing Strand as Mild 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.
RE: Pre-stressing Strand as Mild Reinforcement
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.
RE: Pre-stressing Strand as Mild Reinforcement
Also, see below in the Structural Engineer Magazine on strength of reinforcing
http://ww