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Pre-stressing Strand as Mild Reinforcement
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(Structural)
(OP)
16 Dec 11 10:50I 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?
(Structural)
16 Dec 11 11:11Some discussion in this thread
(Structural)
(OP)
16 Dec 11 12:36Thanks, slick.
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?
(Structural)
16 Dec 11 16:46Lion06,
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.
(Structural)
(OP)
16 Dec 11 16:53The neutral axis depth is always the same for a given tensile force. If you have a very large d (say 40'), then the strain in the steel at crushing of the conrete will be high enough to fail the cable, no?
(Structural)
18 Dec 11 17:59Yes, but that happens with low ductility steel or FRP also if you do the calculations properly. Even Class N steel in Australia and europe (Normal Ductility 5% peak strain) theoretically has this problem for lightly reinforced sections. For normal reinforcing steel calculations most designers simply ignore this check and do not cfalculate steel strain, as the codes do not specifically limit reinforcing tensile strain, though logically they should. In normal situations it does not have a significant effect on ultimate member capacity and is not easy to calculate without computers so codes have ignored it.
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.
(Structural)
(OP)
21 Dec 11 07:45ACI assumes concrete crushes at 0.003 - well, that's the usable strain that we're allowed to assume.
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.
(Structural)
21 Dec 11 10:14What about serviceability issues? Regardless of bond, if grade 270 steel is sized to have the same moment capacity of a section with mild steel, the curvature at service-load moment would be significantly greater than that for mild steel (due to less steel in the cross section). Attached is a sketch of some moment-curvature diagragms of the same section with grade 60 and 270 steel. If high strength steel is used, deflection would govern designs and effectively eliminate the advantages of having high-strength steel.
(Structural)
21 Dec 11 12:56I have not designed a concrete member in quite some time but I seemt to recall there was a maximum YP allowed for mild steel, which was something around 75ksi. Is this still in the code or has it really been that long?
(Structural)
21 Dec 11 13:19Lion06,
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.
(Structural)
(OP)
21 Dec 11 14:41yakpol-
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.
(Structural)
21 Dec 11 18:57Lion06,
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.
(Structural)
(OP)
21 Dec 11 20:20yakpol-
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.
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(Structural)
(OP)
21 Dec 11 20:25yakpol-
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?
(Structural)
27 Dec 11 15:30Lion06,
But what if the strain is above fracture?
(Structural)
(OP)
27 Dec 11 18:05Why is that not what you want? You don't need the concrete to reach 0.003, that's just the max that the code allows you to use. If the steel reaches yield BEFORE the concrete reaches 0.003, then all is good, right?
(Structural)
27 Dec 11 23:23FRACTURE - not yield.
(Structural)
(OP)
28 Dec 11 05:57Who cares? The required moment is reached before then. The required moment capacity is reached b
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.
(Structural)
28 Dec 11 07:59Lion's point IMO is well made if the steel yields before the concrete crushes then the design is ductile. You then limit the capacity of the member to the yield strength of the steel (and verify that the steel yields first).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
(Structural)
(OP)
28 Dec 11 09:12Another way of verbalizing my point is that at some point before the concrete crushes (just at some point, I don't really care when) the steel will reach its full fpu. Then in the nominal moment capacity I can use the full breaking strength.
(Structural)
28 Dec 11 10:11Lion6,
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.
(Structural)
(OP)
28 Dec 11 10:26Yakpol- 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.
(Structural)
28 Dec 11 11:43Lion06,
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?
(Structural)
(OP)
28 Dec 11 12:241. 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.
(Structural)
28 Dec 11 17:01Lion06,
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!
(Structural)
28 Dec 11 17:20ACI 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.
(Structural)
(OP)
29 Dec 11 07:47TX-
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.
(Structural)
4 Jan 12 07:08@Lion,Also, see below in the Structural Engineer Magazine on strength of reinforcing
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PC Strand, the prestressed concrete steel strand, is a twisted steel cable composed of 2, 3, 7 or 19 high strength steel wires and is stress-relieved (stabilized) for prestressed concrete or similar purposes.
According to the number of steel wire in a strand: 2 wire strand, 3 wire strand, 7 wire steel strand and 19 wire steel strand.
According to the surface morphology can be divided into smooth steel strand, scoring strand, mold pulling strand (compact), coated epoxy resin steel strand.
They can also be classified by diameter, or intensity level, or standard.
In the description and list of the table we often see, there are 15-7Φ5, 12-7Φ5, 9-7Φ5 and other specifications of the prestressed steel strand.
To 15-7Φ5, for example, 5 said a single diameter 5.0mm of steel, 7Φ5 said seven of the steel wire to form a strand, and 15 that the diameter of each strand of 15mm, the total meaning is "one The beam consists of 7 strands of diameter 15 mm (each having a total diameter of about 15.24 mm, a dimensional deviation +0.40 -0.20; a diameter of about 5.0 mm per filament).
The general sectional area is calculated according to 140mm ^ 2. The theoretical breaking value is 140 * 1860 = 260.4kN, which can withstand the tension of 156.24-169.26kN according to the prestressing standard of 60% -65%.
Using high-carbon steel wire rod, after surface treatment of cold drawn into steel wire, and then by the strand structure will be a number of steel wire stranded into shares, and then through the elimination of stress from the stabilization process.
In order to extend the durability, the wire can have metal or non-metallic coating or coating, such as galvanized, epoxy resin coating. In order to increase the bond strength of the concrete, the surface can have nicks and so on.
The prestressed strands of the mold are twisted to form a mold compression process, the structure is more compact, and the surface layer is more suitable for the anchor.
Production of unbonded prestressed steel strand (unbonded steel strand) using ordinary prestressed steel wire, coated with oil or paraffin after the package of high-density polyethylene (HDPE) into.
The main characteristic of the prestressed steel strand is high strength and relaxation performance is good, the other when the more straight. Common tensile strength level of 1860 MPa, as well as 1720,1770,1960,2000,2100 MPa and the like intensity level. The yield strength of this steel is also higher.
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