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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 -MPa.
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/. 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/ = 0..
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)/ = 0. 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.
(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)
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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 -MPa 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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The ISO -3, ASTM A416 and ASTM A test standards offer guidelines for testing steel strands. Manufacturers of steel strands are required, through product standards, to test their products using both static and dynamic conditions.
To minimize equipment costs, these tests are frequently outsourced to a service provider. The manufacturers are also externally monitored by independent institutes.
One of the primary quality assurance tests involves static tensile testing with strain measurement. ZwickRoell provides a standard configuration tensile testing machine capable of exerting forces up to 600 kN.
Due to the tendency of steel strands to unwind under tension, ZwickRoell employs special specimen-grip inserts to avert premature failure at unwanted locations.
In the high-cycle fatigue test, the steel strand must endure two million test cycles at a maximum frequency of 20 Hz. Specimen clamping poses a unique challenge in this scenario, as clamping fractures can easily occur.
Image Credit: ZwickRoell GmbH Co. KG
Image Credit: ZwickRoell GmbH Co. KG
In static tensile tests according to ISO -3, ASTM A416, and ASTM A, the steel strand undergoes elongation until it fractures. Steel strands are composed of multiple wires twisted together, and they tend to untwist when subjected to tensile loads.
Throughout the tensile test, both the force applied to the specimen and its elongation are measured. Testing machines designed for high test loads are predominantly utilized because prestressing steel is a high-strength steel.
Tensile tests can be conducted on steel strands ranging from 3 to 20 mm in diameter. Depending on the product or test standard, the gauge length L0 can be, for example, 500 mm or 610 mm. The total elongation can reach values of up to 20 %.
Deflected tensile tests, as outlined in EN ISO -3, can also be performed. In these tests, the moving crosshead of the testing machine is equipped with a through-hole.
Tensile test on rebars and strand wires: non-contact measurement with videoXtens
Video Credit: ZwickRoell GmbH Co. KG
In the high cycle fatigue test (for example, according to ISO -3, XP A 35-045-, FprEN -3:) on steel strands, the specimen must withstand two million test cycles at a maximum frequency of 20 Hz without fracturing.
If the specimen fractures near or in the grip, the test is considered invalid and needs to be repeated. Invalid tests are very costly, as the fatigue test lasts several days. Prestressing steel strands, being highly stiff, are particularly sensitive to notching.
The gripping force imposes an additional load in the clamping area on top of the dynamic test load, which may result in premature specimen failure.
For classic tensile specimens made of solid material, the ends often have larger cross-sections to ensure that the specimen fractures in the ungripped section between the grips. This is not so simple with wire strands.
ZwickRoell has established a specialized specimen grip for steel strands, allowing high-cycle fatigue tests to be conducted without the need for specially attached gripping elements.
These high-cycle fatigue tests are generally carried out using an HA series servohydraulic testing machine.
It is crucial to prevent specimen damage from clamping (such as a pyramid pattern on the jaw inserts) because of the high tensile strength of the individual steel strand wires, reaching up to MPa, and their smooth surface.
Hydraulic, parallel-closing specimen grips with adaptable clamping pressure are, therefore, necessary. The geometry of the jaws must be designed to ensure dependable and homogenous gripping of the specimen, preventing slippage of the specimen during the test.
The product quality of the steel strand specimen is also essential to prevent breakage of the specimen outside the gauge length L0.
During a tensile test, only specimen tears or breaks within the gauge length can be considered a reliable test result. A damaged specimen surface or significantly inhomogeneous tensile strength of the individual wires can result in specimen breakage outside L0, near the jaws.
Clamping the specimen in a high cycle fatigue test on steel strands, following ISO -3, poses a specific challenge due to the relatively easy occurrence of clamping fractures.
The gripping force introduces an additional load in the clamping area to be superimposed on the dynamic test load, which can result in premature failure of the specimen.
For classic tensile specimens made of solid material, the ends often have larger cross-sections to guarantee that the specimen breakage occurs in the ungripped section between the grips. This is not so simple with wire strands. To achieve accurate test results, extensive specimen preparation, including sealing, is necessary.
ZwickRoell addresses this challenge with specially developed specimen grips for steel strand testing, offering the option to test strands without the need for specially attached gripping elements. This development significantly decreases the necessary handling, time, and expense for the user.
The grips incorporate hydro-mechanical and purely mechanical gripping. The latter comprises a conical sleeve into which each end of the strand is fed, along with a counter-cone, and clamped. Much of the gripping force is applied through this method.
The remaining force still needed for the specific test is supplied by classical hydro-mechanical clamping via jaws. Combined, these two gripping principles allow the gripping force to be applied in such a way that clamping fractures are eradicated and steel strands can be tested without being pretreated.
The specimen grips are highly flexible and suitable for a broad range of applications. The variably adjustable hydraulic gripping force can be simply modified from test to test by regulating the oil pressure accordingly.
Image Credit: ZwickRoell GmbH Co. KG
Image Credit: ZwickRoell GmbH Co. KG
Image Credit: ZwickRoell GmbH Co. KG
A fracture in the steel strand commonly causes the deflection of individual wires, posing a risk of damaging a sensor arm extensometer. To ensure safe, reliable, and accurate measurements on highly extensible and elastic materials, ZwickRoell chooses optical extensometers.
The videoXtens 6-680 extensometer, with a field of view of 680 mm and a resolution of 0.6 µm, enables precise strain measurements, even in cases involving long gauge lengths, as seen in tests on steel strands. Key features include:
Additional benefits of this extensometer include automatic recognition of gauge marks on the specimen and the initial gauge length, coupled with the transmission of all data to the testXpert testing software.
Prestressing steel, a high-strength steel, is mainly utilized in the construction of prestressed concrete. The application of prestressing steel facilitates a significant increase in the properties of concrete components for both static and dynamic loads.
The manufacturing of prestressing steel involves a long steel product, and its surface shape varies based on the manufacturing standard. For instance, an individual wire is designed as per the manufacturer standard DIN EN -1, while a steel strand may be designed according to DIN EN -3.
The individual wire exhibits strength values of up to N/mm² and is typically designed in a round shape, whereby trapezoidal cross-sections, referred to as compact strands, are also possible.
Within the general heading of prestressing steel, there are three forms, depending on the product standard:
These diverse forms necessitate high demands on materials testing, and, as a result, various test standards (for example, ISO, ASTM, and so on) are employed to ensure adherence to the required quality standard.
This information has been sourced, reviewed, and adapted from materials provided by ZwickRoell GmbH Co. KG.
For more information on this source, please visit ZwickRoell GmbH Co. KG.
If you are looking for more details, kindly visit Steel Strand for Prestressed Concrete.