Stress-Strain Curve for Prestressing Wire 1

[Psi1118]

Hello All,

I have been searching all over the internet for this information but to no avail. The PCI handbook 6th edition only has a stress-strain curve for 7-wire strand but not wires. Is there any place i can get a stress-strain curve for prestressed wire? I need it to find out my design strength capacity. Thanks!

 

[IDS]

For design purposes you use the E value specified in the applicable design code. You don't need a stress-strain curve.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[hokie66]

Wire is steel. Regardless of the strength, E doesn't vary by enough to matter.

 

[amorrison]

Because of the spiral angle of the wire groupings the "E" of the group will be different than one wire with no angle.
As the wire group stretches the angle of the spiral will change.

 

[transmissiontowers]

I think I disagree with Hokie66. For a steel rod of constant cross section the E for steel is appropriate. For stranded wire there is what I call an effective E (I can't remember the correct term). As load is applied to a stranded steel wire rope or cable there is more stretch as the strands unwind as well as elongate under the load.

You might look for a wire rope handbook. IIRC, SlideRuleEra has a PDF of the book on his web page. Look at this page for wire rope:

http://www.slideruleera.net/miscellaneous.html

Page 30 of the book describes the Modulus of wire rope. I'm not sure if it applies to concrete prestressing strands and I don't do concrete design so take my opinion with a grain of salt.

_____________________________________
I have been called "A storehouse of worthless information" many times.

 

[hokie66]

transmission,
The OP was asking about a single wire, not a strand. Not sure of his application, but that is what he asked for.

 

[transmissiontowers]

Hokie, my apologies. In that case you were correct. Like I said, I don't do prestressed concrete (IIRC that is what the PCI is used for). When he/she cited the PCI, I just assumed a stranded wire because I had never heard of a solid rod used as prestressing steel.

The wire I usually deal with is made of steel core surrounded by aluminum strands and it carries high voltage and gets very hot (on the order of 250°C) so as it heats up it expands and sags down. The all aluminum wire conductor has creep which also adds to the wire sag.

_____________________________________
I have been called "A storehouse of worthless information" many times.

 

[gwynn]

I am a bit confused by the responses here. When dealing with limit states design, and I assume the American equivalent, the stress permitted in strand or wire at the ultimate limit state is beyond the theoretical yield point. The actual stress strain curve is required in order to complete a strain compatability analysis.

In many cases it may not be required, as cracking of the section under specified loads often governs over the ultimate state, but to dismiss it is something never needed is off the mark.

To the OP, your best bet is probably to contact the manufacturer if you can. This is information they should have available.

 

[IDS]

I am a bit confused by the responses here. When dealing with limit states design, and I assume the American equivalent, the stress permitted in strand or wire at the ultimate limit state is beyond the theoretical yield point. The actual stress strain curve is required in order to complete a strain compatability analysis.

In many cases it may not be required, as cracking of the section under specified loads often governs over the ultimate state, but to dismiss it is something never needed is off the mark.

To the OP, your best bet is probably to contact the manufacturer if you can. This is information they should have available.

The design codes specify an Elastic Modulus (usually slightly different for strand and wire) and a yield stress. That's all you need to calculate the force in the strands at ULS. The Australian codes (and I presume others) also give an approximate method which does not require calculating the strain in each layer.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[Psi1118]

Thank you all for the responses, very informative indeed. What I'm working on is a cellular antenna pole which is made out of concrete, we usually work with steel structures so this is a first time for me. I need to calculate the stresses and get the design pole moment capacity. I was following an example from a peer that used to work in this firm but he used a strain-stress curve to do his calculations since he had a strand but what i have is a wire. I tried doing it by calculating my fps but I was having a difficult time so i figured if there was a stress-strain curve out there I could use it and safe some time.

But then again it's just a wire so i dont know how much bond it would have. Does anyone know what my d and dp in the fps equation would be. What i have is not a column since it's more like a hollow tube. I think my d would be from the top of compression fiber to the centroid of the steel and my dp is also labeled in the figure. Please see the figured i attached and let me know if you concur. Thank you all very much for your time and efforts.

 

[BAretired]

Nedri shows properties of prestressing wire (see attached).

BA

 

[dhengr]

Psi1118:
I’m not sure are asking the right question, and you are certainly not asking it in the right place. Why not discuss these issues with a couple suppliers of the kinds of stressing wires you intend to use? They will know their product far better than any of us. I agree with Hokie, if it’s steel the Modulus of Elasticity “E” will be around 29,000ksi. The various wire suppliers/manufacturers might refine this a little for their specific product. They might also talk about creep and relaxation. And, this “E” applies to single wires or wires in a strand, used for pre or post stressing. When you are dealing with strands for stressing or wire rope/cables there is another stretching action called ‘construction stretch,’ which is a function of the lay of the rope and the way it is manufactured. This is usually controlled by prestretching the rope to about 50-60% of its rated breaking strength of the strand or rope. This starts to seat the individual wires in a strand, and/or the strands in a rope to eliminate the mechanical ‘construction stretch’ under load. Again, talk with the manuf’r., they know their product best. Transmissiontowers talks about an ‘effective E,’ because I’m sure their industry has tested for this with each different type of strand; it’s a slightly different application, they use a multi-material strand, different loading and supports, different environmental conditions and don’t likely prestretch their strands.

I’m also not sure that I would get the picture you’ve shown, in starting to look at your problem. You have a conc. tower which I assume is slip formed and can change conc. strengths at different elevations. It has a 15.75' O.D., a 12.5' I.D., 1.6' wall thick; it has 20 - .276" dia. wires, equally spaced and Fpu = 235ksi. I might place those stressing wires neared the O.D., not centered in the wall thick., for better advantage. I would then look at the DL of the tower, its geometry in plan, and ask how much stressing force must I apply so that the tower never goes into tension at the foundation. I don’t think you want the stress picture you show, from some fairly high tension to some conc. stress block, with a linear stress variation. With the variation in wind direction and change from high tension to high compression, you would literally grind the conc. up at the base, in a few years, through fatigue/fracture/granular disintegration. The elastic action of the stressing wires is all/mostly accounted for in the work of the stressing contractor; and once stressed the wires might be grouted.

 

[BAretired]

For this application, Dywidag bars may be a better choice than prestressing wire because they are easier to keep straight during the pour.

BA

 

[hokie66]

I think what you have is a spun concrete pole. I also think it is reinforced with strands, not individual wires. Why do I think that? Because that is the way I have seen it done. The individual wires in prestressing strands are not that large. Spun concrete poles are commodity items, and you should be able to obtain all the information you require, including the calculations, from the manufacturer.

It would have been much better if you had asked for the information you required first, rather than sending us on a wild goose chase.

 

[IDS]

Why does everyone want to over-complicate this?. You don't need to get a stress-strain diagram from the manufacturer. Just use the E value and yield stress given in the applicable design code; that's what they are there for. Why worry about the precise stress-strain behaviour of the steel when the variability and approximations used in the concrete behaviour are an order of magnitude greater?

I don't know what an fps is, but the basis of the diagram looks OK. Just use the E value from the code to calculate the stress up to the specified yield stress, then use the yield stress (not the ultimate).

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[IDS]

I’m also not sure that I would get the picture you’ve shown, in starting to look at your problem. You have a conc. tower which I assume is slip formed and can change conc. strengths at different elevations. It has a 15.75' O.D., a 12.5' I.D., 1.6' wall thick; it has 20 - .276" dia. wires, equally spaced and Fpu = 235ksi. I might place those stressing wires neared the O.D., not centered in the wall thick., for better advantage. I would then look at the DL of the tower, its geometry in plan, and ask how much stressing force must I apply so that the tower never goes into tension at the foundation. I don’t think you want the stress picture you show, from some fairly high tension to some conc. stress block, with a linear stress variation. With the variation in wind direction and change from high tension to high compression, you would literally grind the conc. up at the base, in a few years, through fatigue/fracture/granular disintegration. The elastic action of the stressing wires is all/mostly accounted for in the work of the stressing contractor; and once stressed the wires might be grouted.

The stress diagram is for the Ultimate Limit State; i.e. for calculation of the failure load. Under service loads the section would be entirely or mostly in compression.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[hokie66]

dhengr,
Those dimensions are in inches, not feet. It is a pole.

 

[hokie66]

I was incorrect...the reinforcement is probably wire as the picture shows. Spun prestressed poles I have seen use strands, but apparently deformed wire is used by some manufacturers.

http://www.emcocables.com/catalogs/pconcrete.pdf

 

[BAretired]

I thought the dimensions were in feet too. Had trouble reading the small scale drawing even when it was enlarged. Ignore my remarks about Dywidag bars.

BA

 

[transmissiontowers]

My first question would be, is this a tapered pole? We use them all the time in my business for Transmission lines. We also use them as antenna poles for communications. The best place to start is with the original manufacturer of the pole. There should be a nameplate on the base with the maximum moment capacity.

If it is a constant cross section, then it is a pretty small pole. The manufacturers usually provide a table of allowable moments for every 5 feet along the pole and tabulate the max moment, zero tension moment and a couple of others. The zero tension is the moment where the concrete cracks start to open. This is probably not relevant for free standing antenna poles, but is important for poles that have electrical wire on them. If under everyday conditions the concrete cracks are open, moisture will migrate into the steel strands and start to rust them.

I agree with Hokie, it is most likely a spun cast pole and judging by the picture that may be the top of the pole because it is so small. Our typical tapered poles are 11" to 19" at the tip and around 50" at the base (120' long). They also get about 12,000 psi concrete because there is so much cement.

The pole people use steel tapered forms (top and bottom halves) and place concrete in the bottom half, then close the form and pretension the strands with big hydraulic rams. The the poles are spun and left to cure for a day or two. They remove the forms and relax the prestressing strands and cut them off. The big poles can weigh over 50,000 pounds.

In either case, tapered or straight, the manufacturer should be able to provide the moment table to you. In the worst case, they will analyze their pole for your new loads and tell you if it is good or not.

_____________________________________
I have been called "A storehouse of worthless information" many times.