torque to tension 2

[efsinc]

i need to develop a certain tension in a steel rod with a turnbuckle. has is the correlation between a torque wrench and the rod tension made?
sure appreciate your help.

 

[frv]

Fundamental Mechanics of Materials: PL=EA

 

[frv]

oops.. that's supposed to read PL/EA

 

[rb1957]

T = Pd/5 is a typical relationship between Torque and P for Preload (or Tension)

 

[dhengr]

If you need a really accurate tension value, I’d strain gage the rods. Otherwise, frv is on the right track, but I’d add this to his comments: 1.) support the rods and take all of the sag out of them, 2.) snug up the turnbuckle more than just finger tight, 3.) use the turn-of-the-nut method to estimate your elongation and thus your stress or tension, 4.) work the rods someplace south of Fy.

 

[ToadJones]

nut/bolt torque is not an accurate method for determining tension in fasteners.

 

[ToadJones]

...you could bind up you nuts on the threads and get a wildly high torque reading that is telling you nothing more than your nuts are bound up.
Follow the advice of frv and dhengr

 

[ToadJones]

ahh... I take that back. Turn-of-the-nut isn't all the accurate either, ESPECIALLY when you're talking about a long steel rod.
If I recall correctly, there is something in RCSC about verifying nut turn on any fastener whose length is more than 10 or 12 times the diameter. I'll have to break out RCSC tomorrow...if I chose to go to work.

 

[dhengr]

ToadJ:
“...you could bind up you nuts on the”...., “your nuts are bound up.” What’s that all about anyway? I kinda thought you took care of that problem about an hour ago on another thread. But, you are still the guilty party, as you put it. Please report your findings from RCSC for our edification. I think that’s a slightly different problem than we have here, tell us how and why. I don’t have my FEM turned on at the moment, so I’ll have to await your findings to comment further. Nor do I have a current copy of the RCSC publications so it’ll be interesting to hear what you find.

 

[rscassar]

I wouldn't be specifying pre-tension in rods. The turnbuckle should be used to take out the slack.

 

[ToadJones]

RCSC is a free download

 

[ToadJones]

All I am getting at, is, it will be difficult, in my opinion to use the Turn-of-the-nut method on a turnbuckle located in the center of a steel rod. You will have to some how verify the amount of nut-turn, or how much your turnbuckle has turned with relation to your tension rod. I believe this would entail a solid means of keeping the tension rod from rotating while turning the turnbuckle.
I am not quite sure how that would be done.

Download RCSC and see the comments of table 8.2, which may not even apply since RCSC is for ASTM A325 and A490 Bolts. My reference, in this case, would be in error. See also the commentary of 8.2 with regard to research on bolts longer than 12 db and using the turn-of-nut. This is what I was thinking of when I read the OP.

My apologies directly to dhengr.

 

[ToadJones]

kikflip might have the right idea.
His comment reminds me of guys cranking life lines extremely tight and taking out the catenary....which is dangerous.

 

[dhengr]

Efsinc:
Regarding turn-of-the-nut (the turnbuckle), put a tick mark on each end of the turnbuckle and on each rod at the same locations. One revolution of the turnbuckle w.r.t. the rods (average the two tick marks) means a certain amount of total rod extension as a function of the screw thread geometry involved. Clean the threads well, lub them lightly, try to hold the rods from turning without nicking them up. You will likely be working your threads well below yield, which is not the case for the slip critical joints and high strength bolts covered in Toad’s ref., the RCSC. But, that’s a good ref. for understanding this kind of problem. Those bolts and joints have been tested extensively, and their discussion about tightening up the plys or faying surfaces, prior to applying the turn-of-the-nut, is akin to my comment about supporting (straightening the rod out) the rod to get any sag out of it, and pre-tightening the turnbuckle in this condition. The structural high strength bolts are working at .7 or .8Fult. so there is some thread and bolt stretching and yielding going on with them. And, on larger and longer bolts the thread to body stretch changes, from what’s been tested, so they want you to test against a calibrated tensioning device to set the turn-of-the-nut value for a given tension. Kickflip’s comment deserves some thought too, as it takes almost an infinite force to remove the sag from a spanning rod by tension alone, so again, my reason for saying support it straight, then snug up the turnbuckle.

Otherwise, delta = PL/AE still holds true; and at f = P/A = 24ksi, you should get about .1" extension in the rods on a 10' gauge length. The class and geometry specifics of the screw thread involved will indicate how many turns produce .1" rod extension, i.e. a 3/4" - 10 UNC thread has 10 threads/inch, or one turn/.1" rod extension. You might clamp a 5' long metal bar at one end, atop your rod, so it is supported by your rod, and at the other end use a machinist’s dial indicator to measure .05" bar movement, then compare the two results for you rod tension estimate.

 

[rb1957]

a couple of questions for you guys ...
does "rods" imply rebar ? 'cause certainly you can pretension a free standing turnbuckle (like a tension wire brace)

and IMHO, PL/EA doesn't give you the relationship between torque and preload, 'cause you need to account for friction; no?

 

[ToadJones]

The reason I thought KikFlip comments deserves some attention is, if you tension this rod to somewhere around FY and it receives additional loading, well, then what?
This is what I was getting at with the super-tight life line installations I have seen in the field.
If you crank a life line tight and it approaches being horizontal, the geometry doesn't lend it self well to additional loading from a lanyard. Pre-loading only compounds matters further.

 

[dhengr]

I understood the ref. to the life line or catenary, and at the extreme you would be talking about a tension cable structure. As usual in my old guy fashion, I was offering a first shot at the problem. We are missing a bunch of info. on the whole problem, and the OP’er. seems to have left the building. If the rods are 100' long and oriented or spanning in a horiz. position, then we certainly better give some thought to this issue, because just holding the rods near straight will produce more force than he might want. He has to explain in more detail what he is really trying to accomplish. If they are above a 45° incline and only 30' long, or hanging vert. then the above approach should be pretty good. And, to account for some of this uncertainty, not having done any catenary calcs. on this, was why I said work the rod south of Fy. I was assuming a mild steel rod, not a rebar, maybe 36-50ksi. Mostly, it has to obey Hooke’s Law and work below the yield point, and most of the higher strength steels don’t have a well defined yield point. Otherwise, it seems to me you can preload (prestress) any rod to f1ksi, and then any additional loading just works you further up the stress/strain curve, proportionally, until you get to Fy. Thus, PL/AE relates the force in the rod to its elongation. I think RCSC calls this strain dependant tightening. And, I rejected torque immediately because it is not very reliable for this type of application, for all the reasons suggested earlier, friction, rusty threads, etc. The ME guys have used torque as an indicator of bolt or stud tension or flange clamping force for a long time, and claim some reliability under ideal conditions. Most of our nuts and bolts are not so clean and pretty.

 

[BAretired]

efsinc,

Why must you develop a certain tension in the steel rod? What is your application?

BA

 

[rb1957]

dhengr,
yes PL/EA defines the extension of the rod, but how do you connect P to T, the torque applied, ? wasn't that the original question ??

 

[efsinc]

BA and rb and all,
The application is the post tensioning of the bottom chord of an existing damaged bowstring truss. In addition to the various other repairs, I am intending to utilize a steel round rod each side attached to the heel each end. I can determine the tension force i want developed in the rod but am uncertain how to verify that the contractor has attained it during the installation. I am thinking a calibrated torque wrench and turnbuckle system is the way to go but i am not sure. Thus my original question.
Thanks to all for valuable insight.