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.

 

[BAretired]

Friction on the thread can make torque measurements inaccurate as stated earlier. Also, a turnbuckle is not the best piece of hardware to provide an accurate measure of rod extension because two rods are being stretched simultaneously.

It would be better to snug everything up, place lock nuts at the dead ends, then apply torque to a single nut, first marking where the starting position is, then measuring the elongation. Continue tightening until the desired elongation has been reached.

BA

 

[efsinc]

BA,
thank you. If i am understanding you correctly, the idea is apply a little tension to the rod to pull the truss heels towards each other to snug everything up, to allow all the connections to slip as much as they might, letting all the bolts move to the sides of their holes etcetera. The point reached after this initial 'snugging' movement i would call effectively the point of zero tension in the rod. then lock everything in place at this point except for a single turnbuckle. from this point i would require the number of turns of the turnbuckle to introduce the elongation necessary to achieve my design tension.
am i warm?

 

[BAretired]

Warm, yes and your method would work if you know the pitch of the turnbuckle threads and don't lose track of the number of turns, but I would prefer a visual aid while tensioning. That is why I would mark the position of the nut at the point of zero tension and continue tensioning the rods simultaneously each side of the truss until the elsongations matched the required elongation.

Finally, put a lock nut on each live end.

BA

 

[dhengr]

If you have a wind uplift issue, obviously that has to be accounted for in you repair design. I wouldn’t think that was the cause of broken bottom chord members though, unless your analysis shows how that happens. You have gotten good advice on the wood design and engineering forum, on your other thread. I believe I have participated in a couple other threads on this same subject in the past year or two. But, I’m having some trouble convincing you, on this thread, that a torque wrench may not be the way to go on tensioning these rods. Assuming that you have read and digested the above posts. To more effectively reinforce the trusses, I would consider harping the two rods at each bot. chord. Pick two harp points under bot. chord panel points, and if you can gain access, hook the rod end reaction saddles up over the top corner of the top&bot. chord end joint. That way these saddles are working in bearing on the wood rather than added shear in the end side plates, although that might work too. By harping the rods you add rod tension at the bot. chord over the middle third of the truss and you also add an upward component at the third points (panel points) with the two affects acting as prestressing on the truss. By the way, postensioning in conc. is accomplished by measuring wire extension, not by any torque method. I’ve suggested several ways to gauge the elongation in the rods and thus the tension. If there are a bunch of these trusses, maybe you would strain gauge a few rods to prove your less costly methods.

 

[efsinc]

dhengr,
i think i have been disenchanted with the use of a torque wrench. The direct measurement of rod elongation as discussed in the 2nd 3rd and 4th posts directly above i think will form a part of the final plan.
Regarding the use of a strain gage, i think this sounds excellent, although, honestly, i do not know how to specify one, how to use one, nor how to direct a contractor to obtain and utilize it. the last one i remember was in college some years ago. if you would provide me some direction on this i would be most grateful.

 

[dhengr]

Efsinc:
Read my two posts from 28FEB, for that matter make of hard copy of both of your threads and really study them. You must read btwn. the lines a little bit to see what applies to your specific problem, because until your last couple posts we were guessing at what you were trying to do. But, there are good bits of advice and applicable ideas in a number of the posts in the two threads.

Your local testing lab will do the strain gaging for you, the same people who test your soil, conc., steel and welds, etc. Or, talk to your local Uni. Prof. and get one of his/her Grad. students to do it as a class project. They have the equip. to do this and are experienced at it. It’s not hard to do, we just aren’t proficient at it. Look at my 28FEB11, 14:17 post for answers to several of your questions, and the last para. offers another fairly simple means of measuring rod extension. I don’t know that I would strain gage on every truss, you are just trying to prove your simpler methods for reliability, if there are many trusses involved. This is certainly not an exact process, we are talking +/-kips, not 100's of pounds, and further truss relaxation will change things in fairly short order.

As other’s have said, you will not pull the trusses back together with these rods, you must jack the trusses up and shore them to do most of that, and bring the bot. chords back above a straight line. Then, your thought that you start snugging everything up with these rods is a sound idea, but this is also a judgement call, not an exact number. You have already studied the condition of all the truss joints and you watch them settle back into position; you watch the truss length, movement at bearings, etc. And, you called it ‘zero tension in the rod’ at this point, but that’s no longer really true, I’d call it your started point for the final prestressing, the starting point for your elongation measurement. BA’s comments on nuts and locknuts at the ends is good because that gives you three points at which to do the tensioning, and this will be helpful particularly if you harp the rods. Of course, this assumes you have some working access at the ends of the trusses.

 

[rb1957]

if you don't like the torque wench (i think it's a reasonable tool, just you need to be a little careful in using it), you could use PLI (PreLoad Indicating) Washers, which crush at a predetermined load (and have much less scatter in the load than a wrench, but cost a lot more).

thinking about it, you probably can't use these ... you're tensioning up the rod, to snug up everything, and want to understand how much load you've up into the rod. P = 5T/d is a rough estimate. You could dble check this with witness marks on the rod, to measure the extension (but i expect this'll be so small that the error in measuring would be like the scatter in the torque wrench).

 

[BAretired]

dhengr,

I'm not so sure harping the rods is a good idea. We are talking about a bowstring truss. Under uniform snow load, the top chord is in almost pure compression, the bottom chord in almost pure tension and the web members are almost unstressed. Do you think it is wise to introduce upward reactions at the third points?

If the rods are straight along the bottom chord, there is a danger in introducing too much tension in them. We know that the bottom chord has been damaged, but we don't know where and how badly damaged it is. Is the bottom chord capable of taking the compression which the rods are adding? If the bottom chord buckles, nothing useful has been accomplished.

Perhaps a better plan is to stress the rods until the bottom chord is raised to the same level as the undamaged trusses. Then, when the snow arrives, all trusses will sag more or less uniformaly. Does it snow in California?

The rods should be periodically supported from the truss to prevent sag under self weight.

BA

 

[efsinc]

wow....you all make me proud to be in the engineering community. i'll post more as i proceed.

 

[BAretired]

efsinc,

There is another issue which has not been discussed. If the existing bottom chord is totally disregarded, the addition of the two rods results in a tied arch, not a truss. That means that unsymmetrical snow load will cause bending in the top chord which was not present with the truss.

BA

 

[dhengr]

rb1957.... From sad experience we Structural Engineers just don’t trust torque wrenches on semi-rusty bolts, nuts and faying surfaces, in highly stressed joints. You ME’s and Aero’s have more faith, but usually under more optimal conditions. I’d need to know more about the origin of P = 5T/d as a rough estimate to comment on that. I think most of your torque values are based on considerable testing over fairly specific sizes, joint configurations and load ranges, etc. Intuitively, torque and bolt tension should be related, if all the variables which can foul things up are adequately controlled, but there is plenty of literature and testing out there which muddies that water. Once we grind up a little rust and even out any imperfections in the mating thread profiles, by pre-tightening the nut, and pre-clamping the plates, turn of the nut (or the bolt) is pretty reliable, and it eliminates a few of the variables, still using essentially the same logic, intuition and calcs. And yes, some sort of witness marks should be used to measure turn angle.

BA.... All good points, and you may well be right. But as is usual, we were asked a fairly complex design question and have been given a dearth of meaningful and important info. At least we have peaked Efsinc’s interest now, and he’s back in the bldg. I’ve been working on the assumption that the trusses were overloaded, the bot. chords over stressed and some small uplift might be helpful, but we can’t really answer that right now, in either need, eccentricity or in magnitude. For all the trouble I seem to get into when I talk about computers and computer programs, I would like to see a couple simple computer runs on this truss, under the different loading conditions, along with its construction details, member sizes and dimensions. I don’t know where the structure is or what the load conditions are. In the U.S, that will be another 30 or 40 pages of ASCE 7, Min. Loads reading and calcs.

 

[ToadJones]

if the rods are temporary maybe install a load cell in the rod?

 

[efsinc]

there is no snow. just palm trees and movie stars and a heavy tax load condition.