Verifying Bolt Pretension
Verifying Bolt Pretension
(OP)
Any suggestions on how to verify correct bolt pretension 18 months after installation?
Connections are galvanized and slip critical. In some cases with OVS holes. Erection was completed nearly 18 months ago, and an issue has been raised concerning the bolt installation and whether the appropriate pretension was provided. Bolts are 7/8 A325 and 1 1/8" A490. They were installed by turn-of-the-nut, but most of the marks are no longer visible. And some of the bolts in question clearly lack the appropriate pretension, even though the marks look correct.
I am familiar with the arbitration method provided by RCSC. But even in ideal conditions torque is not a good representation of pretension. Exposure to the elements and loss of lubrication will may torque value scattered.
Thanks
Connections are galvanized and slip critical. In some cases with OVS holes. Erection was completed nearly 18 months ago, and an issue has been raised concerning the bolt installation and whether the appropriate pretension was provided. Bolts are 7/8 A325 and 1 1/8" A490. They were installed by turn-of-the-nut, but most of the marks are no longer visible. And some of the bolts in question clearly lack the appropriate pretension, even though the marks look correct.
I am familiar with the arbitration method provided by RCSC. But even in ideal conditions torque is not a good representation of pretension. Exposure to the elements and loss of lubrication will may torque value scattered.
Thanks






RE: Verifying Bolt Pretension
An expert is a man who has made all the mistakes which can be made in a very narrow field
RE: Verifying Bolt Pretension
rowing...
Your suggestion is similar to the RCSC arbitration. But, this is intended for use relatively close to the installation date of the bolts in question.
(It is curious that RCSC lists numerous variables that can impact the consistency of using calibrated wrench method for pretension. But then provides torque as the only method of arbitration in a later chapter.)
http://www.FerrellEngineering.com
RE: Verifying Bolt Pretension
That is the major drawback of the short thread lengths on A325 and A490 bolts, I looked into a direct tension installer/tester but it needs enough thread to be able to pull the bolt with full load.
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Verifying Bolt Pretension
I remember reading something in RCSC that mentions this.
RE: Verifying Bolt Pretension
http://www.FerrellEngineering.com
RE: Verifying Bolt Pretension
I'm in Australia so the RCSC arbitration means little to me
However if you are looking for a write back to the existing installation for comp, you best choice is to set the lower limit and see how many fail, no one can argue the comp then.
An expert is a man who has made all the mistakes which can be made in a very narrow field
RE: Verifying Bolt Pretension
Snugging out the unevenness of large gusset plates is sometimes a problem with turn of the nut; at which point do you say they are snug and place the starting marks.
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Verifying Bolt Pretension
Would it be possible to un-install each bolt one-by-one and re-install new bolts with load indicating devices (either load indicating washers or bolts).
I am actually surprised by how little construction workers are aware of tensioned bolted connections. When I indicate on the drawings that the bolts are to be TF (tensioned friction), it means nothing to them and they install the bolts as they would for snug tight.
RE: Verifying Bolt Pretension
http://www.FerrellEngineering.com
RE: Verifying Bolt Pretension
I might have misunderstood the context of your statement that pretension and torque are not directly related, but have a look at this site:-
http://www.exprobase.com/Default.aspx?page=841
desertfox
RE: Verifying Bolt Pretension
Read the first few paragraphs of the attached:
RE: Verifying Bolt Pretension
Your reference is based on an ideal installation condition. I agree that if all other variables can be controlled and repeated that torque may provide an acceptable method of verifying pretension. I am familiar with other construction applications which prefer the use of torque values to establish pretension, and use torque controlled bolts to control the maximum pretension.
But, for steel construction torque is not representative of pretension. There are several variables that vary from one installation to the next. Lubrication, degree of contact between connected plies, repeatability of the field equipment, etc.
This said, torque has been the only suggested method to verify completed installations.
http://www.FerrellEngineering.com
RE: Verifying Bolt Pretension
What I was getting at was that snugging has to be repeated, like when you put a wheel back on your car, you snug the first nut, then the third, fifth, second, fourth...and when you get back to the first, it is loose, so you go round again before final tightening. It seems the more bolts, the more times you have to go around.
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Verifying Bolt Pretension
http://www.FerrellEngineering.com
RE: Verifying Bolt Pretension
The only thing I can come up with, is, after you have reinstalled the obvious loose ones, instead of the torque wrench method, back off the nuts lubricate if necessary,and reinstall by turn of the nut using a different color marks.
That doesn't work for the galvanized ones, that needs more thought, if it is even doable, the zinc is subject to galling.
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Verifying Bolt Pretension
Thanks for your response I agree there are a number of varibles that affect the preload.
Here are some accuracies given for various methods:-
Operator "Feel" +/- 35%
Torque Wrench +/- 25%
Angle Torquing (Turn of nut) +/- 15%
Load Indicating Washer +/- 10%
Measuring Bolt elongation +/- 5%
Hydraulic Bolt pretension +/- (1% to 10%)
Strain Gauges / Ultrasonics +/- 1%
Superbolt multi-jackbolt tensioners +/- 5%
Now I notice in one post it mentions backing the nuts off before retightening, if its possible you could measure the contraction of the bolt before retightening and therefore estimate the original preload but i guess that won't be easy.
desertfox
RE: Verifying Bolt Pretension
RCSC does not allow the reuse of galv bolts, right?
I know "touching up" or "retightening" of adjacent bolts is allowed but how much further can you go?
RE: Verifying Bolt Pretension
In my present condition, I cannot verify if the bolts were ever tension properly. 18 months after completed erection, I cannot determine the cause. At this time I am suggesting that all loose bolt assemblies be replaced.
I read an article concerning erection and bolting in windmills. I read that in many cases the bolted connections require inspection on 6 month intervals and re-pretensioning is required. I believe these connections are "pretensioned", but not "slip-critical". Thus the pretension must be maintained, but the resistance to slip is not an issue.
http://www.FerrellEngineering.com
RE: Verifying Bolt Pretension
Michael.
Timing has a lot to do with the outcome of a rain dance.