pretensioned joints

[Ahmmed123]

how do I convert 16 kips preload to tightened torque lb- ft ?. basically, I got the values of the min bolt pretension form table J3.1 AISC manual. but the installer was asking me how much torque lb- ft he need to use in the torque wrench ??

Thank you

 

[azcats]

Snug tight + 1/3 turn.

Unless you have a Skidmore available. Then you could figure out the torque for your specific assembly.

 

[connectegr]

Torque and Pretension are not the same thing!!!!

There are calculations for determining the torque equivalent to a pretension in ideal conditions. (I would share them to avoid confusion) BUT, this does not consider other factors such as; lubrication, surface friction, wrench calibration, etc. RCSC provides several methods for bolt pretension installation. And regretfully calibrated wrench is included, but the commentary also contains a list of variables that effect this process. If calibrated wrench is used the pre-installation verification procedure should be followed. Read the commentary of 8.2.2 carefully before selecting calibrated wrench for pretensioning.

I am not aware of any bolt diameter requiring 16 kips minimum pretension. What is your application? Is RCSC/AISC the appropriate spec?

http://www.FerrellEngineering.com

Providing fabrication and erection efficient structural design of connections. Consulting services for structural welding and bolting.

 

[connectegr]

Sorry a 1/2" A490 bolt does have a 16 kip minimum pretension.

As azcats has indicated turn-of-the-nut is the most common method of pretensioning. This procedure is given in the RCSC 8.2.1

After the bolts are snug tight with all plies in firm contact, the bolt and protruding end are marked, then the nut is turned an additional 1/3 turn. Care must be taken to assure that the unturned part does not rotate.

http://www.FerrellEngineering.com

Providing fabrication and erection efficient structural design of connections. Consulting services for structural welding and bolting.

 

[Ahmmed123]

connectegr : the min for 1/2 is 12 kps at the AISC Manual the PE engineer who aproved my design asked for 16 Kips. I have read the commentary of 8.2.2 but that doesn't tell me what to say to the installer. azcats: I told him that he need to use 1/3 turn but the wrench that he is using required lb-ft torque like the one in the link (

http://www.harborfreight.com/1-2-half-inch-drive-click-stop-torque-wrench-239.html)

I looked everywhere in the RCSC 2009 but It doesn't say how to convert the pretension (preload)kips to torque lb-ft. I was looking online and I found the following:

T = (K D P)/12

T = Torque (ft-lbs)
D = Nominal Diameter (inches)
P = Desired Clamp Load Tension (lbs)
K = Torque Coefficient (dimensionless)

where for my case it's dry and I think It's K= 0.3 (I'll check it when I get to the office in the morning)
D=0.5
p=16000 lb

then T= 200 lb-ft ???!!! this is too much and is that correct???

 

[ishvaaag]

Try a search for Torque Bolts in this forum. T=P·d/5 is quoted at least a pair of times, that would make 133 lb·ft in your case.

 

[WillisV]

Tell him to get a new wrench and use turn of the nut method as required.

 

[Ahmmed123]

willisV: this is not the problem. but is it that hard to get him a number to use in his torque wrench ???

 

[connectegr]

Tell the contractor to use the RCSC installation and pre-installation procedure for calibrated wrench. No "torque" value is prescibed in the RCSC spec, intentionally. The formula noted above is not given in the spec, intentionally. Do not give them a torque value. The contractor should complete the pre-installation verification as described in RCSC. These steps using a Skidmore (tension calibrator) will determine the appropriate "torque" relative to the bolt lot and existing lubrication.

I have never required a pretensioned 1/2 A325 bolt. But with 3/4 A325 and above bolts, I have seen calculated torque values result in pretension 40-50% less than expected.

The RCSC Spec can be downloaded for free on the website.

http://www.boltcouncil.org

http://www.FerrellEngineering.com

Providing fabrication and erection efficient structural design of connections. Consulting services for structural welding and bolting.

 

[oldrunner]

Your contractor can also use Direct Tension Indicators (DTI's)and the torque wrench problem will go away.

 

[rb1957]

"how do I convert 16 kips preload to tightened torque lb- ft ?."

i thought the rule of thumb was T = Pd/5

 

[humanengr]

You cannot convert pre-tension to torque. The torque will vary for any given job site based on site
conditions and the condition of the bolts (lubrication, etc.). The only way (properly accepted way) of obtaining
a torque value is to calibrate the torque wrench on a tension indicating device such as Skidmore.
RCSC specs. provide procedures for pre-installation calibration. The torque wrench must be calibated. Even
turn of the nut must be calibrated to ensure the procedure / turns will result in the required pre-tension.

 

[rb1957]

http://www.skidmore-wilhelm.com/faq.htm

...

this sounds like an expensive solution to the question. do you Need an accurate preload ? is this a fatigue critical joint ??

if it's not Critical, i'd use T = Pd/5 (you can probably find that for yourself, maybe at roymech's site). then using a typical scatter with torque wrenches i'd *1.5 to make sure that the minimum preload developed by the wrench would meet the value i need. then i'd size the bolt to (P*1.5)*1.33 to account for the maximum preload that the wrench might have applied; maximum preload = 16000*1.33*1.5 = 32000 lbs. ie preload from a torque wrench is between 2/3 and 4/3 of the target value.

less scatter is achieved with PLI (PreLoad Indicating) washers, they're 0.9 to 1.1 of the target. in this case the assembly instruction is "torque untill the washer collapses".

 

[msquared48]

Ammed123:

200 ft. lb. is not unreasonable. I used to have to check the mainframe bolts for PEMB structures, and the standard torque for them with a torque wrench was 175 ft. lb.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com