Tightening Slip Critical bolts to the required Tension

[tristan861]

Table 3.1 in (AISC Specs2010) gives the minimum bolts pretension for A325, A490 bolts. Should I use these values when tightening? or do they need to be divided by the slip coeff since I have Slip critical bolts?

Lets say I have 5/8'' bolt size A325, the pretension load given in the table is 19 K. but I have a slip coeff lets say Class A = 0.3, So 19Kip / 0.3 = 63.3 Kips?

Do I get it right?


Thanks,,

 

[KootK]

You don't need to divide the tension by your friction coefficient. Ordinary, pretensioned bolts are not designed to resist any particular bolt shear via friction so there's no need to *scale up" so to speak.

 

[tristan861]

KootK. you got me confused here.

The bolts used in the project are Slip critical bolts (not pretensioned bolts), meaning, shear will transfer via friction.
Do I need to divide the aforementioned 19 Kips by 0.3 in that case ?

 

[DrZoidberWoop]

This is a strangely asked question. If you are a project engineer, you might be asking about "pre-installation verification," where you take 3 bolt assemblies from a lot and tension them to 1.05*Min bolt pretension from AISC Table J3.1 (which is ASTM spec), usually on a Skidmore-Wilhelm machine. If you are a design engineer, the slip-critical strength calc is explained in AISC J3.8. Over the years, changes have been made to slip coefficients and strength/servicability limit states, so use the correct project-specific code from your general notes.

 

[tristan861]

I am asking about the amount of torque or force that the labor should reach when tightening a SC bolt.
Is the pretension-ed and SC bolts tightened to the same amount of torque?

 

[SwinnyGG]

No, you do not multiply anything.

The minimum tensile strength of an A325 bolt is only 27,000 lb; you most certainly cannot install a 5/8" bolt with 63,000 lb of preload.

Pretensioned vs. slip critical doesn't mean the bolts are different- it means the required performance of the joint is different. Bolts are bolts. Joints are snug tight or slip critical or pretensioned.

 

[GC_Hopi]

Refer to Section 8 Installation in the Specification for Structural Joints using High-Strength Bolts by RCSC. This is the reference AISC points to. Usually you are not tightening the bolt to a value but check the requirement based on the method (twist off, DTI, turn of nut) you are using to install the bolt.

 

[tristan861]

Its clear now. Thanks

But I found this formula on calculating the required torque for pretension bolts
T = KPD
Where k is the friction coeff
P clamp force
D diam of bolt.

My point here is that the torque value depends on on the friction coeff. ( coating of the bolt). Is there any references where I can find more about that formula?

 

[dik]

There were lots of articles about it... K is the critical value and this can be very variable.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[azcats]

Friction varies widely between bolt & nut assemblies. There's no code 'k' factor for tensioning structural bolts.

If you want to use torque to tension, you have to test your specific assembly in a Skidmore device to determine the torque for the required pretension.

Otherwise, there are other methods (TC bolts, turn-of-nut) for tensioning bolts in compliance w/ AISC and RCSC.