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# What is the tension in a bolt when "snug tight"?

## What is the tension in a bolt when "snug tight"?

(OP)
I need to calculate the tension in a bolt due to torsion when the bolt is tightened to "snug tight". Is there any formulae or codes (Australian) that states this?

### RE: What is the tension in a bolt when "snug tight"?

(OP)
EDIT: I need this information to input into a Finite Element model.

### RE: What is the tension in a bolt when "snug tight"?

Hand tight plus 3/4 of a turn.

Mike McCann
MMC Engineering

### RE: What is the tension in a bolt when "snug tight"?

Tension is generally treated as zero at snug tight condition.

### RE: What is the tension in a bolt when "snug tight"?

Here is the Microsoft Excel formula for tension at snug tight.

### RE: What is the tension in a bolt when "snug tight"?

Snug tight is defined as the maximum torque exerted by hand using the standard spud wrench that ironworkers use.

Mike... you've described a slip critical torque for a standard grip... (I think), I don't have my charts with me... but more than snug.

Dik

### RE: What is the tension in a bolt when "snug tight"?

From... Guide to Design Criteria for Bolted and Riveted Joints by Kulak et al...

Experience with the one full turn method indicated that it was impractical to use finger or hand tightness as a reliable point for starting the one turn. Because of out-offlatness, thread imperfections, and dirt accumulation, it was difficult and time consuming to determine the hand-tight position. Bethlehem Steel Corporation
developed a modified “turn-of-nut” method, using the AAR studies and additional tests of their own.4.16, 4.17 This method called for running the nut up to a snug position using an impact wrench rather than the fingertight ondition. From the snug position the nut was given an additional ½ or ¾ turn, depending on the length of the bolt. The snug condition was defined as the point at which the wrench
started to impact. This occurred when the turning of the nut was resisted by friction between the face of the nut and the surface of the steel. Snug-tightening the bolts induces small clamping forces in the bolts. In general, at the snug-tight condition the bolt clamping forces can vary considerably because elongations are still within the elastic range. This is illustrated in Fig. 4.18 where the range of bolt clamping force and bolt elongation at the snug tight condition is shown for 7/8 in. dia. A325 bolts installed in an A440 steel test joint. The average clamping force at the snugtight condition was equal to about 26 kip. The bolts in this test joint were snug tightened by means of an impact wrench. This modified turn-of-nut method was
eventually incorporated into the 1960 specification of the council.
For bolts equal to or greater than about ¾ in. dia., snug position provided by an impact wrench is approximately equal to the tightness attained by the full effort of a man using an ordinary spud wrench. For longer or larger diameter bolts, the force produced by this snug load will be less than that for the “standard” case, and for shorter or smaller diameter bolts it will be more. These differences are accommodated in the specification by prescribing the same definition of snug tight...

Dik

### RE: What is the tension in a bolt when "snug tight"?

The RCSC Specification for Structural Joints Using ASTM A325 or A490 Bolts, Section 8.1 defines snug tight as follows:
"The snug-tightened condition is the tightness that is attained with a few impacts of an impact wrench or the full effort of an ironworker using an ordinary spud wrench to bring the connected plies into firm contact."

That being said, at the snug tight condition the tension in the bolts within a given connection will vary from one bolt to another.  The tension will also vary from one joint type to the next and depending on the sequence that the various plies are erected for a given joint type.

But don't let all of this variation get you down.  I usually place more value in the quantifiable portion of the RCSC definition - the full force of an ironworker (say 100# to 150#) using an ordinary spud wrench (16in - 18in long).  On the low end you're looking at 133 ft-lbs and at the high end  you have 225 ft-lbs of torque.

The translation of torque to tension varies widely (surprise surprise) with bolt lubrication, thread type and condition, bolt diameter, bolt length and fastener part (bolt and nut) lot.

Hope this helps.

Tim

### RE: What is the tension in a bolt when "snug tight"?

Mike... I found the data:

bolt grip <= to 4 times bolt diameter 1/3 turn
bolt grip <= to 8 times bolt diameter 1/2 turn
bolt grip <= to 12 times bolt diameter 2/3 turn

Dik

### RE: What is the tension in a bolt when "snug tight"?

As tgattie says, snug-tight is defined as the full effort of a bloke using a proper spud wrench.

If you want to convert this torque into tension you can use these formulas:

http://www.efunda.com/designstandards/screws/fasteners_intro.cfm

### RE: What is the tension in a bolt when "snug tight"?

(OP)
Thank you all for your help

### RE: What is the tension in a bolt when "snug tight"?

This might be of interest. The Steel Construction Institute (UK body) suggested these values:

Bolt size    Spanner lever arm  Approx Torque
[mm]                    [Nm]
M16              460                    90
M20              550                    110
M24              640                    130
M30              730                    160

This is based on assumed 250N force applied by hand.

### RE: What is the tension in a bolt when "snug tight"?

Looks like the table is based on about 200N force, which is wimpy.

### RE: What is the tension in a bolt when "snug tight"?

hokie66,

200N = 20kg (44lbs) approx. I challenge you to exert more than that with nothing but your feet and your weight to push against it.

Just because someone can bench press more than that doesnt mean they can exert it on a spud wrench.

### RE: What is the tension in a bolt when "snug tight"?

There are many references to the turn-of-the-nut procedure above, which I believe to be a good QC procedure for installing slip critical bolts.

In my agency, we used turn-of-the-nut and the procedure includes guidance for snug tight condition.  All values are specified to be -0 to +2 kips tension ...

diameter                snug tight
in kips
half inch                     1
five eighths                 2
three quarters             3
seven eighths              4
one inch                     5
one and one eighth      6
one and one quarter    7
one and three eighths  9
one and a half           10

Many folks get confused about the relationship between tension and torque.  I wouldn't use torque as a measure of bolt tension because the coefficient of friction varies so much from one installation to another.

It is unclear to me how a quality FE model may be obtained for bolting operations.  I have thought about this problem now and then over the years and it is unclear to me how most folks would accomplish it.  I have my own idea, where I would induce a temperature strain on the preloaded components, but that may not produce the results you want.  It sounds good to me.

Good Luck.

### RE: What is the tension in a bolt when "snug tight"?

Wouldn't the tension at snug-tight also depends on the initial out-of-straightness of the plates you're drawing together?  If you had to fight past a fitup gap to get the plies to draw together, isn't that more tension than snug-tight for perfectly flat plies?

Hg

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### RE: What is the tension in a bolt when "snug tight"?

2000 pounds of tension is estimated for snug tight condition when calculating the additional turns necessary for pretension of 90% yield for bolts in critical connections.

### RE: What is the tension in a bolt when "snug tight"?

Here is a statement from Research Council on Structural Connections titled "Specification for Structural Joints Using ASTM A325 or A490 Bolts" (June 23, 2000) 9.1 Snug-Tightened Joints:

"After the connections have been assembled, it shall be visually ensured that the piles of the connected elements have  been brought into firm contact and that washers have been used as required in Section 6.  No further evidence of conformity is required for snug-tightened joints.  The magnitude of the clamping force that exists in a snug-tightened joint is not a consideration."

I believe this is the same in later additions as well.

j

### RE: What is the tension in a bolt when "snug tight"?

jechols has it as i recall (except "piles" should be "plies"). in other words, if it doesn't look like it comin' apart, it's good to go.

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