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Re: Bolt Tightening - Turn of the nut method

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Mr. Raghavan,

Your question is a good one, and deceptively more complex than one might

You are correct to assume that the relationship between 'turn' and 'tension'
or 'compression' is not always linear.  'Turn' is linearly related to bolt
tension ONLY during the elastic portion of bolt tightening.  When a high
strength bolt is tensioned to the levels typical of a slip-critical
(friction-grip) connection, the turn/tension or turn/compression
relationship changes DURING tightening.  That is, equal amounts of turn do
not correspond with equal rises in tension or compression in the bolt's
inelastic region.

Further, the correlation between bolt tension and joint compression is only
sound provided that the plies are in fact together.  It is possible, for
example, to have fully tensioned bolts on a connection in which the plies
are still not in firm and near continuous contact. (i.e. not even snug)

The RCSC Bolting Specification does account for the variability of the
'Turn-of-Nut' method via the differing amounts of turn prescribed for
different bolt lengths and grades.   Thus, provided that one starts from a
'snug' condition with the plies adequately together, the prescribed 'turn'
will develop sufficient tension/compression.

I slightly disagree with Mr. Sprague's comment that Method 1 (turn of the
nut) is a torque type method --- like twist-off bolts or calibrated wrench.
T-O-N is only torque-based to the extent that torque is relied upon as
evidence of 'snug'.  I will grant him that the 'full effort of a man on a
spud wrench' or 'a few impacts with a impact wrench' are torque-based means
to bring the plies together.  However, we do in the spec say that such
efforts MAY attain snug, but they may not as well ---- particularly in
larger multi-ply connections.  Yet, torque plays NO role in the full
tensioning aspect of the T-O-N method.  (Except to say that application of
torsional energy will be required to advance the nut the specified amount of

I agree with Mr. Sprague that, "Of all 4 methods allowed by the RCSC only #4
(direct tension indicator) is indicative of a true bolt tension."  It is my
opinion that the direct tension indicator is a method of inspection rather
than a method of tightening.  That is why we see DTIs used on twist-off type
bolts in buildings, or why states in the U.S.A. use DTIs with a modified
T-O-N method in bridgework.

With respect to a 'monitoring' function for T-O-N, the most common method
for monitoring the T-O-N method is known as 'match-marking'.  Although it is
not required by the RCSC, match-marking is arguably the best way (absent use
of a direct tension indicator) an inspector can verify after the fact that
the specified turn has in fact been applied to each high strength bolt.
Match-marking involves marking both the Nut and the Bolt after 'snugging'
and before full tensioning.  Both marks are necessary to enable an inspector
to verify that the nut has been turned relative to the bolt ----- as it is
often the case that both turn together because no one is holding the head of
the bolt from the other side.  (Sometimes one accidentally holds the wrong
one too.  Easy to do on a big connection, as I've done it.)

T-O-N is elegant and simple in concept.  Were it not for the issues of
'snugging' or 'inspectability', it would likely be the only method needed.

David Sharp
TurnaSure LLC
(disclosure: We make DTIs)

----- Original Message -----
From: "K.S. Raghavan" <ksr(--nospam--at)>
To: <seaint(--nospam--at)>
Sent: Monday, October 08, 2001 2:28 AM
Subject: Bolt Tightening - Turn of the nut method

> Hello all,
> If the "Turn of Nut" method is adopted for monitoring the bolt tension is
> not right  to say that
> the turn indicates the compression of the flanges rather than tension in
> bolt ? In my opinion
> tension in the bolt will be in error depending on the flexibility of the
> flanges if the turn of nut is directly correlated with bolt tension.
> Comments please.  ..  raghavan  (BHEL , India)

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