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RE: Shear-off bolted connection

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Maria

To DTI or not to DTI that is the question.  The choice of one type of
fastener or the other is often a heated discussion and like most such
discussions, you need to look deeper to arrive at the truth.

The DTI connection measures the tension in the bolt while the twist-off
bolt measures the shear in the bolt.  The concern with the twist-off bolt
is that if there is excess resistance from the treads the relationship
between torque and tension will change and the twist off portion will come
off before achieving the correct tension. 

I much prefer to use a direct test as opposed to an indirect test.  The
variability of the torque-tension relationship is something I would rather
not deal with.

The primary argument for the twist-off bolt is that it is faster to
install.  A consequence of the emphasis on speed is the attitude that if
the end twists off there is no problem, which seems to translate into less
concern about quality.  Most twist-off bolts have a button head so that
after the bolt is installed it is not possible to check the torque nor is
it easy to remove the bolt.  If they did not tighten the bolts in the
connection in the proper sequence and one bolt had its tension relaxed you
could not test that using twist-off bolts nor could you re-tighten the bolt
easily.

I believe that there have been some projects where DTI's were used with
twist-off bolts.  This compromise satisfied the engineers concerns while
allowing the contractor to benefit from the ease of erection.

Both systems need to be used in accordance with the manufacturer's
recommendations.  If the attitude is that you do not have to worry as long
as the "test" is passed then you can have problems with either system
although my bias is that this is somewhat less so with the DTI.

There have been some projects where, because the inspector mis-interpreted
the acceptance/rejection criteria for DTI's, the contractor had to do a lot
of additional work.  The manufacturers of DTI's have literature that
clearly explains the testing procedures and acceptance criteria and I would
recommend that this information be provided to the inspectors.

As Harold said, the contractors tend to prefer the twist-off bolts because
their focus is on cost while many engineers prefer the DTI because of an
emphasis on quality.  After having made the previous statement, I must say
that many contractors believe that twist-off bolts improve the quality of
the work.  I am willing to admit that the twist-off bolts are an
improvement over many of the poorer practices in the industry, I just
believe that the DTI produces a higher confidence level.

In spite of my bias, I hope I have set forth the basic issues.  This is one
of these discussions that can go on forever with both parties pointing out
the problems with the other system.  You need to choose one path, but in
each case, you need to follow the manufacturer's recommendations as well as
the RCSC "Specification for Structural Joints Using ASTM A325 or A490
Bolts".

Mark Gilligan