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clamp force reduc. not bolt tension increase

• To: seaint(--nospam--at)seaint.org
• Subject: clamp force reduc. not bolt tension increase
• From: Majid Sarraf <msarraf(--nospam--at)uottawa.ca>
• Date: Thu, 23 Jul 1998 13:46:06 -0400

```In my humble opinion,

The reduction in clamping force is resulted when you apply tension to the
connection. This is an obvious fact substantiated by self-equilibruim of the
bolt-plate assembely. There would be increase in bolt tension, in addition
to pretensioning force, only if you could apply tension directly to the bolt
head and nut, and not through connected plates, or exceedeing the
and pplying tension through connected plates. I also,agree with Charlie,and
I add that usually deformations (shear and bending) of the connected plates,
transferring the load throught shear action, limit the magnitude of the
tension you can apply to the bolts, unless you have farely thick (rigid)
plates.

Majid Sarraf

At 01:12 PM 7/23/1998 -0400, you wrote:
>I've spent too much time on this, but I guess I just don't want to get back
>to work.
>
>There appears to be a misconception regarding the clamping stresses in bolts
>in tension in connections. It has been asserted that applied loads to the
>connection will relieve some of the stresses on bolts, loads up to the point
>where clamping stress equals stress due to applied loads.
>
>My own understanding is that the only way to relieve tension stress in the
>bolt is to remove the strain in it...in other words, let the bolt shorten to
>its original length. As the tension load is applied to the bolt through the
>connected parts, the bolt is not going to  get shorter (thereby relieving
>strain) but get stretched even more. As the bolt stretches more, it is under
>still more stress, a stress in proportion to its dimensions and the applied
>force.
>
>An analogy in prestressed concrete is in the tendons. The tendons are
>prestressed. When a load is applied to the beam, the stress goes up in the
>tendon, not down. Fine, the tendons take advantage of steel's high strength
>and ductility. HS bolts are used in the same way. They have a higher
>capacity for loads in real connections than what one would determin from
>just P/(A x Fu).
>
>Ted
>
>
>

~      ~           ~ ___
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|==========================================|
|                                          |
\               Majid Sarraf             /
\             Ph.D Candidate           /
\   Department of Civil Engineering  /
|        University of Ottawa      |
|           Ottawa, Ontario        |