Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

Re: Bolting issues

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Even though I never used rivets in a design, rivets have one good feature
over bolts in their ability to fill the hole and provide uniform
distribution of forces in the connection. I always wondered how a bearing
bolt connection achieved this.

John Gregg,P.E.
Hurst, Texas
-----Original Message-----
From: Roger Turk <73527.1356(--nospam--at)compuserve.com>
To: seaint(--nospam--at)seaint.org <seaint(--nospam--at)seaint.org>
Date: Saturday, January 23, 1999 11:52 AM
Subject: Re: Bolting issues


>High Strength Bolts (HSB's) started replacing rivets as the preferred
>connection in the 1950's, when I was a student.  In my senior year
(1958-59),
>I wrote a student paper on the then known performance of HSB's,
particularly
>as compared to rivets.
>
>In the beginning, HSB's were permitted to replace rivets on a 1:1 basis;
the
>connection would be designed for rivets and HSB's would be used instead.
The
>performance of HSB's subject to vibration was performed on structures of
that
>arch-conservatism of design, railroad bridges.  Due to vibration, rivets
had
>a history of loosening and had to be replaced periodically.  When HSB's
were
>installed in place of loosened rivets, there was no evidence of loosening
of
>the HSB's even when rivets continued to loosen on the opposite end of the
>bridge in which HSB's were placed.
>
>It was recognized that joint performance of HSB's was due to the bolts
>inducing a high clamping force in the connection and the forces transferred
>thru friction between the parts and not thru shear in the bolts.  Tests
were
>performed with the interface heavily coated with grease and friction still
>prevailed.
>
>It wasn't too long before connection design requirements for HSB's separate
>from rivets was developed, eliminating the 1:1 substitution.  Further
>developments had design requirements for "bearing" (shear) connections and
>"slip critical" (friction) connections.
>
>Do "bearing" connections actually transmit forces by bearing, shear or
>friction?  I think the answer is "Yes."  Witness the "banging building"
>syndrome where large "bangs" are heard and vibrations felt in a building
>when a connection changes from "friction" to "bearing."
>
>A. Roger Turk, P.E.(Structural)
>Tucson, Arizona
>
>Charlie Carter wrote:
>
>. > Majid:
>. >
>. > Here's what I know.
>. >
>. > >> Does <the RCSC Spec. > says how many turns? or how much extra
tension
>. > >> is okay?
>. >
>. > When you install a bolt, you are talking about a range of from 1/3 turn
>. > to 2/3 turn from snug to induce the pretension for the turn-of-nut
>. > method. Other methods use different indicators, but the required turn
>. > will likely be very similar. Rotational capacity tests demonstrate that
>. > high-strength bolts can withstand several turns before failure in
>. > torque-induced tension, particularly A325's, which are quite ductile. I
>. > think any extra tension is okay, as long as the bolt doesn't break
>. > during installation. All you're really doing is over-rotating and
>. > stretching the bolt a little more. But by that time, you're in the
>. > plastic portion of the bolt elongation curve, so even a large
>. > over-rotation only makes a small difference in the installed
pretension.
>. >
>. > Also, when you let go of the wrench, the stress state in the bolt
>. > changes from combined tension/torque to tension only, which is not
>. > nearly as demanding. Add all this up and the worst stresses the bolt
>. > will ever see (if designed to meet the AISC and RCSC Specifications)
are
>. > induced during installation.
>. >
>. > >> excessive pretensioning load must affect shear capcity of the bolt
at
>. > >> "factored loads" not "service loads", specially when shear plane
>. > >> passing through threads
>. >
>. > Shear strength is independent of pretension because any pretension that
>. > existed prior to loading will be released by shear deformations that
>. > occur before failure. That's why the tabulated bolt shear and tensile
>. > strengths are good for any installation condition (i.e., snug-tight or
>. > pretensioned). The shear plane passing through the threads does affect
>. > the shear strength, however. The question of service loads vs. factored
>. > loads is really not applicable. Bolts don't fail at service loads. Slip
>. > occurs just above the service range, but that is an awefully benign
>. > "failure."
>. >
>. > >> I have sound theoretical reasons for why shear-tension interaction
>. > >> exist, and therefore, it must be applicable to bolts as well...One
>. > >> figure <in Kulak's book> (4.17) supports this idea and one does not
>. > >> (4.15)
>. >
>. > You have to distinguish between externally applied tension and
>. > pretension when you talk about shear tension interaction. The
>. > interaction of externally applied shear and tensile forces must be
>. > considered. Interaction between externally applied shear and the
>. > installed pretension is not a design consideration, however, for the
>. > same reason as stated previously (i.e., you'll lose all pretension due
>. > to shear deformations of the bolt shank prior to failure). Kulak Figure
>. > 4.17 is for interaction of externally applied shear and tensile forces
>. > only. Figure 4.15 shows clearly that shear strength is independent of
>. > installed pretension. In fact, it shows that a bolt still has the same
>. > shear strength after 1.5 turns as it had at snug and 1/2 turn!
>. >
>. > I talk too much. (-:
>. >
>. > Charlie
>. >
>
>