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Re: Bolting issues

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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
. >