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Re: Horizontal vessels-Friction forces

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For NONSEISMIC load conditions: Friction from vertical loads at the bottom
of the saddle must be overcome before lateral load is assumed to produce
shear in the anchor bolts.

For SEISMIC load conditions: Apply all horizontal shear forces to the
anchor bolts.

Hope this helps,

Rick Drake, SE
Fluor Daniel, Aliso Viejo, CA


Padmanabhan Rajendran <rakamaka(--nospam--at)> on 03/02/2001 10:56:46 AM

Please respond to seaint(--nospam--at)

To:   seaint(--nospam--at)

Subject:  Horizontal vessels-Friction forces

As far as I am aware of, horizontal vessel, supported
on two saddles, has slotted holes on one of the
saddles (sliding end) and standard holes on the other
saddle (fixed end). Horizontal force of friction due
to thermal expansion/contraction of the vessel is,
then, equal to the product of coefficient of friction
at the sliding interfaces and the vertical reaction at
the sliding end. Consequently, one of the loading for
which the anchor bolt on the "fixed" end should be
designed for is the friction force. Because this
loading is transient, one may use a higher allowable
stress or use a reduced load factor. This is what I
have learned and practiced for about 10 years.

I have changed jobs and the lead engineers (mechanical
and civil) in this company tell me that the above
noted friction force need never be considered in the
design of anchor bolt. Their reason is that the vessel
moves on the sliding end (and, probably on the fixed
end also because the hole diameter on the fixed end is
larger than bolt diameter)and, any friction force that
may develop is so quick and instantaneous that the
bolt will not experience it. The tank in question is a
propane storage tank with an operating weight of 500
kips. Based on my experience I figured that the
friction force will be very large and that in order to
reduce it, I suggested using Teflon sliding surfaces,
such as Fluorogold product. The engineers here say
that Teflon sliding pads do not work well and that the
pads easily come off the equipment. They also told me
that it is common to have both saddles with slotted
holes and that they have never heard of using friction
reducing sliding surfaces on horizontal vessels! Each
of them has more than 20 years with reputed and large
consulting engineering companies, whose design
standards echo what I have been practising.

Rick Drake and Christopher, in particular, (because I
believe that both of you have refinery and pipeline
experience), am I overly conservative in my design
approach and is my design criteria in error?

P. Rajendran

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