In a Project specification i came across the following statement, Could you please explain what it means?

"Support friction resulting from gravity loads must not be considered to provide resistance to seismic force."

Hello Muthu,

Many times the piping is supported by "shoes" that bear upon (but are not welded to) structural steel. These shoe-type supports will be free to slide on the structural steel that they bear upon as the piping moves due to thermal expansion and contraction. There will be some friction as the (typically steel) shoe slides on the (typically steel) structure and the friction will to a greater or lesser degree resist the movement of the shoe (and consequently resist the movement of the piping). When we have these types of sliding supports we usually will include some friction as we develop our Caesar II model (the Mu factor). We will do this because the resisting friction may retard the movement of the pipe enough to cause higher expansion/contraction stresses at the elbows (bends).

My interpretation of your quote from the Project Specification would be that the writer does not want you to "take advantage" of the friction that I described above as a restraint against the displacement of the piping systems under seismic loadings. The writer, it would seem, wants you to design specific seismic restraints to control (limit) the movement of the piping in case of earthquake. In your design, you may not consider the support friction in the design of these seismic restraints.

Regards, John.

Update:

Below, Dave makes reference to the American Lifelines Alliance seismic document and you can find the document here:

http://www.americanlifelinesalliance.org/pdf/Seismic_Design_and_Retrofit_of_Piping_Systems.pdf

Regards, John

Support friction in dynamic calculation have the effect as system damping.
I never take support friction in dynamic analysis !
I think the Pipe Analyst cant take this effect,
because he cant be sure, that friction allways is present. Pipe can move up in some support and then the theoretical friction mechanismus is zero.

The American Lifelines document on seismic design of piping systems states in Section 8.2: "The seismic design should not take credit for the friction forces between pipe and support, which tends to reduce seismic motion of the pipe."

I thought I read a similar statement in ASCE-7 or IBC but I just can't find it now.

Dave,

You are correct. I don't remember the exact paragraph either, but I do remember seeing the "no friction credit" rule in IBC when I had to apply it on a project.

Dear,

The same issue had been earlier discussed in this forum in http://www.coade.com/cgi-local/ultimatebb.cgi?ubb=get_topic;f=1;t=000720#000000.

But, the main problem of the issue is awareness. Are we really serious about protection/integrity of costly systems of process plants we design from a low-probability, but high-risk seismic event!

Just for example, most of us can buy costly flat-screen TV or modern central locking system, but never buy a hammer for breaking window glass and a portable CO2 fire extinguisher to be kept within reach inside a car!

ALA document dated July 2002, referred by our respected mentor John Breen is based on current technology and experience of piping in actual earthquakes and is a guide for seismic design of new piping and seismic retrofit of existing, operating systems.

We can't prevent seismic events, but surely we can limit the damaging effect of the same by proper seismic design or retrofit. If industry best practices dictate 'no-friction credit in seismic design of piping', we at least remain aware!

The extent of damage I had witnessed in Gujarat, India on 26th Junuary,2001 could have been much less, had people designed their buildings and structures as per the existing seismic code! Whosoever had taken care in design, got saved by the grace of Almighty Lord 'Sri Krisna'! The same is true for 'no-friction credit in seismic design of piping' issue.

regards,

sam