Piping seismic analysis, structures influence 1

[carletes]

Dear all!

Regarding dynamic analysis of piping systems with earthquake forces, I see that most structural codes give ground acceleration and response spectrum at ground level depending on the location, but, if I am not wrong, piping system is subjected to the acceleration and movements of its supports, some of wich are attached to structures, and therefore I suppose that the response spectrum of the groung must be "corrected" in some way. Is it so? Is there any code, publication, guide etc that helps in getting such reponse spectrum of the piping supports without carrying out and exhaustive dynamic analysis of the structures?

Any help will be really appreciated.

best regards

 

[Lucifer67]

Apologies for coming in at the end but I couldn't resist.

You have come across a paradox that invariably gets ignored by most engineers; more often due to the difficulties in interfacing and establishing the actual seismic effects.

Referenced within threads are "magic" factors taken from ASCE 7, UBC or alike. When using such factors one must bear in mind that they are based on the primary structural system yielding/behaving in a ductile fashion. If the performance criteria of your "pipe" is such that it must remain elastic then you are goosed - since the raft displacements will govern the design of the pipe.

If you are designing to B31.3 (say) the earthquake is linked directly to UBC, and UBC gives little or no guidance. You then end up with ASCE 7(ish) and you could ferret out a factor, which for the reasons above is rather suspect.

The key to the whole deal is what is acceptable to the Client and Authority having Jurisdiction. One should also bear in mind the "custom and practice" in your respective industry may dictate alternate routes.

You have probably realised that the more you know the worse it gets... I advise to stop reading!!!

Invariably pipes perform well under earthquakes, the rather modest stress limitations in 31.3(3) do not consider plastic hinge formation and all that good stuff; normally the residual capacity is significant.

So in terms of what you need to do....

Be cautious about transitions in rafts, in this respect say from the ground to a piled structure; build in loads of flexibility. So that any settlement etc can be accomodated.

In terms of designing the systems.

1. Establish the performance criteria for the system; you may have 2 earthquake levels to design for typically 1 in 500yrs and say 1 in 5000yrs. In this respect the systems normally remain linear elastic under the lower event and you must ensure they do so!!!

If you have a single earthquake then ductility is the key and you will end up with significant attenuation of motion.

2. Establish with the Client etc what is acceptable; my experience is that normally response spectra or static G will suffice.

3. Failing this you may need to use Multiple Response Spectra, some form of enveloping or Time History.

You would be best shooting for the "quick and dirty method" of input spectra; if necessary make allowances in terms of seismic displacements between structures etc.

There is as you are aware an absence of data in the industry in respect of piping design!!!

The references you will find are rather contrdictory and embroiled in folklore and legend.

Good Luck

Lucifer