Caesar users,

Can I apply a static equivalent siesmic force (coeffiient of G) in static analysis situation, rather than generating the time dependant acceleration spectrum and analysing dynamicaly for siesmic code compliance.

Thanks in advance,
Michael

That kind of depends on your project specifications.

From an "analytical point of view", yes you can determine a static "G factor" and apply this statically. This may or may not yield conservative results. By applying a dynamic load as a static equivalent load you're assuming that the response of the piping system is dominated by a single mode of vibration. If this assumption isn't true, your results are incorrect.

Oakdarcy,

One point I would like to mention. The total force in a direction based on the static push over analysis and by Dynamic analysis should match.

Regards

I'm doing just that right now. I'm following the procedures from IBC-2003 (IBC-2006 has simliar requirements) to develop the equivalent static loads. Also see ASCE-7 (IBC-2003 references ASCE 7-02 and IBC-2006 references ASCE 7-05) and the American Lifelines Alliance document, "Seismic Design and Retrofit of Piping Systems."

See, for example, ASCE 7-02, paragraphs 9.5.3 through 9.5.7, for descriptions of allowed analysis methods for buildings. ASCE 7-05 covers the same material, in much greater detail, primarily in paragraphs 12.6-12.9 and cahpter 13, but also in chapters 16, 19, and indeed in many of the chapters from 12-21. Plan to do a lot of reading! Expect to spend a lot of time at this if it's your first time through it. Because of that, it's very important to check with your contract manager to make sure what the requirements are.

My contract permits the use of an equivalent static laod case. Note that the IBC and ASCE documents referenced above provide guidelines for developing a design response spectrum that is, in some ways, less complicated than what I am going through. (The payback is on the analysis time and particularly on the time spent understanding the dynamic output.)

Also, based on the methodology, I would not necessarily expect the equivalent (horizontal) static loads to match the lateral loads from a dynamic aanalysis. There certainly seems to be no obvious reason why this should be the case, but there is a lot of supporting material that I haven't (and don't plan to) read that may explain how all the math works out.

Richard, Anindya stress and CraigB,

Firstly, thankyou for your responses, your comments have reaffirmed my interpretations of the code readings.

For the majority of the work we do here in australia, a consevative "G factor" is all that is required as per most work contracts and also the Australian Standards.

I guess I was a little vague in my original question, so....

How should I now apply this equivalent "G factor" statically? Should it be applied in the piping input, but I don't see where? Should I apply elsewhere, but I don't how it could be applied outside of the piping input? Can I apply it to the entire model, or do I have to apply it at individual nodes?

Thankyou for your assistance.
Regards,
Michael

If I understood your query properly...then you can input the G value in Piping input spread sheet as Uniform Load.Click the box and input the value as Vector 1,2,3 against Gx,Gy,Gz as per your contract & coordinate system.


Regards

Make sure you go to the "Special Execution Options" dialog and check the checkbox for "Uniform Load in G's".

Craig,

Your original qoute:

"I would not necessarily expect the equivalent (horizontal) static loads to match the lateral loads from a dynamic aanalysis. There certainly seems to be no obvious reason why this should be the case, but there is a lot of supporting material that I haven't (and don't plan to) read that may explain how all the math"

To the extent I know, the concept of Pseudo Acceleration as is used in Time History/Response Spectrum is related to the way static push over load is represented in the Building codes.

Now the elastic resisting force = KX=MW^2X=MA where A=Pseudo Acceleration. Now M=W/g so resisting force= (W/g) A which is similar to the way equivalent base shear is expressed in the Building codes. Now the Dynamic analysis resisting load is a paricular direction should not fall short of this value as this has a sound basis , hence the two should match.

Regards

Another important point if you have any nonlinear restraints such as +Y or supports with gaps, is to formulate the static load cases correctly to comply with the piping code for occasional loads. If you send an e-mail to techsupport@coade.com specifying the piping code you are using I can give you some guidance on setting up the load cases for static seismic.

Anindya

"I would not necessarily expect the equivalent (horizontal) static loads to match the lateral loads from a dynamic aanalysis. There certainly seems to be no obvious reason why this should be the case, but there is a lot of supporting material that I haven't (and don't plan to) read that may explain how all the math"

To the extent I know, the concept of Pseudo Acceleration as is used in Time History/Response Spectrum is related to the way static push over load is represented in the Building codes.

Now the elastic resisting force = KX=MW^2X=MA where A=Pseudo Acceleration. Now M=W/g so resisting force= (W/g) A which is similar to the way equivalent base shear is expressed in the Building codes. Now the Dynamic analysis resisting load is a paricular direction should not fall short of this value as this has a sound basis , hence the two should match.

------------------

We had an in-house seismic analysis seminar yesterday led by the chair of the ASCE 7 Code Committee. His description of this is that the pseudo-acceleration value is used to scale the results of a time history analysis so that the sum of the foundation shear loads match. So you are right, the intent of analysis is to make the two sets of loads match.

Son of a gun, you learn something new every day!

:-)