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mode shapes

mode shapes

mode shapes

When I model 30 story bldg and run the analysis, I noticed that 12 mode shapes were generated with corresponding time in seconds.

I'm just wondering why ETABS generated 12 mode shapes (why not 10, 8, 6 modes etc). Also, is there any possibility that by visually looking at these mode shapes, you can perhaps "sense" that your building is behaving good or bad?

Is mode shapes depends on the static and load combination that you create?

How will I know what particular load case this particular mode shape came from?

CAn somebody shed light on this matter?

RE: mode shapes

Don't take this bad but you should not analyse or design this type of building if you don't have ADVANCE knowledge in high rise building analysis and design.

That includes dynamic analysis as well as many other effects such as podium effect, stay back effect, etc....

Take a graduated courses on those subjects (High Rise + Dynamic Analysis + Earthquake Engineering).

RE: mode shapes

@picostruc. Thanks for your reply. It is true that you do not test the water if you don't have any slightest idea regarding its depth and creatures underneath it.

Regarding structural analysis and design, I bet my bottom pennies even the most experienced structural engineers are  having trouble interpreting the graphical result of "mode shapes". Traditional structural engineers design multi-story bldgs in a manner that can check manually ( checked against total lateral displacement, interstory drift and structural element design etc.,most building that I design(for earthquake and wind analysis) follows this concept from 3 to 20 story high are still standing today... thank god!). With the advent of computer, FEM and dynamic analysis are the talk of the town(which I firmly believe is a great tool for structural engineers.)

In terms of absorbing knew knowledge(FEM and Dynamic analysis), I asked like a child because a child absorbed like a sponge. A child ask question(s) without hesitation, a child doesn't afraid to be misunderstood as long as he/she gets what he/she wants to know. Asking questions should not be interpreted as a sign of weakness(es) but instead as a sign of strength since you do know what's your capabilities and limitations.

I am thankful with this forum regarding invaluable knowledge/information/technique that I learned and hopefully this forum will still thrive for the betterment of the engineering community.

RE: mode shapes

Hi Monchie,

The default mode shape for the dynamic analysis is 12. But we can change the number of modes to get at least 90% (or 80%)of the modal mass participation ratio as per UBC 97 is conncerned.

PicoStruc intention is very precautionary in nature due to complexity of the subject matter. This is not easy to discuss in this forum because of possible miscommunication and misunderstanding. But both of you have valid reasons.

It needs proper training, guidance and long experience to be a high rise bldg analyst and designer.

Thank you for both of you for kind considerations in my thoughts.

RE: mode shapes

I think it is good practice to have your first two modes be X and Y translation modes at or about the period which may be considered "rule of thumb". For a 30-storey concrete shear wall building I would say somewhere between 2.2 to 3.0secs. Maybe higher depending on how you have modelled the foundations.

Try to avoid a building where the torsional mode of vibration is mode 1 or 2.

RE: mode shapes

To answer your original question -- there are two types of modal analysis that can be used in ETABS: Eigen Vector analysis and Ritz Vector analysis.

Eigenvector analysis is the traditional modal analysis.  Essentially, the modes are based on computing a stiffness and mass matrices (taking into account each degree of freedom in the structure), and then solving for the fundamental frequencies.  Load combinations, and load cases have no effect on the eigenvector fundamental modes - they are solely dependent on the mass and stiffness of the structure.  They are deemed the "Fundamental Modes of Vibration" thus directly dependent on only the structure and it's properties.

Raleigh-Ritz Vector analysis is another method of modal analysis.  In ritz vector analysis, starting load vectors (i.e. your dead load, your seismic loading, nonlinear degrees of freedom, etc.) are used to determine the approximate fundamental modes of vibration.  As your starting load vectors are based on your loading, the loading distribution will have an effect on your ritz vectors.

The modes determined by the eigenvector analysis and the ritz vector analysis will not be identical, but are usually similiar.  I believe in most cases CSI recommends the use of Ritz vectors for the analysis.

In regards to mode shapes, with a simplified 3-story 2-D frame with the masses lumped at each story the modes will be as follows:

First Mode -- All masses move in the same direction (i.e. similiar to ASCE 7 pseudo lateral force loading)
Second Mode -- There will be one "kink" in the deflected shape.
Third Mode -- There will be two "kinks" in the deflected shape.

Obviously, once you have a three-dimensional modal that has torsional effects and many degrees of freedom many more mode shapes will be introduced, but typically these translational mode shapes should be dominant for most regular structures.

Keep in mind I'm not an expert on the matter.  I've taken a Dynamics in Structures class taught by Chopra a few years back (his book is a good reference although I'll be the first to admit it is not easy to read through), but haven't done a lot of this recently.

I agree with the others, that asking questions is good and I don't think anyone should fault someone for asking questions.  I ask them all the time.  However, being the design engineer of a 30 story building when you are asking some basic questions in the use of ETABS is a little uncomfortable.  Designing a building of that size is a lot of responsibility and there are a lot of "subtle" check boxes in ETABS that can have large impacts... i.e. assuming you are doing response spectrum analysis you'll have to worry about 1) mass source, 2) scaling of spectrum, 3) modal mass participation factors, etc. which will all have large impacts on the final results.

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