Seismic analysis result gives modes with no mass activated in any direction.

[Anonymous]

Hi there,

I have a performed a Seismic Analysis in Robot of a R.C. 12-storey building with R.C. central core and I am not sure about how to read those results.

As far as I can understand from the results (see below the screenshot and also file attached), in mode 2 there is no mass activated in any direction, as well as in the mode 4 and others. How can I read this? Does this make sense?

Also, in doing a Seismic Analysis (EC8 option), I was meant to run a modal response spectrum analysis, as explained in the Autodesk Robot knowledge page (https://knowledge.autodesk.com/support/robot-structural-analysis-products/learn-explore/caas/CloudHe...). Is this the correct procedure to perform a Modal response spectrum analysis?

Thank you in advance for your help.

Simone

[mustafahesenow]

Hi @Anonymous

Could you attach the model please.

[Anonymous]

Hi @mustafahesenow,

Thank you for your reply.

Find attached the model in the dropbox link.

https://www.dropbox.com/s/1hw379hwl8cqyx6/12-storey%20R.C.%20building.rtd?dl=0

The scope of the analysis is to compare base shear between modal spectrum analysis and lateral force method. Therefore, the design of the columns, slabs, and walls have been neglected at this stage.

As you can see for the modal analysis results, it is also interesting that the first mode shape is translational in the X direction, the second is torsion, the third is translational in the Y direction (files attached). I am not sure about the reason of that. If you could have a look and share your thoughts in regards it would be much appreciated. 

[Simau]

Hello Boncios,

your structure is ideally symmetrical in the x and y directions.
The twist mode will therefore not excite any mass.
I introduced an accidental eccentricity of 5% in x and y.
You will see that the masses excited in twiwt mode are not null.

[Anonymous]

Thank you @Simau for your reply.

That make clear the torsional behaviour.

However, as you could have a look at the model, why do you think I do not have a classic translational displacement in mode 1 and 2?

I have read other forum about that and it seems that this is unusual.

I have tested different solution and the torsional behaviour is given by the presence of the core.

Any feedback on regard would be very helpful

[Simau]

What do you mean by  "classic translational displacement in mode 1 and 2?"

[Anonymous]

 @Simau, I meant that for such a simmetric building, I was expecting to see a first and second mode translational, and I was expecting to see a torsion only at mode 3.

To be honest, I dont know how to interpret this result. Any idea?

[Simau]

Yes of course,

having the 2 first mode in x and y direction is the best for a structure.

You have torsion on the 2nd mode because of a low torsion rigidity.

the more the walls are far from the center, the better will be the behavior of the building ( First and second mode in x an y).

But in your case, torsion mode do not activate any mass. But a so symetrical building is rare, and real mass excentricity can cause significant efforts.

[mustafahesenow]

Hey guys,

Please be noted that the eccentricity has to be defined from the seismic case not from modal analysis parameters .

https://forums.autodesk.com/t5/robot-structural-analysis-forum/strange-stories-reduced-forces-result...

[Anonymous]

Thank you both for your help.

@Simau, I realized that adding shear walls at the perimeter of the building I could provide more torsional stiffness to the building and as a result the mode 1 and 2 appear to be translational, and the mode 3 as a torsion.

Also, I did appreciate your help on defining the eccentricities. Rather than adding them on the modal analysis, I added them on the seismic analysis. Thank you @mustafahesenowustafahesenow