# Evaluation of torsional moment due to the eccentricity between center of mass and center of stiffness

Question:

Hi all,
In a building (structure with a certain number of stories) modeled without the assumption of floor diaphragm (Structure > Control Data > Story > Floor diaphragm do not consider)
We introduce a set of seismic forces by using the automatic procedure (Load > Lateral > Seismic Loads):
- What is the procedure performed by the program to assign the torsional moment due to the eccentricity between center of mass and center of stiffness for each node belonging to the flexible floor?

- Could you also highlight the differences respect to the case in which the the floor diaphragm is considered?
(I am supposing in this case the total seismic force applied in the center of mass is equally distributed at each node of the floor
and the torsional moment is distributed at each node according a sort of "equation of rotational equilibrium")

Best regards

Elia

Hi ,

since there is no reply for the past 3 days, we are closing the ticket. you can reopen the ticket by replying to the same.

kindly let us know if any further assistance is needed

Regards,
Anmol Saxena
Technical Support Engineer

Hi ,

Thank you for replying. Kindly find the model file in the attachments for reference purpose.

kindly let us know if any further assistance needed

Regards,
Anmol Saxena
Technical Support Engineer

Good morning, thank you for the answer.
Looking carefully the images I have a couple of additional questions.
Could you kindly share the model so that I can do some tests and we can discuss in a more effective way by referring to the same model?

Best regards

Elia

Hi ,

What is the procedure performed by the program to assign the torsional moment due to the eccentricity between center of mass and center of stiffness for each node belonging to the flexible floor?

To assign this eccentricity between the center of mass and the center of stiffness you can check on the inherent eccentricity option.

procedure to define the same is stated below

Step 1) define the static seismic Load cases in both the direction X and Y

Step 2) Go to building Control -> check on Eccentricity Ratio -> select DL case and give scale factor as 1 -> add

Repeat the steps for Live load.

Step 3) in story stiffness center, select X_direction load case as EQX and Y_direction load case as EQY.

Step 4) Run the analysis without considering inherent eccentricity.

As shown in Screenshot

Step5) Go to Results tab -> results table-> story-> story eccentricity

Step6 ) Copy the Ecc. Dist. From the tables and paste it into the story data tab.

Step7) check on the inherent eccentricity option in static seismic cases

Now you will be able to see the results including the inherent eccentricity.

- Could you also highlight the differences respect to the case in which the the floor diaphragm is considered?

When floor diaphragms are considered, the total seismic force is applied at the Center of mass of the floor as shown in the screenshot below

When floor diaphragms are not considered the total seismic force is applied at each node of the floor as shown in screenshot below

(I am supposing in this case the total seismic force applied in the center of mass is equally distributed at each node of the floor

and the torsional moment is distributed at each node according a sort of "equation of rotational equilibrium")

Yes, when we do not consider the floor diaphragm the total seismic force is transmitted to each node of the floor, and the torsional moments are distributed at each node according to the equation of rotational equilibrium

I think I was able to answer all the questions, kindly let us know if any further assistance needed.

Regards,
Anmol Saxena
Technical Support Engineer

Hi ,

Thanks for getting in touch. We are working on the query and we will get back to you shortly

Regards,
Anmol Saxena
Technical Support Engineer