Rotational Mass and Translational Acceleration

[lacampos3]

Hi,

I am running a dynamic time history analysis on a pier with a rotational lumped mass at the top and applying a translational acceleration at the base.

The results show support shear, Fx, and support moment, My. I'm trying to make sense of why this happens.

Thanks.

[seismosoft]

Hi,

We probably misunderstood your post, but we cannot quite understand what the problem seems to be here.

Seismosoft Support

[lacampos3]

Well, it seems that if we have a rotational mass at the top, it wouldn't be able to induce any shear(Fx) nor a moment in the y axis (My).

Thanks.

[seismosoft]

Yes, sure, but what is not clear/apparent to us is if you have removed all other possible mass sources from your pier model (e.g. materials self-weight/mass, section additional mass, etc).

If you do remove all sources of mass but the rotational lumped one, then you should indeed obtain null base shear and base flexural moment - we have just tried it ourselves, and this was indeed the (correct) behaviour that we observed.

Seismosoft Support

[lacampos3]

Is there any way you could forward me your program so I can see what I'm doing wrong?

Thank you so much for your help.

[seismosoft]

You have the program already, since you are using it, correct?

If you email us (support@seismosoft.com) your model, we will try to find the time to pay a look at it.

Seismosoft Support

[lacampos3]

Great. I just sent you my program.

Thanks.

[seismosoft]

Dear lacampos3,

We looked at your example and, unfortunately, could not but immediately notice that, as we had anticipated, in addition to your lumped rotational mass, you have also defined material self-weight/mass, which is then associated to your cross-section and then to your element, and thus inevitably leads to the development of translation inertia forces.

In addition, we also noticed that you are applying enormous acceleration values (800 g) to your 60 cm diameter reinforced concrete column, which does look like a units modelling error.

We would also advise you to delete your very far-away non-structural node, which you are not using, or at least to disable its visualisation, so that the rendering can focus/zoom-in on your structural model.

Do read the Help System/User Manual, do check the Verification Report models, and do make use the Wizard.

Seismosoft Support

[lacampos3]

Dear Seismosoft support,

Your first point is well taken. I modified the program and deleted the self-weight of the lumped mass and set the specific weight = 0 in the materials module.

As far as the acceleration units, my accelogram is in g so I multiplied it by 9810 to convert to mm/sec^2 (pg 128 of manual). Would this create problems?

I am still getting the same unexpected behavior even after removing the self weight of the lumped mass and material.

[seismosoft]

- the model you sent us is using m/s^2 acceleration units, not mm/s^2

- after having introduced the changes we reported (we also disabled geometric non-linearities), we managed to run your model without observing the anomaly you mention

Seismosoft Support