natural frequencies difference before and after analyze 1ste

[vesna]

let's say you have created different sections that have tags 1, 2, and 3 and assume that your section has 5 integration points. If at nodes i and j ( the first and the last integration point) you want to assign tags 1 and 3, respectively and for intermediate integration points you want to assign tag 2, then your element is as follow:

element forceBeamColumn $eleTag $iNode $jNode 5 -sections 1 2 2 2 3 $transfTag

[vesna]

To account for panel zone effect you have to model it yourself. Opensees will not do this for you.

If your model is elastic you will get the same results either by using elastic beam-column elements or by using force based beam-column elements with fiber section and elastic material.

[andreascj]

Hi fmk and Vesna,

Could you please explain the reason for decrease in the natural period after gravity (while increase could be attributed to the stiffness reduction due to high gravity loads), referring to an earlier post by sama as follows:

>>>>>I have the same problem, and I used truss elemenets to model rigid diphragm in 2D model. but after the first step the 1mode periode decrease from 1.02 to 0.95
>>>>>I just used equalDOF to model simple connection of my model. ( in one of the bays of my model, beams are pinned to the columns and in the other bays they are connected rigidly(default)). Whats happening here

The reason I am asking this is because, I have also experienced similar problem with a 4-story 3bay by 3bay concrete structure, where the in-plane rigid diaphragm action is simulated using rigid truss elements joining opposite corners of each slab. The periods before and after gravity are 0.84 s and 0.65 s, respectively. Which one is correct?

Thanks.

[andreascj]

Hi Vesna,

Any thoughts on this problem?

Thanks.