difference between periods of models made by seismostruct and etabs

[mehdiharsami]

hi
i'm trying to use seismostruct for ida analysis of my master thesis.
the problem I have is that there is a large difference between periods of my models which are made by etabs and seismostruct, in fact the model made by seismostruct is not stiff enough and has a large unacceptable period. i have checked the loads and masses and two models have the same axial forces in columns, so i guess the problem is because of wrong modeling of my eccentrically braces (EBF) which i want to be in accordance with american codes, note that this codes require the link beam and beams outside the link to be a single member. I would really appreciate it, if you check my model I've been struggling with it for days.
link to my model file : https://ufile.io/4qencgec

best regards to seismostruct team

[seismosoft]

Your model is very big with almost 1000 elements, and obviously, we cannot check all aspects of it.
However, note the following:
1) In the Project Settings> Gravity and Mass you have selected to transform gravity loads to masses. Is this what you intended to do?
2) make sure that the material stiffnesses are equal in SeismoStruct and Etabs, apart from the strengths
3) In most of the FB element classes you have selected 3 integration sections. Maybe you would need to increase this to 4 or 5.
4) make sure that the total weights between SeismoStruct and Etabs are equal
5) not sure what you mean with 'this codes require the link beam and beams outside the link to be a single member.'
6) Most importantly: The diagonal braces are not connected to the beams! For instance brace(+x)214 connects nodes 214 and 22515 but node 22515 is not connected to the beam b215 above. You need to subdivide the b215 beam and connect to the diagonal braces. The short middle sub-element should be modelled with the infrmDB element, which is suitable for short elements.

I would start from point No.6, this seems to be the most important problem of the model.
Seismosoft Support

[mehdiharsami]

thank you very much it was really great help.
best regards