Dear Kay Guanlao,

To give the relevant answer to your question it is necessary to know: (i) the number of stories of your existing steel structure, (ii) type of the floors' slabs with the total weight per sq.m., (iii) hysteresis models used to describe the non-linear behavior of the structural elements, and (iv) the most important point is formation of the dynamic design model of your structure.

Did you take into account the influence of the floors' slabs, of the non-structural elements and of the vertical loads (P-delta) on the overall horizontal stiffness of your building. Otherwise, your results will not be correct.

Best regards,

Prof. Dr. Mikayel Melkumyan (Armenia)

Dear Kay Guanlao,

This period does not seem to be correct, even in high-rise structures. This situation generally is based on local instability of the structure, when the connection between some elements is not correct. The model should be checked. Go to analyze > click on Check model > select all checkpoints. Hence, solve all errors.

Dear Kay Guanlao,

Pinpointing these errors can be really hard in some situations. First the model should be checked which will pinpoint any error based on connection between elements as Pouya Amirchoupani mentioned above. If that does not help your cause, you can try by animating the structure in that particular mode and see for any irrelevant deformed shape.

In his name is the judge

Dear Kay Guanlao

i had similar problem in my modeling for a 11 stroy concrete building, 3 main problem may you have.

one maybe your modeling of diaphragm is not correct ( define deprate diaphragm for each elevation.

second please check your error in etabs maybe you have unconnected point.

third check stability of your model (check maas of each pannel and check stiifness of each structural elements and check your stiffness modifier of your elements).

I hope I helped