Sir,
We are doing a steel structure qualification for the seismic loads.
Description of steel structure :
Steel structure column length is around 15.6m with four floors, with each floor height being 3.9m. Up to the fourth floor (15.6m), the column is made of a box section with two ISMB350 sections.
Above the fourth floor, another steel structure is extended with the column section of ISMB 200.
In the model, the total column length is divided as per the floor height, i.e., 3.9m
Issue :
On analysis, we observe one of the box section column on the fourth floor with a column length of 3.9m is failing.
Now we should consider stiffening in the portion of the column or the entire column length.
Also, one column (ISMB 200) above the fourth floor is failing. Again the same doubt about whether we should consider stiffening in the portion or the entire column length.
It will be a good idea to consider stiffening the entire length of the column. In this regard, lateral bracings can be used. They will allow additional load path for the lateral loads and increase redundancy of the structure.