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Dear SMAQ, have you tried extracting the forces from etabs and designing the column in PROKON? I don't think reducing the effective length is a solution to the problem..

Dear Nustian371, I think Fatima is not asking for the theory or concept behind the captive columns and how they behave. Instead she was asking about the explanation of the limit set by FEMA on such columns.

Got you now. I have not done any practical design or check for Captive Columns but as per my understanding, the height of columns refer to the unsupported length(say L1) and depth means the dimension of column along the wall(Say D1). So you do L1/D1 for captive column. Now take average depth and lengths of columns at that storey and let them be L2 and D2. Now ratio L2/D2 for normal column. You compare both these ratios for the given allowable percentage of 75% i.e., {0.75xL2/D2}>L1/D1. Hope this answers your question.🙂

This is sort of creating a short column in the structure due to architectural features. On a certain storey level you will have your story height and most of the columns are of same length but if any column due to any addition to structure is reduced in size it will cause a short captive column which attract more seismic shear due to stiffness. we try to avoid these elements or detail them adequately. See attached commentary it will explains you much better hopefully.