Saifuddin18

@Hasan009,how did you calculate horizontal spring constant? The Geo-technical Engineer did not give Vertical Subgrade Modulus?

@kHURRAM ALI, that's the code I was talking about. If possible, share the PDF as I couldn't find it in my archive.

@SMAQ, I think you took the moments in wrong directions(Mx instead of My and vice versa) in PROKON[considering column at grid K'8]. Further, how much length have you considered in PROKON and you considered columns to be braced or unbraced?

@kHURRAM ALI, Wa alaikumas salaam at first. Secondly, there is no correlation between the seismic parameters between UBC and IBC. There should be a map developed for your location by your authorities(if general design guidelines are through American Standards and you don't have your own design code). That map will give you the spectral acceleration values at short and long periods. Otherwise, as I remember, there is a standard which gives values of Ss and Sl for international grounds, you can refer it for values in your city and proceed with your design.

@Fatima Khalid, as I said, I have not done such checks personally in a practical project so I can't guarantee what I said but as per my understanding, it should be 4'. If you consider your unsupported length as 6', you are not going to get a Captive column for lengths where your wall height increases which doesn't seem correct when you go through the captive column definition. As per my understanding, if your wall height increases, the column should go near the threshold of captive column. Let me know your take after considering my views.

@SMAQ, Sorry for getting late but I have checked and studied your model, analysis done by etabs seems reasonable to me but the design is somehow not acceptable. Etabs is considering the 'k' value for those columns almost near 3. I suggest you to design those columns in some other program or manually. P.S: When I change the 'k' value to 1 in etabs, the columns passed with 6T16 bars. Would like to know what you did for the solution.

@SMAQ, also, it would be great if you can share your drawing plans and etabs model. That will make it easier to study your issue.

@SMAQ, yes, you can define your own 'k' factors in the program but for that you have to decide your column end conditions first. Etabs automatically calculate the 'k' values based on rotational stiffness of joint considering all connecting members and I presume it to be efficient. There are only few situations in which etabs make mistake with it. However, if it seems to you that etabs is wrong, you can assume the end conditions and do manual design or take help of some secondary program or excel sheet. P.S: Effective length factor value 'k' will be different in both the axes of the members.

@Fatima Khalid But one thing to note here is that the unsupported length we are considering for our captive columns is the one supported with walls. In your case L1=10', D1=18". L2=4', D2=18". 0.75xL1/D1>L2/D2. Which will be true in your case. That means that your columns will be considered captive columns only if the wall height exceeds 7.5'. Would like to here from other group members too about this.

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.🙂

Dear Fatima, I am unable to understand your query, can you please elaborate?

@Simple Structures, well explained, nothing left to say I believe..🤪

Dear QAM, I don't think we should ignore the ratios by any means. If you could possibly share the etabs model, the problem may be figured out easily. Remaining everything seemed reasonable to you except this? This may also be the case. But couldn't say anything without checking the model.