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.

Dear Rifat, you really need to re-read that codal provision. First thing is that you seems to be confused between Strength Reduction Factor, Response Modification Factor and Overstrength Factor. Overstrength Factor is the one you will be using with Seismic Load. Code says that IF you do not consider Overstrength Factor in your seismic load then you need to desgin your soft storey elements by enhancing your storey shear and moments by 2.5 times. That's what I interpret. However, I am open to discussion on this. Also, I am not much familiar with your national code.

Would like to see the reference as I have not been through such clause.

Nope, I mean that your Load Combinations should include 'Omega*Eh' instead of 'E'. System Overstrength Factor and Strength Reduction Factor are two different things I guess.

Dear Rifat, You have to design your structural elements as per the special design combinations wherein you replace earthquake force(E) by Omega*Eh. Note the exclusion of vertical component of earthquake force(Ev) in special design combinations used. Use '1' as your strength reduction factor value if you do not wish to use the code specified values. But why would you do that? Also, would like to know which code are you following? ASCE 7-05?

Amazing! Can't you just return it and ask for the section which you have designed for? PS: I am not aware of the policies in your area!

Dear Talha, You can't just give torsional modifier of 0.01 to any beam. Secondary beams which supports on main beams can be given the torsional modifier as you said. But still, there will be some moment transfer(torsional moment) to the main beam practically. It is best to analyze how much moment you can transfer to main beam before it develop cracks and apply the modifiers accordingly. In case of Equilibrium torsion, there is no other way of load distribution and hence your beam have to take all the torsion which is transferred to it(no modifiers).

Dear Yannick, Both the beams behave similarly. However, while detailing, you detail the inverted beam as an upside down normal T beam. That means, now your bottom tension rebar(in span) can be distributed within the whole flange width. At supports, the flange part of beam will be in compression. Hope this helps.

I am really interested to know the reference of your statement and how we can achieve that in etabs?

Dear Khurram, What I believe is applying vertical component of ground motion is design engineer's decision. As per code, I think there is no clause stating to use vertical component in any particular case. I always use it in all my designs with RSA.