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Regarding column design

 
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rkumar
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Joined: 11 Apr 2010
Posts: 85

PostPosted: Mon Jun 28, 2010 7:02 am    Post subject: Regarding column design Reply with quote

hello Seniours,
 
I have come across this type of questions from clients. Why the steel in last floor column is higher?. Am tired of answering them. In the last floor, as the load is less, but the moment is higher, we get the point in the Ineraction curve in the tensile failure zone , where the steel percentage to be considered is higher, when it is moment resisting frame, if it is non moment resisting frame then the steel will be lower.
 
My question to you, is their is method we can adopt or make any assumption, to reduce the steel in the last floor.
 
Why am asking u is, first floor i got 2006( 6x20#) mm2 steel, but in the last floor am getting am getting 3045(6x25#) mm2 steel, if it is 5 floor resisdential building. so i provided heavier steel in the last floor. So the client is arguing with me. Please seniours guide me.
 
RAKESH KUMAR

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coolestbliss
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Joined: 20 Jun 2010
Posts: 9

PostPosted: Wed Jun 30, 2010 5:03 am    Post subject: Reply with quote

dear
i think top floor columns should not have high steel but instead to some extent ur are ture....if u apply seismic forces u will notice that u will get high steel in second storey than first storey for some columns....just check if ur using some software that the orientation of columns from base to top is same....and just rotate the columns and play with column major axis to check in what direction columns will be stronger....i dont think so that top floor columns will have high steel but it the second storey columns may have....post some more details about project so that i am able to help u clearly!
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vikram.jeet
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Joined: 26 Jan 2003
Posts: 2212

PostPosted: Wed Jun 30, 2010 7:34 am    Post subject: Regarding column design Reply with quote

Dear Er Rakesh Kumar,

This problem is generally faced in top most (roof ) frames at end columns
which may have less vertical load but the gravity moments are about the
same or sometimes ,even more.. Lower floor beams have columns members at
top as well as bottom to resist the floor End moment vis-a vis top(roof) beam  
having column member at bottom only.

However such situation may not be encountered in severe seismic zones
EQ moments will certainly be less at top most storey compared to lower  
floors/ storeys.Design carried out combining( Gravity moments +EQ moments)
may probably not exceed the Lower floors.

Redisrtibution of GRAVITY moments at topmost floor(i.e. roof level) could be
attempted to reduce the reinf in columns to the extent permitted by Codes.
The effect of EQ is least at topmost floor beams/columns and redistribution
in line with IS456 may not be objectionable( for roof beams/columns).

best regards

vikramjeet

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