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Last floor column design
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vijaydshah
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PostPosted: Wed Nov 28, 2012 12:48 pm    Post subject: Last floor column design Reply with quote

Dear sefians
It has been observed from the experience that for all buildings when we do space frame analysis the reinforcement of the last floor is quite high compared to lower stories . This is specifically for the end columns where moment distribution is not possible due to discontinuity of the columns. Providing this reinforcement through out the height of the building do not justify the economy.

Can any one throw light on this subject ?.

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Manoharbs_eq
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PostPosted: Wed Nov 28, 2012 2:09 pm    Post subject: Last floor column design Reply with quote

Of coarse its true, but at lower levels load is governing so steel is higher , its uneconomical to provide last floor higher reinf throughout it should be reduced gradually so to achieve economy reduce it every two floors
On 28-Nov-2012 7:30 PM, "vijaydshah" <forum@sefindia.org (forum@sefindia.org)> wrote:
Quote:
           Dear sefians
It has been observed from the experience that for all buildings when we do space frame analysis the reinforcement of the last floor is quite high compared to lower stories . This is specifically for the end columns where moment distribution is not possible due to discontinuity of the columns. Providing this reinforcement through out the height of the building do not justify the economy.

Can any one throw light on this subject ?.
     


vdshah
     



     


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irfan_ahmed
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PostPosted: Wed Nov 28, 2012 2:10 pm    Post subject: Last floor column design Reply with quote

There are two ways - either provide higher reinforcement for top most story column or while analyzing assign top joint as hinged. However by doing so there are chances that the inter story drift for the top most story exceeds the limit. - from Vodafone
From: "vijaydshah" <forum@sefindia.org>
Date: Wed, 28 Nov 2012 18:18:27 +0530
To: <econf34289@sefindia.org>
ReplyTo: econf34289@sefindia.org
Subject: [E-CONF] Last floor column design

     Dear sefians  It has been observed from the experience that for all buildings when we do space frame analysis the reinforcement of the last floor is quite high compared to lower stories . This is specifically for the end columns where moment distribution is not possible due to discontinuity of the columns. Providing this reinforcement through out the height of the building do not justify the economy.   Can any one throw light on this subject ?.
     vdshah
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SameerDhuri
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PostPosted: Wed Nov 28, 2012 2:44 pm    Post subject: Re: Last Floor Column Reply with quote

Er Vijay Sir,
                       The above problem of heavy steel in Last Floor Column can be tackled by Reverse lapping Heavy diameter Steel Bars in last floor Column with lower dia bars for eg. 20 mm dia bars can be lapped with 16 mm dia bars.
I request Senior Sefians to comment on the appropriate way of dealing this problem especially in cases where there are Swimmimg pools on the roof! which increases the Lateral Force in last floor column.

Best Regards,
SameerDhuri
samsdhuri@gmail.com
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Surendra S
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PostPosted: Thu Nov 29, 2012 2:11 am    Post subject: Last floor column design Reply with quote

Your r right Vijay,

You can go with higher reinforcement at top most floor


rgds
SS

On Wed, Nov 28, 2012 at 9:52 PM, irfan_ahmed <forum@sefindia.org (forum@sefindia.org)> wrote:
Quote:
           There are two ways - either provide higher reinforcement for top most story column or while analyzing assign top joint as hinged. However by doing so there are chances that the inter story drift for the top most story exceeds the limit. - from Vodafone
From:  "vijaydshah"
Date: Wed, 28 Nov 2012 18:18:27 +0530
To:
ReplyTo:  econf34289@sefindia.org (econf34289@sefindia.org)
Subject: [E-CONF] Last floor column design

     Dear sefians It has been observed from the experience that for all buildings when we do space frame analysis the reinforcement of the last floor is quite high compared to lower stories . This is specifically for the end columns where moment distribution is not possible due to discontinuity of the columns. Providing this reinforcement through out the height of the building do not justify the economy. Can any one throw light on this subject ?.

     vdshah
      --
     



     



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sriprakash_shastry
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PostPosted: Thu Nov 29, 2012 4:09 am    Post subject: Reply with quote

Dear Mr. Shah.

There is another angle to your question. What you are saying is absolutely right. But, I also find that if there is a building of 20 floors, I find some engineers detailing only one floor and then repeating the same thing in all the floors.

I dont think this is a right approach. For tall buildings the stiffness of the columns reduce as you keep going to the top. In effect the columns in the lower floors are stiffer than the columns in the upper floor. Stiffer columns tend to attract more force than non-stiff columns. As the force in stiffer columns is more the distribution of forces within the floor are also different.

In a very tall building it may be advisable to have similar reinforcement for every 5 floors, but I dont think it would be economical to have the same reinforcement in beams for all the floors.

Regards,

Sriprakash
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gautam chattopadhyay
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PostPosted: Thu Nov 29, 2012 5:11 am    Post subject: Last floor column design Reply with quote

This happens because at top most storey direct load is much less and moment is very high.

On Thu, Nov 29, 2012 at 9:39 AM, sriprakash_shastry <forum@sefindia.org (forum@sefindia.org)> wrote:
Quote:
           Dear Mr. Shah.

There is another angle to your question. What you are saying is absolutely right. But, I also find that if there is a building of 20 floors, I find some engineers detailing only one floor and then repeating the same thing in all the floors.

I dont think this is a right approach. For tall buildings the stiffness of the columns reduce as you keep going to the top. In effect the columns in the lower floors are stiffer than the columns in the upper floor. Stiffer columns tend to attract more force than non-stiff columns. As the force in stiffer columns is more the distribution of forces within the floor are also different.

In a very tall building it may be advisable to have similar reinforcement for every 5 floors, but I dont think it would be economical to have the same reinforcement in beams for all the floors.

Regards,

Sriprakash
     



     


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Dr. N. Subramanian
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PostPosted: Thu Nov 29, 2012 5:30 am    Post subject: Reply with quote

Dear Er Sriprakash,

I do not follow what you are trying to say! I feel that the beam reinforcement will be almost same in all floors and only the column reinforcement will change.

It is a better practice to have same size of column throughout the height or at least for a few floors and change only the rebars. As Er Shah says, there will be more moments at the top floor and more axial force in the bottom floor. We can have 10 to 15 percent more reinforcement in top floor,
If it exceeds that limit, better to provide that rebars in the bottom columns also. That is the reason for the suggestion of keeping the same size of column. If the column size is reduced, the leaver arm to resist bending will be reduced and hence we may need still more rebars at the top level.

The stiffness of column also depends on the extent of cracking!In IS code there is no provision for that- ACI code gives some formula, I think it also depends on axial load.

Best wishes,
NS
sriprakash_shastry wrote:
Dear Mr. Shah.

There is another angle to your question. What you are saying is absolutely right. But, I also find that if there is a building of 20 floors, I find some engineers detailing only one floor and then repeating the same thing in all the floors.

I dont think this is a right approach. For tall buildings the stiffness of the columns reduce as you keep going to the top. In effect the columns in the lower floors are stiffer than the columns in the upper floor. Stiffer columns tend to attract more force than non-stiff columns. As the force in stiffer columns is more the distribution of forces within the floor are also different.

In a very tall building it may be advisable to have similar reinforcement for every 5 floors, but I dont think it would be economical to have the same reinforcement in beams for all the floors.

Regards,

Sriprakash
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sriprakash_shastry
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PostPosted: Thu Nov 29, 2012 5:47 am    Post subject: Reply with quote

Dear Dr. NS

I have found that in some analytical models that the reinforcement in the beams in the floors lets say between the 15th and 20th floors are lesser than the reinforcement in between the 1st and the 5th floors although the loads on the floors are the same.

I was attributing this to the reduction in the stiffness of the columns. Can you tell me why this could possibly be happening.

Warm Regards,

Sriprakash
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kapildingare
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PostPosted: Thu Nov 29, 2012 6:38 am    Post subject: Reply with quote

Resp Dr Subramanian,

                               Even for six storey building beam forces and hence reinforcement changes with respect to storey height,when we analysis frame for lateral loads.
                                         Please guide me further and I request you to be in eC up to last moment so we can have your guidance.

                                         Thank you,very much.



                                                              Kapil Dingare
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