www.sefindia.org

[vinit6062]

Dear Sir,

As I go through the all the ans. but no one tall confidingly what we have to do If the upper story column steel is higher because increase the steel dia or steel than lower floor it is not understandable..


Thanking you,




Vinit PArikh,(M) 9924626607



Subject: [E-CONF] Re: Last floor column design
From: forum@sefindia.org
Date: Wed, 28 Nov 2012 21:52:31 +0530
To: econf34289@sefindia.org

           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
      --

Posted via Email

[kapildingare]

Dear Vinit,

                     One has to provide the reinforcement according to design forces even it is more than lower storey column ,just no way out.

                    Providing hinge joint is at all no way ,you and me can assume anything but structure is going to behave in its natural fashion only,so by mere assuming ,irrespective of type of construction one can not reduce forces and hence reinforcement in any member.

                                        
                                                           Kapil Dingare

[Dr. N. Subramanian]

Dear Er Sriprakash,

What I wrote was basically for medium tall buildings with less than 10 floors where wind loads do not predominate. When  heavy lateral loads are considered in the design, both top and bottom columns will have heavy BMs, as the bottom columns will have heavy BM due to lateral loads and top columns due to gravity loads!

May be if you provide tapering stiffness in columns, there may be difference in behaviour of beams. But it is better to have the same size of beams, as in DL+LL case the BM distribution in beams need to be same except the top most floor, where the LL will be less but DD may be more due to weathering course.

Regards,
NS

[kapildingare]

Resp Dr Subramanian,

         Sir your comment,

         "But it is better to have the same size of beams, as in DL+LL case the BM distribution in beams need to be same except the top most floor, where the LL will be less but DD may be more due to weathering course. "

             Sir, I agree with you that beams for  all floor except terrace floor will have almost same size and reinforcement when we design it for DL+LL.
                     But actually even for six storey building when we design it for earthquake load , load conditions 0.9 DL+1.5 EL OR 1.5 DL+1.5 EL becomes dominant very frequently compaired to load condition 1.5 DL+1.5 LL.Which even leads to change of location of max. hogging and sagging  moment values and locations with respect to those values and locations with 1.5 DL+1.5 LL load condition.  And then this leads to change of reinforcement for beams with respect to floors.


  (This 1.5 with DL is forcing me to provide more reinforcement in beams and even in columns especially in 1.5 DL + 1.5 EL load condition that's why I had raised that thread of 1.5 with respect to DL(even when there is very less probability of Dl getting increased than estimated) in genaral discussion some days back.)



                                Thank you.


                                                               Kapil Dingare

[Manoharbs_eq]

Sriprakash is right we observe this type of forces in all buildings with lateral loads. previously when we used to design columns for only static cases reinforcement in column would uniformly vary from bottom to top and beam reinforcement would be  same for typical floors.


But when we analyse for lateral loads column will attract more moment if shear walls are not provided or if column is placed at corners, and beam reinforcement changes every two floors.



Rgds
Manohar

[ykalamkar]

Dear Mr. Sriprakash,
This happens usually. I have compared the seismic moments floor wise, in many cases, seismic moments are lesser at top and finally, giving lesser design moments at upper floors.

Thanks & regards
Yogesh Kalamkar



From: sriprakash_shastry [mailto:forum@sefindia.org]
Sent: 29 November 2012 11:17
To: econf34289@sefindia.org
Subject: [E-CONF] Re: Last floor column design



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

Posted via Email

[cckeshav]

Dear Mr Kapil:

The load combination 0.9DL+1.5EL is meant only for stability.. Not for design. As such, beam and column designs which do not have problem with stability can be designed with 1.5DL+1.5EL of other combinations.

C.Channakeshava


Subject: [E-CONF] Re: Last floor column design
From: forum@sefindia.org
Date: Fri, 30 Nov 2012 07:33:46 +0530
To: econf34289@sefindia.org

Resp Dr Subramanian,

Sir your comment,

"But it is better to have the same size of beams, as in DL+LL case the BM distribution in beams need to be same except the top most floor, where the LL will be less but DD may be more due to weathering course. "

Sir, I agree with you that beams for all floor except terrace floor will have almost same size and reinforcement when we design it for DL+LL.
But actually even for six storey building when we design it for earthquake load , load conditions 0.9 DL+1.5 EL OR 1.5 DL+1.5 EL becomes dominant very frequently compaired to load condition 1.5 DL+1.5 LL.Which even leads to change of location of max. hogging and sagging moment values and locations with respect to those values and locations with 1.5 DL+1.5 LL load condition. And then this leads to change of reinforcement for beams with respect to floors.


(This 1.5 with DL is forcing me to provide more reinforcement in beams and even in columns especially in 1.5 DL + 1.5 EL load condition that's why I had raised that thread of 1.5 with respect to DL(even when there is very less probability of Dl getting increased than estimated) in genaral discussion some days back.)



Thank you.


Kapil Dingare

Posted via Email

[cckeshav]

Dear Dr. NS:

In most analytical models, in particular for side and corner columns, the reinforcement required is higher in the last floor due to larger moment and smaller axial load. Many designers make corresponding changes in the reinforcement in the last floor. But it should be kept in mind that during construction, every floor is a last floor (!!) unless due precaution is taken to achieve column continuity before decentering beam supports. As such, the old practice of continuiing the top floor reinforcement (if found higher) into the foundation is better.

C.Channakeshava


Subject: [E-CONF] Re: Last floor column design
From: forum@sefindia.org
Date: Fri, 30 Nov 2012 04:32:36 +0530
To: econf34289@sefindia.org

Dear Er Sriprakash,

What I wrote was basically for medium tall buildings with less than 10 floors where wind loads do not predominate. When heavy lateral loads are considered in the design, both top and bottom columns will have heavy BMs, as the bottom columns will have heavy BM due to lateral loads and top columns due to gravity loads!

May be if you provide tapering stiffness in columns, there may be difference in behaviour of beams. But it is better to have the same size of beams, as in DL+LL case the BM distribution in beams need to be same except the top most floor, where the LL will be less but DD may be more due to weathering course.

Regards,
NS
sriprakash_shastry wrote:  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

Posted via Email

[cckeshav]

Dear SEFIans:

As mentioned elsewhare in my mail, every floor is a last floor during construction. This can be verified by a construction stage analysis. As such reinforcement required for the construction stage governs for end and side columns. Otherwise, specific decentering schemes should be provided which ensures that all the beams in the lower two levels are supported (not repropped) until the current floor is cured.

C.Channakeshava


Subject: [E-CONF] Re: Last floor column design
From: forum@sefindia.org
Date: Wed, 28 Nov 2012 21:52:31 +0530
To: econf34289@sefindia.org

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

Posted via Email

[Dr. N. Subramanian]

Dear Dr Channakeshava,

It is a very good point. Many do not consider the construction stage analysis. We discussed about it earlier, in the context of loading on slabs and removal of props. Concrete international has a few papers on this aspect.

Thanks for reminding. Another important aspect is the design if corner columns, which are critical in the 0.9WL combination, have to be designed carefully. Though the load may be less compared to other columns, they are subjected to bi axial bending and even torsion. I believe they are not researched properly.

Regards,
Subramanian