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Last floor column design
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bijay sarkar
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PostPosted: Sat Dec 01, 2012 6:58 am    Post subject: Reply with quote

Sometime in earlier occasion, it was commented by some expert that all the load combinations including the 0.9DL + 1.5EL shall be considered in design .  We regularly design for 0.9DL + 1.5EL also.

Which one is correct??

with regards,

bijay sarkar
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Manoharbs_eq
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PostPosted: Sat Dec 01, 2012 7:09 am    Post subject: Reply with quote

Er,Tamilarasan



Designing for hinged is acceptable. But if the story sway is less. If the story sway is more we have to use Appendix E of Is code.


Rgds
Manohar
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subhash_mehrotra
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PostPosted: Sat Dec 01, 2012 1:41 pm    Post subject: Last Floor Column Design Reply with quote

Dear All,


As per clause No.7.12.2.1, IS 1893 – 2002:  Tanks, parapets, smoke stacks (chimneys) and other vertical cantilever projections attached to buildings and projecting above the roof, shall be designed and checked for stability for five times the design horizontal seismic coefficient Ah specified in 6.4.2.  In the analysis of the building, the weight of these projecting elements will be lumped with the roof weight.


Due to this, it is observed that steel in columns usually increases in the top most storey and in some cases it increases even more than 100% of the steel required in the floor below.


There is no need to provide same column reinforcement throughout the height.  Starting from foundation the reinforcement can be provided as per design requirements.  To cater for large last floor column reinforcement, it is recommended to increase the steel in column gradually from two to three floors below the top most floor, instead of increasing in one go.


 
Regards,
S C Mehrotra

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tsriharivaishu
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PostPosted: Sat Dec 01, 2012 3:08 pm    Post subject: Last floor column design Reply with quote

Dear sirs, can any body pls assist me to asign wind loads on roof trusses for sheds.RegardsSent from BlackBerry® on Airtel
From: "surya_prakash_k" <forum@sefindia.org>
Date: Sat, 01 Dec 2012 07:52:57 +0530
To: <econf34289@sefindia.org>
ReplyTo: econf34289@sefindia.org
Subject: [E-CONF] Re: Last floor column design

     Dear Dr. Chennakeshava,  During analyses carried out earlier, the dead load induced moments for this construction stage are safe to be taken by columns for last slab condition as long as dead load is not exceeding apprx 40 per cent of total load. However, for light live load structures like car parks and long span structures special analyses need to be done.  Thanks & Regards  K Suryaprakash From: cckeshav   Date: Fri, 30 Nov 2012 18:32:54 +0530 To:  ReplyTo:   Subject: [E-CONF] Re: Last floor column design   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 (forum@sefindia.org) (forum@sefindia.org (forum@sefindia.org)) Date: Wed, 28 Nov 2012 21:52:31 +0530 To: econf34289@sefindia.org (econf34289@sefindia.org) (econf34289@sefindia.org (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) (econf34289@sefindia.org (econf34289@sefindia.org)) (econf34289@sefindia.org (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       This Message and its contents is intended solely for the addressee and is proprietary. Information in this mail is for L&T Business Usage only. Any Use to other than the addressee is misuse and infringement to Proprietorship of L&T Construction. If you are not the addressee please return the mail to the sender. L&T Construction.         --

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Dr. N. Subramanian
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PostPosted: Sat Dec 01, 2012 5:56 pm    Post subject: Re: Last floor column design Reply with quote

Dear Er Srihari,
Please delete your posting here and post it in the general forum as this e-conference is meant fir tall buildings only!

Best wishes
NS
[="tsriharivaishu"]Dear sirs, can any body pls assist me to asign wind loads on roof trusses for sheds.RegardsSent from BlackBerry® on Airtel
From: "surya_prakash_k" <forum>
Date: Sat, 01 Dec 2012 07:52:57 +0530
To: <econf34289>
ReplyTo: econf34289@sefindia.org
Subject: [E-CONF] Re: Last floor column design

     Dear Dr. Chennakeshava,  During analyses carried out earlier, the dead load induced moments for this construction stage are safe to be taken by columns for last slab condition as long as dead load is not exceeding apprx 40 per cent of total load. However, for light live load structures like car parks and long span structures special analyses need to be done.  Thanks & Regards  K Suryaprakash From: cckeshav   Date: Fri, 30 Nov 2012 18:32:54 +0530 To:  ReplyTo:   Subject: [E-CONF] Re: Last floor column design   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 (forum@sefindia.org) (forum@sefindia.org (forum@sefindia.org)) Date: Wed, 28 Nov 2012 21:52:31 +0530 To: econf34289@sefindia.org (econf34289@sefindia.org) (econf34289@sefindia.org (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) (econf34289@sefindia.org (econf34289@sefindia.org)) (econf34289@sefindia.org (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      This Message and its contents is intended solely for the addressee and is proprietary. Information in this mail is for L&T Business Usage only. Any Use to other than the addressee is misuse and infringement to Proprietorship of L&T Construction. If you are not the addressee please return the mail to the sender. L&T Construction.         --

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Dr. N. Subramanian
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PostPosted: Sat Dec 01, 2012 6:08 pm    Post subject: Reply with quote

Dear Er Tamilarasan,

We go in for framed structures for rigidity at joints, which will resist the external forces effectively and provide stability. But by introducing a hinge, just because you want the column to be designed only for AF, we are defeating the basic principle as stated above. Thus you are compromising on stability. Moreover as discussed by Er Mehrotra, once you introduce a hinge, it will become a projected column, which has to be considered specially by IS 1893. Even otherwise, the effective length if this column will be more than 1.0 and due to that the reinforcement will increase. Moreover, how will you detail it in practice? Will you stop the column bars below the joint?

Of course, you can reduce the moments in the column by introducing shear walls in the system, which will reduce the total deflections and hence the BM.

Best wishes,
NS
P.S. I like your name! Smile


Manoharbs_eq wrote:
Er,Tamilarasan



Designing for hinged is acceptable. But if the story sway is less. If the story sway is more we have to use Appendix E of Is code.


Rgds
Manohar
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Dr. N. Subramanian
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PostPosted: Sat Dec 01, 2012 6:14 pm    Post subject: Reply with quote

Dear Er Tamilarasan,
Once you release 100% moments in a joint, it will behave as a hinge! A hinge by definition is one which has zero BM

Best wishes
NS

mtamil wrote:
Dear Manohar

I have used "Hinged" in the sense, 100% moment released and not a hinged support. No way!!.

Can you pl. share your opinion with this correction!!

regards
tamilarasan



Manoharbs_eq wrote:
I don't agree with assigning any support to any structural element other than at the bottom @ foundation level. I suggest we should design the structure as it is (A cantilever) above ground.


we have passed the stage where we use to idolize the structure support as simply fixed, pinned or hinge. At this stage we can model the structure nearest to field condition and even soil springs at bottom to consider SSI.


Rgds
Manohar
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kapildingare
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PostPosted: Sun Dec 02, 2012 2:38 am    Post subject: Reply with quote

Reasp Dr Subramanian,

                                    Happy to see you back on eC sir,Thank You.


                                                          Kapil Dingare
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bijay sarkar
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PostPosted: Sun Dec 02, 2012 7:23 am    Post subject: Reply with quote

Dear Sir,

The extra reinforcements required for the upper floor may be provided bundling the same with the corner reinforcements (4 corners) to get maximum resistance in moments from both directions.

Corner reinforcements contribute maximum in the column capacity in both the directions.  

with regards,

bijay sarkar
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cvrm
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PostPosted: Sun Dec 02, 2012 1:15 pm    Post subject: Last floor column design Reply with quote

UPPER STOREY COLUMN DESIGN

The topic of upper storey columns is recieving significant attention
during the eC. Adequate caution has been exercised by some of the
participants. Many factors can contribute to the stress resultant
demands (i.e., P, V and M) and hence the design of the upper storey
columns; this is experienced by designers as column reinforcement
being much different from those in the lower storey columns. These
factors include:

1. Column size changes::
When the column size is reduced in an upper storey, the
cross-sectional area and moment of inertia change suddently. This
increases the percentage of steel, and even the absolute amount of
steel. It could have been lesser, if the column size in the lower
storey was continued in the upper storey also.

2. Axial versus Bending Effects::
Columns in the storeys close to the bottom of the building undergo
lesser bending, but sustain higher axial load. As one goes up along
the height of the building, bending effects increase upto a certain
height and eventually reduce; the axial load reduces to the top of the
building. Thus, in the upper storeys, the design point lies in the
lower part of the P-M interaction diagram, and hence upper storey
columns are expected to undergo ductile actions. But, in the lower
storeys, the design point lies in the upper part of the P-M
interaction diagram, and hence the lower storey columns behave in the
brittle way. Columns in the intermediate storeys, have both P and M to
be reasonably high, and hence their behvaiour is mixed.

3. Higher modes effects::
Tall buildings have high fundamental T, and their higher mode T is
still high. Design earthquake spectra carry large energy corresponding
to T of higher modes rather than the fundamental modes. Hence, in
modal analysis, the share of higher modes tends to be larger. This
effectively results in the columns of middle storeys receiving higher
moment and axial demand, and the upper storey columns lesser.

4. Reduced gravity load ::
By their mere position, the upper storey columns sustain lesser
gravity load and hence the P-M interaction is diminishing to the top.

5. H/D of the building::
When the aspect ratio of the building is small, the building deforms
laterally in shear-type cantilever mode; here, the upper storey
columns do not see much inter-storey drift. The situation is the
opposite when the building is slender; the building deforms laterally
in flexure-type cantilever mode, and here the upper storey columns
undergo larger inter-storey drift. More interstorey drift implies more
bending of the upper storey columns.

6. Whip-lash effect::
Some buildings will see whip-lash effect at the upper storeys during
earthquake shaking, that has large pulses (as in near field ground
motion) and sudden reduction in column size in upper storeys. In such
cases, the upper storey columns may experience more effects. Some
codes reflect this through an additional concentrated design lateral
force at the top storey. But, this is best noticed when time history
analyses are performed.


In a tall building, some or all of the above factors may contribute to
the structural behaviour of upper storey columns, depending on the
structural configuration of the tall building and choice of locations
of column size reduction in the design process. It would be difficult
to ascribe reasons for reduced/increased longitudinal steel in upper
storey columns to just one of these factors.



C.V.R.Murty

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