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Braced and unbraced column.
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SUKANTA.ADHIKARI
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PostPosted: Tue Jun 17, 2008 4:08 am    Post subject: Braced and unbraced column. Reply with quote

Dear Sefians,

This is a querry regarding braced column. As per IS 456 :2000, " A column may be considered braced in a given plane if lateral stability to the structure as a whole is provided by walls or bracing or buttressing designed to resist all lateral forces in that plane. It should otherwise be considered unbraced".

I was going through the book "Limit state design of RCC structure by unnikrishnan pillai & Devdos Menon ".They have written that if stability index
Q is less than 0.05, the column can be considered as braced as per ACI 318.Stability index Q is a function of axial force,elastic first order lateral deflection,lateral force and height of storey.Suppose we dont have any lateral load resisting element in a frame and after calculating Q we find that it is less than 0.05 ,that means the column is braced.If so it will contradict IS 456 clause.I dont have much knowledge about ACI code.I want to know whether there is any mention of lateral load resisting element in the plane of forces in ACI code , as mentioned in IS code.

Regards,
S.Adhikari

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PostPosted: Tue Jun 17, 2008 7:21 am    Post subject: Braced and unbraced column. Reply with quote

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PostPosted: Tue Jun 17, 2008 8:31 am    Post subject: Avoid test posts and long disclaimers Reply with quote

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PostPosted: Tue Jun 17, 2008 2:22 pm    Post subject: Braced and unbraced column. Reply with quote

Dear Mr. Adhikari,

The stability index, as in the ACI 2005 code, determines if a storey is sway or non-sway. Please refer to clause 10.11.4.2 of the same. It does not determine if a column is braced or non-braced.

Clause 10.11.4.1, on the other hand, mentions that if the increse in end moments due to second order effects does not exceed 5% of the end moments due to first order end moments, the column can be assumed as non-sway.

It has never mentioned the terms "braced" or "unbraced" in these clauses. I do not think the ACI specifications contradicts IS:456 specifications in any way. To be "sway" or "non-sway" is completely different from being "braced" or unbraced".

Regards,
Sudip

On Tue, 17 Jun 2008 SUKANTA.ADHIKARI wrote :
Quote:
Dear Sefians,

This is a querry regarding braced column. As per IS 456 :2000, " A column may be considered braced in a given plane if lateral stability to the structure as a whole is provided by walls or bracing or buttressing designed to resist all lateral forces in that plane. It should otherwise be considered unbraced".

I was going through the book "Limit state design of RCC structure by unnikrishnan pillai & Devdos Menon ".They have written that if stability index
Q is less than 0.05, the column can be considered as braced as per ACI 318.Stability index Q is a function of axial force,elastic first order lateral deflection,lateral force and height of storey.Suppose we dont have any lateral load resisting element in a frame and after calculating Q we find that it is less than 0.05 ,that means the column is braced.If so it will contradict IS 456 clause.I dont have much knowledge about ACI code.I want to know whether there is any mention of lateral load resisting element in the plane of forces in ACI code , as mentioned in IS code.

Regards,
S.Adhikari






Sudip Narayan Choudhury
Devangan, Flat F
9, Vivekananda Sarani
Calcutta 700075
W.Bengal, India
Residence: +91 33 2418 4257
Cell: +91 98307 01642

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PostPosted: Tue Jun 17, 2008 3:30 pm    Post subject: Braced and Unbraced Columns Reply with quote

Braced and Unbraced Columns

Dear Mr. S.Adhikari  

The Stability Index you have quoted from Pillai and Menon's Book is based on ACI 318 code only, which states that " It shall be permitted to assume a column of a structure is non sway if the increase in the column end moments due to second order effects does not exceed 5% of the first-order end moments." In the next clause the an alternative method for determining this based on stability index for a story Q, is given.  

The commentary says " A compression member may be assumed nonsaway by inspecting if it is lcated in a storey in which the bracing elements (shear walls or other types of lateral bracing) have such subtantil stiffness to resist the lateral deflections of the story that any resulting lateral deflection is not large enough too affetct the column strength sustantially." It may be clear that if you do not have bracing elements Q will not be less than 0.05.  

It is interesting to note that IS 800:2007, Appendix B allows you to take the stiffening effect of infill wall panels by introducing a diagonal strut in that storey with area A, defined in the Appendix.


By the way the book by Pillai and Menon is a good book and I would recommend it for anyone designing RC structures.

Best wishes & Regards
Subramanian

Dear Sefians,  
This is a querry regarding braced column. As per IS 456 :2000, " A column may be considered braced in a given plane if lateral stability to the structure as a whole is provided by walls or bracing or buttressing designed to resist all lateral forces in that plane. It should otherwise be considered unbraced".  

I was going through the book "Limit state design of RCC structure by unnikrishnan pillai & Devdos Menon ".They have written that if stability index
Q is less than 0.05, the column can be considered as braced as per ACI 318.Stability index Q is a function of axial force,elastic first order lateral deflection,lateral force and height of storey.Suppose we dont have any lateral load resisting element in a frame and after calculating Q we find that it is less than 0.05 ,that means the column is braced.If so it will contradict IS 456 clause.I dont have much knowledge about ACI code.I want to know whether there is any mention of lateral load resisting element in the plane of forces in ACI code , as mentioned in IS code.  

Regards,
S.Adhikari  


Dr.N.Subramanian,Ph.D.,F.ASCE, M.ACI,

Computer Design Consultants: www.cdcstruct.com
Maryland, USA

See my books at: www.multi-science.co.uk/subramanian-book.htm
www.oup.co.in/search_detail.php?id=144559

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PostPosted: Wed Jun 18, 2008 4:03 am    Post subject: Braced and unbraced column. Reply with quote

Dear Mr Subramanian/Mr Sudip,

I know  book by Pillai and Menon is a good book and the explanation which they have given for limit state design is beyond words.But what one thing which I found was IS code says if stability index Q is less than 0.04 the column is sway column and vice versa.But in the book I have found that same sentence has been used, 0.04 has been replaced by 0.05 and sway replaced by braced and reference was given to ACI 318.I will read out the exactly what has been written in the book., "a column is considered to be braced if the stability index Q is less than 0.05". I also know that that to be "sway" or "non-sway" is completely different from being "braced" or unbraced".But this sentence are a bit confusing.I could not make out if there is no braced system Q will always be less than 0.05. Mr subramaniam please explain in detail


Regards,
S.Adhikari

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PostPosted: Wed Jun 18, 2008 7:23 am    Post subject: Braced and unbraced column. Reply with quote

Dear Mr. Subramanium,

I would beg to differ with the line - "It may be clear that if you do not have bracing elements Q will not be less than 0.05."

Using appropriate bracing elements would indeed bring down the stability index to a value less than 0.05. However, there are other conditions which might bring down ths stability index. Note that as per the PCA notes, the stability index is applicable for a storey and not for a single column. For a single column, the condition is that the increase in moments due to second-order effects should be within 5% of moments due to the first order effects.

The conditions, other than bracing, which would bring down the stability index:

1. The stability elements have substantial lateral stiffness to resist the horzontal storey shears. Thus the deflections due to the horizontal storey shears would be less.

2. The horizontal storey shears are small in magnitude to produce any significant lateral deflections on the storey.

The above two conditions are inherent in the equation of stability index as is defined in clause 10.11.4.2 and given as below:

Q =Pu*o /Vus*lc

Q = stability index
Pu = total factored vertical load
Vus = factoored horizontal storey shear
o = first-order relative lateral deflection between the top and bottom of that storey.

Note that the above equation is not valid if Vus = 0. In this case there are no second order effects and the stability index does not apply.

Regards,
Sudip


On Tue, 17 Jun 2008 drnsmani wrote :
Quote:
Braced and Unbraced Columns

Dear Mr. S.Adhikari

The Stability Index you have quoted from Pillai and Menon's Book is based on ACI 318 code only, which states that " It shall be permitted to assume a column of a structure is non sway if the increase in the column end moments due to second order effects does not exceed 5% of the first-order end moments." In the next clause the an alternative method for determining this based on stability index for a story Q, is given.

The commentary says " A compression member may be assumed nonsaway by inspecting if it is lcated in a storey in which the bracing elements (shear walls or other types of lateral bracing) have such subtantil stiffness to resist the lateral deflections of the story that any resulting lateral deflection is not large enough too affetct the column strength sustantially." It may be clear that if you do not have bracing elements Q will not be less than 0.05.

It is interesting to note that IS 800:2007, Appendix B allows you to take the stiffening effect of infill wall panels by introducing a diagonal strut in that storey with area A, defined in the Appendix.


By the way the book by Pillai and Menon is a good book and I would recommend it for anyone designing RC structures.

Best wishes & Regards
Subramanian

Dear Sefians,
This is a querry regarding braced column. As per IS 456 :2000, " A column may be considered braced in a given plane if lateral stability to the structure as a whole is provided by walls or bracing or buttressing designed to resist all lateral forces in that plane. It should otherwise be considered unbraced".

I was going through the book "Limit state design of RCC structure by unnikrishnan pillai & Devdos Menon ".They have written that if stability index
Q is less than 0.05, the column can be considered as braced as per ACI 318.Stability index Q is a function of axial force,elastic first order lateral deflection,lateral force and height of storey.Suppose we dont have any lateral load resisting element in a frame and after calculating Q we find that it is less than 0.05 ,that means the column is braced.If so it will contradict IS 456 clause.I dont have much knowledge about ACI code.I want to know whether there is any mention of lateral load resisting element in the plane of forces in ACI code , as mentioned in IS code.

Regards,
S.Adhikari


[Image: http://us.i1.yimg.com/us.yimg.com/i/mesg/tsmileys2/23.gif ]Dr.N.Subramanian,Ph.D.,F.ASCE, M.ACI,

Computer Design Consultants: www.cdcstruct.com
Maryland, USA

See my books at: www.multi-science.co.uk/subramanian-book.htm (http://www.multi-science.co.uk/subramanian-book.htm)
www.oup.co.in/search_detail.php?id=144559 (http://www.oup.co.in/search_detail.php?id=144559)






Sudip Narayan Choudhury
Devangan, Flat F
9, Vivekananda Sarani
Calcutta 700075
W.Bengal, India
Residence: +91 33 2418 4257
Cell: +91 98307 01642

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PostPosted: Wed Jun 18, 2008 7:53 pm    Post subject: Braced and unbraced column. Reply with quote

Dear Shri. Adhikari/Mr.Sudip,
<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />  
The IS code provisions (Appendix E-2) are adopted from the ACI code provisions(clause 10.11.4.2), by changing only the limit from 0.05 to 0.04! The second edition of the book (pp.578, example13.1) also refers this IS code limit.  

Sway and No-sway are related to braced and unbraced- A fully braced framework will not sway and vice versa. In fact a frame may be assumed fully braced if there are shear walls or other special bracing elements which are designed to resist all the lateral loads of the frame.

Of course if the columns are short and made very stiff and if the axial force in the columns are not high, the value of Q can be less than 0.05 or 0.04. Consider examples 13.1 and 13.2 given in the book by Pillai and Menon. In Example 13.1 the columns are 300 x 400mm (for a height of storey of 3.5m) and Q works out to 0.0367 (the max. horz. Defn. is of the order of 1.6998 x 10-5). In Example 13.2, for the same frame, the column size is reduced to 250mm x250mm and the value of Q is of the order of 0.0839 > 0.04 (with the defln. as 3.8855 x 10-5).  From this it is clear that if you do not provide shear wall or other bracing elements (which will reduce the cost and result in better performance), you have to provide very big columns in order to reduce the stability index within limits.

Dr. N. Subramanian

Dear Mr Subramanian/Mr Sudip,  

I know book by Pillai and Menon is a good book and the explanation which they have given for limit state design is beyond words. But what one thing which I found was IS code says if stability index Q is less than 0.04 the column is sway column and vice versa. But in the book I have found that same sentence has been used, 0.04 has been replaced by 0.05 and sway replaced by braced and reference was given to ACI 318.I will read out the exactly what has been written in the book., "a column is considered to be braced if the stability index Q is less than 0.05". I also know that that to be "sway" or "non-sway" is completely different from being "braced" or unbraced".But this sentence are a bit confusing. I could not make out if there is no braced system Q will always be less than 0.05. Mr subramaniam please explain in detail  


Regards,  
S.Adhikari

Dear Mr. Subramanium,

I would beg to differ with the line - "It may be clear that if you do not have bracing elements Q will not be less than 0.05."

Using appropriate bracing elements would indeed bring down the stability index to a value less than 0.05. However, there are other conditions which might bring down ths stability index. Note that as per the PCA notes, the stability index is applicable for a storey and not for a single column. For a single column, the condition is that the increase in moments due to second-order effects should be within 5% of moments due to the first order effects.

The conditions, other than bracing, which would bring down the stability index:

1. The stability elements have substantial lateral stiffness to resist the horzontal storey shears. Thus the deflections due to the horizontal storey shears would be less.

2. The horizontal storey shears are small in magnitude to produce any significant lateral deflections on the storey.

The above two conditions are inherent in the equation of stability index as is defined in clause 10.11.4.2 and given as below:

Q =ƩPu*Δo /Vus*lc

Q = stability index
ƩPu = total factored vertical load
Vus = factored horizontal storey shear
Δo = first-order relative lateral deflection between the top and bottom of that storey.

Note that the above equation is not valid if Vus = 0. In this case there are no second order effects and the stability index does not apply.

Regards,
Sudip

Dr.N.Subramanian,Ph.D.,F.ASCE, M.ACI,

Computer Design Consultants: www.cdcstruct.com
Maryland, USA

See my books at: www.multi-science.co.uk/subramanian-book.htm
www.oup.co.in/search_detail.php?id=144559





--- On Wed, 6/18/08, SUKANTA.ADHIKARI <forum@sefindia.org> wrote:
Quote:
From: SUKANTA.ADHIKARI <forum@sefindia.org>
Subject: [SEFI] Re: Braced and unbraced column.
To: general@sefindia.org
Date: Wednesday, June 18, 2008, 10:32 AM

Dear Mr Subramanian/Mr Sudip,

I know book by Pillai and Menon is a good book and the explanation which they have given for limit state design is beyond words.But what one thing which I found was IS code says if stability index Q is less than 0.04 the column is sway column and vice versa.But in the book I have found that same sentence has been used, 0.04 has been replaced by 0.05 and sway replaced by braced and reference was given to ACI 318.I will read out the exactly what has been written in the book., "a column is considered to be braced if the stability index Q is less than 0.05". I also know that that to be "sway" or "non-sway" is completely different from being "braced" or unbraced".But this sentence are a bit confusing.I could not make out if there is no braced system Q will always be less than 0.05. Mr subramaniam please explain in detail


Regards,
S.Adhikari








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SUKANTA.ADHIKARI
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PostPosted: Thu Jun 19, 2008 4:04 am    Post subject: Braced and unbraced column. Reply with quote

Dear Mr Subramanian,

After recieving the explanation I will again ponder on the same question which I have put on the first day of the discussion on braced & unbraced column.As per the latest explanation recieved what I can make out is that, to consider a column to be either braced or unbraced it is not necessary that we need to have special bracing or shear wall,only thing is stability index Q should be less than 0.04 as per BIS and 0.05 as per ACI.Again I am reading out the IS code statement on braced and unbraced column.

As per IS 456 :2000, " A column may be considered braced in a given plane if lateral stability to the structure as a whole is provided by walls or bracing or buttressing designed to resist all lateral forces in that plane. It should otherwise be considered unbraced".

What I can make out is that we need to have a system in the plane of forces which should be able to resist lateral forces,then only the column is braced.But the explantaion which I have recieved is different.

And regarding masonary infill ,we consider the masonary infill as equivalent diagonal diagonal strut of a bracing system.Bracing may be subjected to compression/tension and how a masonary infill will be able to take tension as is tensile strengh of masonary is very less.Also in masonary we have lot of opening for window, door etc.Is there affected accounted in latest version of IS 800:2007.

Regards,
S.Adhikari
















"drnsmani" <forum@sefindia.org>  
06/19/2008 03:07 AM    Please respond to
general@sefindia.org

To
general@sefindia.org   cc
Subject
[SEFI] Re: Braced and unbraced column.




Dear Shri. Adhikari/Mr.Sudip,

The IS code provisions (Appendix E-2) are adopted from the ACI code provisions(clause 10.11.4.2), by changing only the limit from 0.05 to 0.04! The second edition of the book (pp.578, example13.1) also refers this IS code limit.

Sway and No-sway are related to braced and unbraced- A fully braced framework will not sway and vice versa. In fact a frame may be assumed fully braced if there are shear walls or other special bracing elements which are designed to resist all the lateral loads of the frame.

Of course if the columns are short and made very stiff and if the axial force in the columns are not high, the value of Q can be less than 0.05 or 0.04. Consider examples 13.1 and 13.2 given in the book by Pillai and Menon. In Example 13.1 the columns are 300 x 400mm (for a height of storey of 3.5m) and Q works out to 0.0367 (the max. horz. Defn. is of the order of 1.6998 x 10-5). In Example 13.2, for the same frame, the column size is reduced to 250mm x250mm and the value of Q is of the order of 0.0839 > 0.04 (with the defln. as 3.8855 x 10-5). From this it is clear that if you do not provide shear wall or other bracing elements (which will reduce the cost and result in better performance), you have to provide very big columns in order to reduce the stability index within limits.

Dr. N. Subramanian

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PostPosted: Thu Jun 19, 2008 5:00 am    Post subject: Braced and unbraced column. Reply with quote

Dear All,

I would add an interesting observation in this discussion.


(A) IS 456-2000 has provided guidelines for calculation of effective length in Appendix E.  
1. Use (Wood's) Chart fig.26 for columns without sway (braced frame), As per this chart  (L,eff / Lu <=1).  
Use Fig. 27 for columns with sway (unbraced frame), As per this chart
(L,eff / Lu =>1).  

2. Calculate Stability Index Qu. If Qu <0.04, the column is considered as NO SWAY.  
Qu = (Pu/Hs) * (di / h) where
Pu = Factored total axial load of all columns in that storey
Use Factor = 1, Cl. 7.11.1 IS 1893 (Part I) 2002
Loads = D.L, + Reduced I.L. + EQ
EQ Load, based on Analytical Time period (not to use Empirical minimum)

Hs = Lateral Force Acting Within the Storey (Cumulative Storey Shear)
di = Elastic lateral deflection of the column
h = height of the column.

(B) The criteria (di /h) = 0.004 is controlled as per Cl. 7.11.1 IS 1893 (Part I) 2002. Once we put this value in above equation, along with limiting Qu=0.04, we find that for a frame to be NO SWAY frame Hs/Pu < 0.01.  

Now Hs/ Pu = Ah = Z/2 * I/R * Sa/g. (IS 1893 (Part I) 2002)

The max. value of Sa/g = 2.5, I=1 R =5, for all the EQ zones Hs/Pu <0.01, i.e. All such Moment Resisting Frames may be treated as NO SWAY provided drift criteria is satisfied.

For I=1, R=3, Only Zone V frames might demand second order analysis, or to be treated as SWAY Frames.  


K. N. Sheth
Dharmsinh Desai University,
Nadiad

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