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How to calculate exact effective length of column

 
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sahilmujawar
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Joined: 17 Mar 2012
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PostPosted: Tue Apr 03, 2012 7:24 pm    Post subject: How to calculate exact effective length of column Reply with quote

Dear Sefians,
Anybody having any information about how to arrive at exact effective length of column especially long slender column(braced and unbraced).If one cannot arrive at exact effective length then what should be the effective length for columns for designing in practice(thumb rules).Plz reply.
Regards,
Sahil Mujawar
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btgprasad
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Joined: 05 Aug 2008
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PostPosted: Wed Apr 04, 2012 7:13 am    Post subject: Reply with quote

It is very difficult to find exact eff length of column in buildings.

it depends on

  1. relative stiffness of column/beam/slab stiffness and bracing,
  2. type of connection and fixity at the base.
  3. relative magnitude of lateral load and axial load. (stability index)
  4. and other confinements (like column is can be confined in strong walls through out the length then eff length will  be very small)
so for practice ,we can simply follow Annex-E in IS-456. annex-d in IS800-2007

please note that in soft storay columns are not confined and more possibility of sway due to lateral loads,effective length will be more .this may be the reason IS-1893 ask us to design Ground FL columns for more shear(please correct me if otherwise)

rgds,
btg.Prasad.
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Dr. N. Subramanian
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PostPosted: Wed Apr 04, 2012 3:55 pm    Post subject: Re: How to calculate exact effective length of column Reply with quote

Dear Er Majawar,

It is a very good question.  Er Prasad has given nicely the factors that may affect the effective length. Usually engineers take effective length of columns a 1.0L, which may not be correct and may have significant problems in EQ zones.

Table 28 of Annex E of IS 456 gives the values derived from classical buckling theories-but practical cases can not be defined(except cantilevers) within these cases.

For multi-storey buildings IS code has adopted Woods Curves developed in 1974. These two curves can be selected based on whether the Stability Index Q is less or greater than 0.04 (braced or unbraced). Similar provisions are there in ACI Code but instead of Wood's curves ACI has adopted the Alignment Chart developed by Julian and Lawrence in 1959.

Note that the Woods curves presented in IS 456 and IS 800 are identical as buckling phenomenon does not depend on material but of course on L/r, E and boundary conditions. Also IS 800 gives equations for these curves and hence they can be used in software.

I have compared these two methodologies in Chapter  9 of my book on Design of Steel Structures, Oxford University press and shown that both are giving the same results. My friend and mentor Prof. W.F. Chen (who has authored several books and papers) has done extensive work in this are and given several suggestions to improve Julian and Lawrence chart.

It is important to note that  IS 456 suggests the effective length of unbraced columns should not be taken less than 1.2. Also for unbraced columns the effective length can be infinity (for hinged-hinged case), and always greater than 1.0. The provision of Shear walls or stiff columns will convert frames to braced frames.

Best wishes
Subramanian

sahilmujawar wrote:
Dear Sefians,
Anybody having any information about how to arrive at exact effective length of column especially long slender column(braced and unbraced).If one cannot arrive at exact effective length then what should be the effective length for columns for designing in practice(thumb rules).Plz reply.
Regards,
Sahil Mujawar


Last edited by Dr. N. Subramanian on Wed Apr 04, 2012 4:31 pm; edited 1 time in total
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sahilmujawar
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PostPosted: Wed Apr 04, 2012 4:06 pm    Post subject: Reply with quote

Dear Dr N. Subramanian,
Sir if possible can you put forward any research article on effective length of RC column.Plz i would be very grateful to you.
Regards,
Sahil Mujawar

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Dr. N. Subramanian
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PostPosted: Wed Apr 04, 2012 5:23 pm    Post subject: Reply with quote

Dear Er Sahil,

As I wrote, whether it is concrete or steel same Wood curves can be used. The main difference in concrete comes in calculating the moment of inertia I, as concrete columns will crack. A paper appeared in 1990 issue of ACI Structural Journal is enclosed ( See also http://peer.berkeley.edu/publications/research_digests/digests_2006/rd2006-1.pdf and two more papers by Madhu Khuntia and S. K. Ghosh on Flexural Stiffness of Reinforced Concrete Columns and
Beams: Analytical Approach-ACI Structural Journal 2004)

You may be interested in the following papers also-You may google to get copies:
MacGregor, J. G.; Breen, J. E.; and Pfrang, E. O., “Design of Slender
Concrete Columns”, ACI  Journal, Vol. 67, No. 1, Jan. 1970, pp. 6-28

MacGregor, J. G, Design of slender concrete columns—revisited, ACI Structural Journal, Vol. 90, No.3, May-June 1993, pp. 302-309.


Please find enclosed another ACI Structural Journal paper on Eff. length of RC Columns

Best wishes
NS
.
sahilmujawar wrote:
Dear Dr N. Subramanian,
Sir if possible can you put forward any research article on effective length of RC column.Plz i would be very grateful to you.
Regards,
Sahil Mujawar




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sahilmujawar
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PostPosted: Thu Apr 05, 2012 4:59 am    Post subject: Reply with quote

Thankyou for posting paper
Regards,
Sahil Mujawar
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N. Prabhakar
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PostPosted: Thu Apr 05, 2012 7:08 am    Post subject: How to calculate exact effective length of column Reply with quote

Dear Sefians,

As Dr. Subramanian has explained, the design charts given in IS 456:2000 and IS 800:2007 for determination of effective length of columns are based on the technical papers by Dr. R.H. Wood, published in 3 parts in the July/August/September 1974 issue of  The Structural Engineer, Journal of the Institution of Structural Engineers, London.  These papers are copyrighted, but they can be downloaded by the members of the Institutuion from their website www.istructe.org.

In the Excel spreadsheets I have developed in 'RCDesign CD Ver 2.0' and in 'STeelDesign' software (available for a price), given the minimum design parameters, the effective length of columns are worked out easily.  These spreadsheets also in incorporate charts showing the values of effective length ratio for the column-beam frame considered.

With best wishes,

N. Prabhakar, BE CEng(I) MIstructE(UK) MIE (Ind)
Chartered Structural Engineer
Vasai (E)
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NareshBeniwal
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PostPosted: Thu Oct 25, 2012 4:46 pm    Post subject: Re: How to calculate exact effective length of column Reply with quote

Dear Prabhakar Sir,
I am interested to by your software "RCDesign CD Ver 2.0 and STeelDesign"
softwares.
plz tell me how to reach you to buy these.

with regards
Naresh Beniwal
nash.beniwal@gmail.com


N. Prabhakar wrote:
Dear Sefians,

As Dr. Subramanian has explained, the design charts given in IS 456:2000 and IS 800:2007 for determination of effective length of columns are based on the technical papers by Dr. R.H. Wood, published in 3 parts in the July/August/September 1974 issue of  The Structural Engineer, Journal of the Institution of Structural Engineers, London.  These papers are copyrighted, but they can be downloaded by the members of the Institutuion from their website www.istructe.org.

In the Excel spreadsheets I have developed in 'RCDesign CD Ver 2.0' and in 'STeelDesign' software (available for a price), given the minimum design parameters, the effective length of columns are worked out easily.  These spreadsheets also in incorporate charts showing the values of effective length ratio for the column-beam frame considered.

With best wishes,

N. Prabhakar, BE CEng(I) MIstructE(UK) MIE (Ind)
Chartered Structural Engineer
Vasai (E)
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