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kiranklk SEFI Regulars
Joined: 26 Dec 2010 Posts: 34 Location: Banglore
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Posted: Tue Mar 01, 2011 6:27 am Post subject: Design Corner Eccentric Footing on Both Side |
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Hi Please Anybody Tell Me How to Design Corner EccentricFooting on Both Side Without Starp Beam. And Please Suggest me Any Books ForReference and Excel Spread Sheets.
Regards Kiran.
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B.V.Harsoda General Sponsor
Joined: 19 Jan 2009 Posts: 2329 Location: RAJKOT,GUJARAT, INDIA
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B.V.Harsoda General Sponsor
Joined: 19 Jan 2009 Posts: 2329 Location: RAJKOT,GUJARAT, INDIA
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Dr. N. Subramanian General Sponsor
Joined: 21 Feb 2008 Posts: 5538 Location: Gaithersburg, MD, U.S.A.
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Posted: Tue Mar 01, 2011 2:42 pm Post subject: Re: Design Corner Eccentric Footing on Both Side |
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Dear Er Kiran,
It is very difficult to provide ecc. footings with out connecting it to adjacent footing with strap beams, unless it is having very low AF. When it has ecc. on both axes, it is not advisable to attempt such a footing, because footing is very important to any structure, and the loss of footing due to loss of contact may be catastrophic to any building. That is the reason why it is not included in any book. For both side ecc. footing, I suggest you to combine other footings and provide a raft, as we do in lift wells.
Best wishes
NS
kiranklk wrote: | Hi Please Anybody Tell Me How to Design Corner EccentricFooting on Both Side Without Starp Beam. And Please Suggest me Any Books ForReference and Excel Spread Sheets.
Regards Kiran. |
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B.V.Harsoda General Sponsor
Joined: 19 Jan 2009 Posts: 2329 Location: RAJKOT,GUJARAT, INDIA
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Posted: Tue Mar 01, 2011 4:27 pm Post subject: |
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In previous posting Er. Vikramjeet has been Explained nicety as Below:-
"L- Footings is generally required to be provided at the edge of
property line. This footing, if not connected to internal column, is
subjected to very high moment due to eccentricity of load w r t
footing centreline.
It is always preferable to connect this footing with some
internal column through a strap beam so that moment
due to eccentricity is taken care by this beam.For economy
in strap beam, it is necessary to take footing dimension
in direction of eccentricity,as less as possible,say, half the other.
"For edge footing without strap beam,it is difficult to satisfy
the no upliftment condition as redistribution is not permitted
for footings resting on soils.Even if some redistribution is
taken it may give very high value of upliftment of footing
and max pressure after redistribution is quite high(it has to be
less than SBC)
Some designers take advantage
of passive soil resistance(with some FOS) offered by the edge wall
to relieve eccentric moment but in that case edge wall shall
be strong enough to withstand this passive relief moment.
Long back I had seen a paper written by structural expert and author
Sh UH Varyani on the analysis of edge footing w/o strap
beams. This might be available in some of his books."
In Second Post said that
"in my opinion ,try to provide strap beam as per situation
given below in view of L-footing already cast without
strap beam.
i)assuming that columns are not cast above top of footing
In this situation you can provide the strap beam
connecting this column to internal column at
top of footing level .
ii)assuming that columns are cast above top of footing
upto GL/PL
In this situation you can provide the strap beam(tie beam)
connecting this column to internal column at
GL/PL but edge column reinf provided shall be adequate
enough to transfer the eccentric moment upto
strap / tie beam level. You can take advantage of
passive relief from soil(with some FOS) on edge wall on
either side of column .Also the orthogonal cross wall (if it exists)
will help. ( by way of its triangular load component beyond
footing line through dispersion which will provide counter
eccentricity)"
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vikram.jeet General Sponsor
Joined: 26 Jan 2003 Posts: 3835
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Posted: Wed Mar 02, 2011 5:24 am Post subject: Design Corner Eccentric Footing on Both Side |
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Corner Footing
As suggested by eminent members, the corner footing must have
balancing strap beams in both directions to counter the eccentric
moments .These beams must be provided at footing level itself.
Avoidance of strap beam:(costly option)
A] If plinth beams are provided tie-ing the columns in
both directions
(i) The column section of corner column below plinth level shall be
increased appreciably to take care of additional bi-axial moment s
due to eccentricities in both directions. Column Reinf shall be
worked out accordingly.
(ii) Plinth beam in each direction shall be designed for addtional
tensile force =M/Df
M = P*e
e = eccenricity= distance between CL footing and CL column
Df = fondation depth upto plinth level
This tensile force in plinth beam must have a counter force form other side
corner column, otherwise inner columns must share it.
(iii) Column above plinth beam level can have normal reinforcement
B] If plinth beams are NOT provided and roof beams are
available at roof level (i.e next floor level)
(i) The column section of corner column below roof level shall be
increased suitably to take care of additional bi-axial moment s
due to eccentricities in both directions.Column Reinf shall be
worked out accordingly.
(ii) Roof beam in each direction shall be designed for addtional
tensile force =M/(Df +H)
M = P*e
e = eccenricity= distance between CL footing and CL column
Df = fondation depth upto plinth level
H= roof height above Plinth level
This tensile force in roof beam must have a counter force form other side
corner column, otherwise inner columns must share it.
(iii) Column reinf above plinth beam level can can be slightly reduced
since addtional moment (due to eccentric) varies upto roof level.
Other reductions in eccentric moments
(i) Passive less active relief from soil on side faces
Best wishes and regds
vikramjeet
-- ญญ
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B.V.Harsoda General Sponsor
Joined: 19 Jan 2009 Posts: 2329 Location: RAJKOT,GUJARAT, INDIA
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Posted: Wed Mar 02, 2011 8:01 am Post subject: |
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Thank you Very much Er. Vikramjeet ji, Your Knowledge ia appreciable. As rightly said by Dr. NS Sir,due to certain reasons such footings not included in any book ,but considering cost of land & requirement of client, it is not avoidable at all times. In such situations we have to do some thing. It wiil be great work if we can be helpfull in this subject to needful structural Engineers. In past we have discussed several times.Now I request to Er. Vikramjeet ji to submit, Article Regarding L-footing & Corner Eccentric footings in Forum.
Thanks
&
Best Wishes,
B. V. Harsoda
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vikram.jeet General Sponsor
Joined: 26 Jan 2003 Posts: 3835
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Posted: Wed Mar 02, 2011 9:40 am Post subject: Design Corner Eccentric Footing on Both Side |
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Thank you, respected Harsoda sahab.
As informed by you , lot of discussion in this regard has been taken place on sefi.
These footings are not very un -common . IS codes do not provide any guidelines
in this regard .
Books on design/analysis do talk about provision of strap beams and their analysis.
However provision of these footings without strap beams is dealt seldom.The
analysis of L- footings is ,I think,dealt by our respected UH Varyani sahab in his book
on MS buildings which is part of his Paper in ICJ.
I have posted my recent views on L -footings without strap beams purely based
on basics.The basic idea is that eccentricity moment on footing need to be
resisted through either a strap beam (if present) /or through column itself being
tied at certain level above.
As suggested , I would be glad to write on sefi at some future date after noting
the comments of members.
with kind regards
vikramjeet
Thank you Very much Er. Vikramjeet ji, Your Knowledge ia appreciable. As rightly said by Dr. NS Sir,due to certain reasons such footings not included in any book ,but considering cost of land & requirement of client, it is not avoidable at all times. In such situations we have to do some thing. It wiil be great work if we can be helpfull in this subject to needful structural Engineers. In past we have discussed several times.Now I request to Er. Vikramjeet ji to submit, Article Regarding L-footing & Corner Eccentric footings in Forum.
Thanks
&
Best Wishes,
-- ญญ
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binaya shahi SEFI Member
Joined: 03 Jun 2015 Posts: 19
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Posted: Wed Mar 30, 2016 1:22 pm Post subject: width of strap beam. |
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One of my friends told me that the width of strap footing must be greater than the column without referring to any code or book or reasoning. I want to ask if its true coz I don't think it must be true. I have designed a strap beam of 9 inch width and 22 inch depth to connect two columns of dimensions 12"x12". Isn't my 9 inch wide strap beam allowed for 12 inch wide column?
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es_jayakumar General Sponsor
Joined: 24 Nov 2011 Posts: 1408 Location: Cochin
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Posted: Fri Dec 13, 2019 3:32 am Post subject: |
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Dear SEFIans,
Please comment on the rationality of the design of a corner column isolated-footing attached here, that had appeared in this forum at sometime. I was not convinced with the principle / logic of this design.
E S Jayakumar
Warning: Make sure you scan the downloaded attachment with updated antivirus tools before opening them. They may contain viruses. Use online scanners here and here to upload downloaded attachment to check for safety.
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DESIGN OF FOOTING FOR CORNER COLUMN.doc |
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