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Teng's curve
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bijay sarkar
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PostPosted: Mon Jun 27, 2011 10:34 am    Post subject: Reply with quote

Dear Er. Jignesh V Chokshi,

You can decrease the number of iterations as follows :

(i)  I think that there is no need to converge P for locating neutral axis. For locating final NA, convergence of eccentricities ex (=Mx/P) and ey (=My/P) are only required. Thus iteration numbers can be decreased. After determination of NA location, P is to be used for maximum pressure calculation.

(ii) During second look on the Teng's Curves, I deduced all the general equations for the curves of all cases and made a separate computer program for factors K for all cases which are at par with Teng's Curves. I also feel that any shaped foundations can be analysed by these general equations with some required modifications.

regards,

bijay sarkar
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Bilal Lakdawala
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Joined: 04 Apr 2011
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PostPosted: Sun Aug 14, 2011 7:21 am    Post subject: Teng's book available for download Reply with quote

Dear All

I had find the said Teng's book on net which can be downloaded following the link:

http://www.4shared.com/get/3Hyv_YFS/Foundation_Design_-_Wayne_C_Te.html
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mtamil
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Joined: 07 Apr 2011
Posts: 187

PostPosted: Mon Mar 26, 2012 5:06 am    Post subject: Reply with quote

Dear Sefians & Mr. Bijay Sarkar,

It is interesting to note that lot of work has been done on this subject by the sefians. For some time I was searching for methods to design such foundations carrying large biaxial moments and loss of contact.

Right now I am in the process of developing a comprehensive method to deal with such foundations.

The discussions above thrown lot of light to the subject.

May I request Mr. Bijay Sarkar and others to share their work, which will be of great help.

Thanks in advance.
Tamilarasan
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bijay sarkar
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PostPosted: Mon Mar 26, 2012 2:20 pm    Post subject: Reply with quote

Dear Mr mtamil,

I have written a paper on the subject matter and wish to publish it as a paper in any journal. But i am doubtfull whether any journal authority will agree to publish it, as because there are 25 pages in total deducing all the equations for each of the cases (Case II, Case III, Case IV & Case V). Computerisation of the method is also described. Someone may please intimate whether such a paper are acceptable by any journal ?

With Regards,

Bijay Sarkar
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mtamil
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PostPosted: Mon Apr 02, 2012 9:22 am    Post subject: Reply with quote

Dear Mr. Bijay Sarkar,

It’s a very good idea to present a paper on the subject, indeed. I am not sure whether 25 pages will be accepted on not, But I am sure, the subject has the content. Avoiding lengthy derivations may reduce the volume.

I have already developed a MS Excel sheet (+VBA) for design of Rectangular Isolated Footings. The method adopted is iterative & straight forward. This sheet can analyze footings for Base Pressure distribution, Max BMs, SFs, Punching Shear, Overturning & Sliding irrespective of the footing being concentric/ eccentric;  Full contact /Partial contact; Uniaxial/Biaxial. The sheet is being put to use in our design office.

I have searched for analytical solutions for footing with ‘loss of contact in both directions’, mainly to validate the results obtained from the Excel sheet.
Of course, Teng is the natural choice but need to read the curves.

I have found solutions for such foundations in the book "Reinforced Concrete:  Analysis and design – S.S.Ray-Blackwell Publishers". Unfortunately, derivations are not available, in this book.

Best wishes & looking forward for your paper.

Tamilarasan
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mtamil
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PostPosted: Fri Apr 06, 2012 7:17 am    Post subject: Reply with quote

Dear Sefians

The following papers will help in understanding analysis of footings with uplift or partial contact.


1. Analytical Approach to Biaxial Eccentricity - by Eli Czerniak
Journal of the Structural Division, Proceedings of the ASCE (1962)


2. Bearing Pressures for Rectangular Footings with Biaxial Uplift - by Kenneth E. Wilson
Journal of Bridge Engineering -Feb. 1997

We can also derive an anology between reinforced concrete columns subjected to biaaxial bending - post cracked behaviour. The subject is delt in detail in the book Reinforced Concrete Strucutral Elements : Behaviour, Analysis & Design by P. Prurshothaman - Page No. 270.

I have employed Iterative proceedure ( bit heavy on the processor).

Best regards

Tamilarasan
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sukanta.adhikari
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PostPosted: Sat Oct 06, 2012 5:56 am    Post subject: Re: Teng's curve Reply with quote

The discussion on Tengs chart was very interesting...I want to know how much percentage of tension is allowed below footing.Is there any supporting document for  the same.

Regards,
S Adhikari
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mtamil
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PostPosted: Sat Oct 06, 2012 11:41 am    Post subject: Reply with quote

Dear Er. S. Adhikari

Overturning to be checked In the case of footings with partial contact,
 
In most cases the allowable bearing capacity will govern the design, as the base pressure at one end of the footing is high and the other side pressure being zero. If bearing capacitiy is not governing the design then theoretically, we can reduce the footing size, so far as the FOS against overturning is fulfilled.
 
FOS against overturning for footings subjected to uniaxial moment shall be
 
FOS_ot = L/2e ; where e = M/P
 
As we know,
 
FOS_ot is infinite when e =0  (No moment)
and
FOS_ot = 1 when e = L/2 (Footing is in the wedge of overturning)
 

Values of e and FOS_ot are listed below.


e = M/P
FOS against overturning
% contact
Remarks
L/6
3
100%
Min base pressure is 0.
L/5
2.5
90%
10% loss of contact
L/4
2
75%
25% loss of contact
L/3
1.5
50%
50% loss of contact
L/2
1
0%
Footing overturns


Some project specifications allow upto 30% loss of contact (means that FOS_ot = 1.88 ). But anything above 50% loss of contact will violate FOS 1.4 as stipulated in IS 456.

In the case of footings subjected to biaxial moments, FOS_ot shall be calculated (by finding combined eccentricities & point of rotation) and to be satisfied.

It is also evident that the base pressure will be very high at large eccentricities surpassing bearing capacities in normal soils (except hard rock).

Regards
Tamilarasan
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vikram.jeet
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PostPosted: Mon Oct 08, 2012 5:15 am    Post subject: Teng's curve Reply with quote

Admire the content of this posting from Er Tamilrasan. Very useful analysis.

IS codes are somewhat silent on extent of upliftment (loss of contact ) of footing.

As per IRC -78 on Bridge foundations
NO Upliftment is allowed for bridge foundations on soils
For foundations on rock - - - -
20% upliftment is allowed under Normal cases
33% upliftment is allowed under Seismic cases

For structures like Boundary walls , Steel Shed Foundations etc the wind is the  
governing force and upliftment of footing needs to be allowed for sizing the  
foundations economically . No tension(No upliftment ) leads to very large  
sizes/ greater foundation depths.

Maximum base pressure needs also be seen after redistribution of base pressure
when footing is uplifted by certain %.

pmax = 2*P/( B* x )

x= 3*(L/2-e)

L= Length of footing along which BM act on footing
B= width of footing in other dir
P= vertical load
e=M/P




Posting from Er Tamilrasan  

Values of e and FOS_ot are listed below.




e = M/P
FOS against overturning
% contact
Remarks
L/6
3
100%  Min base pressure is 0.
L/5
2.5
90%
10%$B!!(Bloss of contact
L/4
2
75%
25% loss of contact
L/3
1.5
50%
50% loss of contact
L/2
1
0%
Footing overturns




Some project specifications allow upto 30% loss of contact (means that FOS_ot = 1.88 ). But anything above 50% loss of contact will violate FOS 1.4 as stipulated in IS 456.

In the case of footings subjected to biaxial moments, FOS_ot shall be calculated (by finding combined eccentricities & point of rotation) and to be satisfied.

It is also evident that the base pressure will be very high at large eccentricities surpassing bearing capacities in normal soils (except hard rock).

Regards
Tamilarasan  

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sukanta.adhikari
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PostPosted: Tue Oct 09, 2012 8:42 am    Post subject: Re: Teng's curve Reply with quote

Dear Vikramjeet sir,

Will not the type of the structure also decide how much extent of uplift is to allowed...

I raised this question as you rightly said that IS code is silent on this aspect.

Regards,
S Adhikari


vikram.jeet wrote:
Admire the content of this posting from Er Tamilrasan. Very useful analysis.

IS codes are somewhat silent on extent of upliftment (loss of contact ) of footing.

As per IRC -78 on Bridge foundations
NO Upliftment is allowed for bridge foundations on soils
For foundations on rock - - - -
20% upliftment is allowed under Normal cases
33% upliftment is allowed under Seismic cases

For structures like Boundary walls , Steel Shed Foundations etc the wind is the  
governing force and upliftment of footing needs to be allowed for sizing the  
foundations economically . No tension(No upliftment ) leads to very large  
sizes/ greater foundation depths.

Maximum base pressure needs also be seen after redistribution of base pressure
when footing is uplifted by certain %.

pmax = 2*P/( B* x )

x= 3*(L/2-e)

L= Length of footing along which BM act on footing
B= width of footing in other dir
P= vertical load
e=M/P




Posting from Er Tamilrasan  

Values of e and FOS_ot are listed below.




e = M/P
FOS against overturning
% contact
Remarks
L/6
3
100%  Min base pressure is 0.
L/5
2.5
90%
10%$B!!(Bloss of contact
L/4
2
75%
25% loss of contact
L/3
1.5
50%
50% loss of contact
L/2
1
0%
Footing overturns




Some project specifications allow upto 30% loss of contact (means that FOS_ot = 1.88 ). But anything above 50% loss of contact will violate FOS 1.4 as stipulated in IS 456.

In the case of footings subjected to biaxial moments, FOS_ot shall be calculated (by finding combined eccentricities & point of rotation) and to be satisfied.

It is also evident that the base pressure will be very high at large eccentricities surpassing bearing capacities in normal soils (except hard rock).

Regards
Tamilarasan  

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