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COUNTERFORT RETAINING WALL
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es_jayakumar
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PostPosted: Tue Jul 30, 2013 1:41 am    Post subject: Reply with quote

Yes. It is clearly understandable in the STAAD model attached by me. Take "Plate Stress" option from tool bar and study the nature of variation of Mx and My from the coloured contours of BM displayed. Double clicking any plate and selecting Center stresses / Corner stresses will bring the BM values. By this method only I tabulated the values (not attached) and then drew the graph of variation of vertical BM, that I had attached.

It goes without saying that in any slab, the fixed edge behaves as cantilever to some distance. ie, develops hogging BM. In the case of the stem of a counterfort wall, the three edges except the top behaves as cantilever. To how much distance does the cantilever effect extend in such complex (irregular) structures, can be studied from the FEM modeling.

E S Jayakumar
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thirumalaichettiar
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PostPosted: Tue Jul 30, 2013 5:15 am    Post subject: Re: COUNTERFORT RETAINING WALL Reply with quote

cckeshav wrote:
Dear SEFIans:

FEM is based on elastic theory, and hence comparison with yield lines and yield moments are not relevant. Secondly, very few design retaining walls by FEM. The amount of data to be processed makes is impractical. Thirdly, we may have to make a study on mesh size (Please note that even FEM is approximate!) suitable for every problem, as the same depends on the height to width ratio of walls. Fourthly, the FE discretization pattern for shells is tricky (if done manually) and also the interpretation of results. You do need some expertise to do the same as one has to watch for two items, direction of shell normal (whether inword or outword) and shell local directions so that you can interpret the result of moments (Mxx, Myy, etc are in local directions for most FE output). The contours will be misleading if the discretisation of shell elements is not consistent (the local directions many not be same in adjacent elements).

FEM, can not be used as a black box. It is not easy to check the results of FE analysis. The only way a FE result can be checked is using another software and performing independant meshing and data specification. However, classical procedures (commonly used procedures as prescribed by codes of practice and given in text books) give a good idea about the expected results. Again I would like to ask that wherein the code of practice it is mentioned that FE can be used for design of retaining walls or flat slabs? We should not demean the knowledge or the capability of the authors of text books on design of structures (even today the text books both in India and abroad provide the same basic principles).

The most modern and ultimate in analysis would be a full 3D analysis with solid elements, particulary so with reinforced concrete structures as the three dimensions are quite comparable. No software incorporates features for conventional design using 3D solid FE analysis. There are limitations in terms of software, in terms of computer resources, and in terms of design methodologies. What would one do with the stress results obtained from 3D analysis? Can we design RC beams, and Columns with such results? What happens to modeling reinforcements? Even if the reinforcements are modelled, does the results help in design? Our design methodology relies on the fact that concrete cracks in tension and reinforcement takes all the tension. Till such time, the stress in reinforcement is quite small. Only a full nonlinear Finite element analysis which considers cracking of concrete and possibly the nonlinear shape of the compressive stress strain curve can consider these factors. As such, doing a modern and state-of-the art analysis is very subjective.

In conclusion I would like to state that 1. Time tested approximate procedures can not be ignored. For all practical purposes most retaining walls can be safely designed using those methods provided one understands the limitations and makes suitable reasonably conservative assumptions. For example base shear in walls can be still computed using the cantilever assumption whether in counterfort (bottom fourth) or cantilever retaining walls (full height) Side shear in counterfort retaining can still be calculated and all these results match (albeit conservatively) with FE results. 2. Use of FEM (shell elements) for normal routine retaining walls may not be practical. Particularly, for the base slab, we use conventional SBC based procedures which is also used in design of Isolated footings. In FEM it is only possible if winkler model is used along with trials to fix the base width. 3. Commercial softwares should not be used blindly without an understanding of its limitations (All commercial softwares hide behind a standard Disclaimer clause which request the user to verify the results before use). 4. Exact analysis is not always exact because of the of variability in load and material property data for soil and foundations and even concrete.

Dr. C.Channakeshava


Subject: [SEFI] Re: COUNTERFORT RETAINING WALL
From: forum@sefindia.org
Date: Wed, 17 Jul 2013 16:06:04 +0530
To: general@sefindia.org

           many easy assumptions lead to the so called "standard design practices" and reproduced in many handbooks and so called text books of design written mostly by those who never designed themselves. The assumption under question depends on the theory that yield line bisects the angle between slab and the supporting beam (normally 90 deg for ordinary structures but anything in industrial buildings like transfer tower, conveyor floors). The two yoeld lines would then intersect above the base and load on this bottom triangle will be experienced by the bottom edge. while the side edges will experience the trapezoidal figures formed by yield lines. Since top edge is free in counterforted retaining wall, distribution of load is to be studied first. I like to mention that aspect ratio of the wall segment (height/width) will play an important role on magnitudes of bending moment, which was not taken care in past day assumptions.

I strongly suggest that to determine stress resultants in such structures one should go for FEM or FDM in analysis. I have prepared algorithm for various conditions of grillage structures of which 3 edges fixed rest free is for counterforted retaining walls.

Lastly, those who are supporters of past day methods, can say how base shear will be determined? or shear stress is not to be calculated in counterforted retaining walls? We must remember that in early days there was not much importance of shear stress in slabs and walls, interaction between volume of reinforcement and permissible shear stress was unknown, denting action due to shear was totally unknown before 70s.

worshiping past days may give relief in office work but leads to a psychological disease called NOSTALGEA


On Tue, Jul 16, 2013 at 6:42 PM, Akanshu Singh forum@sefindia.org)> wrote:
Quote:
            What i will suggest is that you do a finite element analysis that will help you understand the exact behaviour of the retaining wall you are working on, though it is a common practice of designing the bottom quarter as a cantilever(which will be evident from the moments you get in your analysis). But rather then working on the basic rules, with the availability of softwares one should study the true behavior of the structure.


On Fri, Jul 12, 2013 at 7:40 PM, cckeshav forum@sefindia.org (forum@sefindia.org (forum@sefindia.org)))> wrote:
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I agree with Dr. C.Channakeshava's comment "Commercial softwares should not be used blindly without an understanding of its limitations (All commercial softwares hide behind a standard Disclaimer clause which request the user to verify the results before use).
"

T.RangaRajan.
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es_jayakumar
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PostPosted: Tue Jul 30, 2013 5:47 am    Post subject: Reply with quote

Dear Sirs,

How to get access to a non-commercial FEM package and learn its applications and operation ?  It will be really fantastic.
We have studied FEM for our M.Tech. course in theory only (way back, during 1991) and did not get any hands on experience.

E S Jayakumar
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purushottam145476
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PostPosted: Tue Jul 30, 2013 5:56 pm    Post subject: COUNTERFORT RETAINING WALL Reply with quote

sir can you tell me the thumb rule for designing bean column and slab . how to calculate faster . i m a fresher so wanted to know.


On Sun, Jul 28, 2013 at 10:03 AM, vnacharya <forum@sefindia.org (forum@sefindia.org)> wrote:
[quote]            In general, bending behavior of stem will be, like a cantilever at bottom level (about 1/4th height from bottom), and there after it will be two-way bending.

This behavior can be observed if the whole structure is modeled in FEM.


Regards,

Vishnumoorthi



On Wed, Jul 10, 2013 at 5:56 PM, ramkumark5 forum@sefindia.org (forum@sefindia.org))> wrote:
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purushottam145476
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PostPosted: Wed Jul 31, 2013 2:26 am    Post subject: COUNTERFORT RETAINING WALL Reply with quote

sir can you  answer my question plzzzzzzzzz


On Wed, Jul 31, 2013 at 7:14 AM, purushottam145476 <forum@sefindia.org (forum@sefindia.org)> wrote:
Quote:
           sir can you tell me the thumb rule for designing bean column and slab . how to calculate faster . i m a fresher so wanted to know.



On Sun, Jul 28, 2013 at 10:03 AM, vnacharya forum@sefindia.org (forum@sefindia.org))> wrote:
Quote:
           In general, bending behavior of stem will be, like a cantilever at bottom level (about 1/4th height from bottom), and there after it will be two-way bending.

This behavior can be observed if the whole structure is modeled in FEM.


Regards,

Vishnumoorthi




On Wed, Jul 10, 2013 at 5:56 PM, ramkumark5 forum@sefindia.org (forum@sefindia.org) (forum@sefindia.org (forum@sefindia.org)))> wrote:
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es_jayakumar
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PostPosted: Thu Aug 01, 2013 12:53 am    Post subject: Reply with quote

Your question, I am afraid, is neither pertaining to the subjected topic nor carry enough clarity as to what information you seek from SEFI.
Do your postings in a formal manner, with enough punctuations.

E S Jayakumar
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Rajesh123
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PostPosted: Thu Aug 01, 2013 4:41 am    Post subject: Reply with quote

es_jayakumar wrote:
A study on the variation of vertical bending moment along the height of the stem of a counterfort retaining wall is made using STAAD plate modelling and is attached, with my comments.

E S Jayakumar


Dear Sir,

Sorry for delayed reply. I am still having doubts.

Support condition 'Fixed' in staad is assumption based or imaginary condition. Term fixed depends on amount of fixidity.

In CF wall design, the bottom portion of stem wall is connected to CF of higher depth, whereas top portion of stem wall is connected to CF of lower depth. So, is it correct to adopt same fixed condition throughout stem wall.

Even in codes and text books, the moment factors are given based on three sides completely fixed condition. But in actual, fixidity of stem wall is increasing from top to bottom due to triangular profile of CF.

Seniors please correct me, if i am wrong???


Regards,


ramkumark5
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es_jayakumar
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PostPosted: Thu Aug 01, 2013 7:01 am    Post subject: Reply with quote

Dear Rajesh,

In any design, we are compelled to make some assumptions. In the study I made, I have considered an intermediate bay of a long CF wall. In this case, we can assume fixity over the three sides with fair degree of accuracy. If I had considered the end bay, assuming complete fixity along the three sides may turn erroneous.
Now, fixity is fixity. We arrest all the 6 degrees of freedom at a node. The CF is less stiffer at top agreed. Will the same impart any "softer"  fixity to the stem ? In the case of the end CF and stem, of course, the stiffness of the CF can play some role in the fixity, as the stem can rotate more freely at top compared to bottom. Such minute points are not worth considering in general design.

E S Jayakumar
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Ramakrishna
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PostPosted: Thu Aug 01, 2013 10:23 am    Post subject: Reply with quote

Dear Er. Rajesh,

Pl read teh book, Reniforced design of concrete structures" by Pillai and Menon. You may get a fair idia of the behavior aspects of a counterfort retaining wall
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vaisakhg
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PostPosted: Fri Aug 02, 2013 6:49 am    Post subject: COUNTERFORT RETAINING WALL Reply with quote

Dear Mr. Purushottam
My sincere advice is not to go for thumb rules in the beginning. Concentrate on designing based on standard design procedures and codes of practice.  
Regards,
Vaisakh G
Assistant Professor in Civil Engineering
College of Engineering Kidangur.
http://vaisakhg.blogspot.com/
Phone. +91 9656840794 On Jul 31, 2013 4:46 AM, "purushottam145476" <forum@sefindia.org (forum@sefindia.org)> wrote:
Quote:
           sir can you tell me the thumb rule for designing bean column and slab . how to calculate faster . i m a fresher so wanted to know.


On Sun, Jul 28, 2013 at 10:03 AM, vnacharya forum@sefindia.org (forum@sefindia.org))> wrote:
Quote:
           In general, bending behavior of stem will be, like a cantilever at bottom level (about 1/4th height from bottom), and there after it will be two-way bending.

This behavior can be observed if the whole structure is modeled in FEM.


Regards,

Vishnumoorthi



On Wed, Jul 10, 2013 at 5:56 PM, ramkumark5 forum@sefindia.org (forum@sefindia.org) (forum@sefindia.org (forum@sefindia.org)))> wrote:
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