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Why do designs by different structural engineers vary so gre
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uhvaryani
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PostPosted: Mon Sep 21, 2009 1:23 pm    Post subject: Why do designs by different structural engineers vary so gre Reply with quote

Dear Sefians,
In new colonies in cities,similar buildings are being designed by different
structural engineers with varying results.Say,eight storeyed flats are being
constructed with different foundation systems,varying designs of retaining
walls,different column sizes and steel arrangements and different floor systems.
This is baffling to clients who may be mostly
non-engineersWe,structural engineers,
follow the same system of structural analysis and also follow the same
BIS codes which are mandatory.
Then why should the design-results vary?I request all sefians to
please ponder over this question
and find a satisfactory answer.
with best wishes and regards,
uhvaryani

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vikram.jeet
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PostPosted: Thu Sep 24, 2009 10:15 am    Post subject: Why do designs by different structural engineers vary so gre Reply with quote

Sh UH Varyani ji has touched the very delicate issue. I think reason
for marked variations in design by the different designers( for buildings
in same area )are many:

i) Soil reports being from different agencies which recommend the foundation
systems differently based on their tests

ii) Soil strata can vary leading to different foundation system

iii) Basement or no basement makes lot of difference

iv) Architects for various buildings being different .Buildings are  
planned to suit their whims and fancies and column sizings ,in particular

v) Experience of structural designer is quite important. Less experienced
may choose the arrangement differently leading to variations

vi) Extent of supervision : Poor supervision may compel the designer
to use simpler detailing(without curtailments of bars),shear reinf etc

vii) Ease of construction :S/ designers may give preference
to this and adopt such shapes/forms ---easy to construct
eg Uniform thickness footings instead of Sloped footings  
Columns of same size(or two to three) to have less forms
etc

viii)Designers liking:Designers also have their fancies like architects.
some may prefer large and thick but beamfree slabs whereas others
may like to add beams to divide into small span panels. some may
like to go for ribbed slabs.Like wise for other arrangenments.

There are many more reasons and therefore it may not be possible
to have unanimity on structural arrangement.However ,I think all
SE's are CODE FEARING and designs are done as per standars
of the LAND.

With kind regads to sh UH Varyani ji who is definately a souce of  
inspiration for all SE's at forum

vikramjeet

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ibarua
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PostPosted: Thu Sep 24, 2009 11:19 am    Post subject: Why do designs by different structural engineers vary so gre Reply with quote

24th September 2009

Dear Varyani,

The structural design you do for an eight storied building in Delhi will obviously be different form the one I design in Guwahati. You may use a MR frame system without shear walls, while I, in Zone V,  will more or less be forced to use shear walls. And, since I'm using shear walls, I may be inclined to go in for a flat slab system rather than a beam-slab system. You being in Delhi with easy access to RMC may prefer to use PSC slabs with M35 grade concrete, while I, in backward Guwahati will have to contend with M20 concrete and Fe415 bars. Delhi's soil may be more firm than the soil out here -- that's why I'll have to use a raft while you can make do with individual footings.

What I'm trying to say is that there are too many variables in our business that preclude rigid standardization.

Even in the case of bridges, the superstructure design may be standard for a given span, number of lanes and live load, but the design of the substructure and foundation will vary depending on the particular location.

This is why we engineers must agree to disagree.

Regards & best wishes.

Indrajit Barua.    

On Thu, 24 Sep 2009 uhvaryani wrote :
Quote:
Dear Sefians,
In new colonies in cities,similar buildings are being
designed by different
structural engineers with varying results.Say,eight
storeyed flats are being
constructed with different foundation systems,varying
designs of retaining
walls,different column sizes and steel arrangements and
different floor systems.
This is baffling to clients who may be mostly
non-engineersWe,structural engineers,
follow the same system of structural analysis and also
follow the same
BIS codes which are mandatory.
Then why should the design-results vary?I request all
sefians to
please ponder over this question
and find a satisfactory answer.
with best wishes and regards,
uhvaryani








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Dr. N. Subramanian
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PostPosted: Thu Sep 24, 2009 1:54 pm    Post subject: Why do designs by different structural engineers vary so gre Reply with quote

Dear Shri. Er.Varyani,

You have written an article on this topic in CE& CR.

Though the analysis may produce the same results, the design need not be the same. It is because analysis is scientific and design is creative and may be considered as an art.(although the design steps may be scientific). Some designers like Calatrava or Gehry go to the extreme and produce structures which are not even imagined by many others.

But if the structural form is fixed, e.g. in a typical multi-storey building with beam, slab and column arrangement, the designs will not vary much. Here again, the designers can choose different column layouts and shape of columns, different foundation systems(piles, rectangular,square, or combined footings, raft, etc) some may choose one way slab, another may choose two-way slab, etc.

Regards,
Subramanian

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

Maryland, USA

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





--- On Thu, 9/24/09, uhvaryani <forum@sefindia.org> wrote:
Quote:

From: uhvaryani <forum@sefindia.org>
Subject: [SEFI] Why do designs by different structural engineers vary so greatly
To: general@sefindia.org
Date: Thursday, September 24, 2009, 12:23 PM

Dear Sefians,
In new colonies in cities,similar buildings are being designed by different
structural engineers with varying results.Say,eight storeyed flats are being
constructed with different foundation systems,varying designs of retaining
walls,different column sizes and steel arrangements and different floor systems.
This is baffling to clients who may be mostly
non-engineersWe,structural engineers,
follow the same system of structural analysis and also follow the same
BIS codes which are mandatory.
Then why should the design-results vary?I request all sefians to
please ponder over this question
and find a satisfactory answer.
with best wishes and regards,
uhvaryani
     



     



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PostPosted: Thu Sep 24, 2009 2:40 pm    Post subject: Reply with quote

It is very difficult to produce same design & detailing by two different engineers. We face this problem usually in our office also. specially when there are typical floors. One floor designed by one engineer and another by other will be different with bar combination, curtailment length etc. Ofcourse both are correct!
Yogesh
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suraj
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PostPosted: Thu Sep 24, 2009 2:56 pm    Post subject: Designs differ Reply with quote

Dear All,

Very good question raised by our respected veteran professional that should have a brainstorming discussion I suppose.
Old saying still works that :
No two doctors give the same prescription
&
No two engineers give the same drawing


Building construction is an art by definition in its entirety as envisaged & taught for decades. Many of my friends may not agree that structural engineering is also an art as it is considered based on pure scientific calculations & majority of the engineers world wide call it scientific work.

There was one era when the engineers used to do all activities of the buildings themselves & these were duly regarded. There was no architect statutory designation in our land till 1972.

RCC scenario has changed the entire architecture of the buildings. The brick or stone masonry have been kept on the sequential footings. Structure generates another architecture that cannot be conceieved by an architect I call it structural architecture in addition to aesthetic architecture & both can be married together .

There has been non harmonious working policy between the two professionals devoted to the erection of buildings. Both work on varying approaches. The architect adopts the policy of the author of the designs while the structural engineer is deemed as a support only.

I am sometimes surprised when some good architect after certain professional working failure try to reach some experienced engineer for solution of his trapped building. I have experienced many times with my friend architect to see me any time when the calculations & the solutions to the extension of the existing buildings were required.

I used to give some solutions for example combined footing for constraining boundary conditions & by hard will, the architect complied with as if, this was something miraculous for the site contractor who had never experienced in his life about such reinforcing detailing. Anyway, there was a problem for them & they agreed to do accordingly.

Now the architects are generally educated & they know the value of designs, yet due to giving away a part of fee, they try to avoid the services of the engineer & repeat the previous designs by applying certain variation on the designs. Previously, these gentle professionals used to draw services of certain government civil engineers for a fair designs in few thousand rupees.

Back to the point regarding variations on the designs on various buildings having the common spanning etc, there would be many reasons. As far as a customer is concerned, he may be surprised of course, to see the varying degree of designs.

When we see buildings constructed by the plot owners, it is noticed that very normal columns 225 x 225 mm are used post activity of the brick walls. Rebars 4 to 6 are used in those columns & these go continuously for all levels. This arrangement by the lay contractor keeps the owners happy. When I talked to some neighbour about the quantity of rebars in foundation of my construction, I told that foundation beams have been provided with 6 tonnes steel. He was surprised & told that he had complete his kothi/house in 6 tonne rebars. I explained to him that the span I have designed is more than 11 m & there is basement as well as a water tank below the basement. He was confused.

There is no competent soil investigation approach adopted by the designers particularly architects as well as the profit making  builders or promoters. All builders know the type of soil in a particular area & the experience on previous buildings is used for the new buildings.

Regretful to say that in general, the architects & engineers are not well equipped to construe the report, if anyhow, it is available in details. The geotechnical engineer of the investigating lab may mix up the proposal advise. There may be many factors to his understanding  that could be based on further assumptions in case, the complete information about the project is not given.

Full range soil report may not be ordered to be conducted or only bearing capacity & the settlement criteria would be requested.

Full range investigation costs money & time while the promoter always likes to expedite the project to prove to the investors that the construction is in progress & the public booking progresses fast. The dead lines are declared/milestoned impractical & then the public cries if that is not met.

Nowadays, the practice of including isolated footings is getting obsolete day by day the reason being, the towers are getting constructed in the urban areas & the suburban cities.

Generally, all buildings are more than 4 to 8 levels that do not require the isolated footings or the combined footings. For various reasons, an understanding is working that the plinth beams work well to resist the seismic force, the people believe in that & these are to be provided in any case.

Then the design would definitely alter & lead to a better option in place of independent footing, why not go for a beamed raft? It is an economical proposition in comparison to independent footing keeping in view the general spans of the building which I think do not in common exceed 5 m.

There is another flaw in the thought by the architects, the customer & the promoter to use the higher dia rebars to prove that the structure is too strong & the people are convinced. In case, some SEs recommend 12 mm dia, it is taken as a surprise. When I use 8 mm dia rebars for slabs, the customer, the steel fixer & others take for a surprise & advise that they would not be responsible for any mishap. All what I have told in foregoing, have experienced in my designs practice.

The best approach I understand is that the constructability factor must be taken care of by SEs. Architect intends to see the walls flush with the columns & the same is preferred by the owner but it is not feasible to apply on a good structure, I am surprised to see the resistance by the people to see the columns offset walls & the beams seen in the ceiling. On top of these requirements, the vastu shastra pundits add more fuel to keep the owners at bays & I have seen people changing the complete setting out of the constructed structure.

I was irritated by my real family members who ask me irrelevant questions about their desire to see no structural member by eyes. Vastu shastra is also one that has misrepresented the whole vedic science just for making monies, of course, being a good deal for a green house but, we find in that an ordinary user should not use stone in a house. This means one cannot make a house but a hutment only while this is all misrepresentation by semi qualified pundits.

A beam in the ceiling area adds negative energy in the house that may affect the thought & prosperity. Any columns seen in the covered area would not work positively. There are so many thoughts & principles that would irritate any structural designer. Odd number steps are not acceptable. Stair should be oriented from North ingress to south landing & then to north floor landing. The architects easily support all these & the whole concept is altered causing problems to SEs that makes the simple design a bit complex.

Smaller RCC sections are uneasy to pour & engineer faces the problem on the pouring of concrete. Higher size bars pose the fabrication problems. Constructability of the design is not bothered by any professional & those who bother, adopt moderation on the members. The member should be spacious adequately to include covers, the rebars & keep the pour duly vibrated without any chance of honeycombing or voild formation that were explained in other posts by many writers.

The designs vary due to the considerations of practicality. The average cost of RCC in NCR is about 11000 rupees per cum in case the formwork is repeated to 5 or 6 times. The fear that thicker columns would add to the budget, makes a diferrence. The people are happy to see 25 mm bars in columns & 16 mm in beams while 10 or 12 mm in slabs. They do not bother about our technicalities or the limit state requirement. The rebars used are more than 40 % useless. '

Due to the misunderstanding of the engineering concept, lack of experience & the cost factor alongwith profit making, converge to the varying structural productions. Owners do not know about the bending moment, shear force, deflection or sway, who shall explain to them? sure architect cannot of course? Commercial exercise overweighs technical & engineering & the role of the manager/MBA plays its game art on the concept of reality.

An engineer designing smaller sections is considered a good one irrespective of the structure may fail.

China company constructed RCC 200 m tall chimney failure as broadcast on 24.9.2009, working for Bharat Aluminum, is a live example of worst structural making.

Engineers have to lead themselves for a constructive approach & should involve in educating the public otherwise, we shall only talk among ourselves. Public knows only 6 rods as they call it in column. They know only chowkees for the foundations. They do not know the purpose of columns & beams. These fundamentals must be reached to the national public otherwise, we are not doing our duty to the nation.

The public safety must be taken care of. The buildings have been falling for years together in ours land. The next day, it is forgotten. Failure of any building is part of real safety other than HSE. The lives of the community cannot be played with.

The misuse or abuse of the factor of safety in the material design management must be avoided. the promoters must be made understood that the purpose of this factor is not what it is considered. Consideration when a building is standing, is a safe one is a misconcept that must be clarified to all concerned. Designs may vary from area to area but in one area, depending on the spannig & height of building, a uniformity should exist that is not working currently. I intend to stop at this point for a future continuation with the brainstorming/chintan of the topic.

Thanks indeed for this significant posting.

Reagrds

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Thanks & Warm Regards
IntPE(India)Suraj Singh FIE Civil
Engineering & Arbitration



Last edited by suraj on Thu Sep 24, 2009 6:12 pm; edited 1 time in total
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PostPosted: Thu Sep 24, 2009 3:19 pm    Post subject: Reply with quote

What are the chances that atleast column sizes and location will be same for SAME archtitectural plan given to two different engineers? site condition, soil condition being same but engineer may be at different loacation... Variation will increase for moderate seismic zone like III or IV where designer have option to go for shear wall or to avoid shear wall. One may go for different spacing of columns which will differ the results. I think once columns are different then the entire building will be different.
Yogesh
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PostPosted: Thu Sep 24, 2009 6:40 pm    Post subject: Design Symmetry Reply with quote

Eng Yogesh,

The point is stressed again about what the relation between the architect & engineer exists? There is a possibility should the engineer do the complete design self. But, virtually, the working reality is entirely different. For major project, it is the duty of an architect to work in complete coordination with the structural engineer for the purpose of structural mechanism composition / formation preliminarily but it works rarely. The architect would definitely impose his imagination on the SEs design & the design vary accordingly. No building can be same unless repetition for some housing etc. is conceived. Provision of land use & other factors dicatate the building designs & all the disciplinary requirements are to be met. Planning for all levels may vary & even, on typical floors, some varying inclusions do occur. Loadings may vary floorwise. Elevators, stairs & other services provisions may vary the designs considerably. Spanning may be affected due to these reasons. All simplicity of concepts would go away after having considered all these factors. The concept of the engineer & the other engineers for example, mechanical one would impact the designs. Even then, a similarity can work & be inducted among the framing constitution of the skeleton. Engineer must try his best to keep the dimensions of the columns & beams in safe range simultaneously, without adversely impacting aesthetically the internal as well as external architecture. A point must be kept in view when formulising the frames for calculations that there would be varying impacts by the architectural aesthetic imposition on the structural revealed architecture. How the design allows doing the preparatory works faster, is also a major requirement for a better constructable design? The wastage of the formwork material should also be kept in view while considering dimesnions. An example that 1200 x 2400 ply should be consumed by 300/400 widths or 600 etc. One time Casting height for the column should also be considered that may be 3000 mm or sometimes 4000 mm. Similarly, the downsatnd & soffits of the beams can work. A very good design turns bad at site if it cannot be carried out conveniently by the site personnel. If there are unrequired compexities, these should be removed at the design stage to avoid possible technical queries at the execution stage.

Regards

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PostPosted: Thu Sep 24, 2009 11:49 pm    Post subject: Why do designs by different structural engineers vary so gr Reply with quote

Quote:
It is because analysis is scientific
We analyse a mythical structure as the modelling of an actual structural system has too many variations and analysis can be called scientific only for that particular assumed model.

For example, if the building is analysed as a bare frame by some and others consider all other structural elements, the results are bound to be different.
The earlier 1893 versions have been misused to obtain a very flexible fundamental period and correspondingly lower base shear. In order to have a level playing field, the 2002 version has the provision of minimum base shear irrespective of the modelling adopted.

Interestingly, bare frame models will now [2002 version] give most conservative results of shears and moments in the columns.

Experimentation on actual buildings to determine its frequencies would indicate how poor is the modelling.

We have now introduced a lot of fancy modelling since software is available - it does not mean that we are more scientific or the older analysis is flawed.

We cannot model a horse but only the donkeys of different hues.

Wonderful structures have been indeed built in pre-software era

The previous generation may be shy in informing how they checked a design. Thumb rules should be still valid.

ARC

On Thu, Sep 24, 2009 at 10:36 AM, drnsmani <forum@sefindia.org (forum@sefindia.org)> wrote:
Quote:
           Dear Shri. Er.Varyani,

You have written an article on this topic in CE& CR.

Though the analysis may produce the same results, the design need not be the same. It is because analysis is scientific and design is creative and may be considered as an art.(although the design steps may be scientific). Some designers like Calatrava or Gehry go to the extreme and produce structures which are not even imagined by many others.

But if the structural form is fixed, e.g. in a typical multi-storey building with beam, slab and column arrangement, the designs will not vary much. Here again, the designers can choose different column layouts and shape of columns, different foundation systems(piles, rectangular,square, or combined footings, raft, etc) some may choose one way slab, another may choose two-way slab, etc.

Regards,
Subramanian




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uhvaryani
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PostPosted: Sun Sep 27, 2009 12:40 pm    Post subject: Why do designs by different structural engineers vary so gre Reply with quote

Dear Sefians,
Many thanks to our regular and respected respondents for their
valuable views on this subject.As structural engineering is a science
as well as an art,two engineers may have
different views.As no two doctors agree,so also engineers may give
different designs for a given building.When the soil properties are
different and earthquake zones are different,
it is natural to expect to have different designs.There is no
difference of opinion on this aspect.But,when the site is the
same,then also we find widely varying designs of similar types of
buildings.This is quite disturbing.The main reason may be that some
engineers design buildings only for vertical loads and do not consider
horizontal loads like earthquake and wind.When you go around new
residential colonies,many buildings are coming up with column steel
bars showing up.Columns in some buildings are heavily reinforced,while
in other buildings,columns are lightly reinforced.This clearly
indicates that designers have not considered horizontal loads where
the columns are lightly reinforced.
In an existing design of a 4-storeyed library building,the concerned
designer told me(a long while back) that he had designed the building
for vertical loads only and he had not considered earthquake as it was
only 4 storeys in height.Earthquake causes movement of earth and it
obviously does not know the number of storeys of structures standing
on earth.All buildings of whatever storeys should be checked for
earthquake forces.The difference in steel consumption in similar
buildings is mainly on this account,in my view.
I have written a paper on this subject and it is published in
ICJ,August,2001 and the same is reproduced in my book on
multi-storeyed buildings.
with best wishes and regards,
uhvaryanidesigner

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