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WHY SOME CONSULTANTS SUGGEST AISC THAN IS-LSMDGoto page 1, 2 Next

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TBSPL_6SEFI Regulars

Joined: 17 Jan 2009

Posts: 26

Posted: Tue May 22, 2012 6:34 pm Post subject: WHY SOME CONSULTANTS SUGGEST AISC THAN

IS-LSMD

HI SEFIANS

COULD ANY ONE PLZ EXPLAIN FOLLOWING QUERY

1.WHY SOME CONSULTANTS SUGGEST AISC THAN IS-800-2007-LSMD ?

2.I KNOW THAT LL ARE LESS IN AISC COMPARED TO OUR IS-CODAL PROVISION , AND FACTOR

OFSAFETY IS ALSO LESS THAN OR EQUAL TO ONE IN THE LOAD COMBINATIONS , BT WHY?

3. WHY SOME PEOPLE SUGEEST MIXING OF BOTH CODES IF ONE IS FOR ANALYSIS ANOTHER IS

TO CHECK DEFLCTION CRITERIA. ?

4. ON WHAT BASIS THEY ARE PREFERRING AISC THAN 'IS CODE' , MOST OF THE PEB

STRUCTURES ARE ANLYSED AND DESIGNED WITH AISC. ?

COULD ANY ONE PLZ EXPLAIN WHAT IS THE MAIN ASSUMPTIONS , AND CONEPT BEHIND IT.

THANK YOU

TBSPL_6

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N. Prabhakar...

Joined: 25 Apr 2009

Posts: 206

Posted: Wed May 23, 2012 7:21 am Post subject:

Dear Sefians,

In my opinion, the answers to the queries raised in this posting are the

following:

1. When there is an Indian Code IS 800 : 2007 for steel

design, there is no need for anyone in India to refer to a

foreign code like that of AISC. More than the consultants, it

is the PEB manufacturers who normally specify the American

codes in their competitive offer which is generally accepted

by the client (owner) and the consultant/architect.

2. The main difference between the Indian Code and the other

American Codes is in the classification of the cross-section

of the steel member. As per Indian code, the classes of

section considered for design are Plastic, Compact and

Semi-compact. Class of Slender cross-section, particularly

with thin webs, are not considered for design as the elements

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buckle locally even before reaching yield stress. It is well

known that many PEB manufacturers use sections with very thin

webs in order to reduce the weight of the section and be

economical/competitive in their commercial offers, and these

thin webs do not satisfy the codal provisions of IS 800 :

2007.

3. To use codes of two different country, to suit one’s

requirement or convenience, is not a good engineering

practice, and code of only country is to be used throughout

unless there is no such provision exists in the code one is

using. The analysis part is not normally different between

the two codes, but the codal provisions for the safe

permissible stresses, deflection and other values do

differ. Besides, the properties of the material considered

in the code do vary from one country to the other. This

aspect cannot be easily assessed in the design.

4. As it is said earlier, the main reason to use the AISC

code for PEB structures is due the fact that it leads to an

economical structural solution as compared to the Indian

Code. In the present day cut-throat competition among PEB

manufacturers, the price of the structure that governs in the

end, and not the design considerations. It is possible that

AISC codes are misread and misused to suit their convenience

as many Indian engineers accepting this design are not fully

aware of all the provisions of AISC.

I trust that those who have had the experience of going through the design

of PEB structures will agree with the above observations.

With best wishes,

N. Prabhakar

Chartered Structural Engineer

Vasai (E)

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Dr. N. SubramanianGeneral Sponsor

Joined: 21 Feb 2008

Posts: 4848

Location: Gaithersburg, MD,

U.S.A.

Posted: Wed May 23, 2012 1:33 pm Post subject:

Dear All,

I agree with most of the observations of Er Prabhakar.

But I want to comment on his observation "As per Indian code, the classes of section considered

for design are Plastic, Compact and Semi-compact. Class of Slender cross-section, particularly

with thin webs, are not considered for design as the elements buckle locally even before reaching

yield stress. It is well known that many PEB manufacturers use sections with very thin webs in

order to reduce the weight of the section and be economical/competitive in their commercial

offers, and these thin webs do not satisfy the codal provisions of IS 800 : 2007."

IS 800:2007 has not considered slender sections which are often encountered in cold formed thin

sections, because there is another code IS 801 for this (see page 19 of IS 800 where a note about

this is made). Hence people using cold formed sections can not use IS 800.

IS 801 is still under WSM and currently under revision. God only knows when it will be published

by BIS. Draft code may be ready-Prof Arul Jayachandran of IITM may throw some light on this

as he is heading the committee, I think. May me that is the reason people are using AISC code for

cold formed structures.

Er Prabhakar's comment "the main reason to use the AISC code for PEB structures is due the

fact that it leads to an economical structural solution as compared to the Indian Code" kindled

nostalgic memories. We used to design structures using cold formed sections for TI Metal

sections. My friend Er Vijayaraghavan was there at that time, who is very knowledgeable on RC

as well as Steel design and we used to discuss for hours about the design methods. I used to have

fruitful discussions with a young engineer of their company by name Er Elangovan (I believe he

is with Tiger Steel, another company engaged in PEB, but lost touch with him for 15 to 20 years).

We used to optimize the members sizes by using a IS 801 provision, which will not be normally


2 of 8 22-01-2015 16:26

considered by other designers-I do not have the code here, but I think it is the extra strength

available at the bends of the sections, due to strain hardening effects. My Ph.D. guide Prof.

Ganapathy of IITM, wrote a beautiful explanatory handbook on IS 801, which is still available

through BIS.

Best wishes

Subramanian

N. Prabhakar wrote:

Dear Sefians,In my opinion, the answers to the queries raised in this postingare the following:

1. When there is an Indian Code IS 800 : 2007 forsteel design, there is no need for anyone in India torefer to a foreign code like that of AISC. More thanthe consultants, it is the PEB manufacturers whonormally specify the American codes in theircompetitive offer which is generally accepted by theclient (owner) and the consultant/architect.2. The main difference between the Indian Code and theother American Codes is in the classification of thecross-section of the steel member. As per Indian code,the classes of section considered for design arePlastic, Compact and Semi-compact. Class of Slendercross-section, particularly with thin webs, are notconsidered for design as the elements buckle locallyeven before reaching yield stress. It is well knownthat many PEB manufacturers use sections with very thinwebs in order to reduce the weight of the section andbe economical/competitive in their commercial offers,and these thin webs do not satisfy the codal provisionsof IS 800 : 2007.3. To use codes of two different country, to suitone’s requirement or convenience, is not a goodengineering practice, and code of only country is to beused throughout unless there is no such provisionexists in the code one is using. The analysis part isnot normally different between the two codes, but thecodal provisions for the safe permissible stresses,deflection and other values do differ. Besides, theproperties of the material considered in the code dovary from one country to the other. This aspect cannotbe easily assessed in the design.4. As it is said earlier, the main reason to use theAISC code for PEB structures is due the fact that itleads to an economical structural solution as comparedto the Indian Code. In the present day cut-throatcompetition among PEB manufacturers, the price of thestructure that governs in the end, and not the designconsiderations. It is possible that AISC codes aremisread and misused to suit their convenience as manyIndian engineers accepting this design are not fullyaware of all the provisions of AISC.I trust that those who have had the experience of going through thedesign of PEB structures will agree with the above observations.With best wishes,N. PrabhakarChartered Structural EngineerVasai (E)

Last edited by Dr. N. Subramanian on Wed May 23, 2012 1:39 pm; edited 1 time in total

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Dr. N. Subramanian

General Sponsor

Joined: 21 Feb 2008

Posts: 4848


U.S.A.

Posted: Wed May 23, 2012 1:37 pm Post subject:

Dear All,

in continuation to my posting, I want to discuss one more thing. In India, BIS is making small

codes for different items and making money. We need to integrate the codes. For example in ACI

318, they have integrated Prestressed concrete and EQ provisions. Whereas we have separate

codes for these things. When will we have unified codes? I request those in IS committees to

take up the issue with BIS.

Best wishes

NS


3 of 8 22-01-2015 16:26

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TBSPL_6SEFI Regulars

Joined: 17 Jan 2009

Posts: 26

Posted: Wed May 23, 2012 5:12 pm Post subject: WHY LIVE LOAD IS LESS ?

DEAR PRABHAKAR SIR/ DN SUBRAMANYAM SIR

THANK YOU ALOT FOR YOUR VALUABLE CLARIFICATION

AND ALSO PLZ EXPALIN FURTHER TO MY SECOND QUERY , i.e

2. WHY LIVE LOAD IS CONSIDERED LESS IN THE AISC, COMPARED TO 'IS CODE'

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Joined: 21 Feb 2008

Posts: 4848


U.S.A.

Posted: Wed May 23, 2012 7:34 pm Post subject: Re: WHY LIVE LOAD IS LESS ?

Dear Er TBSPL,

In most of the international codes a partial load factor of 1.6 is adopted for LL and a factor of 1.4

for DL. it is because DL can be calculated precisely than LL. For simplicity IS 875 uses a factor of

1.5 for Both LL and DL.

The LL specified in IS 875 (Part 2) is higher than than those found in earlier load surveys (See

more on this in my book on Design of steel structures, pp. 142)

Best wishes

NS

TBSPL_6 wrote:

DEAR PRABHAKAR SIR/ DN SUBRAMANYAM SIR

THANK YOU ALOT FOR YOUR VALUABLE CLARIFICATION

AND ALSO PLZ EXPALIN FURTHER TO MY SECOND QUERY , i.e

2. WHY LIVE LOAD IS CONSIDERED LESS IN THE AISC, COMPARED TO 'IS CODE'

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sandeep_chauhan

General Sponsor

Joined: 27 Mar 2012

Posts: 110

Posted: Thu May 24, 2012 9:04 am Post subject:

It is a very good question asked by Tata Bluescope Engineer.

I am agree with Prabhakar sir, that we are still using AISC code to analyse PEB Buildings.

I am also working for a Pre-Engineered Building (PEB) & i have Designed more than 100 PEBs in

INDIA.

If i see the codes used for the buildings design by me are:

almost 85 buildings are as per AISC/MBMA/AISI

almost 15 buildings are as per IS-800:1984/IS-875/IS-801

and only one building is as per IS-800:2007/IS-875/IS-801

According to me, the problem in using IS-800:2007 is :

1. It is not a good practice to analyse primary member(portal frame) as per Limit State method

and Secondary member(Purlin, girts,cladding etc) as per Working stress method,in same building.

2. I feel that the Deflection Criteria is not given clearly in IS-800:2007. the load combinations

given in Table-4 for Serviceability are not match with the load combinations given for deflection

check as given in Table-6 of IS:800-2007.

3. Design & Detailing for Earthquake loads as per Chapter-12 is given in Brief. There should be a

Explanatory by BIS for Chapter-12.

Even the book on "Design of steel structures" available in india, does not cover the Chapter-12.

Also i am requesting to Subramanian Sir that please put a Example in our favourite book(DESIGN

OF STEEL STRUCTURES-N.SUBRAMANIAN) for Regid Moment Connection Design, according to

Chapter-12

Regards

Sandeep Chauhan

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Posted: Thu May 24, 2012 12:10 pm Post subject:


4 of 8 22-01-2015 16:26

Joined: 21 Feb 2008

Posts: 4848


U.S.A.

Dear Er Sandeep,

Are not Examples 6.15 to 6.26 moment connections?

Best wishes

NS

sandeep_chauhan wrote:

It is a very good question asked by Tata Bluescope Engineer.

I am agree with Prabhakar sir, that we are still using AISC code to analyse PEB Buildings.

I am also working for a Pre-Engineered Building (PEB) & i have Designed more than 100 PEBs

in INDIA.

If i see the codes used for the buildings design by me are:

almost 85 buildings are as per AISC/MBMA/AISI

almost 15 buildings are as per IS-800:1984/IS-875/IS-801

and only one building is as per IS-800:2007/IS-875/IS-801

According to me, the problem in using IS-800:2007 is :

1. It is not a good practice to analyse primary member(portal frame) as per Limit State

method and Secondary member(Purlin, girts,cladding etc) as per Working stress method,in

same building.

2. I feel that the Deflection Criteria is not given clearly in IS-800:2007. the load combinations

given in Table-4 for Serviceability are not match with the load combinations given for

deflection check as given in Table-6 of IS:800-2007.

3. Design & Detailing for Earthquake loads as per Chapter-12 is given in Brief. There should be

a Explanatory by BIS for Chapter-12.

Even the book on "Design of steel structures" available in india, does not cover the

Chapter-12.

Also i am requesting to Subramanian Sir that please put a Example in our favourite

book(DESIGN OF STEEL STRUCTURES-N.SUBRAMANIAN) for Regid Moment Connection

Design, according to Chapter-12

Regards

Sandeep Chauhan

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N. Prabhakar...

Joined: 25 Apr 2009

Posts: 206


Dear Sefians,

In continuation of my earlier posting on the subject, I would like to add

the following:

The PEB structures what we are discussing are not the structures of light

weight type, low-rise or short span buildings where cold-worked steel

sections can be used. Because these sections are very thin compared to

their widths, buckling at low stress values will result under compression,

shear, bending and bearing. The critical buckling is generally of a local

nature followed by an overall buckling of the member. Because of this

deficiency, the usage of cold-worked steel section for a heavily loaded

compression member is very limited. At best, it can be used as a bending

member of small spans. In industrial type structures, the most popular usage

of cold-worked steel as a structural member is in Z and C shaped sections

for roof purlins and side sheeting rails which are no doubt economical as

compared to hot rolled angle and channel sections. The usage of these Z and

C sections for purlins and sheeting rails is invariably based on the actual

full-scale load tests conducted by the manufacturer of these sections, and

BS 5950 has given empirical equations to check on the size of the members

supplied by the manufacturer.

The PEB structures supplied in India are mainly industrial type, large span

warehouses, factory buildings, etc. For these type of structures

which carry heavy loads and sometime with crane installation, hot-rolled

sections are normally used to avoid buckling failures of the type that occur

in structures with thin cold-worked steel. For PEB structures,

manufacturers prefer to use built-up sections instead of the hot-rolled

sections to arrive at an economical solution. In one industrial structure

with crane, I have come across, the PEB manufacturer has used an I shaped

built-up section made of 496mm deep x 4mm thick web and 220mm wide x 10mm

thick flanges for a column section subjected to axial load and bending

moment. With d/tw ratio of 124 which is more than the limiting value of 42,

it is classified as a slender member as per Table 2 of IS 800 : 2007, and


5 of 8 22-01-2015 16:26

also by other international codes viz. BS 5950,the EuroCode EC3, and AISC

code. This slender section can cause local buckling even before reaching

yield stress which may result overall failure of the structure. While

designing this column section, a well known software is used by the PEB

designers which considers only the overall member strength by satisfying

only the stress requirements, ignoring the aspect of local buckling of the

thin web. No stiffeners are provided to the web as a remedial measure.

This deficiency is mainly because of the designer’s aim in economizing the

size of the fabricated built-up section, ignoring the codal provisions on

the section classification. The above aspect is a very serious matter as

far as the safety and stability of the structure is concerned.

The PEB designers are also accused of mixing too many codes to satisfy the

economic requirement. They calculate the loads as per IS 875, but do the

design as per AISC or AISI ,MBMA, and use welds as per AWS. If they feel

that the steel section is lighter as per one code, they will adopt that

clause of the code and select another clause of another code of another

country for the design of some other part of the same building. Some PEB

designers select some clauses of previous versions of the code and other

clauses of the latest versions. It seems, PEB design teams are on constant

research in the selection of codal provisions of various countries and are

on trials with different clauses. This way of mixing too many codes is

not valid by any means. If the loads and codes are not specified by the

buyer, it is binding on the PEB manufacturer to use the local codes of

practice. The consultants who are proof-checking the design of PEB

structures should do a thorough job, and do not be carried away by the name

of well known software used or to the reference of a foreign code.

With best wishes,

N. Prabhakar

Chartered Structural Engineer

Vasai (E)

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Joined: 21 Feb 2008

Posts: 4848


U.S.A.


Dear Er Prabhakar,

Thank you very much for explaining the situation in detail. I was not aware of that. It is a bad

practice indeed!

Regards,

Subramanian

N. Prabhakar wrote:

Dear Sefians,

In continuation of my earlier posting on the subject, I would liketo add the following:

The PEB structures what we are discussing are not the structures oflight weight type, low-rise or short span buildings wherecold-worked steel sections can be used. Because these sections arevery thin compared to their widths, buckling at low stress valueswill result under compression, shear, bending and bearing. Thecritical buckling is generally of a local nature followed by anoverall buckling of the member. Because of this deficiency, theusage of cold-worked steel section for a heavily loaded compressionmember is very limited. At best, it can be used as a bendingmember of small spans. In industrial type structures, the mostpopular usage of cold-worked steel as a structural member is in Zand C shaped sections for roof purlins and side sheeting railswhich are no doubt economical as compared to hot rolled angle andchannel sections. The usage of these Z and C sections for purlinsand sheeting rails is invariably based on the actual full-scaleload tests conducted by the manufacturer of these sections, and BS5950 has given empirical equations to check on the size of themembers supplied by the manufacturer.

The PEB structures supplied in India are mainly industrial type,large span warehouses, factory buildings, etc. For these type ofstructures which carry heavy loads and sometime with craneinstallation, hot-rolled sections are normally used to avoidbuckling failures of the type that occur in structures with thin


6 of 8 22-01-2015 16:26

cold-worked steel. For PEB structures, manufacturers prefer to usebuilt-up sections instead of the hot-rolled sections to arrive atan economical solution. In one industrial structure with crane, Ihave come across, the PEB manufacturer has used an I shapedbuilt-up section made of 496mm deep x 4mm thick web and 220mm widex 10mm thick flanges for a column section subjected to axial loadand bending moment. With d/tw ratio of 124 which is more than thelimiting value of 42, it is classified as a slender member as perTable 2 of IS 800 : 2007, and also by other international codesviz. BS 5950,the EuroCode EC3, and AISC code. This slender sectioncan cause local buckling even before reaching yield stress whichmay result overall failure of the structure. While designing thiscolumn section, a well known software is used by the PEB designerswhich considers only the overall member strength by satisfying onlythe stress requirements, ignoring the aspect of local buckling ofthe thin web. No stiffeners are provided to the web as a remedialmeasure. This deficiency is mainly because of the designer’s aimin economizing the size of the fabricated built-up section,ignoring the codal provisions on the section classification. Theabove aspect is a very serious matter as far as the safety andstability of the structure is concerned.

The PEB designers are also accused of mixing too many codes tosatisfy the economic requirement. They calculate the loads as perIS 875, but do the design as per AISC or AISI ,MBMA, and use weldsas per AWS. If they feel that the steel section is lighter as perone code, they will adopt that clause of the code and selectanother clause of another code of another country for the designof some other part of the same building. Some PEB designers selectsome clauses of previous versions of the code and other clauses ofthe latest versions. It seems, PEB design teams are on constantresearch in the selection of codal provisions of various countriesand are on trials with different clauses. This way of mixing toomany codes is not valid by any means. If the loads and codes arenot specified by the buyer, it is binding on the PEB manufacturerto use the local codes of practice. The consultants who are proof-checking the design of PEB structures should do a thorough job, anddo not be carried away by the name of well known software used orto the reference of a foreign code.

With best wishes,

N. PrabhakarChartered Structural EngineerVasai (E)

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