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[TBSPL_6]

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

[N. Prabhakar]

Dear Sefians,

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

1. 当我有一个印度的代码S 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 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. 使用代码的两个不同的国家,以适应�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)

[Dr. N. Subramanian]

Dear All,

I agree with most of the observations ofEr 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 IITMmay 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 friendEr Vijayaraghavanwas 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 nameEr 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 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. guideProf. Ganapathy of IITM, wrote a beautiful explanatory handbook on IS 801, which is still available through BIS.

Best wishes
Subramanian

[Dr. N. Subramanian]

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

[TBSPL_6]

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'

[Dr. N. Subramanian]

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

[sandeep_chauhan]

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

[Dr. N. Subramanian]

Dear Er Sandeep,

Are not Examples 6.15 to 6.26 moment connections?

Best wishes
NS

[N. Prabhakar]

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.

钢结构在印度主要是提供dustrial 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按表2是800:2007,也被其他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)

[Dr. N. Subramanian]

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