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sandeep_chauhan
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Joined: 27 Mar 2012
Posts: 122
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 Posted: Fri May 25, 2012 3:30 pm Post subject: |
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Dear Prabhakar Sir,
I am fully agree with you.
Mostly PEB menufacturers are doing the same thing as you mentioned.
I feel that, there should be a joint discussion between all PEB manufacturers to follow only & only latest Indian codes in all future peb designs in india.
Thanks & Regards
Sandeep Chauhan
Sr-Engineer Design
| N. Prabhakar wrote: | 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 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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N. Prabhakar
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Joined: 25 Apr 2009
Posts: 474
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| Posted: Sat May 26, 2012 5:44 am Post subject: |
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Dear Er Sandeep Chauhan,
I appreciate your posting on the subject as you are being honest in accepting the present scenerio on the design of PEB structures. Your suggestion for all the PEB manufacturers agreeing to do the PEB design as per Indian codes only is no doubt a good one, but it requires a concerted effort by everyone involved in it, particularly of the owners of PEB manufacturers . If there is an association of PEB manufacturers or that sort of organisation, your proposal can be suggested there.
With best wishes,
N. Prabhakar
Chartered Structural Engineer
Vasai (E) |
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sriprakash_shastry
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Joined: 23 Mar 2010
Posts: 50
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| Posted: Fri Jun 29, 2012 11:13 am Post subject: |
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Dear Dr. NS,
I have something to contribute to this very juicy and exciting topic. Here are the problems we are facing in the new IS800:2007.
1. We are talking only about hot rolled slender sections here for which there are no provisions in IS800:2007 as clearly stated by the code in clause number 3.7.3(c) on page 19.
2. Comparing this with AISC 2005 wherein the design of slender elements is calculated considering the reduction factor Q for buckling effects.
3. Let us go item by item
a. Flexure in Flange for Compact Sections b/t formulas as per IS800:2007 is 9.4e and in AISC it is 0.38sqrt of (E/Fy).
b. For Semi Compact sections b/t formula in IS800:2007 is 13.6e and AISC allows 0.95*sqrt(KcE/FL). This allows a whopping saving of nearly 30% in the section sizing.
c. For Slender Sections the comarison is as stated in points 1 and 2 above.
d. For flexure in WEB for Compact sections in IS800 h/tw is 105e and in AISC it is 3.76*sqrt(E/Fy).
e. For semi compact sections h/tw as per IS is 126e and as per AISC it is 5.7*sqrt(E/Fy). This again leads to a lot of economy.
4. There is no clear span to depth ratio given to distinguish between a beam and a plate girder. Normally we take a Span to Depth ratio <12 as a common practice.
5. Most PEB buildings are structures where wind is the governing force. Having said that Clause 12.1 states that "Frames which form part of a gravity load resisting system but are not intended to resist lateral earthquale loads need not satisfy this section". Compared to this the AISC adopts a different approach wherein the ductile detailing is governed by the response reduction factor.R.
6. Clause number 12.8.4.1 of IS800 which states that column sections need to be plastic. Why?
7. L/r of bracing should be less than 120 as per cl.12.7.2.1. In PEB's the bracing is tension only member and does not carry compression force. They are also not broken in the centre.
8. No provision for reduction of base plate thickness by provision of stiffeners.
9. Another critical clause relates to deflection of crane rails in a Dead + Wind case wherein only 10mm is permitted.
These are but a few points raised. Im sure if an experienced PEB designer writes on this topic many more points will be added to this list. When we ask for a quote using IS800:2007 we get a quote but in the terms and conditions we get "Chapter 12 ignored". "D/t upto 170 without stiffeners" and "b/t ratio upto 23.1". "Secondary design as per AISI-96" etc. We as designers are actually scared of the new code. We dont know what are the boogey traps in this code. In the US large hot rolled sections are easily available off the shelf. Here we are stuck to our ISMB 600. The moment it is insufficient we have to add flange plates to it and then the calculations can make you go crazy.
Code writers should consider these aspects and provide solutions for this and other aspects of the code and that too on a war footing basis. The code is already 4 years old and still there are a lot of issues that need to be addressed.
BR
Sriprakash |
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tigran mamikonyan
SEFI Member
Joined: 23 Apr 2013
Posts: 2
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| Posted: Thu Apr 25, 2013 12:20 pm Post subject: dear Mr. N. Prabhakar |
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I have exact same situation as you had,
I shaped member
h=450 mm
t=4 mm
Seismic zone Armenia , to understand which kind of seismic force, just give you the ground motion))) it is 0.4g=400cm/sec^2)))))
If you Go in seismic provisions even for AISC 341-10 You will see in table D-1 which kind of limitations they gave)))
http://www.aisc.org/WorkArea/showcontent.aspx?id=29248">http://www.aisc.org/WorkArea/showcontent.aspx?id=29248
This is the link of seismic provisions , please have a look))) |
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sandeep_chauhan
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Joined: 27 Mar 2012
Posts: 122
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| Posted: Wed Feb 10, 2016 4:32 pm Post subject: |
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Respected Subramanian Sir,
As per IS:800-2007 Page-19- " The design of slender web elements may be made as given in 8.2.1.1 for flexure and 8.4.2.2 for shear". So can we design Slender web with this clause ?
Regards
Sandeep Chauhan
| Dr. N. Subramanian wrote: | 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 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 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 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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someshwar ganti
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Joined: 06 Feb 2010
Posts: 80
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| Posted: Fri Apr 20, 2018 1:50 pm Post subject: Structure does not KNOW THE CODE to fail in |
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There can be many arguments on the design code, but finally the structure DOES not know the code to FAIL in.
as per Zamil Steel Design Manual
hw/tw <= 180
but where as as per IS it is 42
at this rate ALL THE BUILDINGS DESIGNED BY ZAMIL STEEL SHOW HAVE FAILED.
or
OUR CODE is over conservative |
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