conf on indian seismic codes

7/31/2019 Conf on Indian Seismic Codes

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e-conference on Indian Seismic Codes(January 26 - February 8, 2002)hosted byNational Information Centre of Earthquake EngineeringIndian Institute of Technology Kanpur

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Steel Stuctures

S.R. Satish Kumar [Monday, January 28, 2002 11:15 AM]

Subhamoy Kar [Wednesday, February 06, 2002 8:57 AM]

Anand Ghaisas [Wednesday, February 06, 2002 3:38 PM]

Vijay Patil [Thursday, February 07, 2002 10:55 AM]

Subhamoy Kar [Thursday, February 07, 2002 1:26 PM]

Anand Ghaisas [Thursday, February 07, 2002 7:08 PM]

S.R. Satish Kumar [Monday, January 28, 2002 11:15 AM]

Dear Colleagues,

I had quick glances at the emails flooding my computer and was happy to notethe interest shown by all of you in the design issues and codal provisions. I mustcongratulate the organisers for providing the opportunity to all of us.

I would like to add a few comments/suggestions in brief;

1) everyone seems to agree that there is an urgent need to get the code revised.Unfortunately, this is not in our hands but what we can do is to publicise ourcollective opinion so that it will be heard at the right quarters. I hope theorganisers will take a lead in this regard with our full support.

2) there are also several questions regarding the rationality and applicability ofthe codal provisions in the current and proposed draft codes. On one hand,questions like the effect of wall stiffness may be difficult to answer satisfactorilybut on the other hand questions on design methodologies may be explainedmore easily. The best way to do this will be to bring out a document/handbookwith explanatory notes and worked examples. A good summary report of thiseconf may also be of great use

3) Another rather dangerous development which I see as a consequence of theBhuj earthquake is that people are trying to do things which are well beyond


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their scope. The other day, a final year architectural student dropped into myoffice saying that he is trying to do the seismic design of a ten storeyedbuilding. To my surprise, I found that he was not aware of shear force andbending moment diagrams. There is also a tendency to oversimplify seismicdesign in the form of dos and donts. My suggestion/request to all of you is to

discourage such things.

4) Lastly, I would like to comment on seismic behaviour of steel structures.Although, steel as a material is ductile, steel structures need not be ductile. Asteel frame with simple connections like angle web cleats, will have noresistance to lateral loads. It is also well known that concentric bracings tend tobuckle and give poor performance. Perfectly brittle failure of steel has also beenobserved. But notwithstanding these problems, steel structures can giveexcellent performance IF properly designed.

Currently, our IS800 is in working stress method which does not provide arational factor of safety against collapse. On the other hand, Plastic Theorywhich gives better predictions of the collapse load, does not provide a means toverify if the deformations are within limits. Hopefully, if the code is revisedinto a Limit State Method, and if some additional clauses are incorporatedtowards the design for seismic loads, we may be able to arrive at morerational performance factors. Until then, we should continue to use the infoavailable in IS 1893 and rely on our own judgement of the performance of ourstructure.

In other words, the codal provisions should be used as guidelines and not asa means of guaranteeing safety against earthquakes. This statement isapplicable not only to steel structures but to all types of structures as well.

With best wishes for the econf,Satish

Subhamoy Kar [Wednesday, February 06, 2002 8:57 AM]Hi Gentlemen...

This E-conference is intended to discuss various aspects of IS:1893 and IS:13920.Prima facie this write-up on earthquake resistant design of steel structures mayappear slightly offbeat. Because there is no dedicated code of practice on thissubject like IS:13920 (For RCC structures) and present IS:1893 does not addressspecific requirements for aseismic steel design. But IS:1893, being the generalcode (as its title suggests) for earthquake resistant design of structures, shouldinclude the subtleties of aseismic steel design. I guess, this aspect might beincluded in the Part-4 of revised IS:1893, which is expected to come out at a later


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date. I would like to take the opportunity of this conference to summarize a fewground rules for design of steel structures in high seismic zones.

So far the development of earthquake resistant design in India was mainlycentred on RCC structures. As the steel structures are constructed primarily in

industrial sector, which is much organized compared to real estate business. So,contrary to the later one, steel structures are engineered to varying degrees andalso constructed with better workmanship as project is executed by qualifiedConsultants and Contractors. That's the reason why comparatively less failure isreported in this sector. But we must consider the fact that earthquake of greaterintensity than 2001 Gujarat earthquake is not unexpected on the Indian plate andalso the actual earthquake force is likely to be much higher than Design BasisEarthquake (DBE) force even if site specific acceleration spectra is adopted indesign. So, the steel structures also need careful attention, especially when theyare located in high seismic zones. Besides this, distribution of gravity loads on

the structure dictates its exposure to seismic load and hence the importance ofaseismic design / detailing.

For example, a Boiler structure in power plant has got weight of all the Boilercomponents (including steam drum) supported on the suspension girders at top(Approx. Elevation 50 - 60m). Heat Recovery Steam Generator (HRSG) is anotherexample. Similarly, the Deaerator structure supports the entire equipmentweight (Approx. 350Ton) on its roof. These are a couple of examples where thestructures are subjected to severe seismic load, governing the design. Followingare the few general guidelines by adopting which seismic performance of these

kinds of structures can be improved.

1) Providing moment resistant frames instead of braced frames. This willincrease flexibility considerably to attract less seismic force. However thestructure is to be qualified against the horizontal deflection (drift) criteria. Also, aP-Delta analysis should be performed to assess the effect of large eccentricity ofvertical load induced during earthquake.

2) Generally built-up I-sections are used as columns in heavy steel structures foreasier fabrication and member connections. But these I-sections are inherentlyweak about their minor axis, which compels the design engineer to bracecolumns in the weaker plane. Typically concentric bracing is observedeverywhere in India, which has got extraordinary stiffness. If the bracing systemis made eccentric it will be more flexible. To explain it further, the two diagonalmembers of a Chevron bracing typically meet at the bottom of beam / strut atthe upper level (at a single work point). If they are connected to two differentpoints below the beam instead of making them concentric (i.e. there will be twodifferent work points), then the flexural stiffness of beam will come into play, as


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the structure is subjected to lateral load. This will reduce the seismic load onstructure due to reduced stiffness of braced frame. However, this idea can not bedirectly applied for HRSG, because its structure like a hot gas duct made ofstiffened wall plates. So, only portal frame is possible in cross direction.

3) Using clip angle connections instead of end plate connection. This increasesductility of the connections because deformation of clip angle under load isprominent than that of end plate.

4) Shop welded - site bolted connections are generally preferred by Consultantsto the all welded connections due to the following reasons :

a) It is difficult to ensure the quality of site weldingb) Welded connections are brittle and prone to fatigue failure.

In most of the cases the bolted connections adopted in India is of bearing type.For this type of connections the bolt stem should be in contact with the innersurface of bolt-hole, as the shear forces are transferred by bearing of bolt on thehole. Therefore IS:800 recommends bolt-hole to be marginally bigger than thebolt diameter as required just to insert it. In reality, it is noted many a times thatthe bolt-holes kept in the two structural members does not match at theconstruction site, which makes the site personnel to drill oversize holes forinserting the bolts. If a few bolts in a bolt group are housed in over-size holes,they weaken the entire connection substantially and may lead to prematureconnection failure under occasional higher loading like in the case of earthquake.

This problem can be tackled by providing High Strength Friction Grip (HSFG)connections or Slip Critical connections (As described in AISC Manual of steelconstruction). For these types of connections, oversize holes are not harmful asthe connection forces are transferred by friction between the contact surfacesinstead of bearing on the bolt. Also, these connections have superior fatiguestrength and hence can perform best under earthquake shaking.

5) Now-a-days, a few leading construction contractors in India are equippedwith state-of-the-art technology and tools for welding. Also, they have a goodnumber of qualified welders and very firm on-site quality assurance plan. So,gradually all welded construction is coming to practice. Until now I had beenassociated with the engineering of four such large projects. In the case of such allwelded construction, ductility can be built into the connections by clearlydemarcating "No weld zone" in the connection elements at the junction. For abeam or column splice, it means any welding will start at a distance of 25mm oneither side of C/L of splice (i.e. total 50mm) and extend up to the requireddistance. Similarly, for moment connection the welding in the top and bottomconnection plates will start at 50mm from the face of column. By leaving this


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50mm "No weld zone" at the junction it is possible to prevent annealing of steelin that area. So, the structural steel in this "No weld zone" retains its originalmechanical properties (percentage elongation) and thus imparts ductileproperties to the connection. This is not possible when structural members arespliced or connected by butt welding without any splice / moment plates and

hence not desirable.

6) Typically, for steel structures there are two predominant failure modes - (a)buckling (elastic instability) of members and (b) connection failure. Very rarelythe steel members are found to be snapped due to tensile force and occasionallycracks are found to have developed due to fatigue under cyclic loading.Recognizing the two basic failure modes as stated above, the buckling part isduly taken care of by the Design Engineer. But the second important aspect, i.e.the connection design is left to the discretion of fabricator. Although fabricationdrawings are made to show the connection design, often the concepts used in

analysis and member design are not properly translated in the connectiondesign. Sometimes the design drawings show a few typical connections, but theyare indicative only. They do not specify the actual number of bolts, weld length,angle size, plate thickness etc. Actually, the connection design is much morecomplex and rigorous than structural analysis and design. In this context, thefollowing books may be referred to-

a) Structural steelwork connections, by G.W.Owens & B.D.Cheal, a Butterworthpublication.b) Joint in steel construction, published by The British Constructional Steelwork

Association Ltd. (BCSA)

Probably, due to this reason in United States, connection design is done by thestructural engineer himself and the work is not entrusted to Fabricator or anyother third party. This is a statutory requirement over there. However, what Ifeel is that as good engineering practice, we (The structural engineers) shouldperform the design of at least some major connections of the main frame andindicate those in the design drawings with due weightage to the connectionelements. Importance of the connection may be judged by the role it plays on thestability of the main frames and be selected accordingly for design. A guidelinein consonance with the above may be incorporated in the Part-4 of upcomingIS:1893 for important structures in high seismic zones.

Thanks and regards.

SUBHAMOY KAR


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Anand Ghaisas [Wednesday, February 06, 2002 3:38 PM]Dear Shubhamoy:

I appreciate your efforts in discussing the important topic for StructuralSteel Construction.

I have a few points to make :

(1) Moment Connection Vs Braced ConnectionAs rightly pointed out, the Moment Connection type Frame is promising thanthe Braced type by utilizing the FLEXIBLE portion of the spectrum. Howeverdue to functional limitations in the Industrial Structures ( Due to restrictions onequipment/piping deformations) , many a times one has to go for a BracedFrame.

(2) As regards Eccentrically braced connections, the basic reason is to allow forductility and superior in-elastic behaviour by utlizing the Short Beam Segment (also called a Link) as the Primary zone of Elasticity. As regards the ElasticStiffness the reduction would be marginal when compared with theConcentically Braced Frame.

(3) Bolted Construction Vs Welded ConstructionThis needs further discussion with respects to several aspects such as Design ,Detailing, Constructabilty and Maitainability. Many of Indian clients preferwelded connection as Maintainance of Bolted Structures is found to be

cumbersome.

It is interesting to note that Bolted Construction shall allow us utilizing a higherpercetage damping (typically 4%) as compared to Welded Construction(typically 2%). This means that the Spectrum ordinates are much lower.

Lack of good construction practices for Bolted Construction should not lead toREJECTION of this type of connection. Instead there is a need forIMPROVEMENT in fabrication technology.

The use of Slip Critical Connections as the BEST alternative in BoltedConstruction appears to be incorrect. In fact use of such connections isPROHIBITED due to the very fact that "Earthquake motions are such that slipcan not be prevented in all cases." I fact use of Oversize/ Long Slot holes is alsoPROHIBITED by the American Practice on Seismic Detailing.

A very detailed discussion on the above topics is availiable in : SeismicProvisions for Structural Steel Buiildings by American Institute of Steel


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Construction.by Task Committee 113.

With Best Regards

Anand Ghaisas

Vijay Patil [Thursday, February 07, 2002 10:55 AM]

Dear Mr. Subhamoy Kar,

Thanks for mentioning about steel structures it was becoming more of a concretee-conf.It was interesting to go thro your mail on Structural Steel Design for EQ. Therewas a definate need for it to be mentioned in this e-conf. I had asked about it onthe second day of e-conf without any response.

I dont know whether you had been mentioning of strictly Industrial Structuresbut I am interested in knowing for building structures.

I have a series of questions / doubts

What is ductile detailing for steel structures??

According to the present EQ code the performance factor depends of the framingsystem in which if I have a steel structure with bracings( frame resisting only

25% of the forces) the performance factor is 1.3 where as in case I have a purely amoment resisting frame with dutile detailing (nowhere the code mentions whatis the meaning of ductile detailing for the steel structures) the factor is 1.0. Due toa lot of factors of site welding etc. as you had also mentioned in your mail toachieve a 100% moment resisting junction is a bit doubtful.

IS:800 permits simple, semi-rigid and rigid designs. What would be the impact ofthese type of designs for EQ conditions is not clear from the code.

You are right I sections are weak along the minor axis for which I thought we can

go for Box sections. By using Box secitons a lot of problems of column stiffness inboth directions is solved. Only in case of box sections the connections become aproblem. I have not come across a tech paper/book which would give the basisfor connections for hollow box sections.

To make Moment resisting frames esp. if columns are I sections the minor axis ofthese columns do not contribute much. In which case bracings turn out to bemandatory to control horz deflection. Bracings are probably the best way to take


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care of Horz forces be it EQ or wind. However in case we try to have a momentresisting frames without bracings the consumption of steel goes up drastically.

It is so surprising the in our country very less material is available of steelstructures which makes it impossible to design an economical building in Steel.

Meanwhile i just thought of mentioning that this e-conf is about to end. So if ucould be kindly mark a Cc: to me( [email protected] ) of your reply.

Furthermore I wish to ask you whether I can keep in touch with you directlyafter the e-conf.

Vijay Patil

Subhamoy Kar [Thursday, February 07, 2002 1:26 PM]

Dear Anand,

Thanks for the complementary note. I would like to clarify the following.

(1) Moment Connection Vs Braced ConnectionTypically, power plant structures those were used as reference to my earlierdiscussion are not very slender. Instead they are of squat type, i.e. having a goodwidth along with the height. Hence lateral deflection could be controlled withmoment resistant frames to satisfy the criteria of equipment support. However,

this may not be the same for tall refinery structures (Say, for 12m X 6m X 90mCCR structure in Jamnagar Refinery) wherein, as you have rightly said, one hasto go for a braced frame.

(2) Eccentric bracing systemEccentric bracing connection need not always give rise to a "Short beamsegment". Eccentricity may be increased to even one third of the beam span. Iguess, this will result in good flexibility in elastic range also. In this context "TallBuildings" by B.S.Taranath (A Mc.Graw Hill publication) may be referred to.

(3) Bolted Construction Vs Welded ConstructionI appreciate your views on this comparison. I put forward the suggestion ofadopting slip critical connection as a preventive measure against weakening ofbearing type connection by drilling oversize holes, which is highly undesirable. Idid not mean to encourage over-drilling by adopting slip critical connections.You will agree that it is difficult to improve the fabrication technology overnightto rule out this ground reality, as it costs money.


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I will try to find the literature referred by you and wish to consult it. I don'tknow any clause in AISC or UBC which prohibits usage of slip criticalconnections in high seismic zones (May you please suggest?). My observation isactually contrary to that. Let me try to explain. It may be true that always slip cannot be prevented in case of earthquake. But I understand that as per AISC

manual, slip critical connections are to be checked and qualified for their bearingstrength also in addition to the frictional resistance. The idea is to prevent grossfailure even if any occasional slip occurs between the contact surfaces due toincreased load. To this subtle extent, slip critical connections are different fromHSFG connections and that's how AISC has guarded the slip critical connectionsagainst possible slip occurring during earthquake. Also, in this context, one maylook into section 5.5.1 of "Structural steelwork connections", by G.W.Owens &B.D.Cheal (a Butterworth publication), where it states, "Bearing capacity in thepost-slip condition will be a function of plate thickness * for most practicalconnections it will not govern."

My last humble submission is that all of us know slip critical connections aresuperior considering their greater ductility, higher damping factor and increasedendurance against fatigue. SO MANY MERITS OF THIS CONNECTIONSSHOULD NOT GET OVERSHADOWED BY ONLY ONE DISADVANTAGE.Instead the structural engineers should work out a solution for this, in the spiritof AISC.

Hope I could clarify my views. Also, I convey my sincere thanks to you forgiving me an opportunity to revisit my earlier comments.

With best regards.

SUBHAMOY KAR

Anand Ghaisas [Thursday, February 07, 2002 7:08 PM]

Dear Subhamoy:

Thanks for further discussion on the topic.

I wish to quote a portion of the Commentary by the AISC Committee.

Quote

Bolted JointsThe potential for full reversal of design load and likelihood of inelasticdeforamtions of members and / or connected parts necessitates that fully


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tensioned bolts be used in bolted joints in the Seismic Force Resisting System.However earthquake motions are such that slip can not be prevented in all cases,even with slip critical connections. Accordingly these provisions call for boltedjoints to be proportioned as fully tensioned bearing joints with faying surfacesprepared as for Class A or better slip critical connections. That is bolted

connections shall be proportioned with design strengths for bearing connectionsas long as the faying surfcaes are still prepared to provide a minimum slipcoefficeint of 0.33. The resulting nominal amount of slip resistance will minimizedamage in more moderate seismic events.

Unquote

THus the design procedure recommends use of Bearing Connection strengthonly. Also further recommendations regarding noraml size or short slotted holes

ONLY, are given in view of large in-elastic deformations expected.With Best Regards

Anand Ghaisas

conf on indian seismic codes

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