Joined: 21 Apr 2008
|Posted: Mon Nov 03, 2003 7:59 am Post subject: Answers from the Moderator to questions raised during the eC
The answers to the questions raised upto October 19, 2003, have been posted as Article #8, on the "Articles" page on the SEFI website (www.sefindia.org). The answers have been provided by Prof. Kalyanraman, and the document is in PDF format (75 KB only), for easy readability.
The answers are also appended at the bottom of this email.
ANSWERS FOR THE QUESTIONS IN E-CONFERENCE
Question: Section 3, Clause 188.8.131.52 of draft code states,
Where the Limit States Method can not be conveniently adopted; Working Stress Method (Section 11) may also be used. I request code committee members to elaborate this provision for clarification. - Jignesh
Ans: This clause has been introduced to permit working stress method (WSM) design as an alternative method wherever LSM can not be adopted by an engineer as a transition from WSM to LSM.
Question: I support the views of Mr. M.K. Mishra, that we should have some reference material too, and having an explanatory handbook is very good idea. Now i come to the point as per clause 3.7 the code has defined four types of cross sections. How will we come to know that which type of section falls under which category?- Mc.Upadhyay
Ans: A commentary is being prepared to supplement the code, which will be published by INSDAG. In the commentary, references to specialist literature on different topics would be presented in addition to explanation and background material for all the code provision. This commentary will be finalized after the draft code is adopted. The categories under which the different sections will fall are given in Table 3.1 and Fig. 3.1.
Question: I feel that IS: 800 should have explicit provisions for earthquake effects as IS: 1893 is primarily meant for RCC structures. In particular, the punitive clauses of IS: 1893 should not be applied to steel structures. The empirical formulae for fundamental period [which is used for defining minimum base shear] are NOT applicable to steel structures. The modeling of steel structures could be far more accurate than RCC structure as the properties of section [EA, EI, GJ] can be accurately determined. Further, it is likely that steel structures would be "an engineered construction" and therefore let the forces as obtained from analysis be taken for design without imposing arbitrary penalties. There is also a case for larger reduction factors for steel structures as compared to concrete construction!! The reduction factor of 5 is mostly unachievable in RCC whereas it could be achieved in steel. Hence to have overall effective seismic factor [a combination of Z, I, R, T] of the same order, R for steel structure could be larger, say, 5 & 6 instead of 4 & 5. - A.R Chandrasekaran
Ans: Section 12 is giving explicit provision for design of steel structures to resist earthquake effect. However, IS: 800 cannot override the provisions given in IS: 1893. The empirical formula address uncertainties in modeling such as stiffness of infills and is applicable to steel structures also.
Question: Some attention may also be given to composite construction in which the vertical elements are mainly steel or steel encased with concrete. - A.R Chandrasekaran
Composite concrete/steel design, which is one thing that makes steel economically feasible, has been entirely left out of the code. .-Vipul Ahuja.
Ans: IS: 800 contains provisions for design of structures made of only steel. The code for composite construction (IS: 11384) contains provision for the design of steel-concrete composite structures. At this time this code provisions are based on allowable stress design and are also very cursory. IS: 11384 also needs to be revised in tune with IS: 800.
Also note that concrete was treated by LSM in IS:456 and WSM in IS:11384.
Question: I would like to highlight the impact of the revision to IS: 800 as per the draft that is in circulation, with respect to the other codes which are not being revised, specially the following
**806-1968 Code of practice for use of steel tubes in general building construction (first revision)
**4000-1992 Code of practice for high strength bolts in steel structures (first revision)
**4923-1997 Hollow mild steel sections for structural use (second revision)
**11384-1985 Code of practice for composite construction in structural steel and concrete.
My concerns are as follows
1) How do we deal with the incompatibility generated by using LSM (Limit State Method) in the revised IS: 800 and the WSM (Working Stress Method) still in force in the other codes.
2) Since there are a lot of references to hollow sections in the draft revisions (unlike the earlier 1984 version), what would be the criteria for decisions, in case of conflicting guidelines with IS: 806 & IS: 4923. This issue is particularly relevant as the popularity of hollow sections is on the rise. -Pankaj
Ans: Codes like IS: 802, 806,4923,11384 will also have to be revised to Limit state format. Pending such revision, the designer can use WSM or LSM as per convenience. Codes, which give only section properties (IS: 4923(Hollow section), Parallel flange sections) or describe other aspects such as installation of bolts etc, are not conflicting with the revised Draft. However, the difficulties pointed out will be faced by designers until all these codes are also revised. Since IS: 800 is the mother of all these codes this has been revised first. It is easy for designer to reconcile the differences in these codes in most cases, since the commentary to IS: 800 will contain explanation to the provisions in IS: 800.
Question: The amount of accompanying live load should be given as a fraction of major live load.-Upadhyay
Ans: In combinations involving live load and earthquake load only the earthquake load should be calculated considering a fraction of the live load as per IS: 1893. Whereas when two different live loads are considered (say floor live load and crane load etc.). The accompanying load and the main load are independent of one another. However, since the probability of both live loads being above their maximum characteristic values at the same instance of time is very low the partial safety factor for the accompanying live load is prescribed a lower value.
Question: Should the block shear failure be considered only for Plates and Angles (Sec 6.4) & why we are considering gross area in one mode (shear or tension) with net area with another mode, why not net areas in tension and shear simultaneously, which will give minimum strength. .-Upadhyay
Ans: Block shear has to be checked for in bolted and welded end connections wherever such block shear modes of failure such as at end connections of plates, angles, channels, I section in tension as well as at shear connection of coped ends of beams to beams/columns. Once either area under shear or under tension in the block shear critical section reaches ultimate stress that section may rupture even before the other section reaches the ultimate stress. Both sections may not reach ultimate value simultaneously as observed in experimental and analytical studies.
Question: Clause 184.108.40.206 in page 28 may be reworded as follows: "Where the Limit State Method cannot be conveniently adopted, Working Stress Method (Section 11) shall be used -Prabhakar
Ans: 'Shall' instead of 'may' in the above class is prescriptive and is not appropriate. A designer is given freedom to overcome the inconvenience by some means other than by following working stress method.
Question: The Partial Safety Factor of 1.05 for load cases with crane (CL), given in Table 5.1 in page 48, is very small indeed. This may be a typographical error. In industrial type structures, dead and live loads on roof etc. are usually very small, and crane loads combined with wind load are critical in the structural design in most cases. This Partial Safety Factor needs to be revised to 1.5. -Prabhakar
Ans: In such cases, crane load is the leading live load to be multiple by 1.5 and roof live load etc will be treated as accompanying live loads.
Question: In Table 5.3 in page 51, the types of cladding mentioned need to be defined, particularly with reference to the types of cladding used in India. -Prabhakar
Ans: Brittle claddings are AC sheeting, fiber concrete sheeting unreinforced masonry etc which crack easily under larger deflection metal claddings are under elastic cladding. The commentary will contain such explanation we will include this in section 1.2 under terminology.
Question: The figures shown in page 70 for the lacing details of columns are inadequate. The code should show all the figures (Figs. 4A to 4D) given in the National Building Code of India 1983, Group 2, Part VI Section 6 Structural Steel, for more clarity.
Ans: After referring NBC the committee will decide whether to put in code or commentary
Question: Section 10 deals with connections between steel sections only. Clause G.5.2. In pages 197 and 198 on holding down bolts is also very sketchy. These do not deal with various types connections of steel sections to concrete, such as with expanding type bolts and insert plates with anchors, which are used in large numbers in industrial type structures. In most design offices, usually two separate groups operate for the design of steel and concrete structures. This practice has become more universal. The steel designer considers only the strength of steel while designing the end details of steel members and usually does not bother about the strength of the concrete to resist the end loads. The concrete designer designs the supporting concrete members only for the loads furnished by the steel designer, usually not verifying the design of steel end details given by the steel designer. In these circumstances, sometimes failures do occur in concrete at the interface of the connections. These connections are a specialised subject on its own, and the latest literature on the subject may not be available to many structural designers in India. Hence, a guideline should be given in the code on this important subject.
Ans: Such anchors in concrete and their pullout/shear strength depend upon the failure characteristics of RC member, local reinforcement detail in the RC member etc. Hence, they are based on the knowledge relevant to RC code and beyond the scope of IS: 800.
Question: I understand, what Jignesh wanted to say was that the subject clause in its present form would create confusion and controversy, especially with the projects involving peer review. I think just a slight re-wording of the clause will not solve the problem. The phrase "conveniently adopted "may see varied interpretation by different agencies and engineers. Instead, the choice of adopting WSD or LSM should be clearly given to the purchaser. When purchaser does not specify anything, the right of choice should remain with the design engineer. At this juncture, when both the design methods are retained, I feel, code should not exhibit a bias towards any particular method. For example, API-650 (Welded steel tanks for oil storage) stipulates two methods for design of shell. Here the right is given to the purchaser to decide which method he wants to be adopted. Similarly, AISC has got two parallel documents for ASD and LRFD. But AISC does not advocate for either of these.- Subhamoy Kar
Ans: The case of choice between the WSM & LSM in reference to AISC & API is not relevant to the draft IS: 800. As in AISC the total amount of documentation for both the methods is on almost equal footing, while in the draft IS: 800, WSM covers only 4 pages. This fact in itself implies that WSM is a stepbrother, and should be avoided almost in all cases. So in fact draft IS: 800 vocally advocates (without actually saying so) that only LSM should be followed, and in case some young & inexperienced engineer (who cannot understand the complex issues) is designing some small & irrelevant structure, he may use WSM as a rough guideline or a thumb rule. I am tempted to cite the example of the moments & shear coefficients given in clause 22.5 in IS: 456 for continuous beams, which start with the phrase " Unless more exact estimates are made....". Now no engineer uses these, and if someone uses these coefficients, he will be challenged left & right by everyone. I feel that same is the case for WSM in the draft IS: 800. - by Pankaj
IS: 800 draft clearly desires the designs to be based on LSM. The WSM method is only a less desired option, being made available as a transition option, just as IS: 456-1978 had such provisions for RC as a transition document.
Question: I strongly recommend an ELABORATE subject index. That's what sets it apart as a world class code. It makes it so much easier to use. This hard work is well worth it. Please restrict the use of the word "should" in the code. It becomes difficult to implement. Use of "shall' or "must" is preferred. Just referring to "specialist literature" to the designer leaves him out in the cold. Particular references should be given (and when possible particular extracts given in footnotes or commentary). Publication of a commentary along with this code is of utmost importance. See AISC or ACI formats. Our code language attempts at places to explain parts that normally should be given in the commentary. This will also make for simpler reading and accessing information.-Vipul Ahuja
Ans: We will try to introduce a subject index. It is proposed to bring out a commentary by INSDAG, which will contain full references to specialist literature.
Question: Very little attention has been paid to earthquake (ductile) connection design. Even though earthquake standards (IS 1893-2002 etc.) have been mentioned yet nothing has been said about ductility-refer lessons learnt from Northridge earthquake.-Vipul Ahuja
Ans: Provision of Section -12 require ductile detailing and hence certain provisions to the effect. A separate code/special publication may have to prepared for giving more elaborate recommendation for ductile detailing as in reinforced concrete (IS 13920).
Question: "Wide flange" shapes are now beginning to be manufactured in India-any impact on any design procedures or figures & tables (Appendix I) shown in the code?
. -Vipul Ahuja
Ans:The provision can be used even for wide flange shapes.
Question: 3.5.4. What about combination of blast load? Also definition of blast (shock spectra etc.)? Is Accidental loading same as blast load (table 5.1)? If so why not keep the same terminology? Also see 3.5.6. -Quantify or supply equations to calculate "bumper impact" load or refer to particular references. These kinds of issues are big time-takers in design offices. How about rain load (not to be confused with ponding)-see IBC? Usually an alternate drainage path (in the form of a direct spout that is not prone to clogging) is provided that spills the water directly outside. This spout is above the level of the proper drainage. If the regular drainage path is clogged, water will only accumulate to the level of the alternate drainage and no more. However this accumulated water must be accounted for in design. Collapses have been reported when alternate drainage systems have not been provided. Also Flood load can be incorporated for areas subjected to inundation.-Vipul Ahuja
Ans: The accidental loadings are those that are not expected to act on a structure normally unless an accident has occurred. They are blast loading due to either gas cylinder bursting or terrorist attack, ramming of large vehicles on piers of bridge etc. The accidental load to be designed for should be specified by the purchaser/client or IS: 875. Other specific detailing for drainage etc are outside the scope of IS: 800.
Question: 5.2.2 Limit State of vibration (as in floor vibration). Cl. 5.6.2 only advises broadly but gives no specifics. Appendix D should be referenced to from the appropriate places.-Vipul Ahuja
Ans: Appendix D gives some provisions for design against Floor vibration and will be referenced in Section 5.6.2.
Question: 220.127.116.11 I have a strong suspicion the character "Ë" is a typo in this electronic version-it is replacing many symbols.(Note this ASCII symbol "Ë" is a "rectangle" in "WORD") .-Vipul Ahuja
Ans: Everything appears normal in an view of the website. Hence we do not understand
Question: 7.5 Does this section also apply to unequal leg single angle struts? . -Vipul Ahuja
Ans: Yes some additional are proposed to the clause to take into account unequal angles.
Question: 8.6 Instead of giving requirements for thicknesses of web it would be better to present the various limit states for which the design is being checked. That way it is simpler for the designer to understand and has fewer chances of making mistakes. Also, is there a section that deals with concentrated forces on flanges and webs-limit states of flange local bending, Web local yielding, web crippling, web sidesway buckling, web compression buckling etc.? -Vipul Ahuja
Ans: Effects on concentrated load on web/flanges are discussed under 8.7 section classification (section 3.7) takes care of local buckling requirements of all elements.
Question: Design rules of box girder should be more explicit-perhaps refer to other parts of the code when appropriate (like b/t ratios etc.). Also it may not be practical to put additional horizontal stiffeners in box girders referred to by 8.7.13 via 8.8-is that really the intent. . -Vipul Ahuja
Ans: Box girder webs and flanges may require stiffness (vertical and horizontal) depending upon their slenderness, as per section 3.7.
Question: 10.4.7 Prying forces cannot be developed for bolts with prestress for the "T" section shown. -Vipul Ahuja
Ans: Prying forces can be developed even if pretensioned HSFG bolts are used .
Question: Cl 10 & Appendix G: Design of the connections such as end-plate connections, continuity plate & doubler plate design procedures are also not covered. . -Vipul Ahuja
Ans: Only some typical details in Annexure G and guidelines in section 10.6 to 10.12 are given in the code. For more information on connection text book and handbooks should be referred to.
Question: When we were discussing about the 0.9DL+ 1.5 EL combination as when it should be accounted for, Prof. Sudhir Jain wrote: The confusion on this account comes from IS: 456 wherein it is mentioned that this combination is to be considered when stability against overturning or stress reversal is critical. So if this comb. Has to be considered necessarily why don't we delete the same note at least in IS 800, which is there in the Table 5.1. -Upadhyay
Ans: The note in table 5.1 will be modified to reflect that 0.9 factor should be used whenever dead load contributes to stability against overturning or causes reduction in
stress due other loads, such as wind uplift in roof trusses.
Question: The subject clause calls for incorporation of base stiffness in to analytical model. The stipulations under serial no a) and b) warrants: 1) Modeling of pedestal below the column base.2) Pedestals are to be assigned with stiffness relative to the column stiffness, depending on attachment.3) Earlier draft included modeling of pedestal bottom end, which probably is omitted because it related to foundation. I understand that these might have got a research background. But I anticipate a problem due to these stipulations. i) In a large and complex 3-D model, with varying elevation of beams and ties along two orthogonal directions, column stiffness calculation will be a very time consuming and laborious task.ii) Design will proceed in an iterative manner, to qualify each and every member. Every time one column size is changed, the base stiffness is to be recalculated. iii) Boundary conditions are not specified in code and no reference is indicated, as to how the pedestal end will be idealized. So, even if the column base stiffness is accurately modeled, but boundary conditions are not appropriate, then the purpose of such elaborate exercise will b defeated. It will only increase the burden of design engineers. As illustrated under serial no. i) And ii) above, I feel these stipulations will impose a great deal of complexity in day-to-day design work. I might have felt so because of my strong prejudice about traditional support idealization, i.e. fixed base or pinned base. But I did not find such clause of base stiffness in couple of International codes, I have worked with. I was wondering whether, they recognize the practical difficulty in doing base stiffness modeling.- Subhamoy Kar
Ans: Assuming the steel column to be hinged to fixed at the base plate in the FEM model is not appropriate, since this assumes the pedestral and foundation to be rigid. The IS:800 Draft proposal is to include this pedestral and foundation also in modal appropriately. If the dimensions of the pedestral are not available , may the pedestral stiffness may be assumed as given in section 4.3.4. Regarding boundary conditions the committee will decide whether to put in code.
Question: In section 6.3.3 the draft code gives two expressions for b
b = 1.4 - 0.35 (w/t) (fu / fy ) (bs / L)
= 1.4 - 0.52 (bs / L)
In the first expression is it fu / fy or fy / fu ? It is found that the second expression is simpler and gives values similar to the first one. Hence the first expression may be removed. - N. Subramanian..
Ans: The correct equation in section 6.3.3. should read as
b = 1.4 - 0.076 (w/t) (fy / fu ) (bs / Lc) or more simply
= 1.4 - 0.54 (bs / Lc)
where w and bs are as shown in Fig 6.2
Lc = Length of the end connection, i.e., distance between the outermost bolts in the joint along the length direction or length of the weld along the length direction
First expression is more accurate and the second one is simpler and hence both are given. The second equation can be used for iterative design and the first equation can be used as a check in final step.
Question: Similarly section 6.3.3 gives two expressions for finding the design strength Tdn, governed by tearing at net section. For simplicity only one expression may be given in the code. Perhaps you may give both in the explanatory handbook, which will be prepared by you at a later date. - N. Subramanian.
Ans: In the preliminary design a designer may use the second equation to calculate net area to be provided, whereas first expression is more accurate and can be used for checking the design strength after preliminary design.
Question: In section 10.3.2, for calculating nominal shear capacity of bolt, the code suggests to take the shear strength as fu /sqrt (3). Whereas the British code suggests that the value be taken as lesser of 0.69 times the yield strength and 0.48 times the ultimate strength of fastener. The value suggested by the code will result in less number of bolts. Will it be safe? . - N. Subramanian..
Ans: Using the partial safety factor,1.25 for bolts given in Table 5.2, the corresponding values workout to be 0.46 as against 0.48 in BS and 0.64 as against 0.69 in BS, therefore safety will be ensured
Question: In the course on IS 800 at Kolkata, you said that commentary of the code is under preparation and will be brought out by INSDAG soon .my request to you is to please make it sure that a exaustive list of references (for each section of the code) is included in the commentary, apart from the explanations of relevant clauses. This will be very much useful. If possible, the background reports prepared for Draft IS 800 may also be uploaded on the SEFI site. with personal regards.-Mohan Gupta
Ans: These comments will be taken into consideration
Question:. Many times for architectural reasons the compression members cross section needs to be varied uniformly over the height or length. Is there a general formula which can be used to find out the effective length or the slenderness ratio of such members in which the compression member is say round tubular section with diameter d1 at one end which increases to diameter d2 at centre and then reduces to diameter d3 at the other end.-Vijay Patil
Ans : Books on stability (Timoshenko & Gere ) give the effective length factors for such non-prismatic column members.
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