Joined: 21 Apr 2008
|Posted: Tue Nov 11, 2003 6:57 am Post subject: Closing statement
The third and final set of answers are posted in Article #8, as before on the Articles page of the website (www.sefindia.org). They are also appended at the bottom of this email.
This brings us to the conclusion of a long special eConference on the draft IS 800.
Considering the time required to read through the draft document which is a major eparture in terms of philosophy, analysis and design procedures, it was very encouraging to see the number of people who have contributed in a small and large measure to the discussions.
For the statistics lovers, we had a total of 77 postings with 13 messages discussing crane loads, 12 discussing composite sections, 3 on composite sections, 11 messages discussing seismic issues and a sprinkling of messages on many very important topics. The issues discussed during the eConference went into many finer points of the code and there was an active debate on some of these topics.
However, was it a case of missing the woods for the trees?
There didn’t seem adequate reaction to the magnitude of the changes recommended. Only time will tell us if we are ready to grapple with so many changes at once.
Prof Kalyanraman and Prof Satish Kumar have been very kind to respond to the major issues brought out in the e-conf. We look forward to such more interactions between the industry and the code-writers which will ensure that our codes are more realistic and practical.
We would like to specially acknowledge the participation of the following who have taken the time out to read the draft code and contributed significantly in bringing out issues - Rudra Nevatia, H.S.Nagle, Mihir Joshi, Vijay Patil, Sundar Chandramouli, Narendrakumar, Jignesh Shah, A R Chandrasekaran, B. J. Gupta, Amit Ghangurde, S.P.Srinivasan, Vipul Ahuja, Upadhyay, Prabhakar, Subhamoy Kar, N. Subramanian, Mohan Gupta, Mishra M K, Alokes Guha, Anjan Kumar De.
I am sure the issues have helped the rest of silent participants to get a deeper insight into the proposed code revisions. More discussions on the draft IS 800 may continue in the main Discussion Forum.
On this closing note, we would also appreciate input regarding any special topics that you would like to discuss threadbare. Do you think we are having too many e-conferences and you cannot cope with it? Or are you enjoying them even if you do not actively participate?
What more would you like to see at the SEFI portal?
With warm regards,
Alpa Sheth and Pankaj Gupta
REMAINING ANSWERS FOR THE QUESTIONS RAISED DURING
Question: I think the notional horizontal loads are given only to check the sway stability of the rigid jointed frame i.e. we have to calculate the deflection of each storey, for these notional hor. loads with gravity loads and check the criteria given in Cl. 4.1.2 c) . We don't have to design the structure for these hypothetical loads, so there is no question of considering other loads with these notional hor. loads. But i would like to know from moderators & others whether the deflection given as per Cl 4.1.2 c) is absolute or relative.-Mukesh Upadhyay
The term notional horizontal loads (NHL) first comes in the Cl 4.1.2(c)in which we are deciding the type of rigid jointed frame (whether sway or non sway), after applying NHL and comparing the deflection of each story with the given expressions. the Cl. 4.3.6 also says To check the sway stability of the frame subjected to gravity loads, notional horizontal forces should be applied.
So it is nowhere written that we should design the str. for these NHL as suggested by you (The sway stability check is done as per combined forces requirement of section 9.3, which is a strength check) If this is the intention it should be included in the table 5.1 given as load combination.
Now if a rigid jointed frame comes out to be non-sway as per Cl. 4.1.2 (c) for NHL whether it will be considered as non-sway for other lateral loads (WL/EL) also or I have to determine the type of frame for these loads separately.-Mukesh Upadhyay
Ans: If only gravity load is considered in an elastic analysis, P- effect is not accounted for. Hence notional load should be considered only with gravity load to account for magnification of moments due to P-Δ effect. This is what one means when we say sway stability.
Further notional load is also used to classify frames into sway non-sway frames. This classification is necessary to calculate the effective length of column is frames as suggested in Annexure D.
Question: Why are we using minimum eccentricity of 100mm for the design of columns as per Cl. 126.96.36.199 why not actual eccentricity. It is not clear from the Cl. 188.8.131.52 whether this ecc. is applicable for simple construction (i.e. Bending members assumed pinned at ends As in Cl. 4.3.5 ) or for rigid jointed frames too.-Mukesh Upadhyay
Ans: Even in simple connections, where only shear is expected to be transferred normally some eccentricity of load is encountered due to connection details. Hence the minimum eccentricity required..
Question: This is with reference to section 12 (Design and detailing for earthquake loads) of the draft IS: 800, its correlation with IS: 2062 and availability of steel products. Aseismic design heavily relies upon post-yield inelastic response of the structure, especially for SMF, as it is evident from the response reduction factor (R). It implies that if the design criteria for SMF are followed, then steel sections would be able to undergo plastic deformation at specific locations to form a predicted mechanism. Therefore, such frames are capable of collapse prevention and life safety performance, even if they are designed for relatively less seismic load. As per IS: 2062, Fe 410W Grade-A steel is to be adopted only for non-critical applications (Refer to Foreword), where there is no risk of brittle failure. Otherwise, Grade-B steel should be used, wherever the structure is subjected to severe fluctuation of stresses. The risk of brittle failure cannot be accepted by any means in aseismic design. This warrants usage of Grade-B steel strictly. But in reality, I understand, manufacturers (irrespective of what their catalogue shows) are producing Grade-A steel in general and Grade-B steel is not readily available. As a result, we find only Grade-A steel is used to build structures in India, except for plates having thickness > 20mm. All the structural components (except large crane girders) are of Grade-A steel, including the lateral load resisting system of the structure. So also the connections adopting plate thickness lower than 20mm. For illustration, this applies for large span trusses, columns, bracing etc.I was just pondering over ..
1) Does not this go against the criteria of earthquake resistant design by enhancing the risk of brittle failure? Here also, the structure is subjected to severe fluctuation of stresses in the inelastic range
2) If the above understanding is incorrect, than how the usage of Grade-A steel is justified in lateral load resisting system and the connections, in view of earthquake resistance? My concern is, if the material of construction (as available) does not support or not compliant with design philosophy prescribed by the code, then adhering to the codal provisions will not yield the desired result.-Subhamoy Kar
Ans: We will exclude use of grade A under seismic SMF and fatigue load condition.
Question: Appendix D: I believe the heel drop impact test is no longer used as a criterion for floor vibration—especially where floors are very light. For one it is skewed even by the added mass of the people witnessing the test. There has been far more research into this topic. Code writers should try to incorporate the state of the art information. Prof. T.M. Murray’s work has been well recognized in this area (refer AISC web site and search for floor vibrations). Floors subject to aerobic dancing should be very much within the scope of the code & not excluded. . -Vipul Ahuja
Ans: The code provision given for floor vibration is for normal use. If special situation such as the one mentioned above have to be designed for specialist literature such as the one mentioned above has to be referred to.
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 pedestal and foundation to be rigid. The IS: 800 draft proposal is to include pedestal and foundation also in the model appropriately. If the dimension of the pedestal are not available the pedestal stiffness may be assumed as given in Section 4.3.4. We will discuss in the committee regarding the boundary conditions, in the absence of any detailed procedure based on theory or test
Question: Present Steel design code IS: 800:1984 is having guidelines for crane load combinations (Article 184.108.40.206). Requirement of this provision was necessary in year 1984 since at that time prevailing loading standard IS: 875:1964 did not have similar provisions. In subsequent revision IS: 875 part 2 -1987 cl.6.4 introduced similar provisions with little more elaboration Hence, IS: 800 provisions have become superfluous and shall be removed from subsequent revisions. To my surprise I find that IS: 800-draft code has retained original provisions. A close look at both brings out small difference between two Footnote below article 220.127.116.11 of IS: 875 states "Lateral surge force...However, if there is only one crane in the bay, the lateral and longitudinal forces may act together simultaneously with vertical loads” On the other hand in Draft version of IS: 800 article 3.5.4 talks about lateral and longitudinal loads not to act simultaneously and has no such rider for one crane in the bay situation. In my opinion, crane load combination shall be left for IS: 875 to decide and shall be removed from IS: 800 which essential is for steel structure design. Also any future revision in IS: 875 may create further anomalies between two provisions. -Jignesh
Ans: We will discuss in the committee and see if we can remove all the details of crane loading from IS: 800 and refer to IS: 800-1987(Part II).
Question: As pointed out by Amit, the values of r1 and r2 have to be either both positive or both -ve. There cannot be two different cases of r1 alone being -ve and r2 alone being -ve. Something is wrong here.- S.P.Srinivasan
Ans: In Table 3.1 there is typological error in second case it should read as “if r1, is positive”.
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