e-conference on Indian Seismic Codes (January 26 - February 8, 2002) hosted by National Information Centre of Earthquake Engineering Indian Institute of Technology Kanpur ================================== Codes S. Sankaran [Monday, January 28, 2002 8:30 PM] Krishnamurthy Pandurangan [Tuesday, January 29, 2002 12:03 PM] Sunil Mahto [Tuesday, January 29, 2002 2:40 PM] Ramakanth Ingle [Monday, January 28, 2002 3:24 PM] Jitendra K Bothara [Tuesday, January 29, 2002 12:00 PM] Subhamoy Kar [Tuesday, January 29, 2002 12:14 PM] Vidyut Gandhi [Tuesday, January 29, 2002 6:53 PM] C.V.R. Murty [Wednesday, January 30, 2002 12:32 AM] Jitendra K Bothara [Wednesday, January 30, 2002 1:10 AM] Jitendra K Bothara [Wednesday, January 30, 2002 3:19 AM] R Rathi [Wednesday, January 30, 2002 11:09 AM] Alpa Sheth [Wednesday, January 30, 2002 1:05 PM] Alpa Sheth [Wednesday, January 30, 2002 4:36 PM] C.V.R. Murty [Wednesday, January 30, 2002 7:29 PM] Sunil Mahto [Thursday, January 31, 2002 9:41 AM] Durgesh C. Rai [Thursday, January 31, 2002 2:55 PM] T.K. Ghosh [Thursday, January 31, 2002 3:18 PM] Prachee Dhavlikar [Thursday, January 31, 2002 7:16 PM] Rupen Goswami [Thursday, January 31, 2002 8:04 PM] Dipak Shah [Friday, February 01, 2002 8:31 AM] Subhamoy Kar [Friday, February 01, 2002 6:06 PM] Arvind Jaiswal [Friday, February 01, 2002 11:19 PM] Arvind Jaiswal [Saturday, February 02, 2002 8:55 PM] Ramesh P. Singh [Sunday, February 03, 2002 10:49 PM] Prakash kadam [Sunday, February 03, 2002 4:28 PM] Arvind Jaiswal [Sunday, February 03, 2002 10:22 PM] S.P. Srinivasan [Sunday, February 03, 2002 9:02 PM] T.K. Ghosh [Monday, February 04, 2002 12:01 PM] D B Joshi [Monday, February 04, 2002 6:01 PM] S.P. Srinivasan [Monday, February 04, 2002 9:30 PM] S.P. Srinivasan [Monday, February 04, 2002 9:36 PM] Chinmay Gorantiwar [Monday, February 04, 2002 10:24 PM] Pramod Sahasrabuddhe [Tuesday, February 05, 2002 12:34 AM]

Arun Bhalaik [Monday, February 04, 2002 10:12 AM] Durgesh Rai [Tuesday, February 05, 2002 11:53 AM] M. Hariharan [Tuesday, February 05, 2002 8:40 PM] C. Ravi Kumar [Tuesday, February 05, 2002 11:18 PM] Arvind Jaiswal [Wednesday, February 06, 2002 10:04 AM] Reji Zachariah [Wednesday, February 06, 2002 4:40 PM] Gopal [Wednesday, February 06, 2002 6:34 PM] Hiren Desai [Wednesday, February 06, 2002 9:29 PM] S.P. Srinivasan [Thursday, February 07, 2002 8:42 AM] Jitendra K Bothara [Thursday, February 07, 2002 11:31 AM] S.P. Srinivasan [Thursday, February 07, 2002 12:45 PM] Vinay Gupta [Thursday, February 07, 2002 4:40 PM] Vinay Gupta [Thursday, February 07, 2002 4:41 PM]

S. Sankaran [Monday, January 28, 2002 8:30 PM] Dear All Interesting e conference. Please keep up. During my interaction with many designers I understood the following; 1. Many are unaware of the revised draft IS 1893.Unless published they don't give due weightage. 2. The significance of Response reduction factor R is not appreciated by many average designers. Many are unaware of the fact that more R means more inelastic deformation after the event and it is to be highligted in the code. An approximate expression is needed to know the ductile deformation in a structure which is deflection dependent. Many think that the structure is elastic during the earthquake. I hope the code relies upon the energy absorbing capacity in elato-plastic range in recommending R. This shall be specified. 3. If we need only elastic response should we take R=1 irrespective of material and type of construction? This needs clarity. The forces may be higher but the initial shape of the stucture is recoverable. This may be important in some sensitive structures which are deflection dependent. Regards. Good going indeed. s.sankaran

Krishnamurthy Pandurangan [Tuesday, January 29, 2002 12:03 PM] dear organizers, we wish to pose the following queries on the clauses of IS 1893-1984 1. clause 3.11 gives that the predominant direction of earthquake force is horizontal - is it always true?

2. clause 3.15 says that vertical component of earth quake forces need to be considered under specific situations and is not precise to in which type of structure such forces need to be considered 3.clause 3.12 says that the soil should not lose strength , but is not the actual situation and in sandy soils liquification may occur and the code is silent in these situations 4.appendix clause F3.1 the damping factor given for materials is not specific and the ranges given may generate wide range of seismic forces and hence we suggest that the damping factor should be more specific. 5.clause 3.1 value of elastic modulus for dynamic analysis may not be correct 6.In figure 2, average acceleration spectra , max value of natural period of vibration in seconds is maximum of 3.0 and in our view the natural period of buildings may go above 3.0 . in this case the chart needs modifications 7.in article 5 for elevated buildings, the water tanks are assumed to be single degree of freedom structures. 8.in article 5.2.4 full or empty conditions of checking of water tanks is specified. is that sufficient 9. in the article 5.2.3 the explanation given for the calculation of defections is not clear 10.In clause 4.2.1 c> building height greater than 40 is given we suppose that it should be greater than 90. 11. in figure 5A , the figure given is for the building and it would be better that a three dimensional drawing is given or the title may be changed to building instead of frame to avoid ambiguities. Krishnamurthy

Sunil Mahto [Tuesday, January 29, 2002 2:40 PM] Sir, Please accept my good wishes for this wonderful idea of an e-conference on earthquake engineering/ codes. 1. As most of us are aware, the provisions of IS:13920 for ductile detailing are the same for all Zones where ductile detailing is applicable. Would it not be more logical to have a less stringent requirement for the zones with lesser seismic coefficient? Ductile detailing increases reinforcement consumption by about 20 - 30 % which is considerable. 2. What is the ductility level (in terms of ratio) sought to be achieved by the specified recommendations?

3. It is to be noted that the IS:1893-1984 is under revision. The draft itself, is constantly undergoing revisions and difference of opinion between experts still persist. In such a scenario, it is not advisable to adopt the draft for design purpose unless and until the revisions are accepted and formally incorporated. Moreover designers will be facing the problem of having to redesign everytime a draft code is revised. A distinction has to be made and understood especially between research and actual project execution which have many other constraints such as cost and time. 4. In my opinion, based on reports of the recent Bhuj earthquake, structures which were designed as per the existing IS codes suffered minimal damage. Therefore more than anything else, the engineering community should pressure the authorities for strict compliance to and enforcement of Indian Standards wherever applicable. Any amount of research and technical deliberations will not be fruitful unless allout effort is made for ensuring that recommendations are not flouted. SUNIL MAHTO

Ramakanth Ingle [Monday, January 28, 2002 3:24 PM] Sir, I have following few querries: 1. The locations of the tie beams shall be specified in the code. In case of hard soil, the distance between footing top nad plinth beam may not be more than 1.5m. Whether the plinth beams can be considered as tie beams? 2. Draft code suggest number of modes to be used in the analysis should be such that sum of total modal mass of all the modes considered is at least 90% of the total mass. What to do with the balance mass? The balanced mass may be considered as static correction to total response. Code is sailent about this. 3. The clause regarding to e = 1.5 esi +/- 0.05 bi is applicable to all buildings i.e. symmetric and unsymmetric or not. In case of sym bldgs whether e = 0.05 bi shall be considered or not, being 0.05bi as accidental eccentricity. 4. In many softwares, we can supply the actual mass centre and stiffness centre is evaluated by software, whether torsion correction given in code is still required to do in this case or not? Enough for today Regards Dr R K Ingle

Jitendra K Bothara [Tuesday, January 29, 2002 12:00 PM] Hi, It is in response to quary from Dr. Ingle, Of course, different code requires the addition (+/-) of accediantle eccentricity over the calculated one. Even though we "calculate" mass centre and stiffness centre manually or by

computer there remains many uncertainties associated with these centers. Some are actual stiffness distribution, different degree of stiffness degradation of laterl force resisting system during the inelastic response of the building, variation in material property, distribution of mass etc. There also remains uncertainty in tortional ground motion. Jitendra K Bothara

Subhamoy Kar [Tuesday, January 29, 2002 12:14 PM] Hello... This is in continuation with the mail from Dr. R K Ingle included below. I have a humble clarification about point no. 2 of the mail. During any dynamic or pseudo-dynamic analysis the progress of mass participation becomes very slow after 90% mass participation is achieved. This convergence depends on the boundary conditions and global stiffness of the structure. In any case 100% mass participation is not possible as because the structure is fixed at some boundary and the small fraction of total masse located near those supports will not get excited as the masses located far from supports. That is the reason why it takes a large number of iterations to converge mass participation from 90% to 95%. Also, it is to be noted that once 90% mass participation is achieved, the higher modes will have appreciable effect on the response of the structure. For example, if the total mass participation at 'n' th mode is 90% and at 'n+1' th mode is 95% (it is never so high!), then we can say 5% mass participation is contribution of 'n+1' the mode. But as a result of this, the combined response of the structure calculated by SRSS method is given by SQRT(90^2+5^2) = 90.138. So, the increase in total structural response is marginal. This is occurring due the reason that modal responses are not combined by absolute summation and instead a total statistical response (SRSS or CQC) is determined and used in structural analysis. To my understanding, the code restricts the number of modes up to 90% of mass participation as because, the rest of the mass is not expected to have appreciable effect on the total dynamic response of structure. Thanks and regards. SUBHAMOY KAR

Vidyut Gandhi [Tuesday, January 29, 2002 6:53 PM] Sir, at the end of clause 7.8.1 ( IS-1893 ) there is a note that for irregular buildings lesser than 40meters in height Dynamic analysis is recommended. It means that even in town where normal building does not go beyond 30 mt. & irregularities exists both in plan & elevation ; Dynamic analysis should be preferred. is there any way to design by static method by making some corrections? Dynamic anlysis in underdeveloped region will be a costly affair.

what do you suggest? vidyut gandhi

C.V.R. Murty [Wednesday, January 30, 2002 12:32 AM] Four wonderful points from Mr.Mahto..... 1. Would it not be more logical to have a less stringent requirement for the zones with lesser seismic coefficient? Yes... absolutely desirable!! But, the Indian seismic design scenario is still in its nascent stage. We need to get comfortable with (a) Good Gravity Load + Wind Load Design, and (b) Good Basic Seismic Design (with Seismic Coefficient Method to begin with), in that order. Then, we can have an Indian Code that will have more than one type of ductile detailing. Ductile detailing increases reinforcement consumption by about 20 - 30 % which is considerable. Is this % of the "structure alone" or of the "structure plus finishes"? Some say the "structure alone" costs only 30% of the whole. We need many consultants to speak up their numbers, and only then we will know if the additional cost of providing seismic resistance is indeed considerable!! Hope other consultants will also give their experiences. 2. What is the ductility level (in terms of ratio) sought to be achieved by the specified recommendations? An overall structure ductility of 4-6 will be the ideal situation. But, we need to do a large number of experiments on prototype structures/subassemblages designed with Indian seismic code and understand the ductility that can be achieved. The limited experience in our laboratory at IIT Kanpur with RC beam-column joint sub-assemblages designed with IS:13920 detail shows that it is around 6-8 (some details in ICJ, Vol.75, No.4, April 2001). But, the "overall structure ductility" with this "subassemblage ductility" will be smaller. More work needs to be done to know how much "overall structure ductility" will be available..... 3. The draft itself, is constantly undergoing revisions.... Just a clarification. All the inputs into the fifth revision of IS:1893 (Part1) on General Provisions and Buildings are frozen, and the manuscript has been sent for printing. There are no more changes that are underway. So, the latest draft of late 2001 (if you have one) is as good as final. So, please dont hesitate to discuss the draft provisions. 4. Therefore more than anything else, the engineering community should pressure the

authorities for strict compliance to and enforcement of Indian Standards wherever applicable. An addition:: The structures are being built by the "engineering community" and not the "authorities". So, why pressure authorities....., why not itself first? ....... The issue is not "engineering community" VERSUS "authorities"....!! A better way of looking at things is:: "engineering community" WITH "authorities"....!! For every structure, if the OWNERS want SAFE structures, they can get them, else they will have only CHEAP ones, which may not be safe. Thus, owners with the assistance of authorities must ask for safe structures first and economical ones later....... c.v.r.murty

Jitendra K Bothara [Wednesday, January 30, 2002 1:10 AM] Dear Friends, 1. I agree with Prof. Murty about the nascent stage of aseismic design and construction so major thrust should e on consolidation of it. 2. According to my experience, up to 3 storey RC framed building additional cost earthquake resistance comes out something like 10% over the normal construction cost for ordinary building. In case of masonry building with brick in cement mortar and RC floor and roof slab it comes around 5-6% additional cost if detailed according to Indian standard. Jitendra K Bothara

Jitendra K Bothara [Wednesday, January 30, 2002 3:19 AM] Dear friends, let me do some more clarification that I missed in last mail. According to my experience, additional cost for eathquake resistance of up to 3 storey RC framed building comes out something like 10% over the normal construction cost. In case of masonry building with brick in cement mortar and RC floor and roof slab it comes around 5-6% if detailed according to Indian Standard. Of course I am talking of structural costs only (excluding finishing cost) and ordinary residential buildings with span in the range of 3-5m. Jitendra K Bothara

R Rathi [Wednesday, January 30, 2002 11:09 AM] Congratulations to NICEE (IIT Kanpur) and every body for this very educative e-conference. I have a query on the Parameter for Soil Profile Type SA to SF as defined in UBC 97 - Table

16 J. A similar parameter Sa/g is being suggested in 1893 draft code to account for the Soil profile type. Are these parameters totally independent of the foundation system? If so, is there any other parameter to account for the different foundation systems like isolated footings, raft , pile foundations etc in calculating Design Base Shear for the structure? Regards R Rathi

Alpa Sheth [Wednesday, January 30, 2002 1:05 PM] Murty,

>1. Would it not be more logical to have a less stringent requirement for the zones with lesser seismic coefficient? > Yes... absolutely desirable!! But, the Indian seismic design scenario is still in its nascent stage. We need to get comfortable with > (a) Good Gravity Load + Wind Load Design, and > (b) Good Basic Seismic Design (with Seismic Coefficient Method to begin with), in that order. Then, we can have an Indian Code that will have more than one type of ductile detailing.

Murty, I tend to agree with you. ACI, UBC which presumably are not in the nascent stage do not have different types of ductile detailing for their zones 3 and 4. But they have diff criteria for their Zone 2. The Zone 2 requirements are similar to IS 4326 ductility requirements ie Intermmediate Moment Resisting Frames. The IS 13920 requirements of capacity design, Clauses 6.3.3 and 7.3.4, are not applicable to UBC Zone 2. These clauses are not quite the same but seem to be derived from the philosophy of those for UBC Zone 3 and 4. In my humble opinion, one of the interpretations for no difference in ductility criteria of IS Zones IV and V could be that the lesser seismic risk of Zone IV as compared to Zone V is already being acknowledged with a lower seismic coefficient. However, the ductility which is called upon in Zones IV and V during the event of an earthquake remains the same. Hence the ductile detailing rules do not change. But our Zone III requirements of IS 13920 for 5 storeys and higher continue to flummox me! Take a 6 storey building, resting on v hard strata, regular etc. Let's compare the UBC Zone 2A with our IS 1893 Zone III As per UBC, Zone 2A, An intermemediate MRF of 6 storeys (20m) will have a seismic coeff factor =0.15*1*1.6/8 =0.03W (I'm skipping the steps) For a 6 storey frame structure as per IS 1893, in ZONE III, T=0.6, seismic coeff for such a case = 1.0*0.75*0.04=0.03W

Thus I.S. Zone III and UBC ZOne 2A have a similar seismic coeff for the same structure. (This ofcourse is a gross simplification!) Would it not be more logical then, to accept ductility requirements of IS 4326 for Zone III without those backbreaking clauses 6.3.3 and 7.3.4 of IS 13920? In Zones I and II, I suppose there is little seismic risk and the overstrength factors and the factors of safety (besides inherent nominal ductility) can themselves take care of the larger demand during the earthqauke and hence additional ductility provisions are not called upon.

> > Ductile detailing increases reinforcement consumption by about 20 - 30 % which is considerable. Is this % of the "structure alone" or of the "structure plus finishes"? Some say the "structure alone" costs only 30% of the whole. We need many consultants to speak up their numbers, and only then we will know if the additional cost of providing seismic resistance is indeed considerable!! Hope other consultants will also give their experiences.

I would agree about this increase. The increase of 15-25% steel in numbers works to about 8%- 15% of the RC cost and varies from 3% to 8% increase on the structure plus finishes cost- depending on which part of the country you are working. I would want to know from designers, how much is the increase in cost due to conformance to IS 13920 CLauses 6.3.3 and 7.3.4. assuming the ducitlity criteria of IS 4326 are being provided in any case?

> >2. What is the ductility level (in terms of ratio) sought to be achieved by > > the specified recommendations? > An overall structure ductility of 4-6 will be the ideal situation. > But, we need to do a large number of experiments on > prototype structures/subassemblages designed with > Indian seismic code and understand the ductility that > can be achieved. The limited experience in our laboratory > at IIT Kanpur with RC beam-column joint sub-assemblages > designed with IS:13920 detail shows that it is around 6-8 > (some details in ICJ, Vol.75, No.4, April 2001). > But, the "overall structure ductility" with this "subassemblage ductility" > will be smaller. More work needs to be done to know > how much "overall structure ductility" will be available.....

Have you done any tests for only IS 4326 ductile detailing? WHat kind of ductility ratios do you come up with? Thanks and regards, Alpa

Alpa Sheth [Wednesday, January 30, 2002 4:36 PM] A clarification. In my earlier post, when I referred to the requirements of IS 4326, I was referring to clauses of IS 4326 -1976 and not the 1993 revision. Apologies if there was any confusion in this matter. Thanks, Alpa

C.V.R. Murty [Wednesday, January 30, 2002 7:29 PM] Alpa:: Thanks for your note.

> Would it not be more logical then, to accept ductility requirements of IS 4326 for Zone III without those backbreaking clauses 6.3.3 and 7.3.4 of IS 13920?

Wonderful thought!! I agree with you that it is desirable to have:: (a) Zones I and II:: No special ductility provisions (b) Zone III:: No back-breaking provisions of IS13920-1993; and simple IS4326-1976 type provisions should do. (c) Zones IV and V:: Major ductility provisions of IS13920-1993. The basic concern is the seismic zone map. We are not yet sure that zone III means moderate level risk in FUTURE EQs. We may say, zone III means MSK VII level shaking in PAST EQs. So, once we have a consistent seismic zone map, considering possible future events on known faults also, we can have the 1976 provisions for zone III...!!

Have you done any tests for only IS 4326 ductile detailing? WHat kind of ductility ratios do you come up with?

We did these experiments in around 1995, so we were concentrating on the "new" IS:13920-1993 only. As I said, we tested only RC exterior subassemblages and they gave ductility around 6-8. Of course, the ductility values will reduce at the overall structure level!! warm regards... murty

Sunil Mahto [Thursday, January 31, 2002 9:41 AM] Dear Mr. Murty, 1. The % increase quoted by me was for increase in reinforcement. The overall increase in cost of buildings works out to approx. 8-10 %. 2. As per Park & Paulay (reference), the ductility ratio in normal buildings without ductile detailing is approx. 5. Considering this to be O.K. one would expect that with ductile detailing provisions, this ratio would be considerably higher than 6-8 mentioned by you.

3. The help of authorities (with some biting teeth) within or without the engineering community is essential to take care of the black sheep in our engineering fraternity. For this administrators have to be given teeth to bite. I dont think our bureaucrats will want to give any such power to the engineers. Hence it is they who will have to look after the enforcement part- ofcourse with the help of the engineering committee. Regards SUNIL MAHTO

Durgesh C. Rai [Thursday, January 31, 2002 2:55 PM] Replying to Ramanuj Rathi on Soil factors In the draft 1893 the soil site effects are treated in the same fashion as in the UBC-97. However, it has only three soil types: hard (rock), medium and soft, with three different relations of Sa/g w.r.t. natural time period (T), one for each soil type. The factor Sa/g is not new: in the current 1984 code, there is only one relation for Sa/g which is modified by multiplying a soil-foundation factor (beta) which is constant for all natural periods of the spectrum and its minimum value is unity (i.e. no increase in design base shear). In other words, its independency with T means that shape of design spectrum remains same. This beta is explicit function of a foundation system, i.e., isolated footings vs. raft foundation. A closer look at the table for beta suggests that larger values are endowed to footings which are more vulnerable to differential settlement. However, the observed strong ground motion data on various soil sites indicates that local soils tend to change the shape of spectrum from that of at bedrock/rock sites. That is, their influence (usually amplification of spectral values) is predominant over a certain period range which usually lies in the velocity sensitive region of the spectrum. Large spectral velocities have been observed on deep alluvium sites in comparison to rock sites. Thus, the current thinking is to de-link the soil types from the foundation types, while specifying the design base shear (i.e. design spectrum). The settlement vulnerability of the structure has insignificant effect on the earthquake-induced inertia forces. The effect of foundation structure may be treated separately as a part of soil-structure interaction. Also refer to paper by Sudhir Jain on proposed 1893 draft code and commentary, which is available among the conference's resource materials at the NICEE web site Durgesh Rai

T.K. Ghosh [Thursday, January 31, 2002 3:18 PM] 1. As per Cl 7.3.4 of IS 13920, the design shear force for col shall be max. of that as per analysis or the elastic col shear based on the moment of resistance of the joining beams framing into the col. Logic of considering moment resistance of the joining beam is not understood since the maximum shear to which the col can be subjected even during plastic hinge formation at top and bottom ends is the sum of moment of resistances of the two ends of the col divided by its

length. This shear may be more than that as per codal provision if the col is stiffer than the joining beams and vice versa. 2. As per 7.3.4 of SP-34:- when a column at a particular floor level is smaller in crossection than the col immediately below it and the offset is more than 75 mm, the longitudinal bars are to be terminated at the floor slab and separate dowels to be used whereas as per cl. 7.2.1, lap shall be provided only in the central half of member length which is contradictory. This can be avoided by restricting the reduction of col section within 75 mm offset which may lead to provision of a much higher section than required in the upper floor, specially when bottom floor col section requirement is high because of some specific requrement like movement of crane or similar. tk ghosh

Prachee Dhavlikar [Thursday, January 31, 2002 7:16 PM] Dear organizers and colleagues, I would like to make a suggestion: The recent earthquake at gujrat made me ponder over many points. Malpractices and igorance of earthquake detailing were among the main factors responsible for the disaster. Which could have been avoided. But an important point which I want to mention is that of soil structure. I live in Pune city,which comes under the zone III as well as Ahemdebad. Though the zoning has been done with respect to the earthquake intensity, it does not at any time consider the soil characteristics. To elaborate it further, Pune lies on the Deccan Plateau which has Basalt underlying. Whereas Ahemedebad sandy soil underlying it. If we consider liquefaction of soil, the effect of similar magnitude earthquake in Pune and Ahemedebad will have different implications on the behaviour of superstructure. Zoning therefore needs to be based also on the soil characteristics. Or while doing analysis, ie IS1893 or IS 13920 should mention an additional check to account for this effect. I have mentioned this points in one of my papers published in an journal in Pune. Unfortunately I have never had the time to investigate or study this matter in detail. It would be interesting if someone does this study, so as to quantify or rather qualify the suggestion I have done. Information would be most welcome Regards Prachee Dhavlikar

Rupen Goswami [Thursday, January 31, 2002 8:04 PM] Mr. K. Pandurangan, The responses to your queries on some of the clauses in IS: 1893 - 1984 are as follows:: >1. clause 3.11 gives that the predominant direction of earthquake force is horizontal - is it always true? No, it is always not true! There may be a vertical component of it also. However, the

additional (+/-) vertical force (along gravity) is generally taken care of by the factor of safety against gravity loads that are already accounted for in the design. Here, our main intension is lateral load resistant design, and hence, vibration in horizontal direction is considered. >2. clause 3.15 says that vertical component of earth quake forces need to be considered under specific situations and is not precise to in which type of structure such forces need to be considered It clearly says where "stability" is a criterion, vertical component needs to be considered. Moreover, for structures having very large spans, the additional moment may be large due the vertical component, and in such situations vertical component becomes important. >3.clause 3.12 says that the soil should not lose strength , but is not the actual situation and in sandy soils liquification may occur and the code is silent in these situations No, the Code is not silent on this issue. Please refer to Note 3, Table 1 of the Code. >4.appendix clause F3.1 the damping factor given for materials is not specific and the ranges given may generate wide range of seismic forces and hence we suggest that the damping factor should be more specific. Yes, it results in a wide range of force values if Response Spectrum Method is used, but the exact damping in a structure is also difficult to ascertain. However, 5% of critical is the damping in most of the general concrete structures, and the new draft Code gives the spectrum for 5% damping to be used for such structures. >5.clause 3.1 value of elastic modulus for dynamic analysis may not be correct I think you are refering to the elastic modulus of concrete which is usually taken as that given in IS: 456. However, in my understanding, a lower value accounting for cracked concrete should be used. >6.In figure 2, average acceleration spectra , max value of natural period of vibration in seconds is maximum of 3.0 and in our view the natural period of buildings may go above 3.0 . in this case the chart needs modifications Yes, in some special cases, the period may go above 3.0, and the new dreaft Code has it upto 4.0 sec. >7.in article 5 for elevated buildings, the water tanks are assumed to be single degree of freedom structures. The water tanks when not "full" will definitely not behave as SDOF. The sloshing of the liquid should also be accounted for, and the sloshing mode of vibration will also play a role in the overall response of the structure. However, the two periods are generally well apart, and the problem may be considered as that of two uncoupled SDOFs.

>8.in article 5.2.4 full or empty conditions of checking of water tanks is specified. is that sufficient . In full condition, maximum restoring force is obtained, while in empty condition, restoring force is minimum, and a check against tension can be done. It seems to be sufficient. >9. in the article 5.2.3 the explanation given for the calculation of defections is not clear I think, all we need here is the stiffness (k=F/delta) of the system. The period is 2*pi*sqrt(m/k), and this reduces to the formula given in the Code, if a lateral load (F=m*g) equal to the weight of the system is applied. >10.In clause 4.2.1 c> building height greater than 40 >is given we suppose that it should be greater than 90. Yes, it should be greater than 90m. >11. in figure 5A , the figure given is for the building and it would be better that a three dimensional drawing is given or the title may be changed to building instead of frame to avoid ambiguities. Yes, it would be best to have a 3D figure explaining the concept. rupen goswami.

Dipak Shah [Friday, February 01, 2002 8:31 AM] Hello Friends, Namskar & Good Morning, Dear Mr. Rai, The Final Draft Code defined soil profile as > "The soils which have shear wave velocity greater than 750 m/s and where the soil depth is less than 50 m and the overlying rocks are stable deposits of sand, gravel or stiffer clay is considered rocky site. The soils which have shear velocity less than 750 m/s including sites where the soil depth exceeds 50 m and the overlying soil types are stable deposits of sands, gravels or clays is considered soil site" [As I know this statment is removed from code !] How to determine the soil profile? What should be the depth of soil investigation? Dipak Shah

Subhamoy Kar [Friday, February 01, 2002 6:06 PM] Hi Gentlemen...

This refers to point no. 1 of Mr. T.K.Ghosh's mail (included below). I guess, this aspect of seismic detailing has been tabled by many others in the past few days. Clause no. 7.3.4 of IS:13920 stipulates that for RCC columns, apart from the shear force obtained from structural analysis, the same shall also be determined on the basis of beam moment capacities and compared with the former one. The higher value out of these two shear forces shall be considered in design. To my understanding this codal provision stems from the requirement that building columns should stand upright even after formation of plastic hinges at the beam-ends. If we consider free body diagram of the beam column joints, moments developed at the beam-ends will induce a total unbalanced moment on the joint. To have a joint equilibrium, this unbalanced moment is to be resisted by the upper and lower segment of the column. This phenomenon have been explained in Figure * 8 of IS:13920. This idea should be understood in the light of CAPACITY DESIGN and not as the design under working loads due to earthquake. To explain it further, a shear capacity is being built into the column section so that it does to fail (in shear) even when the beams have reached their limiting moments. This genesis of this capacity design is logical, because the actual seismic force in reality may be much higher than the calculated seismic force. I appreciate the Design Engineers' concern to follow this stipulation, as the project schedule and sequence of work hardly permits to exercise this option. But the engineering justification of this work can not be ignored. I think the project Owner's are also required to be considerate to the Engineers (in terms of time) if they really want safe, sound and healthy structures. Thanks and regards. SUBHAMOY KAR

Arvind Jaiswal [Friday, February 01, 2002 11:19 PM] Dear friends, For ductile failure of a multistorey building due to an earthquake, it is necessary to design the structure to have strong column and weak girder concept. This concept ensures that a plastic hinge is formed in a beam rather than column and column remains elastic as for as possible. (This also calls for the importance of splicing of longitudinal reinforcemnt.) Such a provision of strong column and weak girder was present in the draft of IS: 13920, but it had been removed from the final 1993 version. Any particular reason for not including at that time? Is it not desirable to have the clause? What is contained in the proposed final code? with warm regards......... Arvind

Arvind Jaiswal [Saturday, February 02, 2002 8:55 PM] This is in response to Mr. Mahto "1. As most of us are aware, the provisions of IS: 13920 for ductile detailing are the same for all Zones where ductile detailing is applicable. Would it not be more logical to have a less

stringent requirement for the zones with lesser seismic coefficient? Ductile detailing increases reinforcement consumption by about 20 - 30 % which is considerable" Arvind's response: I would be very cautious with our experience of Latur, the earthquake struck in the zone which was not an active zone as per seismic zoning map. Secondly The cost of normal frame with reinforcement works out to say approximately Rs. 1000 per Sqm. (Where frame consists of foundation, plinth beams, columns, roof slab, beams and staircase etc.) Now this is with consumption of steel of approximately 32 to 37 Kgs / Sqm. Now if this has to go up by say 20 % then the steel consumption will be 38.5 Kgs to 44.5 Kgs. This increase in cost of 44.5 Kgs-37.) Kgs works out to Rs. 7.5 x 23 = Rs. 172.50. This is an increase of about 17.25 % above the basic frame cost of Rs. 1000 per Sqm and if we take basic cost of construction with finishes, electrical, sanitary etc. to Say Rs. 4900.00 per Sqm then it works out to be an increase of only 3.5 % of the total cost of construction on account of ductile detailing and extra consumption of reinforcement. Hence I feel we professionals should not make a hue and cry on this small increase for additional safety and gauranteed performance in future shaking. The clients have to be convinced about very marginal cost increase upto maximum say 5% only and assured benefits will outplay the extra cost incurred! Mr Mahto wrote: " 3. It is to be noted that the IS: 1893-1984 is under revision. The draft itself is constantly undergoing revisions and difference of opinion between experts still persists. In such a scenario, it is not advisable to adopt the draft for design purpose unless and until the revisions are accepted and formally incorporated. Moreover designers will be facing the problem of having to redesign every time a draft code is revised. A distinction has to be made and understood especially between research and actual project execution which have many other constraints such as cost and time." Arvind's response: The word "change" brings natural resistance from human reflex action. But this does not mean we should stop thinking, stop adopting new ideas, stop improving. This is a never ending process. But I agree with you on one point that there will always be difficulty in certification of structures built with the previous codal requirements once the code is revised. This needs more attention from our friends. with warm regards.........Arvind

Ramesh P. Singh [Sunday, February 03, 2002 10:49 AM] Response to Mr. Shah's query How to determine the soil profile ? One of the crude methods is to make a borehole and study the soil characteristics along the borehole. Depending upon the construction project (in terms of size), one may carry out seismic refraction or resistivity soundings for soil information. > What should be the depth of soil investigation ? It all depends on the size of the project. The maximum depth of the soil investigation should be upto 500 m for a mega project. However, a broad geological/geophysical knowledge of the area should be known before a project and its site is decided. Ramesh P. Singh

Prakash Kadam [Sunday, February 03, 2002 4:28 PM] hi, Reduced live load during seismic- In industrial structures, %live load considered for seismic analysis should be for entire floor .For a floor normally designed for a generalised live load of 7.5 kn/sqm say 50% load considered for seismic analysis means total load of 1350 kn for total floor area of say 360 sqm of area which is ridiculously high. Wl meet again. take care. prakash kadam

Arvind Jaiswal [Sunday, February 03, 2002 10:22 PM] This is in response to Mr. Kadam who wrote: Reduce live load during seismic-In industrial structures, %live load considered for seismic analysis should be for entire floor .For a floor normally designed for a generalized live load of 7.5 kn/sqm, say 50% load considered for seismic analysis means total load of 1350 kn for total floor area of say 360 sqm of area which is ridiculously high. Arvind's comment: It is required to investigate the provisions in codes of other countries. Our friends from NZ, US, UK please offer your comments.

Also I would like to bring to the notice Note 2 in IS: 1893 after Clause 4.1.1 Design Live Loads which says "The proportions of live load indicated above for calculating the horizontal seismic forces are applicable to average conditions. Where the probable loads at the time of an earthquake are more accurately assessed, the designer may alter the proportions indicated or even replace the entire live load proportions by the actual assessed load." This means designers have been given some flexibility. But how to accurately assess probable loads at the time of an earthquake? Please clarify. with warm regards.........Arvind

S.P. Srinivasan [Sunday, February 03, 2002 9:02 PM] Hello everybody, Effect of similar earthquakes on cities and villages are different, both in terms of lost human lives and in terms of shattering of economy. Should we not have a factor to take these into account? Why don't we introduce an additional factor for seismic coefficient? This factor could be 1.3 for the four cities of Delhi, Mumbai, Kolkatta and Chennai, 1.2 for other major cities like Bangalore, Hyderabad, Pune etc, 1.1 for smaller cities and towns and 1.0 for villages and wilderness? Regards, S.P.Srinivasan

T.K. Ghosh [Monday, February 04, 2002 12:01 PM] Sir, Ref : Cl 6.3.3 & 7.3.4 of IS:13920 1. Referred expressions for shear forces are based on the assumption of weaker beams and stronger col philosophy. But in practice, for a multi storey frame, beams normally have more ultimate moment carrying capacity than the cols. Reasons being:- a) Under elastic analysis with earthquake forces, the moment in the beams & cols are almost of the same magnitude under EQ load With DL & LL. b) Cols are subjected to compressive force in addition to moments thereby requiring les reinforcement area per face compared to that of beams.

Thus it is felt that either there should be a specific codal provision to ensure more ultimate moment carrying capacity of cols than that of beams in each joint to keep the expression under referred clauses valid or else these clauses need to be modified. Regards tk ghosh

D B Joshi [Monday, February 04, 2002 6:01 PM] I wish to understand IS:1893-2001 [Draft], when read along with 1997-UBC. Ref. Cls. 1612.2 and 1612.3. of UBC. I understand that 'E', Seismic Forces as determined by Codal Procedures, is of the nature of Limit State. Meaning that no load Factor has to be applied to 'E'. Similarly for Allowable Stress Design Method 'E' has to be divided by 1.4 in the load combination. This provision will reduce Design Seismic Forces and help in achieving economy. As I understand that revised IS Code Procedures are in line with UBC. How do we draw parallel? I request author of Revised IS Code to throw light on this matter. Wishing you well, D B Joshi

S.P. Srinivasan [Monday, February 04, 2002 9:30 PM] Hello everybody, The Draft IS 1893 deals with liquefaction of soil under the clause on 'Increase in Permissible Stresses'. I hope it has been moved to a separate section in the final printing of the code. Note 3 under Table 1 contains a table indicating desirable minimum N values (in liquefiable strata) for various seismic zones and at various depths. In this table, Zone V is not mentioned. I would like to know whether Zone V has been included in the final version of the code. And what are the relevant details for Zone V? Also there is no mention about normal structures in Zone II. Now, as I understand, when liquefaction occurs below founding level, no analysis or design or detailing is going to save the building. It appears as if the code is almost implying, 'do not worry about liquefiable layers below residential buildings in Zone II; if soil liquefies, let them collapse'. I think that the Zone II parameters given in the table should be made applicable both to normal buildings and important buildings. Any comments? Regards S.P.Srinivasan

S.P. Srinivasan [Monday, February 04, 2002 9:36 PM] Hello everybody, All engineers know that when a rebar has a bent shape and is stressed, it imparts local bearing stresses on the concrete at the location of the bend. IS 456 contains a clause for calculating this stress and checking for safety. If the bend is as per the provisions of IS:2502, the stresses will be within permissible values. However, when we give full anchorage to reinforcement at discontinuous ends, the bars have to be extended well beyond the bend. This results in high stresses in the bars at the bend. This in turn increases the bearing stress in concrete at the location of the bend. The only way to solve this problem is to increase the radius of curvature of the bar such that the calculated bearing stress is less than the allowable value. This is a simple calculation, but is not done by many engineers. This aspect has to be highlighted in IS 13920 since integrity of the beam-column junction is important. Figures 1, 5, and 9 of the above code should show the bars bent with a large radius. Regards S.P.Srinivasan

Chinmay Gorantiwar [Monday, February 04, 2002 10:24 PM] dear sirs, Sir in second draft copy of is-1893 also doesn't include any method for calculating the time period infill frame with brick masonry with opening Door/window. V. Thiruvengadam in 1984 gives the multiple strust anology method. But results are to much differ than experimental results.Then is these method is able to use for analysis purpose. Chinmay Gorantiwar

Pramod Sahasrabuddhe [Tuesday, February 05, 2002 12:34 AM] I am listing down my questions/suggestions. A) 1893 (present as well as draft) 1. Is zoning done on the basis of probability of occurrence of earthquakes? UBC zoning is done on 10% probability (if I recall correctly) in 50 years. What would be equivalent zoning of India if same standards are used? 2. Is there any move to publicise data of recent earthquakes? Data on earthquakes is kept in closed garbs. Currently you have to approach Mausam Bhavan of Delhi to get any data and it is not easy to get it (I have tried it without success). At any time there is more data available on USGS site than any other Indian site. Does any one have seismograph of Bhuj Earthquake? Have we tried simulation of this earthquake on existing buildings?

3. Soil parameters (no more questions than what has been discussed) in the draft code seem to be inadequate (only two classifications). 4. Time period: The basic time period of any structure depends on its stiffness and mass (matrices in complex system). The new code seems to have only one factor height which poorly represents stiffness (as there is no width consideration) and no mass considerations. Although infilled frame structures are stiffer (than open structures) these are also heavier (thereby increasing the time period). Can't we have a better time period formula than what is proposed? The formula which is being proposed is similar (nearly same) to UBC. Can we put in same logic (where light partition wall is a practice than heavier infilled walls) and formula? 5. Should this time period be made compulsory requirement? If one takes into account both stiffness and weight of infills should it not be taken as better approximation? 6. I fail to understand the load factors given in code. As I understand the load factors represent the probability of occurrence of any load. Currently we have 1.2 factor for combination of dead+live+earthquake loads and 1.5 factor for combination of dead +earthquake load. Does it mean that the probability of occurrence of earthquake is much higher when all live load is gone? Or does it mean that buildings should survive when everyone has vacated the premises? B) 13920: (can this code be renamed as ductility and sizing code?) 1. Ductility criterion: If one tries to economize the structural cost (as it is one of the prime objective of structural engineers) one must reduce grade of concrete. (as you have to provide higher percentage of reinforcement in higher grade of concretes). Does this make sense? Generally one specifies a grade of concrete however what you get (thanks to better technology) is higher grade concrete than specified. Should one ask contractor to demolish a part of building if one gets higher strength? (as the resultant building will not be ductile!). 2. 135 degree hook: Can't there be better alternatives to this? These hooks create nuisance for vibrators. There should be alternatives like a) welded links b) links with higher laps which are tied by designed binding wire. 3. Cross tie: This forms third layer in the reinforcement. Does one increase cover to take care of this? Or should one sacrifice fire criterion at these little points? The cross tie is required to prevent buckling of main column link. Can one have improvements in the detailing of the cross tie? (such as welded or better tied normal tie link). If one calculates axial load carrying capacity of main column link and adopt 2.5% criterion (as in steel design) to fulfill this requirement, one can design a strong binding wire to take the load. C) On Bhuj Earthquake: (since many of the contributors are from the region my questions are addressed to them and other researchers). 1. Ahmedabad Scenario: What is the statistical distribution of collapsed buildings. a) storey wise b) design wise (i.e. unengineered buildings, RCC buildings designed without horizontal

load considerations, RCC buildings designed with 1893 load provisions, RCC buildings designed with 1893 provisions and detailed as per 13920) c) What is their rough percentage in each criterion. Was there any strong evidence of collapses due to soil, no plinth/tie beams, very poor quality concrete? 2. Kutch Scenario: Statistics on similar lines as above.

Arun Bhalaik [Monday, February 04, 2002 10:12 AM] "In storage and warehouse occupancies, a minimium of 25 percent of the floor live load shall be applicable" UBC 1630.1.1 (1997). Alltough I have used higher percentage of Live Load in consultation with the Owner. Arun Bhalaik

Durgesh Rai [Tuesday, February 05, 2002 11:53 AM] Mr Dipak Shah wrote: > The Final Draft Code defined soil profile as > "The soils which have shear wave velocity greater than 750 m/s and where the soil depth is less than 50 m and the overlying rocks are stable deposits of sand, gravel or stiffer clay is considered rocky site. The soils which have shear velocity less than 750 m/s including sites where the soil depth exceeds 50 m and the overlying soil types are stable deposits of sands, gravels or clays is considered soil site" [As I know this statment is removed from code !] > How to determine the soil profile ? What should be the depth of soil investigation ? You are referring to the 2000 draft. The 2001 draft has 3 soil types -- rock or hard soil, medium soils and soft soils -- characterized according to N values of SPT at the founding level. N is greater than 30 for hard soils, between 10 and 30 for medium soils, and less than 10 for soft soils. In fact, it is following the same definition of soils as given in the Table 1 of the current code for allowable bearing pressure. The characterization based on shear wave velocity is dropped in the subsequent revision! Durgesh Rai

M. Hariharan [Tuesday, February 05, 2002 8:40 PM] You may refer to American Petroleum Institute's Publication, API RP 2A _ Recommended practice for... Designing Fixed Offshore Platforms. There are three response spectrum curves for the three types of soils. The spectral ordinate peaks at 2.5 (normalised Sa/g) and drops at 0.8/T, 1.2/T and 1.8/T for the three types of soils.(Rock, Shallow strong alluvium and Deep

strong alluvium respectively), where T is the period in seconds. There is no soil effect for periods less than 0.32, 0.48 and 0.72 seconds for the three types of soil. This may be of some use. M. Hariharan

C. Ravi Kumar [Tuesday, February 05, 2002 11:18 PM] Dear Sir, is clause 3.3.1 (permissible increase in material stress) & clause 3.3.3 (permissible increase in allowable bearing pressure of soil) applicable when seismic forces are combined with dead load only or is it applicable only when it is combined with both dead & live loads. Regads C. Ravi kumar

Arvind Jaiswal [Wednesday, February 06, 2002 10:04 AM] This is in response to following comment: "I fail to understand the load factors given in code. As I understand the load factors represent the probability of occurrence of any load. Currently we have 1.2 factor for combination of dead+live+earthquake loads and 1.5 factor for combination of dead +earthquake load. Does it mean that the probability of occurrence of earthquake is much higher when all live load is gone? Or does it mean that buildings should survive when everyone has vacated the premises?" Arvind's comment: 1. The code specifies the use of elastic design (working stress method) permitting an increase of 33 1/3 percent in the normal working stress in materials (concrete, steel, wood etc.) when the effects of earthquake load are combined with other normal dead and live loads. 2. Since the increase of permissible stresses cuts into factor of safety, it is natural that load factor should be reduced proportionately as compared with normal dead and live loads by dividing 1.33 with warm regards.........Arvind

Reji Zachariah [Wednesday, February 06, 2002 4:40 PM] Sir, As per code we have to use load combination of 1.5 DL + 1.5 EQ load. Why this is required? When earth quake load acts there is a load P and Moment M Hence stress = P/A + M/Z

with safety of 1.5 = (P/A + M/Z ) * 1.5 We get the same from 1.5 EQ which is the 1.5 times the actual load condition with out any LL. With 1.5 DL, we are adding another P/A unnecessarily. as below 1.5 DL + 1.5 EQ = P/A + P/A + M/Z Suppose we are simulating with a software package 1.5 of EQ is only what we need. That means 50% safety over actual stresses. Please comment. Can we also reduce the factor of safety considering the increase of material strength by 33%? (As commented by Mr Aravind), I would like to get more comments on this. Thereby reducing 1.5 / 1.33 to 1.13 only Please comment. Regards, Reji Zachariah

Gopal [Wednesday, February 06, 2002 6:34 PM] Hi Folks ! The Base shear reduction due to ductility (mu) is not uniform throughout the frequency range of structures. (a) For low frequency structures the force reduction is 1/mu (Displacement equality) (b) For Intermediate frequency structures it is, 1/sqrt(2.0 mu-1) (Energy equality) (c) For high frequency structures it is, 1.0 (No force reduction) If that be the case, why we provide a uniform reduction of 1/mu, irrespective of the frequency of the structure vis-a-vis the peak frequency content in the spectrum. (Both UBC as well as Indian Draft codes) Can some one clarify ? Gopal

Hiren Desai [Wednesday, February 06, 2002 9:29 PM] According to IS 13920., Shortest dimension of the column shouldn't be less then 300 mm if C/C span of beam framing into it is more than 5.0 meter or unsupported length of column is more than 4.0 meter. I would like to know basic reasons behind this provision, as well as if one has to provide lesser dimension of the column, what extra precautions in design as well as detailing is required?

Hiren Desai

S.P. Srinivasan [Thursday, February 07, 2002 8:42 AM] Hello everybody A) In an earlier mail I had indicated that the proposed IS 1893 is silent about Zone V parameters regarding liquefaction. While the table does not mention about Zone V, the paragraph under clause 6.3.4.2 treats Zones III, IV and V together. Hence probably, whatever is given for Zones III & IV in the table are applicable to Zone V also. B) From the 2000 version of the draft 1893 code, I find the following information regarding occurrence of liquefaction of soils: (1) Soil is loose sand or soil falling under classification SP (2) Soil shall be in submerged condition (3) The minimum N-values which will prevent liquefaction are given I have the following querries: (i) Soil classification SP (as given in IS 1498) does not match the requirements for liquefaction given in published literature. The requirements indicated in literature are (a) The 10% size of the soil particles generally are reported to be between 0.01mm and 0.25mm (b) The coefficient of uniformity is reported to be between 2 and 10. Should IS 1893 give independent requirements for liquefaction instead of indicating SP type of soils? (ii) The code is silent about non-liquefaction settlement of soils. If dry loose sand is subject to seismic vibrations, it can settle considerably. This can lead to unacceptable differential settlements and impair the resistance of the building to seismic forces. Should IS 1893 give guidelines to be adopted under such conditions? I request the experts' reply to these queries. Regards S.P.Srinivasan

Jitendra K Bothara [Thursday, February 07, 2002 11:31 AM] Hi friends, The NZS4203 (Loading Standard), gives "area reduction factor" and "live load combination factor for ultimate limt state" for calculation of seismic live load rather than live load reduction percentage for ultimate limit state. The seismic live load is product of basic live load (similar to IS875 (Part II)), area reduction factor and live load combination factor for ultimate limit state (0.4-0.6 depending upon occupancy). Area reduction factor is function of area under question and occupancy and two formula are given.

Regards, Jitendra K Bothara

S.P. Srinivasan [Thursday, February 07, 2002 12:45 PM] Hello everybody I request clarifications for the following additional queries on liquefaction 1) A very thin liquefiable layer cannot create structural problems. What is the minimum thickness of liquefiable layer which can create problems? 2) The N-values indicated in IS 1893 are uncorrected or corrected values? If they are corrected N-values, what corrections are to be applied? Regards S.P.Srinivasan

Vinay Gupta [Thursday, February 07, 2002 4:40 PM] Dear Mr Hiren Desai In response to yr mail dated 6th Feb, 2002 please note that the minimum column dimension giveg in IS:13920-1993 is based on the international practices. For example cl 21.4.1.1 of ACI:318-95 has the same provision. Moreover, as per good engineering practices where longitudinal beam reinforcement extends through a beam-column junction, the column dimension parallel to the beam reinforcement should be a minimum of 20xdia of the largest beam longitudinal reinforcement (see cl 21.5.1.4). The 300mm dimension of column will barely satisfy a 16mm dia bar, which is usual for 5.0m span beams. Hope, your doubts are clear now. VINAY GUPTA

Vinay Gupta [Thursday, February 07, 2002 4:41 PM] Dear Mr S P Srinivasan With reference to your mail dated 7th feb, 2002 please note that Liquefaction is a very vital problem, which requires a combination of Seismic shakings, poorly graded cohensionless soin and water table. Both present code as well as the new draft code indicate the parameterscausing liquefaction very briefly and are mostly not adequate to assess the liquefaction potential. A better estimate of the same can be using the book Ground Motions and Soil Liquefaction During

Earthquakes by Seed H Bolton and Idriss I M wherin D-50 value of the soil is considered for the purpose. As a consultant I would like to advise that in the cases of liquefiable substrata it ideal to provide deep foundation such as pile or well foundation. VINAY GUPTA