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IS 800- Sections 1-3

 
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alpaseth at giasbm01.v...
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PostPosted: Tue Oct 28, 2003 5:52 am    Post subject: IS 800- Sections 1-3 Reply with quote

Dear All,



I am going to follow the request by some on SEFI for Section wise discussion of the draft IS 800 code.

I confess to not having read the code completely as yet and I may at times be jumping the gun for issues which are clarified later in the code, so please do bear with me.

My first email is to do with Sections 1 to 3.



FOREWORD

I find that a foreword is missing. For a code which recommends a sea change from the way we have been designing steel, this is a MUST. We are moving from ASD (or working stress design) to limit state design and various concepts need to be expanded. I am also not sure why we are not going  into LRFD instead of  Limit State.

The foreword needs to elaborate the departures from the earlier IS 800-1984, the change in the philosophy of design and the whys and hows of it. Without this explanation and a concise  summary of the changes proposed, it is very difficult to understand the full impact of this revision.



SECTION 1



a)       There is a very expanded terminology as compared to the IS 800-1984.



I had suggestions for  the following definitions:



i)    Compact Section: While the definition is quite succinct, it needs to be expanded in view of the  importance of the  concept being discussed. There may be an appendix elaborating the concept with a diagram showing the bending stress distribution for different classes of sections- Compact vs. semi-compact vs. Slender.

ii)             Design Load/Factored Load - "A load value obtained by multiplying the characteristic load with a load factor, which is generally greater than one."

The definition suggests that load factor is generally greater than 1.0 We know that this is not true in load combinations of dead with wind or earthquake and this part of the sentence ".(load factor), which is generally one" may be removed.

iii)            Instability - "The phenomenon which leads to the inability of an element, member or a structure to carry further load due to eccentric deflection lateral to the direction of loading and vanishing stiffness."

There could be  other reasons of instability than eccentric deflection and this part of the definition from "due to eccentric deflection lateral to.."  may be deleted.

iv)            Period of Structural Inadequacy - This should be renamed  as "Period of Structural Inadequacy for Fire"

v)              Semi Compact Section- Comment same as for Compact Section

vi)            Stability Limit State - A limit state corresponding to the loss of static equilibrium of a structure by excessive deflection transverse to the direction of predominant loads.

Again, the last part of the definition from "by excessive deflection.."  may be eliminated as loss of static equilibrium may occur due to other reasons also.






Section 2.



2.2.4 Properties:

I was happy to see that the physical properties have been added  in a consolidated manner and one no longer has to scrounge around searching for them.



The draft code has done away with conformance specifications for washers, concrete and other materials. I wonder why. I for one would have been happier if they had included more materials such as  studs used in composite construction ( such  as flooring with metal decking and  concrete topping).



Section 3



Clause 3.1.2.2
"Where the Limit States Method can not be conveniently adopted; Working
Stress Method (Section 11) may also be used."

I agree with Jignesh and others on SEFI who have requested this clause to be a bit less ambiguous. It should clearly state that "As far as possible the Limit State Design Method should be used" and "Only in exceptional cases, where the Limit State Method cannot be conveniently adopted; Working Stress Method (Section 11) may also be used."



It is quite  evident that the code writers would like LSDM to be the design methodology in the years to come and  the intent of retaining ASD (WSD) is only  to give transition time to people to switch over.  

Clause 3.5 Load Combinations



I agree with Jignesh that the entire details for crane loading may be removed for here and reference be made to IS 875 part 2 for crane loadings. This should also include, as he has mentioned elsewhere, that some % of lifted load for wind and earthquake load needs to be considered as against the 3.5.5 clause implying that no lifted load need be considered. Also as Vipul has brought out, the IS 875 should  specify the bumper impact loads therein.

On the same lines I think blast loading  should  be addressed separately in a code for blast resistant design and referenced herein.



Clause 3.7.2



As Amit has stated, the terminology "internal elements" and "outside elements" need to be explained with figures.



Table 3.1 Limiting Width to Thickness Ratios



This table is unnecessarily complicated and confusing and could be cleaned up to make it more readable. The reading material provided in the refresher course (organised by INSDAG, IE(I) etc. in some cities such as Mumbai)  on Proposed Revision of IS 800 has a cleaner table (Table 10) for limits on Width to Thickness Ratios and perhaps this could be used in the code  instead.





Regards,



Alpa

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