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interpretations on IS:1893

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rkingle at rediffmail.com

PostPosted: Tue Sep 21, 2004 4:59 am    Post subject: interpretations on IS:1893 Reply with quote

I have few observations and some of my interpretations on IS:1893:

1.  Cl 3.24.1 Fundamental Natural Period ( T1 ) is defined as first (longest) modal time period of vibration.  This is very much true in many cases and normally in buildings. If the structure is modelled using FEM (i.e. plate / shell elements), it may happen that we may get some beginning few modes, which are not contributing to the response. Actual first mode contributing towards the response may be defined as fundamental natural period. This can be judged from the modal mass excited in the mode.

2.  CL 4.17 Principal axes.  Can the code through some light in obtaining the principal axes of an unsymmetrical building.
a.  It may be obtained as axes defining maximum and minimum moment of inertia. I.e. for L shape building, it may be u-u and v-v axes.
b.   I think it may be proper to study the direction of the fundamental mode shapes (each in principal direction) of the building. This information can be used in defining the principal axes of the building for further calculations.

In case this is true, all the unsymmetrical buildings in plan will have to be designed for 3 component motion of earthquake.

3.   Cl 4.25: Weak storey is defined based on lateral strength of building. It seems that it is moment carrying capacity of the lateral forces resisting elements.  

4.  CL Can we define the orthogonal horizontal direction of the L shape building parallel to each flange direction and design all the elements using single component of earthquake at one time. Whether another designer taking horizontal axis similar to u-u and v-v axes of an angle section and thus defining all the column elements not oriented in orthogonal horizontal direction, designs the elements for 3 comp of motion, is wrong?

5.  Table 7: Building with shear walls includes a. Buildings having shear walls and frames and b. Building with duel system. The R factor is same for both systems. (sr no vi and vii).  SR NO viii – xi also defines R value same for a and b above.

6.  Cl 7.6.1 and cl 7.6.2 gives relationship to calculate approximates fundamental natural time period of building without and with infills. Whether it is possible to define some % of infill which draws line between the two extreme cases. This may help in optimization some designs.

7.  CL 7.7.1: Design lateral force at floor i Qi is function of square of hi. In many codes the power of hi varies from 1 to 2, depending upon the shape of the mode shape (i.e. linear or parabolic). Can we have similar values for the power of hi.

8.   Cl 7.8.1 suggests buildings with plan irregularities as defined in Table 4 cannot be modelled for dynamic analysis by the method given in, then why cl 7.9.3?

9.  CL 7.8.3: Time history method is also recommended by code. Whether it is possible to supply atleast three time histories for the IS spectra?  This will be useful in getting floor response spectrum for designing the equipment supports.

10.  CL Modal Combination: The modal combinations formulae are quite common and they can be only referred in the code. The details on formulae can be omitted. ( Only suggestion).

11.  CL Building with regular or nominally irregular plan configurations may be modelled as a system of masses lumped at the floor levels with each mass having one DOF, that is lateral displacement in the direction under consideration. … I hope code is not suggesting the shear beam model for analysis of such buildings. As most of the building beams are not rigid in bending hence such modelling will not be correct. Also why CL  It is well know that in Response Spectrum method,  un = [S(n)/ 2n]max n. Once we get displacement vector, the forces can be evaluated using matrix method of structural analysis. Yes, this method was OK in old days when computer were not there and many used to do hand calculations using calculators.

12.   CL 7.9 Torsion:  I think this is the clause which discourages many structural designers not to perform earthquake analysis of buildings. It is not clearly spelt out. There is need to spell out the torsional correction if we are using either plane frame models / using CL for analysis and performing three-dimensional analysis. Many software can take care of eccentric masses. The torsional correction at each floor and each directions is somewhat lengthy and the effect (i.e. in % of lateral loads) may be less in many cases. Also a simple statement “However, negative torsional shear shall be neglected”, involves more combinations.  There is need for someone to come up with few solved examples for unsymmetrical buildings, to cater the need of all the designers.

13.   Torsion correction in case of flexible diaphragm??

14.   General:

i.  The code is salient about P-delta effect. For some structures it may be critical.
ii.  I do not know whether we should talk about mathematical modelling of the buildings in this code. I think some where it should find place.
iii.  Soil structure Interaction is not addressed with more space.
iv.  There are two damping levels suggested by Newmark & Hall. We have single Damping ( i.e. 5% for rcc structures) whether we use LSM or WSM for design of structures.

R K Ingle

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