Rudra Nevatia ...
Joined: 26 Jan 2003 Posts: 215

Posted: Tue Sep 21, 2004 10:55 am Post subject: Prof. Ingle's Questionnaire 


Dear Prof. Ingle,
I couldn't resist taking this test so here I go with my answers (and some counter questions):
Quote:  Q1. 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.
Quote:  A1. Shouldn't we expand the definition to make it clear that there are
 multiple fundamental periods depending on the direction e.g. two lateral, one vertical and one torsional for buildings. Weeding out of 'spurious' modes is best left to software.
Quote:  Q2. 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 uu and vv 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.
Quote:  A.2 What happens if orientation of principal axes changes from one storey to
 next? If the real concern is about decoupling of modes, I vaguely remember a paper doing some mathematical jugglery to achieve this.
Quote:  Q3. 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.
Quote:  A3. Or is it the lowest values considering *all* modes of failure  shear,
 flexure, bond..?
Quote:  Q4. CL 6.3.2.2 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 uu and vv 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?
Quote:  A.4 This one is a googly.

Quote:  Q5. 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.
Quote:  A5. I believe Note 4 of Table 7 makes things clear.

Quote:  Q7. 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.
Quote:  A7. Parabolic variation (power 2) applies to elastic deformation and linear
 variation (power 1) applies to postyield deformations.
Quote:  Q8. 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 7.8.4.5, then why cl 7.9.3?
Quote:  A8. Proposed modification deletes Cl.7.9.3

Quote:  Q9. 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.
Quote:  A9. Synthetic accelerograms can be generated for a given reponse spectrum.
 Please see "Dynamics of Structures" by Clough and Penzien.
Quote:  Q11. CL 7.8.4.5: 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 7.8.4.5? It is well know that in Response Spectrum method, un = [S(&#61559;n)/ &#61559;2n]max &#61542;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.
Quote:  A11. The code refers to global degrees of freedom. Individual members may have
 any number of degrees of freedom.
Quote:  Q12. 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 7.8.4.5 for analysis and performing threedimensional 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.
Quote:  A12. The magnifier on static eccentricity is applicable to plane as well as
 space frameanalysis. The proposed revision gives an option to use either of "all floor" or "single floor" definition. As a further simplification, maxima of all floors can b used conservatively. The statement regarding neglecting negative torsional shear need not increase the number of load combinations. Please see solved example at http://rudranevatia.com
Quote:  Q13. Torsion correction in case of flexible diaphragm??

How did I fare, Prof. Ingle?
Regards, Rudra Nevatia
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