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Prof. Ingle's Questionnaire

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Rudra Nevatia

Joined: 26 Jan 2003
Posts: 215

PostPosted: Tue Sep 21, 2004 10:55 am    Post subject: Prof. Ingle's Questionnaire Reply with quote

Dear Prof. Ingle,

I couldn't resist taking this test so here I go with my answers (and some
counter questions):

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.

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.

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 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.

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.

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

A3. Or is it the lowest values considering *all* modes of failure - shear,
flexure, bond..?

Q4.  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?

A.4 This one is a googly.

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.

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

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.

A7. Parabolic variation (power 2) applies to elastic deformation and linear
variation (power 1) applies to post-yield deformations.  

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, then
why cl 7.9.3?

A8. Proposed modification deletes Cl.7.9.3

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

A9. Synthetic accelerograms can be generated for a given reponse spectrum.
Please see "Dynamics of Structures" by Clough and Penzien.

Q11.  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.

A11. The code refers to global degrees of freedom. Individual members may have
any number of degrees of freedom.

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 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

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

Q13. Torsion correction in case of flexible diaphragm??

A13. None.

How did I fare, Prof. Ingle?

Rudra Nevatia

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