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Re: Effective Section - ACI 318

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Dear Friends,

I would be thankful if some of you could explain the background to the
clause of ACI as quoted below:

The specifications of ACI 318 RM - 02 for moment of
inertia of members are
beams    0.35 Ig (gross moment of inertia)
columns  0.70 Ig
walls uncracked 0.70 Ig    cracked  0.35 Ig
Flat slabs and flat plates   0.25 Ig
Is it based on analytical/experimental research or is it empirical ? Could
you suggest any References for me
to become more familiar with this topic ?

I have some experience of system identification studies -
trying to fit analytical model to match experimental results.
In particular for a RCC framed structure (TG framed foundation) we carried
out extensive experiments after the structure was built and operated under
actual loads.
The structure was analysed as a space frame as well as by using 3D solid
finite elements. The gross section of the concrete was used with static
modulus of elasticity.
The best fit was obtained when using an effective factor of 0.65. In one
sense, ACI 316 gave us confidence that the effective factor should be less
than unity but it was not possible to assign different factors for beams and
to confirm ACI values. 3D analysis could predict frequencies and mode shapes
near the operating range of frequency (50 Hz) and space frame analysis could
not do so.

A R Chandrasekaran

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