Stiffness modification factor Building response 1

[KenStructural]

As per section 10.10.4.1 of ACI318 refers to stiffness modification factor for compression and flexural members/elements, particularly for columns 0.7Ig indicates it won't crack. I find this factor a bit unconservative to capture the actual response of the building, is it okay if i make a parallel run considering 0.7I for columns and then compare the factored moments to craking moment (modulus of rupture) simillar approach to walls. Attached is the protion of ACI318 for your reference

 

[JAE]

Keep in mind the derivation of the cracking moment of a column depends on P/A also....not simply the moments by themselves.
Note that ACI's Ie value must be adjusted a bit if you have axial compression or tension.

ACI defines cracking stress as fr = 7.5 sqrt(f'c). Some folks prefer a lower value (5.0 sqrt(f'c))

ACI then sets Mcr = fr(Ig)/yt.

But with axial, you would adjust fr. If compression exists, then the cracking moment is higher (more moment required to overcome the axial to create tension on the face).
So the adjusted formula - Mcr = (fr + P/a)Ig/yt. Keep the sign convention such that a compressive P value is positive. The higher Mcr will increase the Ie value.

Also - the stiffer the column the more load it will soak up in the moment frame. So going lower on the stiffness (less than 0.7Ig) won't necessarily be more conservative and could be less so.

The use of these factors (0.7 for columns, 0.35 for beams) is not precise and any attempt to nail down an exact stiffness in a member might simply be fantasy.


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[KenStructural]

Hi JAE,

Thanks for the reply, going back to my question these means that the effective stiffness factor for column is not precise and can be calculated nor reduced per equation 10-8.

Thanks for the input,