Expert: Arshad Khan - 1/26/2012

Question
Dear sir,
as i understand from ACI code, you need reduced stiffness for 3 purposes:-
1) Vertical Deflections
2) Horizontal Deflections
3) Slenderness

Now if we use chapter 10 slenderness reduction values, its expressed in terms of Inertia. For frame we can directly put it in ETABS but for areas in ETABS there is no I. There is only axial, bending and shear modifiers. So how we can relate ACI CODE Ig with ETABS modifiers.

Answer
Hi Waseem,

Before asking me a question I would encourage you to please search my past answers, where you would have found:

QUOTE:

Subject: section modifier in  reinforced conc. shear walls

Question: QUESTION: dear sir
 I want to know your opinion about assign modifiers and its value on f11,f22,f12,... when we use shell elem. for modeling R.C.Shear wall to act againest earthquake.

until this time i use .35 for f22 & .85 for f12.for others i use 1.
 
thank you

ANSWER: Hi ,

This depends on the code you are designing to, but in the Middle East we have been using:

For walls:
m11=m22= 0.7 for uncracked analysis
m11=m22= 0.35 for cracked analysis

For beams:

f11= f12 = f22 = m22 = 0.35

and for slabs:

m11= m12 = m22 = 0.25

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And also:

QUOTE:

Question:
According to ACI31895 if P-Delta analysis is used, it's necessary to consider the cracking effects in calculating deformation and internal forces of the elements.
In Etabs, we assign the cracking modifiers to f11  and f22 .we would like to know if it is necessary to assign cracking modifiers to both of f11and f22    .
Another question is that in" L" and" T" shape walls how can we assign the cracking modifiers  with consideration that the local axis of two parts of walls are different.


Answer:
Hi ,

For high-rise buildings, in order to avoid complications with differential axial shortening, it is recommended that you only modify m11 and m22 for walls, but can modify all four (f11, f12, f22, m22) for coupling beams.

For T and L shaped cores, treat all walls in each plane separately as they provide stiffness mainly in-plane rather than out-of-plane, so each part of the T or L will only contribute in one orthogonal direction.

Check this forum for more on this subject:  http://www.eng-tips.com/viewthread.cfm?qid=141414

Allexperts.com

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