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Cracking Moment Mcr

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Greetings,
 
I am emailing with questions regarding Cracking moment, Mcr and Modulus of rupture Fr issues with regard to ACI 318.

 

  • R10.11 of ACI 318 says “If the factored moments and shears from an analysis based on the moment of inertia of a wall taken equal to 0.70 Ig indicate that the wall will crack in flexure, based on the modulus of rupture ………”. My interpretation of this statement in the commentary would lead me to compare the factored moments versus the cracking moment based on the modulus of rupture at strength level.
  • However, 18.3.3 talks about classification based on modulus of rupture at service level forces.

          Chapter 14,14.8.2.4 talks about phi Mn > Mcr, where Mcr is given using fr

 

  • In his textbook, Dr. McGregor (Page 63, Fourth Edition) states that the 7.5 √f’c is used for deflection and 6 √ f’c is used for strength calculations (ACI 11.4.2.1). In Dr.Wang’s textbook, phi Mn is compared to Mcr. (please see attached file)

 As you can clearly see, I am quite confused as to what value of Fr is to be used at service level forces and what value of Fr is to be used at factored forces. Is it phi Mn > 1.2 Mcr for strength level or 1.0 Mcr?

 

Correct me if I am wrong, shouldn’t a limit state such as modulus of rupture be at design level forces (factored)?

 

I am really hoping that someone will help me understand this part of the code.

 

Also, how does your office preliminarily size shear walls? Do you take into account the cracking under service loads or are they based on gross moment of inertia? Do you size your walls for deflections based on a 10 year wind?


Thanks and regards,

Anantha


 
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ANANTHA NARAYAN, E.I.
Structural Engineer
Bliss and Nyitray Inc.
Miami, FL - 33134


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