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RE: Question regarding Cracking Moment

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Here are my thoughts, based on ACI 318-05:
1. In the Notations and Definitions, the modulus of rupture 'fr' is referenced to  Thus 'fr' is defined in the code as equal to '7.5(f'c)^0.5', regardless of what other references propose.
2.  'fr' is like 'Ec'; it is a property of the concrete and has the same value regardless of whether applied loads are factored or unfactored.
3.  It is acceptable to compare service loads with 'fr' or factored loads with 'fr', depending upon the circumstances and goals.
Bill Sherman

From: Anantha Narayan C.K. [mailto:anant_27(--nospam--at)]
Sent: Thursday, June 14, 2007 3:15 PM
To: seaint(--nospam--at)
Subject: Question regarding Cracking Moment

Here is my question once again.
I have questions regarding Cracking moment, Mcr and Modulus of rupture Fr issues with regard to ACI 318.
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.
Correct me if I am wrong, shouldn?ft a limit state such as modulus of rupture be at design level forces (factored)?
I am really hoping that you can help me understand this part of the code.
Also, what are your office standards as far as drift limits for buildings. We typically use H/400 to H/500 for 10 year winds and 3/8" max for interstory drifts. I would love to hear more suggestions/opinions.
Thank you all

Structural Engineer
Bliss and Nyitray Inc.
Miami, FL - 33134

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