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# RE: Question regarding Cracking Moment

• To: <seaint(--nospam--at)seaint.org>
• Subject: RE: Question regarding Cracking Moment
• From: <William.Sherman(--nospam--at)CH2M.com>
• Date: Fri, 15 Jun 2007 07:27:18 -0600

Here are my thoughts, based on ACI 318-05:

1. In the Notations and Definitions, the modulus of rupture 'fr' is referenced to 9.5.2.3.  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
CH2M HILL / DEN
720-286-2792

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

Folks,

Here is my question once again.

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

• R10.11 of ACI 318 says ?gIf 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 ?c?c?c?h. 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 (which as you stated is a limit state) 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 ?ãf?fc is used for deflection and 6 ?ã f?fc is used for strength calculations (ACI 11.4.2.1). In Dr.Wang?fs 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.

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
Anantha

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

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