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RE: CMU Wall Effective Flange Width

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I think the intent here is to treat the wall like you would a T-beam in reinforced concrete with effective flanges. If your bar spacing is greater than 48" than you would be limited to 48" for the effective flange width regardless of the size or number of bars you put in one cell. For your example of two reinforced cells at 6'-0" OC I would interpret the code to require you to use an effective flange width of 36".
-----Original Message-----
From: Chris Banbury [mailto:cbanbury(--nospam--at)nicholson-engineering.com]
Sent: Tuesday, March 11, 2003 5:44 AM
To: seaint(--nospam--at)seaint.org
Subject: CMU Wall Effective Flange Width

I'm analyzing a wall for a 15' high, single story, 8" CMU, hip roof, residential structure for out of plane bending under wind loads.  Two (2) vertically reinforced cells are located 6' on center.  Would it be inappropriate to assign an effective compression flange width of 56" (48" + 8") or even 72" rather than 48" (6*T)?  While I would expect ACI 530 to be pretty clear on this, it appears that the MDG uses this kind of rationalization in assigning effective flange widths greater that 48" in the design of pilasters where 6*T is permitted on either side of the pilaster.  It appears that the a literal interpretation would penalize the use of double reinforced cells spaced more than 48" o.c..  The ACI 530-02 Sec. 2.3.3.3 further complicates the matter by specifying the effective flange width "per bar" which would seem to indicate that the effective flange width could be much larger than 48" for an 8" wall where two adjacent reinforced cells are used.  In short, it appears that if a bar is placed in two adjacent cells located 6' o.c. that 'b' would be limited by 's' or 72" not '6T per bar'.
 
Christopher A. Banbury, PE
Vice President
Nicholson Engineering Associates, Inc.
PO Box 12230, Brooksville, FL 34603
7468 Horse Lake RD, Brooksville, FL 34601
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www.nicholson-engineering.com