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RE: CMU Wall Effective Flange Width

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I have forwarded this question to a member of the ACI 530 comittee and this is the response......

 

Jim K.

 

Yes, very interesting question. It brings up

what appears to be an inconsistency in MSJC.

Section 1.9.4.2.3 permits 6t each side of the pilaster to be considered

effective as the flange. THerefore, the total compression width per

pilaster, is 12 t plus the web width (when there is no movement joint at

the pilaster). Yet, Code 2.3.3.3.1 limits the compression width per bar

to 6t total. It does seem to be justifiable to use 12 t plus the web

width when the pilaster is reinforced, because of Section 1.9.4.2.3.

When the pilaster is in-wall, as in the case of your questioner, it

doesn't make sense that the rules should change. On the other hand,

what is the difference between an in-wall pilaster and a single grouted

reinforcing bar in the wall? Should there be different rules for

unreinforced and reinforced pilasters?

This is an issue for MSJC to resolve.

-----Original Message-----
From: Chris Banbury [mailto:cbanbury(--nospam--at)nicholson-engineering.com]
Sent: Tuesday, March 11, 2003 7:44 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
(352) 799-0170 (o)
(352) 754-9167 (f)
www.nicholson-engineering.com