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RE: ACI 318-08 Section 14.8 Alternate Design of Slender Walls

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I think the person at your seminar got it wrong.
 
It is my understanding that the 05 cracked inertia calculation involved with the P-Delta analysis of thin walls (like tilt-up panels) was too conservative and was not even representative of the test results conducted for the Green Book. Here is a summary of what I'm talking about:
 

The most significant difference between the UBC provisions and ACI 318-05 is the ACI Code's use of Branson's equation for Ie to account for the effect of a cracked moment of inertia instead of using the UBC bilinear load-deflection.  In addition, the value for Mcr used in Branson's equation was set at the traditional ACI value.  In light of these differences and in consideration of the fact that slender walls don't behave in accordance with the long-standing ACI deflection equations it appears that the current ACI 318-05 Code will likely be revised.  It should also be noted that neither the Yellow Book, the Green Book, nor the SEAOSC Slender Wall Task Group Report discuss the lower cracking moment Mcr used by the ACI or the bilinear moment-deflection equation provided above.

 

Recent research has also identified significant limitations with Branson's equation for Ie when applied to thin concrete members with a central layer of steel.  Branson's equation, first published in 1965, was based on large test beams with a ratio of gross inertia to cracked moment of inertia (Ig/Icr) of 2.2.  When this ratio exceeds a value of about 3 the use of Branson's equations leads to poor predictions of deflection.  Slender concrete walls are far above this limit, with common ratios ranging from 15 to 25 for single layer reinforced walls and 6 to 12 for double layer reinforced walls, therefore the actual deflection is under predicted.  The main culprit for this under prediction is the lack of proper consideration of tension stiffening in Branson's equation.  Recommendations to replace Branson's equation with a more accurate equation incorporating tension stiffening effects have recently been proposed.

 

Comments obtained during the late 2007 development of ACI 318-08 concluded that the 1997 UBC equations match the test data well, but the ACI 318-02 equations (that were unchanged in ACI 318-05) did not correlate well with the test data and typically underestimate service load deflections. In fact a comparison between the test date and the current ACI 318-05 indicates that the cracking moment is overestimated by 26% on average.  The revised equations proposed, and assumably included in the ACI 318-08, produce a moment-deflection curve that is identical to the UBC prediction and more closely matches the original test data but still conservatively underestimates Mcr by 16% on average.


________________________________

From: Andre Sidler [mailto:asidler(--nospam--at)hotmail.com]
Sent: Tue 10/28/2008 6:53 PM
To: seaint(--nospam--at)seaint.org
Subject: ACI 318-08 Section 14.8 Alternate Design of Slender Walls


SEA Listers -
 
I sat in on a webinar on Tuesday morning and I was informed that the calculation of the P-delta and secondary moment in Section 14.8.4 of ACI 318-05 is unconcervative and that the brand-spankin' new ACI 318-08 has corrected this.  Is there anyone out in the SEA Listserve universe that would be kind enough to send me a PDF file of section 14.8 of the ACI 318-08?  I can't bring myself to purchase this edition just yet; I'd rather wait until IBC 2010 becomes the law of the land.
 
Second, why doesn't ACI issue a supplement to ACI 318-05 to correct this?  ASCE issued a supplement for ASCE 7-05 that made some modifications.  It seems that if this method is as unconservative as we were led to believe in the webinar then this should be taken care of immediately.
 
Cheers!
 
Andre J. Sidler, S.E., P.E.
Quantum Consulting Engineers
Seattle, WA
 
asidler(--nospam--at)hotmail.com

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