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RE: ACI Chapt. 22

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I’m not sure if I’m addressing your question correctly.  As I understand it you want to know if the eccentricity of the axial force can be outside the middle third of the wall thickness.  I would look at this as the virtual eccentricity of the load P caused by moment.  If this is what you’re asking then I believe the reason you must stay within the middle third of the wall thickness is so  that there is no tension force in the wall.  When the virtual eccentricity exceeds 1/6 from the centerline of the wall there is tension stress in the concrete.





From: Richard Calvert [mailto:RichardC(--nospam--at)]
Sent: Tuesday, March 17, 2009 1:41 PM
To: seaint(--nospam--at)
Subject: ACI Chapt. 22


I’m trying to check a plain (unreinforced) concrete basement wall for a large point load.  In Chapter 22 of the ACI-05 there appear to be two ways to approach this.  The seemingly obvious approach is with Eq. 22-14 – but when considering the soil pressure and backing into the resultant vector, it does not occur within the middle-third of the wall, as required by section 22.6.3.  Is this what it being stated by this section? So my next step is to use the equations per 22.5.3, as instructed in 22.6.3.  but I find that the use of A1 instead of Ag gives a much lower allowable compression.  The use of A1 in the context of a wall doesn’t make much sense to me.  Is it acceptable to use Ag ILO of A1 in this case?


In other words, is the correct calculation:


  1. 22-14
  2. 22-14 + 22-2,3 (unity)
  3. 22-5 + 22-2,3
  4. 22-5(w/ Ag ILO of A1) + 22-2,3




Richard Calvert, EIT 

Project Engineer