Masonry Wall Design - Axial Stress

[BadgerEngineer]

Per MSJC 2005 Section 3.3.5.4 - Equations apply to wall with factored axial stress of .20 *fm or less.

My question is - what happens when the axial stress is greater than .20*fm?

This was previously addressed in the MSJC 2002 which stated deflection criteria applied to walls with axial stress greater than .05*fm and less than .20*fm. Per Section 3.2.5.5. (MSJC 2002) - walls with axial stress greater than .20*fm shall be designed in accorance with provisions of section 3.2.5.4 which references strength design. (Similar to axial stress less than .05*fm) with no deflection requirements.

Can anyone offer any insight on this? Possibly I'm missing something very basic.

Thanks!

 

[BadgerEngineer]

One additional item that I forgot to add...

MSJC 2005 Commentary reads: “There are currently no strength design provisions for axial stress greater than .20*fm.”

Does this mean wall thickness or fm MUST be increased if axial stress is greater than .20fm for strength design?

I've reviewed the MDG-5 by the National Masonry Society as well and they simply state "No guidance is given for axial stress greater than .20fm"

Our firm had previously used the provisions in the 2002 code to design walls with axial stress greater than .20fm. Possibly this isn't allowed anymore?

 

[haynewp]

As it states "no strength design provisions", have you considered ASD design?

 

[BadgerEngineer]

Yes, I have considered ASD - It is too conservative for this example.

This wording in the code is very unclear to me for the following reasons:

1. Previously (In ACI 530-02) Deflection design and P-delta effects need only be considered for walls with axial stress beteen .05fm and .20fm. Why is this not considered for walls with greater than .20fm?

2. In ACI 530-05 they revise the above statement to check deflection for all axial stress less than .20*fm and give no direction for axial stress over this limit.

3. What is the purpose of the .20*fm axial check? Slenderness is already included in the Pn equations. It seems the .20fm would always control and there would be no reason to check your Pn. (Therefore vertical rebar would not contribute to your compressive strength)

My question is this - if you have an interior wall with negligible (10 psf) lateral load. Do you still need to be held to the .20*fm stress limit? This equation falls under the "Out of plate loads" section so I'm not sure.

 

[Lion06]

This does seem a little odd. Every wall must be checked for out-of-plane loads. Even interior walls as you mention.

 

[haynewp]

Yes, you have to still meet the 0.2*f'm. You have to use ASD for your example.