Masonry Wall Out-Of-Plane Capacity

[RFreund]

The following is in regards to MJSC design code for masonry walls for ASD design (not slender-walls).

Say you have a wall section and you apply your axial and moment forces simultaneously. So you iterate to find the depth of the neutral axis (similar to a concrete beam-column). Now you can find your compression force based on the maximum allowable masonry compressive stress which is typical F'm/3. Then you find your nominal compressive strength based on the compressive pressure and triangular area. You have already satisfied the requirement of not exceeding the maximum allowable compressive force for combined axial and flexure.
Now, when determining the allowable axial capacity. Do you reduce your nominal compressive force or compressive stress that was previously found when considering flexure. Or do you ignore the moment and apply the reduction factor based on the entire wall being in compression. Then compare this to the axial force?

Thanks in advance!

EIT

http://www.HowToEngineer.com

 

[IsaacStructural]

I've read it a couple times and I'm having some trouble following your question here.

What version of the code are you looking at? I've got 530-11 in front of me. Look at section 2.3.4.2.2 on page C-98. Does this answer your question?

2.3.4.2.2
"The compressive stress in masonry due to flexure or due to flexure in combination with axial load shall not exceed 0.45f'm provided that the calculated compressive stress due to the axial load component, fa, does not exceed the allowable stress, Fa, in section 2.2.3.1."



Licensed Structural Engineer and Licensed Professional Engineer (Illinois)

 

[Jerehmy]

No, that's what the interaction equation is for.

 

[RFreund]

Thanks for looking at it.

Take a look at the attached. I think it may help clear things up. The analysis is the same you would do to determine a P-M interaction graph or to analyze a concrete column. Basically iterate the neutral axis until the forces balance.

@Jerehmy - the interaction equation is a 'simplified' method. This procedure is directly accounting for the combination of axial and flexural stresses. However interaction method is more appealing at this point but none the less the question stands.

EIT
www.HowToEngineer.com

 

[RFreund]

Sorry the link for the attachment broke.

Here it is again.

EIT
www.HowToEngineer.com

 

[DST148]

@RFreund> You are right. However, I prefer to draw Moment-Axial interaction diagram independent of slenderness effects, i.e. h = 0. Determine pure axial compression
capacity Po by applying appropriate R factor and stability checks (P <= Pe/4). Then superimpose this Po on the moment axial interaction diagram. The resultant diagram will look
flat / horizontal at the top, something similar to concrete interacton diagrams.

 

[RFreund]

@DST Thanks for chiming in. That's what I had thought. I was second guessing myself as I had a previous spreadsheet that I believe was from one of Max Porter seminars. It looked like he was multiplying the nominal section strength by the reduction factor even though it appeared the notes were saying that you did not have to do that (basically that it was a check against axial load only). I have formated by interaction diagram like you have described.

I want to complete a Strength design one though as ASD is kind of odd in the sense that there are no provisions for stability (P-delta effects). I've been meaning to blog about that...

EIT

http://www.HowToEngineer.com