Moment Magnifier Method for Masonry Walls

[JoshPlumSE]

Looking at the 2013 MSJC code for "wall design for out-of-plane loads" there are some new provisions that did not exist in previous versions of the code.

Section 9.3.5.4.2 is the traditional out of plane wall design. Axial stress is limited to a maximum of 0.2*f'm. But, if you're h/t ratio is greater than 30, the axial stress is limited to 0.05*f'm.

However, section 9.3.5.4.3 gives a new alternate method that used a moment magnifier where your wall moment are magnified by a factor along the lines of 1/(1-Pu/Pe). Very familiar to those of use who have done P-little delta calculations in steel or concrete or such. But, still very much new to the MSJC.

Now, the question / concern that I have is that this new alternate method has no limits on it. We've been limited the axial force in masonry walls for the last 30+ years.

Now, I've run some numbers and the 0.2*f'm approximately corresponds to the Pe value, so this isn't really any less conservative for shorter walls. But, for slender walls (h/t>30), the allowable axial load has gone up by a lot.

Does anyone have any thoughts on these new provisions? What are you doing for slender masonry walls? Are you still limiting the axial forces, or are you using these new code provisions to bravely go where we haven't been allowed to go for 30+ years?

 

[JoshPlumSE]

There is surprising little documentation / explanation on these new code provisions. At least not that I could find. I've ordered a copy of the latest Masonry Design Guide to see what it says. But, there isn't much else out there. I had expected Structure magazine or The Masonry Society to have published articles talking about this new procedure, where it came from and about where it deviates from the traditional provisions.

 

[allgoodnamestaken]

I don't have my masonry book with me at the moment, but the canadian CSA codes have been using an at least similar (not sure if identical atm) moment magnifier method for masonry design since 2004, and tends to produce conservative designs.

http://www.nrcresearchpress.com/doi/pdf/10.1139/L06-167

 

[mike20793]

We actually brought this code issue up to MSJC in the middle of last year. You are right, the slender walls can take what seem to be an unrealistic amount of load. We have some higher ranking MSJC members at my firm and they are looking into the issue, but recommended sticking with the upper limit for now. We found, in general, that the slender walls weren't in that many projects and the design with the upper limit was still reasonable. It likely won't be addressed in the next code issue, either.

 

[JoshPlumSE]

Mike -

Thank you for the follow up. For what it's worth, I'm not directly challenging the validity of the new procedure. I'm really just surprised that such a major change hasn't gotten the publicity I would have expected.

Plus, I would have expected some commentary about why the axial force restrictions are no longer deemed necessary for the new procedure.

 

[RFreund]

I don't have an answer, but I am equally surprised/interested by this. More so the lack of announcement/explanation than the actual change itself. I think the change makes sense. The original limits came from testing but maybe now that they are changing the equations to match that of concrete they have decided such limits do not need to apply.

Since we're on the subject I had submitted questions about masonry walls and stability some time ago:
Masonry Stability LINK

Although this new provision may make programing the wall capacity a bit easier, atleast now there wouldn't be a double iteration (with deflection)... I think..

EIT

http://www.HowToEngineer.com