Moment Magnifier Method for Masonry Walls
Moment Magnifier Method for Masonry Walls
(OP)
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?
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?






RE: Moment Magnifier Method for Masonry Walls
RE: Moment Magnifier Method for Masonry Walls
http://www.nrcresearchpress.com/doi/pdf/10.1139/L0...
RE: Moment Magnifier Method for Masonry Walls
RE: Moment Magnifier Method for Masonry Walls
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.
RE: Moment Magnifier Method for Masonry Walls
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
www.HowToEngineer.com