Masonry Wall Deflection

[memphiseng]

I'm designing an interior 8" NLB CMU wall, and I'm not sure why my deflection in the RISA-3D model is so large.

As a cantilevered interior wall I've got:

8" CMU
48" c/c grout & reinforced
DL = 70 plf downward from a metal stud/gyp. board wall on top of the CMU wall
LL = 5 psf lateral interior wall load (I've seen some say they analyze this as a WL, code seems vague and treating it as a LL is more conservative)

There's also a 6'4" X 7'4" double door opening in the center of the wall.

I've fixed the bottom nodes of the wall, and after running the analysis I'm getting a joint displacement at the top of the wall of 3.542 in. This seems unrealistic, and doesn't agree with the results of analyzing the wall per TMS/ACI, per TEK 14-11B, and per Tedds which all agree on a service load deflection of 0.022 in.

I'm comfortable based on the hand calcs with the design, but unsure of why the RISA 3D model is giving such a large deflection. Does anyone have experience with results like this in a similar condition, and knowledge of what might be wrong with the model?

 

[JoshPlumSE]

Tough to tell from the information provided. Some thoughts:
1) RISA is likely converting the wall into an "equivalent thickness" for shear and using that for the analysis.
2) Ideally, RISA would use an orthotropic plate element formulation so that the thickness used to derive out of plane bending stiffness is different than the in plate stiffness. That's what the program does so that the axial stiffness of a wood shear wall is based on the studs in the wall, but the shear stiffness is based solely on the sheathing.
I believe the program is using orthotropic plate elements, but I cannot be 100% certain. If not, you really need to harass them to update this as the out of plane stiffness is important.
3) The program does a slender wall calculation (after the analysis). Taking the moment from analysis, comparing them to Mcracked, then deriving an Iequiv and re-deriving the moments and deflections (including P-little delta effects). It iterates this up to 100 times to come up with the final out of plane deflections and moments.
4) The program also has an Icr factor in the Advance tab of the wall panels spreadsheet. This is used to account for cracking and reduces the stiffness of the wall in the desired direction.

 

[memphiseng]

Thanks for the suggestions.

I've unchecked the option for using cracked sections under the Concrete tab of the Model Settings. Mcr based on TMS is 7.3 kip-in and Mser is only 3.0 kip-in in this case, so the wall needs not be analyzed as a cracked section. Also realized I've assigned the fixity at the base from the Model View and not the Wall Panel Editor, which for whatever reason previously led the program to assign the fixed base condition as a pin at the end joints.

This brings deflection at the top down to 0.25 in. which is more realistic and within 0.007h, but still an order of 10 higher than following TMS/TEK 14-11B.

With only 70 plf dead load on top of the wall, in my calculations I stopped in the second iteration as with such a small deflection in the first iteration the second deflection with p-little delta effects did not add 0.001 in. (well within 5%) to the deflection.

If RISA is calculating a larger deflection in the first iteration, the p-little delta effects might be larger and require more iterations, but there's still very little DL on the wall. The discrepancy I'm sure is in the first deflection calculation, though I'm not sure how/why RISA arrives at a deflection 10X the prescribed method per code. If my solution wasn't supported by the TEDDs solution, I'd probably assume I'd dropped a zero in a calculation but that isn't the case and I've been through this multiple times now.

I've uploaded a copy of the model if anyone wants to peek under the hood and see if they can determine the issue.

If nothing else, at least I'm whittling away at this one.

 

[JoshPlumSE]

Also realized I assigned the fixity at the base from the Model View and not the Wall Panel Editor, which for whatever reason previously led the program to assign the fixed base condition as a pin at the end joints.

That would do it. So, that explains the majority of your issue.

Next thing to test is compare the FEM joint deflections (0.216") from the analysis to the p-little delta amplification you get from the out-of-plane detail report (0.25). You can see that most of this deflection is coming from the FEM analysis directly. And, the deflection doesn't change much when you turn off P-Delta from the analysis either.

I'd recommend breaking this down into it's simplest form. Look at deflection formulas for beams under a uniform distributed load. And, compare those the TMS / TEK results and teh RISA results.

I believe you will realize that the formulas in that TMS / TEK are based on a simply supported wall instead of a cantilevered wall.... Something like a deflection of 5wL^4 / (384EI) whereas the model you've created is a true cantilever and should be something more like wL^4/ (24EI). That means your model would be more than triple the deflection. And, that's before we even consider the effect of the opening and how that load increased deflection in the adjacent regions.

That should get the RISA numbers pretty close.

PS It looks like you have the latest version of the program. Why post this to Eng-Tips as opposed to contacting RISA support directly?

 

[memphiseng]

I believe you will realize that the formulas in that TMS / TEK are based on a simply supported wall instead of a cantilevered wall.... Something like a deflection of 5wL^4 / (384EI) whereas the model you've created is a true cantilever and should be something more like wL^4/ (24EI). That means your model would be more than triple the deflection. And, that's before we even consider the effect of the opening and how that load increased deflection in the adjacent regions.

In my calculations I followed TMS/TEK method, but the moment and deflection equations for a cantilever with uniform distributed load were used ( Mmax = wl^2/2 and DELTAmax = wl^4/(8EI) ) so that shouldn't be responsible for the discrepancy in results. I haven't considered the effect of the opening, as you say, but I wouldn't have thought it responsible for such a dramatic increase in deflection. I at least feel like the RISA results are much more realistic now, and well within limits as I would expect.

How are you isolating the FEM joint deflections results? The fact that the deflection doesn't change much with P-delta would agree with my calculations, and it appears the discrepancies are resulting mostly from an idealized analysis versus a more complete and rigorous analysis.

In regards to contacting RISA support directly, I was already here doing some reading and just assumed (mostly correctly) that my issue stemmed from a bone-headed error that someone here would recognize quickly and straighten me out.

 

[bones206]

I took a look at your model. One weird thing I noticed is that the wall region R2 isn't correct. If you go to tools --> delete wall panel regions and re-run the model, it fixes that issue. I don't think it affects the deflection results though.