Partially Grouted CMU Wall

[MDStruct]

Hi all. I am stuggling with finding some information. I am working on some shear wall designs and am trying to find equations to help me with the flexural capacity/required steel for in-plane flexure on partially grouted shear walls. More specifically I cannot find good equations on how to find k and j. Tek 14-7B gives an equation k=SQRT[2ρn+(ρn)[sup]2[/sup]]-ρn but says this is for when NA is in the face shell, meaning out of plane bending. I found some others that assume a fully grouted wall, but cannot find something addressing partial grouting. What should I use for this? Solve for k using a rho determined with b=2*t[sub]faceshell[/sub]]? Should I just set j=0.9 and be done? The thing is we have walls with pretty low loading, so I'm almost certain they'll all work just fine, but I need some way to prove it. Any resources on partial grouting would be helpful.

 

[RFreund]

A partially grouted wall would be like a concrete "T-Beam". If the compression block is in the face shell then you treat it as if it were fully grouted. Which really means that the you have the full width for your compression block. If you find that the N/A is not in the face shell then your analysis assumptions are incorrect and need to use the "T-beam" approach.

I tried to disucs this here for stength design:

http://howtoengineer.com/masonry-walls-partial-grout-out-of-plane-str/

There is also an ASD link.
Don't trust the spreadsheets, verify them.

EIT

http://www.HowToEngineer.com

 

[MDStruct]

Thanks RFreund, but my issue is In-Plane flexure, not out-of-plane.

 

[msquared48]

Have you checked the wall tables in Amrhein?

Mike McCann, PE, SE (WA)

 

[mike20793]

In plane design is very similar (if not identical) to reinforced concrete beam design where you will have the tension force resisted by each reinforcing bar. Most software will only consider the boundary elements to resist in plane bending because the exact configuration of the reinforcement isn't always known. I believe QuickMasonry considers all the bars, but RISA only uses the boundary element width that you define.

 

[RFreund]

Oops, sorry.

I've wondered about in-plane as well. I have seen it done a couple different ways:

1.) Consider an exact arrangement. You have "moment arms" and areas to all the grouted cells. Your compressive stress only acts on grouted or mortared areas. I've been trying to complete an algorithm for a spreadsheet/mathcad calc for this. It is more time consuming than I originally hoped and thus have not finished it.

2.) Consider an "equivalent" solid area. Basically you have a solid area at the ends if you have boundary members (flange) (this may also be admitted). Then for the web you assume some sort of equivalent wall thickness based on grout spacing (or you just have a web).

3.) Ignore the interior bars and design only the boundary members.

EIT

http://www.HowToEngineer.com

 

[Guastavino]

I've never been a fan of applying ASD design to Masonry. LRFD is much more intuitive to me personally. You can create a spreadsheet that does strain profiles, relates it to stresses in each bar and iterates to find a centroid based on equating the forces. It's quite complex. It's intellectually stimulating and helps you understand everything, but it's still a shotgun approach. Honestly, whatever you do, go back and do the old 30 second method to check your numbers. That being 15ft shear wall with (2) #5 bars at each end should have a capacity around:

0.31*2*60ksi*0.9*15ft*0.9=450k-ft plus or minus.

But more onto your point. If you want a complex analysis, do the iterative spreadsheet.

1. Your variable that changes is your centroid depth.
2. You set your strains in bars equal to the strain based on compression crushing strain of masonry, location in wall, and centroid.
3. Get stress in bars based on strain.
4. Get force in bars based on stress*area.
5. Compare your force in bars, compression force in wall, etc. and make sure it all equals zero by iterating. I use solver in Excel. (Although I use RISA to design the walls and then use my 30 second method to check it).
6. Smile and realize that the wall will perform differently then what you've calculated, but you're conservatively covered!

I might have left out some steps above, but generally that's what you've got.

 

[XR250]

Make sure to check the horizontal shear stress in the wall in addition to the bar stress.

 

[MDStruct]

Thanks for your replies all.

msquared - I do not have that book.

Mike - I am actually doing this by hand.

RFreund - I ended up doing something similar.

Sorry if my responses seem short, I am just replying all at once because I didn't see any of these until right now. I spoke with my engineer and used a method similar to what RFreund suggested.

Let me know your thoughts on this method because I'm new to this....

I looked at the in-plane section as a "solid rectangle" of sorts. I solved rho=As/(b*d) by using b=2*t[sub]faceshell[/sub] and As=reinforcing at the end of wall. So I was able to find a rho to plug into my equation for k listed in my initial post.