Masonry Wall Design for Out of Plane Loads at Openings 2

[SteelPE]

I have been asked to seriously consider designing a building with masonry bearing walls. A sample elevation is attached.

The required code is IBC 2015 with ACI 530-13. I am wondering how to design the piers in-between the windows. Without windows my wall has an ultimate moment of 3,038 +/- ft-lbs/ft at mid height. I have an ultimate moment of approximately 2,700 ft-lbs/ft at the head of the opening.

I am leaning towards designing the masonry above the opening to span between the head of the opening and the roof support. I would then have a bond beam at the head of the opening to put a point load on my pier (see attached diagram). I would then design the pier to design these loads (including any gravity loads calculated the same way).

This seems like a logical analysis, but I would just like the opinions of others to see how they would approach the design.

 

[SteelPE]

Agree with those above. 30ft+ is common for 12" CMU in my area. We always specify rebar positioners be used, and have had good luck with rebar placement.

https://wirebond.com/category/rebar-positioners

I can't say that I have ever recommended using those before. They look like a pretty good product. I took a look at a Figure 8 for 12" CMU and I noticed that they product is 10" wide. I always thought 12" block had a face shell of 1.25"..... That would place the opening at 9 1/8". How do they get something that is 10" wide into a space that is 9 1/8"?

How often do you install these? Are you talking every 4'-0" +/- or is this something that you would want every 8"?

 

[phamENG]

It sits in the mortar bed joint. I've seen a typical 192 bar diameters used. Don't ask where that came from...

 

[SteelPE]

192 bar diameters, how do you thread that needle? Using a #6 bar you are looking at every 12'-0" o.c. ..... if you use a #5 you are looking at every 10'-0" o.c.

 

[KootK]

Making the positioners work, I think, is largely predicated upon several of the unpopular block construction rules being followed. If one adheres to the maximum 5' single grout lift, then I'd think that installing positioners at the top of each lift before it's grouted, would reliably get the job done.

 

[phamENG]

It's a "not to exceed" number - so the mason has to round down to coursing. In a design with especially critical rebar placement like yours, bringing it down to something much tighter would certainly make sense. Will you be using bond beams at 5'4"? If so, requiring one at each bond beam would seem reasonable and would make inspections convenient.

 

[SteelPE]

If we were to attempt this, we would be placing a bond beams at approximately 10'-0" o.c. This would put a bond beam at the window head and 10'-0" above the head (just shy of mid height between the window head and joist bearing. I am not fully on board with the the beam at 20'-0" yet.

 

[bones206]

Here’s a concise summary of code requirements for grout pour heights and lift heights:

http://www.masonryinstitute.com/wp-content/uploads/GroutPourLiftLimits.pdf

 

[southard2]

When dealing with masonry between large window openings sometimes its the compression side that actually controls when you use spread reinforcement bars (1 on each face). You can sometims pick up more strength simply by filling another adjacent cell with grout. Sometimes you can increase f'm and get it to work. My standard was f'm = 1500 psi for the longest time. But I've noticed for quite some time now that most of the block I get submitted for approval is also capable of achieving f'm = 20000 psi so I've started switching over to that on some of my projects. You'll notice that you'll get more bang for your buck say from switching from a #5 to #6 that way.

Regarding rebar placement I agree it is usually pretty bad. Strangely I got lucky the last few years and it hasn't been entirely all bad. But still in general I try the best I can to just use bars in the center of cells. I'd rather place #5s at 8" o.c. in the center of cells for four feet between windows than only grout four cells and spread the bars. Sometimes I have found that you can use 2 bars in the center of each cell and that will be enough. I will try anything to avoid having to spread the bars. But here in Florida its unavoidable a lot of the times.

John Southard, M.S., P.E.

https://www.pdhlibrary.com/

 

[SteelPE]

One last question, what you would use for a distance from outside face of block to edge of reinforcing (I guess you could say edge distance but this would include the face shell of the CMU) for 12" block? I was thinking about using 2 1/2 inches (1 1/2" for face shell thickness and 1" of cover with grout). Does this seam reasonable?

 

[phamENG]

If you're going to use rebar positioners, look at those. Most of them have set distances for placing the bars, so you'll be limited in where you can put them.

 

[SteelPE]

So as predicted above, we are getting killed by factored axial stress with the limit being 0.05f'm for h/t>30. On the backside of the building we have top of joist at 30'-10" and we are using LH series joists. This puts bearing at 30'-5" and requires us to limit factored axial stress to 0.05 f'm. The shear mass of the wall including the openings attracted to the piers is killing us on stress (even if we solid grout the piers).

We have asked, and they refuse the lower the backside of the building. They also refuse the increase the height of the foundation. So I am left with either:

a) using a high strength block
b) using 12" deep seats on the joists to bring bearing down to 29'-10" making h/t 29.83

I am more than likely going to choose option B.... but how dumb is this limit? I understand why they have it, but in my case I am going to "cheat" my way out of it.... and when I do, I suddenly increase my factored axial stress 4x. You would think the drop would be gradual vs dramatic.

 

[bones206]

Not sure what masonry strength you need, but there’s a push in the industry to try to get engineers to specify higher strengths because it’s readily available nowadays. southard2 was implying the same thing. f’m of 2500 or 3000 psi should be achievable without too much difficulty.

 

[SteelPE]

I started with 2,000 psi block but ended going up to 2,500 psi block. I understand about the push, but if I can get h/t<30 then there is a huge difference in factored axial stress limit. Much greater than anything you can get out of increasing the block strength.