Unreinforced Masonry Wall Stresses

[sshields]

When an unreinforced masonry wall goes from no grouting to partial grouting, the compressive stress from any axial load (live or dead), should go down, as An increases correct?

In lieu of this, if I want to maximize my vertical compressive stress to offset tensile stress from flexure in the grout (from out of plane loading) it seems that adding grouting would not be the solution.

At the moment my wall is failing because the flexural tensile stress in the grout is above the limit, now, by grouting some cells, it actually has an offsetting effect on the net stress because the additional grout lowers my compressive stress, which I was subtracting off my flexural tensile stress, however it does lower my flexural tensile stress as well... the key is figuring if flexural tensile stresses lower enough such that the decrease in compressive stresses don't put me in an overstress condition still ... per table 2.2.3.2 in the ACI 530-05.

After my analysis I found that as I add grout, the wall tensile stresses do go down as the section modulus increases, however, the net effect from the dead load stress doesn't help my overall net tensile stresses as much. Therefore, if I were to fully grout my wall I'm finding that I still need to add steel reinforcing.

Is this an appropriate analysis?



Scott

 

[JLNJ]

When you add grout you add area. The effect on the net compressive stresses in the face shells is nil.

For simple low rise situations I would neglect self-weight altogether when looking at the flexural stresses in a wall.

You are finding out that hollow CMU doesn't have much flexural strength. It sounds like you will need make yours a reinforced wall.

 

[BAretired]

I didn't think unreinforced masonry walls were permitted by code anymore. They were when I started practicing, but that was back in the horse and buggy days.

BA

 

[DaveAtkins]

You are neglecting one thing--the allowable flexural tensile stress in unreinforced CMU increases as you add grout--see Table 2.2.3.2 in ACI 530-08. So you do get a benefit from grouting, besides the increased section modulus.

And now I will be climbing into my horse and buggy;-)

DaveAtkins

 

[sshields]

@ Dave...


/Facepalm. I was just looking at that. Now the note that describes interpolation, as it referes to percentage, what do people typically do, take the full length of wall ... count up grouted and ungrouted cells, and do the division and interpolation from there. Or is there a different way that is more standard. I guess that seems the most obvious to me.

Scott

Scott

 

[a2mfk]

Where is this wall and why aren't you reinforcing it? I'm with BA, I thought these weren't allowed anymore. And if it is existing, I have them add reinforcing to hollow CMU if we are altering the structure. URM in seismic areas is obviously bad news, and in wind you may very well have no axial load because of roof uplift if this is single story.

 

[GerryBertier]

Where in IBC and/or ASCE 07 is URM not allowed? I'm in a SDC A&B and design plain unreinforced, hollow masonry walls on the regular.

 

[BAretired]

I don't have the latest code but I believe there is still a chapter in the Canadian Masonry Design Code entitled "Empirical Design for Unreinforced Masonry". To use that section of the code, there are a number of limitations which makes it unattractive for many buildings, but it is still legal.

BA

 

[DaveAtkins]

Unreinforced CMU is still allowed.

DaveAtkins

 

[a2mfk]

I inspect older residential CMU structures all of the time here in Florida, many being lightly reinforced if at all. It takes a very small amount of settlement or expansion/contraction to cause cracks in the mortar. Unreinforced CMU relies on the mortar bond for all of its strength in lateral bending unless you have enough compressive stress to overcome the tensile, which in modern construction almost is never going to happen on low rise (that I can think of). This is why I am shocked it is still allowed in the US and would never personally sign off on URM. Plus reinforced CMU responds much better to other stresses, such as those from differential foundation movements and expansion/contraction stresses.

Curious as to where URM design is still occurring and what are these structures?

 

[GerryBertier]

We do mainly low rise load-bearing masonry schools. OWSJ floors w/ concrete on form deck. OWSJ roof w/ metal desk. Generally not higher than 2 stories.

95% of the existing schools are unreinforced load bearing masonry. Many dating back to the 1910's.

 

[a2mfk]

Gerry- where? 8" CMU?

 

[GerryBertier]

Mainly 8" CMU. But up to 12" CMU. Sometimes 8"+4" composite masonry walls. I'd say 50% of the exterior walls end up requiring some sort of reinforcement.

Shear walls are Plain Unreinforced Masonry. Interior non load bearing are unreinforced.

Chicagoland area.

We detail #4 Dowels @ 48" OC for the required mechanical connection to the foundation per IBC 2009. We didn't do this up until IBC 2009. No mechanical connection hasn't created any problems. Concrete masonry is concrete, foundation wall is concrete. Empirically no problems. We've had lengthly discussions concerning this and its hard to justify to the owner (since we haven't been for some time). Ultimately we decided it would be a good practice (and to code obviously).

Existing schools tend to be 2'-0" multi wythe load bearing brick. All of the 8" load bearing masonry my firm has done in the last 20 years is holding up well and we haven't had any problems.

Most of the owners we work with over long periods of time. We work with architects which tend to rotate school districts throughout time, but we work with most of the architects, so we end up seeing our work down the road.