Shear stress in hollow masonry

[mfstructural]

I have a question regarding calculating the shear stress on a hollow masonry CMU wall. I know that for a partially grouted wall the section resisting shear is "T" shaped (the grouted cell with the face shell). My question is for a hollow masonry wall is it just the face shell resisting shear? so the bd is equal to 12"x1.25" (face shell thickness).

 

[SteelPE]

I would imagine the part resiting shear would be the parts that are mortared together. So in unreinforced masonry calculating the shear resistance based upon the face shell only would be correct if not slightly conservative.

 

[mfstructural]

One more question: Would you take only one face shell or both face shells since fv = V/bd. The reason I ask is because I've seen examples worked out where only one face shell is assumed to resist the shear and not both face shells.

 

[steellion]

I would use both face shells. Logically, each face shell is a plane capable of resisting shear.

 

[msquared48]

I would certainly agree since both sides should be mortared.

Actually, I find it hard to imagine a scenario when one side would not be mortared.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[mfstructural]

I agree...just asking because of the example I saw. I would also use both face shells.

 

[dhengr]

In the example you saw, check to see that the wall did not have a lateral loading too, wind for example, or a moment on it which might make the compressive loading on one of the face shells fairly low. In this case the bond/tension, or shear cap’y. might be low enough to warrant its being neglected in total shear cap’y. calc. Absent that type of condition I would consider both face shells to be acting in shear in the plane of the wall.

 

[mfstructural]

Ahhh...yes. There is a moment on the wall. So yes, I would understand just using one face for that case.

 

[msquared48]

OK... But if that's the case, and I am assuming the moment is due to a load transverse to the wall, then there are two things to consider:

1. Technically, the presence of such a moment should require at least the minimum amount of vertical reinforcing steel and some grouted cells at a minimum of 4 to 6 feet on center, which will increase the allowable shear to the wall. as neither concrete nor mortar are good in tension, and

2. The presence of mortar on only one side will cause the wall to bow away from the mortared face unless spacers are used on the unmortared side. Not very appealing to me...

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[DST148]

@mfstructural: assuming the wall is reinforced.....
For out of plane loads we take the net crosssectional area of the wall. For in-plane loads we take (b . d), where b is the net effective width of the partially grouted masonry, and d the effective depth.