masonry wall openings and out of plane loads 1

[dilbert318]

When designing a cmu wall with door/window openings in a high seismic area how should you distribute the lateral load from the area above the opening to the sides of the opening? How wide of a strip on either side of the opening do you assume will take the load?

 

[csd72]

The window will have lateral load too.

Yes, the method sounds correct.

 

[msquared48]

As for the width of the strip, as wide as you need, including the load from the strip width you choose too, to install the required vertical steel for the strip. Just have to run the numbers...

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[dilbert318]

No building code requirements and/or NCMA standard?

So if I have a 30' long wall with with an 8 foot wide door opening, I can just assume the load is taken by a 4' wide strip on either side of the door and select steel accordingly?

 

[msquared48]

Or smaller if that works. I commonly get away with 16 to 24", depending on the vertical and lateral loading combinations.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[dilbert318]

Ok - thanks for the help.

 

[ron9876]

I usually use about half the height of the wall as the maximum width. My logic is that if you saw a failure at this condition it would likely fail at about a 45 degree angle from the foundation and roof.

There would have to be a limit. In my opinion it wouldn't be reasonable to count on reinforcing steel that is say 10 feet away from an opening in an 8 foot tall wall.

It has always bothered me when codes don't address conditions such as this which is very common.

 

[sandman21]

6 times the wall thickness is what is typically used, I have been told there is a code section other than, ACI 530-05 Sec. 2.3.3.3.1, but I have not found it.

 

[Den32]

Ron and Sandman's logic sounds reasonable (although couldn't find ACI 530 reference - I only have 530-08 so maybe it has been revised?). I agree that you wouldn't want to go too far out - the masonry would have to also span horizontally to get the load there.

I looked in Masonry Design's guide (2nd edition) and the examples either have 'piers' with set dimensions, or they chose a dimension for the effective width without explanation. Also looked quickly at NCMA tech notes and nothing jumped out.

 

[dilbert318]

Thanks - I could not find anything either, you would think that someone would have created a standard since this is a common situation.

 

[a2mfk]

I have always done a similar method as others have described.

For wind I have always taken half the opening load to the jamb, plus the trib for the jamb, plus half the distance between the jamb and my resumed normal filled cell spacing. I rarely exceed 48" o.c. for minimum reinforcement though I know some engineers do, I have a hard time believing the CMU via mortar bonds (no intermediate bond beams) will span any further. This is probably conservative but...

With some buildings if the openings are large, wind loads are high, and maybe you also have floor loading above causing eccentric moments, we end up using 16" tie columns at many jambs. Also, if you have eccentricity at the floor/roof level to account for, this also gets concentrated in your jambs.

A more accurate procedure may be FEA similar to how PCA wall and other tilt programs work. However, unlike tilt which has a homogeneous thickness/rigidity, partially grouted CMU has a varying stiffness which would have to be accounted for in a program. I don't know of any such program to date.

 

[sandman21]

That section was moved to 1.9.6.1 ACI 530-08. That is not the section that allows the use of 6t, only one area in the code with 6t. The UBC had a section regarding flanged walls and 6t each side of the web. I believe that the 6t width evolved out of these design assumptions.

 

[a2mfk]

I think you are exactly right sandman in regards to effective flange width. The compressive stress distribution model is limited by this width, the same way you are limited in composite beam action by the compressive flange of the concrete slab...