Flush/In-Wall Pilaster & Masonry Wall Design

[SVaughn]

Hi All,

I am having difficulty in designing a masonry wall to work for my given situation. I am very green when it comes to masonry construction and am quickly finding out that it is not as simple as I originally thought it to be. I have the following situation, 8" cmu walls that are approximately 17.06' tall and are taking a horizontal pressure due to wind of 35.68 psf. The pilasters to take the wind loads are 13.45' on center, therefore the 48" wide in wall pilaster is to take a 480 plf line load located at the center along with an axial load of 9k. With that, the pilaster is to take a pressure of 480/4 120 psf. This of course describes the worst case. My wind loads are quite high as I have a high basic wind speed of 155 mph. Please see attached diagram for clarity. I am under the impression that an 8" cmu wall is not going to work even grouted solid with vertical reinforcing at 8: o.c.. As always, any and all help is greatly appreciated.

Thanks,
Shaylon

 

[hokie66]

If the wall must be flush, i.e. without projections, I think your best approach would be to use the whole wall spanning vertically, with a girt (horizontally spanning beam) at the top to take the load back to the girders. In high wind loading areas, choose the robust approach without too much "refinement".

 

[SVaughn]

Hi all,

I am still struggling with my problem as noted above. One piece of information that I failed to mention is that I cannot use the roofing material for any shear transfer, therefor I do not have a diaphragm. I have been working on this on and off for the last three weeks now and still am not knowing how to get this to work.

hokie66, I don't quite understand how spanning the wall vertically to a bond beam to transfer the load back into the pilaster. I'm sorry, I am very green when it comes to masonry design.

Thanks for everyones help,
Shaylon

 

[hokie66]

I didn't say to use a bond beam. You would need to use something stronger. I was thinking of a steel beam to transfer the load into the roof structure.

You can never use "roofing" for a diaphragm, although structural roof deck is often used in that manner. Roofing is to shed water.

You have to have a way to tranfer lateral loads at the roof level. Perhaps a horizontally spanning truss? As you have only talked about the wall itself, it is difficult to give advice on the whole structure.

 

[SVaughn]

Hokie66,

I'm sorry, I misread your previous post about the horizontally spanning member. What you are saying makes more sense to me now.

I must also apologize for misspeaking about using the "roofing" material as a diaphragm. I guess what I should have said is that I am not able to use the metal deck system being used on this project as a diaphragm. We are using a 4in thick isulated "sandwich" panel which does not include any allowable shear values in the tecnical data.

Please see the attached framing plan to give you a better idea of how the structure is laid out.

Thanks,
Shaylon

 

[msquared48]

For a 155 mph wind, the loading of 34.68 psf would be in the ballpark for a flat roof and Exposure "B" wind, but too low otherwise.

You might consider a two way spanning wall for the wind loads here - as in a two way flat slab - with both CMU pilasters and steel WF wind beams.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[MiketheEngineer]

If you can/want - put the pilasters outside the building and cover them with dirt??

 

[msquared48]

?????????????????????!!!!!!

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[SVaughn]

msquared48,

I have a qz = 45.17 psf with my current configuration. 35.86 psf is the worst case on any portion of the wall once the internal/external pressure coefficients and the different zones are under consideration.

Thanks,
Shaylon

 

[SVaughn]

Sorry MiketheEngineer, I have to agree with msquared48. I'm not sure what you are talking about or how that possibly makes any sense.

Thanks for the suggestion though,
Shaylon

 

[MiketheEngineer]

I assumed this was a foundation wall - maybe a mistaek on my part??

 

[dhengr]

Talk with the supplier/manufacturer of the 4" insul. panels. They have some shear capacity, as a roof diaphragm because they likely have plywood or OSB skins top and bottom, Edge framing, and maybe splines at their mating edges, and in those winds they must also be held down (fixed) to the roof framing against uplift in a substantial way. We can’t see your sections and details in the plan you show, but I think Hokie is suggesting putting some fairly stiff members, beams or trusses in the plane of the roof framing, which will pick up and transfer the wall loads into the roof system. Maybe you could x-brace in the plane of the top flanges of the stl. beams and drag and distribute that wall load the way you want it to go. Maybe that x-bracing could be diagonal flat bar stock right on top of the beam flanges. Maybe you have to reconsider the thickness of your CMU walls. Why not frame that whole roof with bar joists spanning the 36' direction, and then use plenty of x-bridging in the long direction, btwn. joists? You end up with a more uniform gravity load on the CMU walls and a more uniform transfer of lateral loads.

 

[hokie66]

I wouldn't even try to use those panels as a diaphragm. Just ignore the panels for lateral loading and add in the members required to make the roof work as a braced building. That would mean a member all around the perimeter, and diagonals to form trusses in each direction. May not be the way it is typically done in the US, but it works. We don't have built up roofing on metal deck in Australia, so our solutions generally incorporate horizontal trusses in the roof plane.

 

[msquared48]

I agree with Hokie here. Some insulated panel systems are rated, but likely not for the diaphragm loads your building will see. Others insulated panel systems are not.

And by the "sandwich" panel term you mention, I assume you have 1/2" CDX ply on both faces with foam in between with edge 2X splines for nailing attachment?

Professionally, I would not use these panels in a commercial application, only residential, and only with tested and rated systems, both for shear and vertical load.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[dhengr]

Hokie:
Your way was about where I was going, but I’ve been accused of having some trouble with serious commitment. Oh... that’s a different subject.

 

[SVaughn]

msquared48,

The "sandwich" panels I am refering to are made of two metal panels with some insulating material in between them. See the attached PDF for the system we are using.

hokie66,
I am working on getting your method to work. I like this idea.

Thanks everybody. When I get something figured out or need further assistance I will post on this thread again.

Thanks again,
Shaylon

 

[msquared48]

Two comments here structurally relating to this system:

1. The screws used to transfer any shear are awfully long and seeing a lot of bending. I'm not sure how much restraint, if any, can be clunted on from the foam layer.

2. The upper level of metal decking, dut to the high rib, may not offer much lateral resistance. that may be concentrated in the lower layer, depending on its gage and the ability of the screws and foamm to contain it from upward movement.

I still stand by my original comments here.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[concretemasonry]

msquared-

I think Mike was referring to projecting pilasters and not internal pilasters that can be used on 10" or 12" walls. The projecting vertical sections are often objectionable for interior function purposes and external architectural purposes, but they do define control joints although they can be hidden by a good architect/detailer. - One of benefits for thicker walls in some markets where unit weight is not cost detriment.

Dick



Engineer and international traveler interested in construction techniques, problems and proper design.

 

[Garfus]

If you span the CMU vertically from footing to bond beam and
then X-brace with cable or steel rods across the roof. This is
like a prefab metal building roof and would not depend upon the
panels for shear transfer. There is no way you will get 8"
CMU to work at your wall height. Might just as well figure 12".
10" is almost impossible to find these days.