PEMB With Masonry Walls

SmithJ · Oct 2, 2003

Hi All,

I have a structural consultant who is wanting to use a Pre-Engineered Gable rafter system on 12" (and 8&quot

CMU Walls (instead of steel columns). The left Sidewall will be a 24' high, 12" CMU Wall and the right Sidewall will be a 14' high, 8" CMU Wall. The width of the building is 50' and the ridge occurs at 15' from the left Sidewall (Non-Symmetrical Gable).

My initial reaction was that this was impossible, but after giving it some thought I believe that if the connections from the Gable rafter to the CMU Walls were pinned, It might be able to work.

My concerns are as follows:

a) How does one take care of the thrust that will occur at the top of the CMU wall due to the fact that the roof profile is Gabled?

b) Standing seam roofing sheets will be used in the roof system thus eliminating any diaphragm action. How will wind loads against the sidewall be taken out of the building since there is no longer any frame action? The owner wants to avoid a cantilever CMU wall with moment transfer at the foundation.

Any ideas or thoughts that you might have will be greatly appreciated. Does anyone know where this has been successfully done in the past?

Thanks in advance.

JS.

JAE · Oct 2, 2003

You could take out the lateral wind/seismic forces on the wall in a number of ways. Here are a few:

1. Provide a horizontal truss system, laid flat, running across the width of the building and located along the bottom chord of the trusses. This would be required on each end of the building. You can set this truss depth to be either one or two truss spaces so your truss chords align with the bottom chords of the interior truss and the top of the wall. So the load path for a "piece of wind" would be to hit the wall, run up to the top of the wall, enter the horizontal truss through a wall-to-horiz truss connection, and then proceed down the truss to either side shearwall.

2. You could try to establish a diaphragm ceiling if the architecture will allow it.

3. The truss manufacturer may have some means of establishing a diapragm system in his roof under the metal roofing with diagonal braces of some kind. With this, though, you'd need a lot of coordination and also some diagonal braces from the top of the CMU wall to the top chord of an interior truss to transfer that wind up to the roof level and into these brace systems.

Lutfi · Oct 2, 2003

I like to add to JAE the following:

I live in Florida and for a long time I avoided doing this type of construction. It failed miserably, for many reasons some of which the lack of horizontal systems that JAE talked about above, after Andrew.

I simply construct a rake wall. Build up the CMY wall and step it as much as possible. Then cap it with a concrete tie beam. I have developed a detail that I use on all of my projects that requires this gable end.

Truss guys do not care except for designing their own trusses because the responsibility of designing the entire structure is the engineer of records’.

If you will let me know, I will be more than happy to share my rake wall detail. Please let me know.

Good Luck

JAE · Oct 2, 2003

Lutfi is right about the responsibility lying with the EOR and not the truss manufacturers. On smaller buildings the concept of the wind load path is ignored and the problem doesn't show up until an Andrew does!

SmithJ · Oct 3, 2003

Thanks for your responses. It seems we share similar concerns about the lateral load capability of the system. I will look into JAE's suggestions.

Lufti, Thank you for your offer. I would certainly like to take a look at your rake detail if you are willing to share it. My Email address is smithj19761@hotmail.com. Please feel free to contact me at that address.

Another concern I had was that under gravity loading, there will be some horizontal thrust at the tops of the masonry wall. If I model the top of masonry wall as pinned connections, I believe I will be overestimating this thrust since in actuality the wall will deflect a little.

I am thinking of using horizontal springs to model this joint. Are there any ideas or suggestions on how best to accomplish this?

Thanks.

JS.

jheidt2543 · Oct 3, 2003

Lufti, I would like to see your rake detail too. Please either send it to jheidt2543@aol.com or post a link to it in your next message.

Thanks in advance for your effort.

Lutfi · Oct 4, 2003

I will send the detail on Monday. I can send it in CAD-2002 or PDF format. Please let me know your preference before Monday. The detail is not that large and can be re-drawn easily.

Have great week end.

SmithJ · Oct 6, 2003

Lufti,

CAD 2002 works fine for our system.

Thanks again.

JS.

Lutfi · Oct 7, 2003

I sent the sketch in PDF format. Please remember that this one detail was for one particular project. You may use it or tailor it to specific project.

Good Luck

jheidt2543 · Oct 8, 2003

Lufti,

Thanks for the detail. It seems to be easily constructed, but I wonder what the Architects and/or Owners say about the exposed concrete vs. the exposed block wall. I suppose the concrete could be covered by wood trim or EFIS?

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