CMU Fireplace Chimney
CMU Fireplace Chimney
(OP)
Starting work on a a 4'x7'x24' CMU chimney with two flues. Does anyone have any suggestions or resources they would recommend? My biggest concern will be lateral loads due to seismic, the dead weight of the CMU stack will be rather substantial.
This chimney is for a traditional log home, not your usual stick frame construction. Even though the chimney will be attached to the wall I don't yet know how much lateral bracing I can or cannot assume from this connection. The additional lateral load to the log framed shearwalls is probably another factor worth considering and I'm not sure the original engineering took this into consideration. Another major issue is the settling of the logs and the differential movement this creates in the wall relative to the chimney.

This chimney is for a traditional log home, not your usual stick frame construction. Even though the chimney will be attached to the wall I don't yet know how much lateral bracing I can or cannot assume from this connection. The additional lateral load to the log framed shearwalls is probably another factor worth considering and I'm not sure the original engineering took this into consideration. Another major issue is the settling of the logs and the differential movement this creates in the wall relative to the chimney.

A confused student is a good student.






RE: CMU Fireplace Chimney
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: CMU Fireplace Chimney
Mike McCann, PE, SE (WA)
RE: CMU Fireplace Chimney
RE: CMU Fireplace Chimney
Thank-you for the reference in the IBC Woodman88, that will be the first thing I review before getting too involved in this project.
Originally the chimney was to be stick framed with a cultured stone facade, however the owner decided he wanted a "real" wood burning fireplace. Fortunately, he made this decision prior to pouring the footing, so he does have a fairly substantial footing under the fireplace footprint (11'x8'x18"), which calcs out for a little over 100,000 lbs of dead load at @ 1500 psf. However, the masonry and engineering was not part of the original plan and engineering. This one is going to be interesting and challenging.
A confused student is a good student.
RE: CMU Fireplace Chimney
Here is what CMU looks like:
A confused student is a good student.
RE: CMU Fireplace Chimney
A confused student is a good student.
RE: CMU Fireplace Chimney
Mike McCann, PE, SE (WA)
RE: CMU Fireplace Chimney
Mike McCann, PE, SE (WA)
RE: CMU Fireplace Chimney
IRC or IBC has typical details. Residential chimneys are one of those things that they just do. If you tried to detail it, you'd go insane. I have pictures somewhere of the remains of a hunting lodge that burst down. The stone chimneys are still there. Free standing for 30'. On the top of a mountain. For 40 years.
There will be some level of support from the house itself, although I wouldn't worry about the connection. A metal strap should give you a decent tension force and account for differential movement.
When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.
-R. Buckminster Fuller
RE: CMU Fireplace Chimney
RE: CMU Fireplace Chimney
Manstrom I would tend to agree with you but the client has already been told by another engineer that he would need to have ugly braces off of the roof helping stabilize the chimney and that they would need to somehow allow for the shrinkage of the logs etc...
My first thought when he described what he wanted was to engineer the chimney and its footing as a freestanding column. From the bottom of the footing to the top of the chimney will be approx. 32'.
A confused student is a good student.
RE: CMU Fireplace Chimney
RE: CMU Fireplace Chimney
Mike McCann, PE, SE (WA)
RE: CMU Fireplace Chimney
RE: CMU Fireplace Chimney
RE: CMU Fireplace Chimney
RE: CMU Fireplace Chimney
Here are some notes from the building official to the owner:
A lot of this seems prescriptive in nature and really the scope of the mason or architect as many of you have inferred.
A confused student is a good student.
RE: CMU Fireplace Chimney
I agree with Hokie66 - make it free-standing.
The space over the windows & doors is intended to accommodate shrinkage of the logs. 6 logs losing 1/4" each due to shrinkage adds up to 1.5". This also means that the roof will be at least that much lower as time goes on. The foundation for said chimney will also settle as you load it up with the CMU/stone as well as some soil settlement over time.
Connecting the chimney to the structure (or vice versa) would be foolish as it will lead to problems in the future. Detail all connections between the chimney and structure such that they can accommodate some differential horizontal movement and a lot of differential vertical movement.
Just MHO.
Ralph
Structures Consulting
Northeast USA