15 ft CMU chimney design method
15 ft CMU chimney design method
(OP)
We have a job that requires us to design a 15 ft free standing cmu chimney, dimensions 3' x 7'-8" x 15'. In this case seismic shear will govern so I have to design this chimney to resist the lateral seismic force.
My question is related to the design and or actual response of a 15' tall cmu chimney to ground shaking.
I believe that the vertical reinf should be designed to resist the induced moment in the cmu due to lateral forces. Therefore I intend to design a 1' wide by 15' tall section of chimney and apply a seismic force at the center of mass and design the vert reinf based upon the moment.
My boss wants me to design the horizontal reinf. He wants me to design a 1' tall x 7'-9" wide section of chimney with a distributed seismic load over the face. Basically a simply supported beam, except the loading is not vertical buy horizontal.
Now I believe that this is the wrong method.
I think that as the chimney starts to shake the upper portion of the chimney will want to snap or whip into and that the vertical reinf will need to be designed to resist these forces.
Any thoughts would be greatly appreciated.
Erik Gibbs
My question is related to the design and or actual response of a 15' tall cmu chimney to ground shaking.
I believe that the vertical reinf should be designed to resist the induced moment in the cmu due to lateral forces. Therefore I intend to design a 1' wide by 15' tall section of chimney and apply a seismic force at the center of mass and design the vert reinf based upon the moment.
My boss wants me to design the horizontal reinf. He wants me to design a 1' tall x 7'-9" wide section of chimney with a distributed seismic load over the face. Basically a simply supported beam, except the loading is not vertical buy horizontal.
Now I believe that this is the wrong method.
I think that as the chimney starts to shake the upper portion of the chimney will want to snap or whip into and that the vertical reinf will need to be designed to resist these forces.
Any thoughts would be greatly appreciated.
Erik Gibbs






RE: 15 ft CMU chimney design method
I don't see why you want to design a one foot wide strip when you have a closed tube which will cantilever as a unit.
RE: 15 ft CMU chimney design method
I think you may find designing a single vertical CMU wall for the anticpated loads very difficul on uneconomical. You may have better luck using the gross section of the CMU. In which case you need to consider the out of plain seismic foces iparted to the wide face of the chimeny (very importana if you are applying a large stone veneer). However, i recommend following the minimum horizontal reinforcement requirments for SDC D, this will often control the rebar size and spacing hoizontally.
CMU firplaces, lots of bars, lots of grout be very diligent in your detailing at the lintel and hearth slab. And remember to check stress at all the elevation sections, especially the one through the fire box.
RE: 15 ft CMU chimney design method
RE: 15 ft CMU chimney design method
That's my two cents...
Cheers,
YS
B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...
RE: 15 ft CMU chimney design method
I recently had a similar condition. Some people in my office told me I was crazy for the reinforcing I came up with, even though it was nominal. I designed the structure as vertical cantilever beam.
At the end of the day I supervised a fair amount of the construction and the mason was not shy about telling me I was crazy and that we never saw so much steel in a chimney.
RE: 15 ft CMU chimney design method
Mike McCann
MMC Engineering
RE: 15 ft CMU chimney design method
One more comment: You need to include the added flexural capacity (ie: increased bending moment capacity) of the chimney due to the vertical load from self weight.
I do this for all my retaining walls and most masonry or concrete applications: Code dependant, the calculations should look something like:
Mn = Ms + Mp
where Ms = Asfy(d-a/s)
& Mp = Nn(d-a/2)
Might help quite a bit, and is often forgotten because we do so many beams which naturally do not get this component!
Regards,
YS
B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...
RE: 15 ft CMU chimney design method
The wide face of the needs to consider the out of plane seismic effects, as young structural stated its a compoiste shape and needs to be have the reinforcing to transfer the stresses between the compresssion and tension sides.
The weight of the chimeny needs to be consider in the design but you may find this to controling load case. I typically end up with the compresses stress controling due to the limited section at the base. In these cases i try and bring up a concrete section to above the fire box, capped with a slab. The masons can then work off the slab and fill in between the two slabs.
Your seismic requirements and specific configuration may not require as much detailing and refinement.
I've attached a picture i took yesterday onsite. You can see the concrete collars for attchment of steel and the small cmu piers at the fire box where i have a lot of grout and steel.
RE: 15 ft CMU chimney design method
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RE: 15 ft CMU chimney design method
Good luck with the repairs;
I'd recommend grinding, power cleaning and then either concrete patch with SSD (surface saturated dry condition for the existing concrete) or an industrial grade sawcut sealant (for your driveway).
Patching the masonry in the chimney pretty much necessitates repointing the block to get a good quality and durable result.
Regards,
YS
B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...