15 ft CMU chimney design method

[shacked]

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

 

[hokie66]

Of course you are correct in that the chimney has to be reinforced vertically as well as horizontally. The primary reinforcement will be vertical. Is your boss a structural engineer? If so, I think you may have misunderstood.

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.

 

[JJMT]

CMU Chimneys are a black box of mystery

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.

 

[civilperson]

The structure is not a wall but a closed rectangle with an enormous Moment of Inertia about the axis. Designed as a vertical beam with 3' depth, (7'-8" width), it needs nominal reinforcement.

 

[youngstructural]

Hokie66 and civilperson are spot on in my opinion: The structure will tend to act as one large cantilevered beam. Given that cantilevers deflect quite a bit, I always try to use intermediate floors as props; Since you've got a free-standing chimney (as best as I understand), you don't have this luxury, so you'll need to be careful with your calculation of Icr and Ieffective in your deflection check for sway at the top of the stack. And bear in mind that you need to ensure the structure can act composite, ie: You need reinforcement continuous about the corner to satisfy the longitudinal stress from Tau = VQ/IT

That's my two cents...
Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[tdsull]

Black box of mystery is dead on.

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.

 

[msquared48]

I also agree with Hokie and Civilperson, but would add that to calculate the I, the effective thickness of the wall depending on the number of cells grouted need also be included.

Mike McCann
MMC Engineering

 

[youngstructural]

Good catch msquared; It's kind-of there is other comments, but good to flag specifically.

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...

 

[JJMT]

The top portion can be designed as a large rectangluar box but the challenge is when you get to the fire box where your gross section changes. I deal with a lot of double sided fire boxes and end up with a very odd section that requres diligent detailing around the lintel and down into the hearth slab, where i redistribute the loads.

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.

 

[cvg]

I'm not a structural engineer, however I experienced a 7.3 and then 6.7 earthquake in my home in the same day (Landers and then Big Bear quakes in California). I was not at the epicenter of either one, but in between both. Besides the broken glass and items falling to the floor in every room, the only structural damage I was able to ascertain caused by the quakes was a crack in my chimney and another in my driveway. I wished the builder would have overdone it with the reinforcing when he built my chimney. It is very difficult to go back and repair it.

http://earthquake.usgs.gov/regional/states/events/1992_06_28.php

 

[youngstructural]

Sorry to be picky there cvg, but it would require a lot more than a mason overdoing it with the reinforcement in order for the total amount of damage due to a ULS quake to be less... In your case I can see why you'd want more though! It is too bad...

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...