CMU Fireplace Chimney 1

[medeek]

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.

A confused student is a good student.

 

[woodman88]

If you are designing under the IBC have you read the SECTION 2111 MASONRY FIREPLACES?

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.

 

[msquared48]

I would have to see the proposed placement in plan wry the existing walls to comment on the lateral. As for the settlement, if the structure has been in place for two years, most of the 3 to 6 inches of anticipated settlement should have occurred already.

Mike McCann, PE, SE (WA)

 

[jheidt2543]

Check the website for the Brick Institute of America, they have a lot of information on fireplace designs with typical details.

 

[medeek]

As you can see from the picture it is brand new, they are about to put the roof on. I was quite impressed with all the measures in place for settlement throughout the structure, jacking screws at the posts, 4" of clearance above all the windowss and doors and even interior partition walls have special screws/ slots for gyp. board etc... in place to deal with the projected 4"-5" of settlement.

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.

 

[medeek]

Is it just me or does it seem like split face CMU is not the most attractive chimney for this application? For a log cabin like this I would think a more rustic looking stone or rough cut rock veneer would be more suitable. Granted split face is better than standard CMU.

Here is what CMU looks like:

A confused student is a good student.

 

[medeek]

Something along these lines but on the exterior:

A confused student is a good student.

 

[msquared48]

Large river rock is the classic look. Maybe CMU with a river rock veneer?

Mike McCann, PE, SE (WA)

 

[msquared48]

Ever see Dale Prenike's video of his log cabin construction in the Alaska wilderness. One of the most interesting videos I have ever seen. Never get tired of watching it. I may have spelled the last name wrong, but I'm sure you can Google it. Has a great section on the construction of his river rock chimney. Still there to this day and part of the Alaska parks system now.

Mike McCann, PE, SE (WA)

 

[manstrom]

I think you are thinking too much.

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

 

[Sawbux]

Make sure you have provisions for the log structure to settle. Looks like you have already done so with the doors and windows.

 

[medeek]

I saw another picture with exactly that, CMU with a rock veneer, it's not my call but I'm going to suggest that to the mason and client. Thank-you MSquared48. He was already planning on veneering the inside but why not the entire chimney, it only makes sense.

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.

 

[hokie66]

I would make it freestanding, isolated from the wooden structure.

 

[msquared48]

A chimney is normally not structurally engineered. They are covered as standard details and are self supported for all loadings.

Mike McCann, PE, SE (WA)

 

[JedClampett]

Have you looked at NCMA TEK Manual 17 (

https://marketplace.mimeo.com/ncmapublication#name=17)?

It says it costs $2.15, but if you go onto a CMU manufacturer's website, they're usually linked for free.

 

[JedClampett]

Sorry, I meant 03-07A, not 17.

 

[XR250]

I agree with the others. Let the mason figure it out. I think I have engineered one residential chimney and it was a PIA.

 

[medeek]

Trying to find a free copy of NCMA TEK 03-07 looks like it has a lot of the information I need.

Here are some notes from the building official to the owner:

[pre]There is no detail for the chimney
portion. There are also not reinforcement details for the hearth or
chimney. Examples would be things like combustion air, tight
fitting doors, damper location, thickness of walls of combustion
chamber, reinforcement call outs, hearth extensions, firebox dimensions,
firebox and flu lining material, etc... If you plan on covering the CMU
block with a "half stone" or similar veneer we will need the attachment
details of that material to the CMU wall as well.[/pre]

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.

 

[RHTPE]

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