Designing wood-framed chimney with brick veneer

[Labs763]

Hello all,

I am curious how others are designing residential wood framed chimneys with brick veneer. I designed a new house, and the homeowner was previously going to install a freestanding CMU or prefab Isokern chimney. This was to be an exterior chimney outside of the back porch framing. During construction they have discovered they cannot afford it and want to switch to wood framed that is laterally supported by the roof structure. They also want to increase the size and cantilever height (11ft above roof line). My question is how engineers generally design the cantilever frame? This is in a high wind (145 mph) and high seismic zone.

I am probably overconservative on my design, but I have always sized the chimney studs to cantilever without the help of wall sheathing. I do not recall dealing with a chimney cantilever this tall before though. I can get two full height stud packs on the back side of the chimney, but everything else is discontinuous. Even with PSL or LVL stud packs, cantilever studs do not work for deflection. The walls are not wide enough to meet min. shearwall requirements, but it is obviously a fully sheathed box that offers a lot of stiffness. Is there anyone here that does this regularly with no issues? I would prefer switching to a steel frame for this, but want to gauge if that is out of line/overly conservative.

 

[XR250]

I would def. be using the sheathing. What about reinforced masonry? Is there enough room for that?

 

[HouseBoy]

I too would def use the sheathing.

On the back side it looks like you have wide wall at the bottom portion with a narrower "cantilever" projecting up. I'd probably try to run the studs full height (using LSL or LVL -OR- indicating a specific lap pattern for multiple studs. Sometimes I use LVL material as a mast (i.e. 3 1/2" LVL in a 2x4 wall using as large a depth as I can)

On the front wall it looks like you will want to use a "transfer beam" and then somehow get the lower portion of the wall with the giant opening in it (so maybe the main roof of the house will help) to provide lateral resistance.

In the "front to back" direction, looks like a similar "transfer beam" situation. Not sure from the sketches if the rest if the house can be used for lateral support but that's what I would look for and try to tie the chimney to the house at the roof level.

I'd expect substantial hold-down anchors at all discontinuities.

 

[XR250]

I'm curious if this is something most engineers even bother detailing or is it something that "just works" regardless. Seems like you could burn up a significant amount of fee on this one component.

 

[dik]

For heating? or just decorative? Does the chimney have to extend so high above the roof?

-----*****-----

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[Labs763]

Thank you for responses. See below for responses to your comments:

1) It is for heating (wood burning), but does not need to be this tall. The owner/architect raised it about 4 feet after I issued construction drawings.

2) Yes there is (or was) enough room for masonry. It was original intended to be a freestanding CMU chimney, but the cost came in too high.

3) Agreed on using LVLs, but as noted, they do not meet deflection requirements without considering effects of sheathing.

3) I am not sure if other engineers detail this - I am hoping others will give input. Can I just defer to "building code requirements"? The building inspector has been finding a lot of issues with the construction thus far (ie not matching the drawings), so I predict they are going to question the chimney and ask me to sign off on it. And yes, it is taking up an absurd amount of time/fee.

4) I see that the consensus is to use the sheathing. My question is how this meets building code? If it was wood siding I would not be too concerned, but brick has really tight deflection requirements. And this is at the edge of an open porch, so the roof diaphragm is already cantilevering out past the house to this point.

So no one thinks this should be steel framed? 10ft cantilever with brick veneer??

 

[dik]

Is it enclosed? How does it draw air? Often chimneys draw hot air from the house. Our fireplace was enclosed and drew air from the outside. It could be framed with CFS and strap 'bracing', with brick veneer fairly easily. Height not good for seismic loading.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik