CMU Fireplace & Chimney

[shacked]

I am working on a project for a client that has a design from a landscape Architect that calls for a freestanding fireplace for their residence.
The firebox is 6ft tall x 7ft wide x 33 inches thick with a 3ft wide x 19 in thick x 7.5ft tall chimney on top and covered with 1.5in stone & 1in mortar mix. See attached pdf.

Since I do not do a lot of this type of design I just wanted to get some thoughts on what my calculations are coming up with.

I am using light weight 6in CMU for the chimney and based on seismic loads (Seismic design cat D)I am using #4 @ 8" o.c. with #3 closed ties @ 8" o.c.

This is supported on a 13in thick conc slab.

But for the firebox I can't seem to make 8in CMU work due to the large overturning moment at the base, so now I am thinking that the whole firebox should just be poured in place concrete with a masonry chimney.

Just curious what some of your thoughts are?

Thanks for your time.

 

[shacked]

Sorry, just a hello to get this post back to the top

 

[KootK]

In a high seismic region like California, making a tall, slender, heavy thing work for overturning tends to be a problem. What is the nature of the failure mode that's causing you grief? If it's just global overturning, can you not address that by simply increasing the footing width? I've done fireplaces in California in concrete in the past but that was always where I was using the fireplace construction as a shear wall for a building proper.

 

[shacked]

I don't have a problem with the overturning since I sized the footing to account for this.

The problem may be that I am trying to design the bottom firebox 8inch CMU wall section to resist the total bending moment per foot created by lateral load to the chimney & firebox. I can't use 10 or 12" block due to size limitations therefore I would need #8 rebar spaced at 8in o.c. in 8in fully grouted cmu.

This seems like a little too much steel, so I am trying to design the firebox differently. Note that the fireplace uses a self contained self venting pre-manufactured gas firepplace.

Maybe if I design the chimney to be supported on the 13inch concrete slab on top of the firebox, then resolve the moment developed in the slab into uplift forces on each side of the slab that is anchored into the walls parallel to the lateral load. Then I could design the firebox for only the bending moment due to the firebox effective seismic weight, therefore reducing the total steel.

Any thoughts on this design method?

 

[KootK]

I'm not confident that I understand your proposal based on your verbal description. Regardless, this is how I'd be looking at the design.

 

[shacked]

Maybe this describes my thought process better. See attached

 

[shacked]

I think this is what you are also describing KootK

 

[KootK]

Oh yeah, work that box shape to your advantage. Trying to get it all done using just the back wall in out of plane bending would simplify detailing but has little chance of succeeding in a high demand situation.

 

[shacked]

Ok, thanks for the confirmation. I was about to design the whole firebox out of concrete, then I checked with a concrete contractor that I work with from time to time and he said that masonry block is used all the time for smaller projects like this, so I started thinking of redistributing the moment.

Now it is a detailing issue.