Masonry Chimney for Beam Support

[TRAK.Structural]

Are there any code provisions (residential) that prohibit the use of brick masonry chimneys for support of beams (likely multi-ply LVL)??

I have a project where the owner wants to do a rear addition and vault the ceiling. The new ridge happens to line up with an existing fireplace chimney so I am trying to decide how best to support the ridge beam at this end. I'm not sure that I'm even comfortable with this concept aside from code provisions because I don't really have any info on the construction of the chimney other than the outside geometry. Leaning towards framing a support structure adjacent to the chimney for the ridge support, maybe a king truss. What would you do?

 

[XR250]

In my area, they don't like wood on chimneys. I use i-beams in this case or sometimes Flitch beams with the wood cut back and a bearing plate.

 

[TRAK.Structural]

In my area, they don't like wood on chimneys. I use i-beams in this case or sometimes Flitch beams with the wood cut back and a bearing plate.

Steel makes sense.

Do you see many chimneys that are a minimum of double wythe masonry? In concept I'm not seeing a great way to resolve uplift reactions if the ridge bears on the masonry for support; allowable tensile stresses are small.

 

[phamENG]

You'll have to check clearance to combustible rules. The idea, as I understand it, is to ensure combustible materials outside the flue are far enough away so they won't heat up to their ignition temperature if there's a fire inside the flue. But yeah, even you obey them the local AHJ may reject it anyway.

 

[XR250]

Steel makes sense.

Do you see many chimneys that are a minimum of double wythe masonry? In concept I'm not seeing a great way to resolve uplift reactions if the ridge bears on the masonry for support; allowable tensile stresses are small.

Honestly, I never check that. There is usually a bunch of dead load above it.

 

[TRAK.Structural]

Honestly, I never check that. There is usually a bunch of dead load above it.

In my case I think there will only be a foot or 2 of chimney above the new ridge so not a significant amount of load to hold it down.

 

[JAE]

Is the brick chimney to be left exposed to view in the new room? If not can't you furr out a new wall in front of the chimney and install a new column there (hidden by the new wall furring?)

 

[TRAK.Structural]

Is the brick chimney to be left exposed to view in the new room? If not can't you furr out a new wall in front of the chimney and install a new column there (hidden by the new wall furring?)

The clients preference is to keep the brick exposed.

 

[TRAK.Structural]

As an alternate to using the chimney for support, what about a "bent" steel beam to match the roof slope like the sketch below??

 

[XR250]

In my case I think there will only be a foot or 2 of chimney above the new ridge so not a significant amount of load to hold it down.

Code may require the chimney to be extended as I think they have to be 3 ft. min. above the roof

 

[phamENG]

You'll likely need steel columns in the wall to control deflections, but something like that can work. I've done it before where I didn't have any place for posts under a 50' long gable ridge. Broke it up into 3 spans with a similar setup. I had a floor, though, so I was able to make a giant, webless truss and thrust at the base wasn't a concern.

 

[TRAK.Structural]

You'll likely need steel columns in the wall to control deflections, but something like that can work. I've done it before where I didn't have any place for posts under a 50' long gable ridge. Broke it up into 3 spans with a similar setup. I had a floor, though, so I was able to make a giant, webless truss and thrust at the base wasn't a concern.

If the "bent" beam were a true pin-pin then yes there would be some thrust to deal with, but if the numbers are reasonable can't you just let the ends of the beam slip/deflect horizontally to remove the thrust component?

Steel columns and then providing foundations to resist thrust seems like it could get out of hand. On the previous job you mentioned did you make a moment connection at the beam/col joint and essentially make it a rigid frame with a pinned base?

 

[phamENG]

If the numbers are reasonable, sure, but I doubt they will be. Making that fit in a reasonable depth (for residential) AND having small enough deflections is going to be very tough. And slip connections don't work well in residential where everything is hard fastened to everything else with rigid (and brittle) drywall and or plaster all around. Will it behave as a slip connection? Yes. Because the wall will deflect out with it.

did you make a moment connection at the beam/col joint and essentially make it a rigid frame with a pinned base?

No. I had an occupied attic to work with so I incorporated a beam in the floor. That's what I meant by a webless truss. I was able to resist the thrust in that as a tension member rather than have to deal with horizontal movement. The connections were a bit hard to get right, but it worked out in the end.

 

[fp23]

If it were me I would avoid framing into that chimney. I would do a flat beam at N-S walls plate height in front of the chimney and a king post down to it from the ridge. Or a bent steel beam. that way you might be able to avoid adding load to the existing foundation too

 

[TRAK.Structural]

If the numbers are reasonable, sure, but I doubt they will be. Making that fit in a reasonable depth (for residential) AND having small enough deflections is going to be very tough. And slip connections don't work well in residential where everything is hard fastened to everything else with rigid (and brittle) drywall and or plaster all around. Will it behave as a slip connection? Yes. Because the wall will deflect out with it.


No. I had an occupied attic to work with so I incorporated a beam in the floor. That's what I meant by a webless truss. I was able to resist the thrust in that as a tension member rather than have to deal with horizontal movement. The connections were a bit hard to get right, but it worked out in the end.

Good points and I don't disagree.

I can/will check the numbers, but I'm not sure that any kind of column that can fit in a 2x4 wall will give me much resistance to deflection at the beam/col joint from thrust. Maybe I can get them to use 2x6, or sell it as an architectural feature to have a little bump out on the interior around columns.

I think I could detail something reasonable to keep the top plate continuity in tact where a frame like this would interrupt it, but what about at the base of the wall? The steel column probably wants to bear directly on the foundation stem wall but if possible I'd want to try to resist the lateral reaction with the wood framed floor somehow. Any ideas on that?

 

[TRAK.Structural]

If it were me I would avoid framing into that chimney. I would do a flat beam at N-S walls plate height in front of the chimney and a king post down to it from the ridge. Or a bent steel beam. that way you might be able to avoid adding load to the existing foundation too

This could work, the post would obstruct the view of the chimney brick and the beam across the front of the chimney kind of goes against the vaulted concept of having all open area, but cost of other more complicated options might steer the client towards a solution like this.

 

[XR250]

Maybe try some parallel or near parallel chord scissors trusses instead of the ridge?

 

[TRAK.Structural]

Maybe try some parallel or near parallel chord scissors trusses instead of the ridge?

Good suggestion!