Residential Chimney & Fireplace - Shear Wall Connection 1

[skuhlman123]

Hello,

I am designing a residence and I would like to use the chimney as part of my lateral force resisting system. Does anyone have details showing a typical diaphragm-masonry chimney connection and a collector-masonry chimney connection? Based on my review of the IRC, a minimum clearance is required to prevent damage to wood members (which makes sense), but I am confused on how designers maintain this clearance and provide an adequate load path. I apologize if this is a simple question but I had some difficulty finding resources on line that discussed chimney detailing as a member of the lateral force resisting system.

Thanks,
skuhlman123

 

[msquared48]

Metal strap between the double top plate and reinforced concrete / CMU structure of the chimney - i.e., a drag link that can function in either compression or tension.

Mike McCann, PE, SE (WA)

 

[oldestguy]

I trust you know that chimneys expand when in use. If there is a liner that really moves up and down.

 

[skuhlman123]

Thank you for your quick responses. I appreciate the feedback but was hoping for a little more clarification

msquared48,

Could you clarify how the compressive forces are transferred? I thought metal straps only transfer tension forces and flex underneath compressive forces. Are you saying that you use metal straps on each side of the wall and one will always be in tension when the load is transfered?

oldestguy,

Could you elaborate on your comment, especially on how it relates to loadtransfer between walls and the chimney? Are you recommending that I account for thermal expansion along with the shear forces when designing the metal strap?

 

[KootK]

I believe that we generally try to isolate our chimneys from the adjacent structure when possible. See this thread for additional info. Creating hard attachments to something that gets hot and experiences significant temperature fluctuations can be tricky.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[XR250]

I believe that we generally try to isolate our chimneys from the adjacent structure when possible. See this thread for additional info. Creating hard attachments to something that gets hot and experiences significant temperature fluctuations can be tricky.

Every old house I have seen torn into has never had a gap between the framing and the masonry. Never have witnessed an issue. I imagine the chimney does not actually get that hot outside and the expansion is minimal.

 

[thaidavid40]

I have to agree with @XR250 - there's no telling how many old houses I have seen where floor and roof joists rest in pockets on the outside of the brick chimney coursing. That said, I don't know that modern chimneys are built with as much "beef" as the older ones. However, I can't imagine that the lateral thermal expansion of the chimney itself would be enough to compromise adjacent resilient wood framing.
Dave

Thaidavid

 

[KootK]

Depends on the liners etc. Most of what I think I know about acceptable chimney construction comes from IRC. I don't dispute what has been done in the past. Often the situation is reversed and you're needing to tie the chimney to the diaphragms for seismic support. Lateral expansion is not my concern as the dimensions in play are small relative to thermal COE. Vertical might concern me in a taller building I suppose.

I'd think that you'd need to satisfy exception #1 below in order to make a go of this. Connecting with steel hardware across any gap might make some sense rationally but I'm unsure how well that would be received by AHJ. Metal being a rather good conductor after all.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.