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Re: Chimney Flues

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Nels,

Glad to see we are on the same wavelength...

Most of my chimney work these days consists of rebuilding existing and the masonry base with metal flue is a common approach for me. I appreciate the update on the spring type strap as it had just seemed to disappear from my perspective. Conceptually, I had hopes for it, but didn't know how it was going to work. I still find contractors that try/want to build to the old standard that cities were freely distributing before Northridge, but as I tell the contractors and property owners.... someone finally started listening to us engineers. One of my first projects out of school was to design a major restaurant with 2 large masonry chimney/fireplaces, and I became very aware of the problems of connecting the two dissimilar materials and wondered for almost 20 years why cities kept distributing the old standard.

Steve P

Nels Roselund, SE wrote:

Steve,

That has to be an anchorage detail that is pre-Northridge-Earthquake, or
that is not appropriate for seismic zone 4.

I don't believe that there is a reasonable way to design a light-framed wood
residence with a masonry chimney in seismic zone 4 -- it certainly does not
fit into the conventional light-framing category.  The chimney/fireplace
assembly is so rigid that it tends to become a shear wall.  The assembly is
so massive that it not practical to design the plywood shear walls to brace
the chimney.

One way to build a house with a conventional masonry fireplace is to build
the masonry to a level below the diaphragm [it can cantilever from a
reasonably sized foundation independently of the rest of the house], with a
light-weight flue in a light-framed chimney shaft built on a bondbeam on the
top of the masonry.  Where the appearance of real masonry is needed for the
chimney, real bricks can be filleted (including L-shaped corner pieces) and
applied as a veneer adhered to stucco on the chimney framing.

Additions to houses with existing masonry chimneys need to include masonry
or concrete shear walls in order to provide seismic bracing that has enough
rigidity to protect the chimney during strong shaking.

I don't believe the spring-anchor ever went any where.  The fine tuning
needed to make the anchor match the dynamic characteristics of a specific
house/chimney combination was probably more than anyone wanted to try to
solve, and each would most likely have to be one of a kind.

Nels Roselund
Structural Engineer
South San Gabriel, CA
njineer(--nospam--at)att.net




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