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Re: Masonry fireplaces and chimneys, Fp calcs

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It has become impractical to design a new house with a masonry chimney --
the seismic response of the rigid masonry chimney and the flexible
light-framed house are require one of two approaches: make the chimney a
shear-resisting element to brace the house, or else make the shear-resisting
system of the house rigid enough and strong enough to brace the mass of the
chimney.  I suppose that is the reason we see so few new houses with masonry
chimneys metal flues enclosed in a light framed shaft are very commonly used

 I have used these approaches:
1.  Apply the provisions of Chapter 34, which allows repairs that comply
with the Code without requiring the entire structure to comply with the
code.  In southern CA, this is not interpreted by building departments to
allow rebuilding a damaged chimney without analyzing and strengthening the
lateral load resisting system of the house -- but it may be worth a try in
your locality.
2.  Rebuild the masonry fireplace and chimney with a top elevation that is
below the ceiling/roof diaphragm, so that the masonry structure does not
interact with the lateral load resisting system of the house.  The flue is
then a stainless steel assembly [such as manufactured by Metalbestos]
enclosed in a light-framed [wood or metal studs] shaft.  If the appearance
of the brick chimney is important, the old bricks may be filleted to 1" or
3/4" thick units [including 'L-shaped' units for the corners] that are
applied to the shaft as an adhered veneer over stucco.

Nels Roselund
Structural Engineer
South San Gabriel, CA

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