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Re: Chimney Flues

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Thanks for the info Nels.

I think I might be okay if I can block the cantilevered deck and drag into the shearwall with a strap plate ... that way I don't have to rely on the Swiss cheesed plywood to deliver the load to the walls. (Light bulb turned on right after I sent my post)

It is also unclear from the drawings if the joists go into the chimney area, most of the plans show the framing stopping before the chimney shaft but there are a couple where the joist line crosses into the shaft.... If the joists didn't go into the shaft, the plywood is unblocked and very weak.Think I'll play it safe and recommend the solid blocking and the strap.

Also, how are other people handling things like 50% decrease in Gyp. Bd. shearwall capacities coupled with about 30-50% increase in base shear (potentially gyp. bd. walls are 100% overstressed in existing structures) Are you calling for removal of sheathing and re-sheath w/ plywood? Or do you put plywd over the (e) gyp. like at a perimeter firewall?

-gerard
SF, CA

Thanks,
-Gerard


>>> njineer(--nospam--at)att.net 06/26/01 04:06PM >>>
Gerard,

The UBC rules for chimneys and fireplaces are in Chapter 31, in volume 1 of
the 3 volume set.

The chapter includes a chart for sizing flues based on the chimney height
and fireplace opening size.

I've seen old fireplace/chimney systems with more than one fireplace served
by a single flue, and I don't see a rule against that in the UBC.  The rules
imply a flue serving a single fireplace, but it doesn't seem to be stated
explicitly.  My concern would be the possibility of an odd draft condition
drawing smoke from the chimney into a room with an unused fireplace, though
it seems unlikely since smoke should rise in the flue rather than fall into
a cold fireplace.

James Amrhein wrote "Residential Masonry Fireplace and Chimney Handbook"
published by Masonry Institute of America.  It covers everything you need to
know about building the kind of fireplace and chimney that has become
conventional in the western U.S.

Also check out http://www.rumford.com for information on the Rumford
fireplace design -- quite different form the conventional design, plus lots
of general information on fireplaces and chimneys.

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


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