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Tall Chimney Design

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I'm designing a non-structural component of an old building that will
replicate a decorative brick chimney.  The structure will be a reinforced
concrete shaft, floor to top, with brick masonry veneer above the roof only.
The chimney is to project 9 feet above the roof.  The center of gravity of
the total chimney reinforced concrete chimney structure will be below the
roof.

Using the 1997 UBC, Chapter 16, Formula 32-2 gives the anchorage force at
the roof based on using hx = hr.  Because the c.g. is below the anchorage
point, I have selected ap = 1.0 from Table 16-O, item 2.B(2).

Is it the intent of this Section that the seismic force on the portion of
the chimney that cantilevers above the roof, for the moment in the shaft at
the attachment elevation at the roof, is to be based on ap = 1.0, or should
the cantilever moment at the upper point of restraint be based on ap = 2.5
because the seismic force develops above the attachment elevation?

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




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