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Re: Tall Chimney Design

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Nels,

If I follow everything you describe correctly, the procedure would be as
follows (my interpretation, from the Blue Book)

ap = 1.0
Rp = 3.0

Calculate force based on hx = 0, compare to minimums

Calculate force based on hx = hr

Perform a weighted average based on Fp min (hx = 0 or Fp min, whichever is
larger) and Fp max and the height of the center of mass relative to hr

Distribute the resulting Fp avg over the element and calculate your
reactions, moments, etc.

Next I would consider the anchorage force.  Specifically speaking from the
code the calculated force would be the anchorage force, but if the element
is fairly flexible and similar to a wall in expected behavior I would bump
up the anchorage force to ap=1.5 if the anchorage is to a flexible
diaphragm, the same as per section 1633.2.8.1.  Not a direct application but
something worth looking at.

Paul Feather PE, SE
pfeather(--nospam--at)SE-Solutions.net
www.SE-Solutions.net
----- Original Message -----
From: "Nels Roselund, SE" <njineer(--nospam--at)att.net>
To: <seaint(--nospam--at)seaint.org>
Sent: Wednesday, July 24, 2002 1:53 PM
Subject: Tall Chimney Design


> 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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