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Re: Masonry fireplaces and chimneys, Fp calcs

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No wonder material yards have had trouble selling FHA straps!

Steven A.
Los Angles


sacharya wrote:

> Elements and components transfer forces to the LFRS. Since your chimney is a
> self-supporting structure because it is not attached to the roof diaphragms,
> it would be appropriate to use UBC 1634, nonbuilding structures.
>
> Suresh Acharya, S.E.
>
> -----Original Message-----
> From: Thor Matteson, SE [mailto:matteson(--nospam--at)yosemite.net]
> Sent: Monday, February 24, 2003 9:22 PM
> To: SEAINT
> Subject: Masonry fireplaces and chimneys, Fp calcs
>
> A couple of new Code editions have come along since I moved from the worst
> seismic zone to the second-worst, whereupon wind forces came to govern most
> of my lateral designs.
>
> In the 1991 UBC, the equation for figuring F-sub-p for seismic force on a
> portion of a structure was :  Fp=Z*I*Cp*Wp
>
> In Zone 4 for things besides cantilevered  parapets, this typically gave a
> value of Fp=0.3*Wp.  (0.225*Wp in Zone 3.....)
>
> The 1997 UBC and the beloved 2001 Calif. Bldg. Code give one formula for
> Fp=4*Ca*Ip*Wp, which gives a value in Zone 3 of Fp=4*0.36*I*Wp/1.4  for ASD,
> or Fp=1.03*Wp for typical soil and importance factors.
>
> This more than quadruples the design lateral force, from what I can tell
> after reviewing the SEAOC Blue Book and triple-checking myself....  What I
> am hoping is that someone on the List  will really embarrass me by saying,
> "you forgot to multiply by the 'Omicron-Theta-prime-sub-nought factor' that
> will bring your forces back down so Fp=0.38*Wp."
>
> I just designed a replacement chimney for a house--the original suffered
> from a car crash--and the footing needed to be 9 x 11 feet to resist
> overturning.  The existing house has skip-sheathing on the roof, so I could
> not anchor at the roof diaphragm level.  The contractor does not want to
> undermine half the house to pour this enormous footing.  ("Good-old-days"
> footing size would have been about 4 x 5 feet, 1/5 the concrete and
> excavation)
>
> I envision the Fp from masonry chimneys  exceeding the base shear for the
> entire house in some cases.  For this particular chimney, if  I actually had
> a roof diaphragm, I would need to strap the chimney to the roof framing for
> about 10,000 pounds.
>
> Please tell me whether I have finally gone crazy on my own, or we have all
> gone crazy together.
>
> Thor Matteson
>
> http://www.shearwalls.com
>
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