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RE: Formula for Fp with Braces

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My interpretation is that the reduced Rp value of 1.5 only applies to
the "anchorage", not to the whole system.  See also footnote 14 to Table
16-0; for this similar situation "the design forces for the anchors" are
increased by a factor of 2.0, not for the whole mounting frame. 

William C. Sherman, PE 
(Bill Sherman) 
CDM, Denver, CO
Phone: 303-298-1311
Fax: 303-293-8236
email: shermanwc(--nospam--at)cdm.com
 

> -----Original Message-----
> From: Markajohn(--nospam--at)cs.com [mailto:Markajohn(--nospam--at)cs.com] 
> Sent: Tuesday, July 05, 2005 8:28 PM
> To: seaint(--nospam--at)seaint.org
> Subject: Formula for Fp with Braces
> 
> Hello fellow Engineers:
> 
> Here is an issue that I think will come up for me soon.  I 
> was recently involved in designing seismic bracing for some 
> process pipe runs.  The pipes are supported on trapezes 
> (all-thread hangers and a unistrut cross member) hung from 
> the underside of a concrete slab.  On about every other 
> trapeze, there would be a 45 degree kicker running up from 
> the cross member to the underside of the slab and attached 
> there with a shallow (less than eight diameters) expansion bolt.
> 
> Section 1632.2 of the 1997 UBC gives a formula for Fp and a 
> requirement that Rp is 1.5 for shallow expansion anchors.  Rp 
> is usually 3.0, so this requirement has the effect of 
> doubling the Fp.  
> 
> So my non-engineer client will say, "no problem, just double 
> up on the number of expansion bolts at the top of the brace.  
> But this doubles the force on the rest of the brace too 
> including the hanger rod and its' attachment to the underside 
> of the slab which then needs to be increased.  
> 
> I know some engineers use the doubled Fp to design only the 
> shallow anchor attachment at the top of the brace but just 
> use the reduced Fp (as if Rp were 3.0 instead of 1.5) to 
> design the rest of the brace.  
> 
> I know this is not per the code, but what's wrong with it?  I 
> wouldn't want my client to call me a code monkey.
> 
> The answer will have a legal aspect to it as well as an 
> engineering aspect.
> 
> TIA
> MJ

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