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Formula for Fp with Braces

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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.


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