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[SEAOC] Re: [SEAOC] Basic Roof Rafter Analysis Question

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If you proved that wind governs , did you actually
check the existing lateral system for wind, or are you assuming it is o.k(
fat chance it works)
I think it would not be prudent to make such an assumption.

>Mark (Shake4Bake)
>It's not whether I choose to design for wind or seismic. Wind will govern
>because it will yield the largest lateral force against the building when
>the numbers are run. Even if an earthquake is a greater threat, the
>resulting load at 18.6% of the structures weight will not exceed the force
>of a 70 mph wind in exposure C.
>The structure is designed for the worst case - regardless of wind or
>What my comment meant is that the difference in weight is not sufficient to
>cause the lateral weight of the structure to govern over the current wind
>load. For example:
>Assume a 17 psf wind load
>17.4*(8/2+5)=156.6 plf
>Now assume seismic -
>0.186((20psf*8ft/2*2+30ft*15.5psf)=116.25 plf
>Therefore wind still governs at 156.6 plf.
>The seismic considered the dead load at 15.5 psf for the roof with a
>lightweight tile (7.0 psf) and 20 psf for the exterior stud walls with
>stucco finish. There is quite some room to increase the roof load before
>the lateral due to seismic will govern. This is typical of tract homes. The
>40' depth of diaphragm is a bit excessive for a house this size and is
>usually around 30 feet front to rear.
>In the transverse direction seismic will more often govern due to the depth
>of diaphragm, but this is not the critical direction since the distributed
>diaphragm force will be directed into more shear wall than in the
>longitudinal direction.
>So, to put it to rest, seismic is not this critical issue here, but rather
>the gravity load and it's effect out-of-plane on the exterior bearing walls
>(which becomes a function of how well the rafter ties work) and deflection
>of the rafters (which is not as critical on a gabled roof). Therefore, it
>the rafter is designed within allowable bending stress and the intermediate
>support is positively connected to an interior bearing wall - the roof
>should be considered safe.
>Dennis Wish

Tarek Mokhtar
consulting engineer