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# [SEAOC] Basic Roof Rafter Analysis Question

• To: <seaoc(--nospam--at)power.net>
• Subject: [SEAOC] Basic Roof Rafter Analysis Question
• From: "DW" <wish(--nospam--at)cyberg8t.com>
• Date: Tue, 13 Aug 1996 00:35:11 -0700

```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
seismic.
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
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

...

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