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Roof Truss Wind Pressure Coefficient

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I have observed fabricators of pre-engineered wood roof
trusses routinely using the Primary Frames and Systems
Pressure Coefficients from Table 16-H of the '97 UBC,
instead of using the Elements and Components Coefficients,
to calculate wind uplift forces.  The difference is
significant (i.e. 0.7 vs. 1.3).  Foot note 2 of the table
says, "For tributary areas greater than 1,000 sf, use
primary frame values".  Under that criteria, a 24" oc wood
truss would have to span 500 ft, before it qualified for the
lower Primary Frames Coefficient.

I have now observed this practice in two high wind states
(Alaska and Hawaii), with calculations done by major truss
fabricators, using software supplied by two of the major
truss plate manufacturers.   I am wondering what other
engineers consider to be the appropriate pressure
coefficient for wood trusses, and if using the primary
frames coefficient is common in other areas.

Monty A. Hart
Anchorage, Alaska



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