Temperature Factor

In the NDS 2005 Edition, Section 2.3.3 states that the "reference design values shall be multiplied by the temperature factors, Ct, in Table 2.3.3 for structural members that will experience sustained exposure to elevated temperatures up to 150 degrees F (see Appendix C)." In this context, what length of time is meant by "sustained exposure?"

The NDS® Commentary section 2.3.3 states:

"Temperature adjustments in the Specification apply
when structural members are exposed to temperatures between
100°F and 150°F for extended periods of time, such
as in industrial applications in which structural members
are in close proximity to or in contact with heated fluids
used in manufacturing processes. In general, adjustment of
reference design values in the Specification for temperature
should be considered for applications involving sustained
heavy dead or equipment loads, or water immersion, or
wet or high moisture content service conditions, when
sustained or frequent extended exposure to elevated temperatures
up to 150°F will occur.

"Use of lumber or structural glued laminated timber
members in applications involving prolonged exposure to
temperatures over 150°F should be avoided. Where such
exposures do occur, adjustments for both immediate and
permanent strength reductions should be made. Permanent
effects should be based on the cumulative time the
members will be exposed to temperature levels over 150°F
during the life of the structure and the strength losses associated
with these levels (183). Roof systems and other
assemblies subject to diurnal temperature fluctuations from
solar radiation are not applications that normally require
adjustment of reference design values for temperature."

Here's a link to the NDS Commentary for more information:

http://www.awc.org/pdf/NDSCommentary2005.pdf