ASD/LRFD Manual - American Wood Council

146 M16: FIRE DESIGN
Compression Members (Tables
M16.2-3 through M16.2-5)
Structural: D+L R s F c C p-s C D C M C t
I f = Moment of inertia using crosssection
dimensions reduced from fire
exposure
C M = Wet service factor
Fire:
D+L 2.58 F c C p-f
C t = Temperature factor
where:
D = Design dead load
R s =
2.03 I
I C C
s M t
f
(M16.2-3)
L = Design live load
R s = Design load ratio
F c = Tabulated compression parallel-tograin
design value
C p-s = Column stability factor using full
cross-section dimensions
C p-f = Column stability factor using crosssection
dimensions reduced from fire
exposure
C D = Load duration factor
C M = Wet service factor
Buckling load ratio tables were developed for standard
reference conditions where: C M = 1.0; C t = 1.0
NOTE: The load duration factor, C D, is not included
in the load ratio tables since modulus of
elasticity values, E, used in the buckling capacity
calculation is not adjusted for load duration in
the NDS.
Tension Members (Tables M16.2-6
through M16.2-8)
Structural: D+L R s F t A s C D C M C t C i
C t = Temperature factor
Fire:
D+L 2.85 F t A f
The calculation of C p-s and C p-f require the designer
to consider both the change in compression area relative
to compression parallel-to-grain strength and the change
in buckling stiffness relative to buckling strength. While
these relationships can be directly calculated using NDS
provisions, they can not be easily tabulated. However, for
most column fire endurance designs the mode of column
failure is significantly influenced by buckling. For this
reason, conservative load ratio tables can be tabulated
for changes in buckling capacity as a function of fire
exposure.
Structural (buckling): D+L R s π 2 E min I s / < e
2
C M C t
Fire (buckling): D+L 2.03 π 2 E min I f / < e
2
where:
D = Design dead load
L = Design live load
R s = Design load ratio
F t = Tabulated tension parallel-to-grain
design value
A s = Area of cross section using full crosssection
dimensions
A f = Area of cross section using crosssection
dimensions reduced from fire
exposure
C D = Load duration factor
where:
D = Design dead load
C M = Wet service factor
C t = Temperature factor
L = Design live load
R s = Design load ratio (buckling)
E min = Reference modulus of elasticity for
column stability calculations
I s = Moment of inertia using full crosssection
dimensions
R s =
2.85 A
A C C C
s D M t
f
(M16.2-4)
Load ratio tables were developed for standard reference
conditions where: C D = 1.0; C M = 1.0; C t = 1.0
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