ASD/LRFD Manual - American Wood Council

ASD/LRFD MANUAL FOR ENGINEERED Wood Construction
145
M16.2 Design Procedures for Exposed Wood Members
For members stressed in one principle direction, simplifications
can be made which allow the tabulation of load
factor tables for fire design. These load factor tables can
be used to determine the structural design load ratio, R s , at
which the member has sufficient capacity for a given fire
endurance time. This section provides the rational used to
develop the load ratio tables provided later in this section
(Tables M16.2-1 through M16.2-10). For more complex
calculations where stress interactions must be considered,
use the provisions of AF&PA’s Technical Report 10 with
the appropriate NDS provisions.
Bending Members (Tables M16.2-1
through M16.2-2)
Load ratio tables were developed for standard reference
conditions where: C D = 1.0; C M = 1.0; C t = 1.0; C L-f
= 1.0
The calculation of C L-s and C L-f require the designer
to consider both the change in bending section relative
to bending 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 beams exposed on
three sides, the beams are braced on the protected side.
For long span beams exposed on four sides, the beam
failure is influenced by buckling due to lateral instability.
When buckling is considered, the following equations
should be used:
Structural: D+L R s F b S s C L-s C D C M C t
Structural (buckling): D+L R s E min I yy-s / < e C M C t
Fire:
D+L 2.85 F b S f C L-f
Fire (buckling):
D+L 2.03 E min I yy-f / < e
where:
where:
D = Design dead load
D = Design dead load
L = Design live load
L = Design live load
R s = Design load ratio
R s = Design load ratio (buckling)
F b = Tabulated bending design value
S s = Section modulus using full crosssection
dimensions
S f = Section modulus using cross-section
dimensions reduced from fire exposure
C L-s = Beam stability factor using full crosssection
dimensions
C L-f = Beam stability factor using crosssection
dimensions reduced from fire
exposure
C D = Load duration factor
C M = Wet service factor
C t = Temperature factor
E min = Reference modulus of elasticity for
beam stability calculations
I yy-s = Lateral moment of inertia using full
cross-section dimensions
I yy-f = Lateral moment of inertia using
cross-section dimensions reduced from
fire exposure
R s =
C M = Wet service factor
C t = Temperature factor
2.03 I
yy- f
I C C
yy-s M t
(M16.2-2)
M16: FIRE DESIGN
Solve for R s :
R s =
2.85 S f C L- f
S C C C C
s L-s D M t
(M16.2-1)
16
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