ASD/LRFD Manual - American Wood Council

ASD/LRFD MANUAL FOR ENGINEERED Wood Construction
119
Structural Glued Laminated Timber
Fires do not normally start in structural framing, but
rather in the building’s contents. These fires generally
reach temperatures of between 1,290°F and 1,650°F. Glued
laminated timber members perform very well under these
conditions. Unprotected steel members typically suffer
severe buckling and twisting during fires, often collapsing
catastrophically.
Wood ignites at about 480°F, but charring may begin
as low as 300°F. Wood typically chars at 1/40 in. per
minute. Thus, after 30 minutes of fire exposure, only the
outer 3/4 in. of the structural glued laminated timber will
be damaged. Char insulates a wood member and hence
raises the temperature it can withstand. Most of the cross
section will remain intact, and the member will continue
supporting loads during a typical building fire.
It is important to note that neither building materials
alone, nor building features alone, nor detection and
fire extinguishing equipment alone can provide adequate
safety from fire in buildings. To ensure a safe structure in
the event of fire, authorities base fire and building code
requirements on research and testing, as well as fire histories.
The model building codes classify Heavy Timber
as a specific type of construction and give minimum sizes
for roof and floor beams.
The requirements set out for Heavy Timber construction
in model building codes do not constitute 1-hour fire
resistance. However, procedures are available to calculate
the structural glued laminated timber size required for
projects in which 1-hour fire resistance is required (see
NDS 16.2 and AF&PA’s Technical Report 10 available at
www.awc.org). The minimum depths for selected structural
glued laminated timber sizes that can be adopted for
1-hour fire ratings are given in Table M16.1-6 for structural
glued laminated timber beams.
To achieve a 1-hour fire rating for beams whose dimensions
qualify them for this rating, the basic layup must be
modified – one core lamination must be removed from the
center and the tension face augmented with the addition of
a tension lamination. For more information concerning the
effects of fire on structural glued laminated timber, refer to
APA EWS Technical Note Y245 or AITC Technical Note
7. For determining fire resistance other than 1 hour, see
NDS 16.2 and AF&PA’s Technical Report 10 available at
www.awc.org.
Table M16.1-6
Minimum Depths at Which Selected Beam Sizes Can Be Adopted
for One-Hour Fire Ratings 1
Beam Width (in.) 3 Sides Exposed 4 Sides Exposed
6-3/4 13-1/2 or 13-3/4 27 or 27-1/2
8-1/2 7-1/2 or 8-1/4 15 or 15-1/8
8-3/4 6-7/8 or 7-1/2 13-1/2 or 13-3/4
10-1/2 6 or 6-7/8 12 or 12-3/8
10-3/4 6 or 6-7/8 12 or 12-3/8
1. Assuming a load factor of 1.0 (design loads are equal to the capacity of the member). The minimum
depths may be reduced when the design loads are less than the member capacity.
M16: FIRE DESIGN
16
American Forest & paper association