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Fire Endurance
Dimension lumber joists and rafters have a long history
of solid fire endurance performance. The first fire endurance
assemblies developed for wood frame structures were
performed using dimension lumber structural members.
Further evidence of this exists in the codes where
calculating fire endurance assemblies is allowed. Times are
assigned for the contribution of wood-frame construction in
fire assembly calculation sections (i.e. Section 720.6 of the
2000 International Building Code). Wood floor and ceiling joists,
16" on center, have a time of 10 minutes assigned to them.
For additional information, refer to Component Additive
Method (CAM) for Calculating and Demonstrating Assembly
Fire Endurance, Design for Code Acceptance No. 4, from the
American Forest & Paper Association at www.awc.org.
Properly designing a building for fire safety means
faithfully executing building code regulations. This means
breaking up a building into fire-resistant compartments. With
compartments and an efficient protection system in place,
fires can be localized and suppressed easily. To prevent fire
from spreading from one compartment to the next, the codes
require finished assemblies be built to withstand full fire
exposure without major damage and, at the same time, act
as barriers to heat transfer.
Standard fire tests measure the fire endurance
performance of a variety of structural assemblies and
boundary conditions that make up compartments. In North
America, ASTM Standard E119 sets forth the conditions of the
test and the interpretation of the results. Test results are
measured in terms of the assembly’s ability to withstand a
severe fire for a period of time. Performance times are
measured in hours: 1-hour rated; 2-hour rated; etc. The
codes reference these hourly requirements for various
building construction types and occupancies.
The major source documents for dimension lumber fire-endurance
assemblies are the Fire Resistance Design Manual
published by the Gypsum Association, and the Fire Resistance
Directory published by the Underwriters Laboratories, Inc.
(UL). The major source document for metal plate connected
truss fire-endurance assemblies is the Metal Plate Connected
Wood Truss Handbook, section 17 Fire Performance of Trusses
and section 18 Sound Transmission and Fire Resistance Rated
Truss Assemblies, found at www.woodtruss.com.
There are numerous fire-endurance assemblies detailed in
these source documents. These assemblies include different
options, such as the direct application of gypsum, or the use
of resilient channels, insulation or suspended ceilings. They
range in performance from 45 minutes to 2 hours, providing
flexibility for any project need. The most common dimension
lumber and truss fire-endurance assemblies are detailed in
Figures 1 – 5.
Wood has out-performed non-combustible materials in
direct comparison fire tests. As illustrated above, a 2x4
timber tie maintained more of its original strength under
higher temperatures and for a longer period of time than
did aluminum alloy or mild steel. This is because of wood’s
unique charring properties, which actually protect it from
fire. As such, wood can be an excellent performer under fire
conditions, contrary to prevalent concerns over its
combustibility.
Flame Spread
Flame spread ratings are often confused with fire
endurance ratings. Flame spread is solely a measure of the
surface burning characteristics of a material. A low flame
spread material will not necessarily improve the performance
of a fire endurance assembly.
Flame spread requirements are typically used by the code
for interior finish materials. Different maximum flame spread
rates are permitted depending upon the building occupancy,
location of the material in the building, and the presence of
sprinklers. In certain cases within the code, wood structural
members with a Class A flame spread rating can be treated as
non-combustible material.
Generally, the building code flame spread classifications are:
| Class |
Flame Spread
Range |
Locations |
|
I or A
II or B
III or C
|
0-25
26-75
76-200
|
Enclosed vertical exits
Exit access corridors
Other rooms and areas
|
Southern Pine paneling and lumber thicker than 1/4" fits
into the Class C flame spread range. Southern Pine has a
flame spread rating of 130-195 based on the ASTM E84 test
method according to Underwriter’s Laboratory, UL 527,
May 1971, Test Report 64S197.
For additional information, refer to Flame Spread
Performance of Wood Products, Design for Code Acceptance
No.1, from the American Forest & Paper Association at
www.awc.org.
Smoke Developed Index
A Smoke Developed Index was also measured for various
wood products. This Index has a value of 100 for Red Oak.
In none of the products tested did the Smoke Developed
Index exceed 450, a limiting value used in building code
regulations.
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