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Fire Endurance, Sound Transmission
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 721.6 and Table 721.6.2(2) of the
2006 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 sources for dimension lumber fire-endurance
assemblies are the Fire Resistance Design Manual
published by the Gypsum Association, the Fire Resistance
Directory published by the Underwriters Laboratories, Inc.
(UL), and Section 720 and Table 720.1(3) of the 2006 International
Building Code. The major source for metal plate connected
truss fire-endurance assemblies is found at www.sbcindustry.com.
There are numerous fire-endurance assemblies detailed in
these sources. 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. The most
common dimension lumber and truss fire-endurance
assemblies are detailed in Figures 1 - 5 to the left.
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.
Sound Transmission
Sound transmission ratings are closely aligned with fire
endurance ratings for assemblies. This is because flame
penetration and sound penetration follow similar paths of
least resistance.
Sound striking a wall or ceiling surface is transmitted
through the air in the wall or ceiling cavity. It then strikes the
opposite wall surface, causing it to vibrate and transmit the
sound into the adjoining room. Sound also is transmitted
through any openings going into the room, such as air ducts,
electrical outlets, window openings, and doors. This is
airborne sound transmission.
The Sound Transmission Class (STC) method of rating
airborne sounds evaluates the comfortability of a particular
living space. The higher the STC, the better the airborne
noise control performance of the structure. An STC of 50 or
above is generally considered a good airborne noise control
rating. The following table describes the privacy from each
STC rating:
| STC Rating |
Privacy Afforded |
|
25
30
35
40
45
50
55
|
Normal speech easily understood
Normal speech audible but not intelligible
Loud speech audible and fairly understandable
Loud speech barely audible but not intelligible
Loud speech barely audible
Shouting barely audible
Shouting not audible
|
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.
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
|
Most tested wood products, including Southern Pine, have
a Flame Spread Index of 200, making them acceptable under
current building codes for a wide range of interior finish uses.
Commerically available fire retardant treatments for wood
and panel products can reduce the Flame Spread Index to 25
or less. 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.
None of the products tested exceeded 450, a limiting value
commonly used in building code regulations.
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