Fire & 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 2009 International Building Code). Wood floor and roof 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 Wood Council.
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 Underwriters Laboratories, Inc. (UL), and Section 720 and Table 720.1(3) of the 2009 International Building Code. The major source for metal plate connected truss fire-endurance assemblies is available from the Structural Building Components Association.
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.
Wood has out-performed non-combustible materials in direct comparison fire tests. As illustrated above, a 2×4 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 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|
|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 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
|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. Commercially 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 Wood Council.
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.