Fire Resistant Walls - Beyond the Ratings.
Fire has been both friend and foe to mankind. Confined and controlled, it warms dwellings, powers machinery, and makes production of new materials possible. When it escapes controlled confines, fire destroys lives, property, and businesses. Examples of the destructive potential of uncontrolled fire range from historic fires which virtually destroyed great cities such as Rome, London, and Chicago, to more recent urban-wildland interface fires in Southern California (Reference: "Urban-Wildland Interface Fires - The Case for Non-Combustible Construction," Masonry Today, Vol. 6, No. 1, Summer 1996). Events such as these have prompted people to review causes, evaluate means of minimizing reoccurrence, and institute provisions for fire protection. Elements of fire protection that can minimize loss of life and property include use of non-combustible building materials, use of fire-resistive building assemblies, installation of automatic detection devices and sprinklers, and development of improved fire fighting techniques. Fire protection provisions of modern building codes are a rather complex blend of these active and passive fire protection requirements, with an increasing reliance on automatic detectors and sprinklers to assure life-safety. However, the role of non-combustible building materials and fire resistive assemblies in fire protection provisions must not be overlooked or diminished.
Fire resistance is the ability of a material or an assembly to withstand fire or give protection from it. Walls may be required to provide a barrier to the spread of fire or perform structurally when exposed to fire, or both. Model codes reference the ability of a material or assembly to maintain its particular fire-resistant properties as its fire resistance rating, expressed in hours. Fire-resistance ratings have traditionally been determined by standardized fire testing performed in accordance with ASTM E119, Standard Methods for Fire Tests of Building Construction and Materials. However, due to the wealth of data that has been compiled through years of ASTM E119 testing, codes today recognize analytical methods for determining fire resistance ratings (see "New Standard for Calculating Fire Resistance" in this issue of Masonry Today).
It is important to remember that the term, "fire resistance rating" is a legal term utilized by model codes to regulate building construction. While the ratings are based on the same fire test exposure, assemblies having an identical rating but made form different materials often perform quite differently. For example, a 1 hour fire resistance requirement can be achieved utilizing wood stud construction faced with gypsum board on both sides or with 4-inch thick concrete masonry construction. The difference in system integrity between the two, however, is very apparent. Wood-frame construction adds fuel to a fire while the non-combustible concrete masonry system does not. Because of this, masonry construction will continue to exhibit greater structural fire resistance than its wood counterpart. In fact, the structural fire resistance of a masonry wall will typically exceed its barrier fire resistance. Therefore, a masonry wall will normally continue to carry a load, even after its established fire resistance rating period has been reached.
The disparity in performance that is permitted between these assemblies is largely attributed to the test conditions established in ASTM E119. The endpoint for determining the fire endurance of a wall assembly is determined by the time required to reach the first of any of the following:
As noted above, the structural fire performance of masonry walls typically exceed heat transmission endpoints. This is often not true for wood or steel frame construction.
- Ignition of cotton waste due to passage of flame through cracks or fissures.
- A temperature rise of 325oF (single point) or 250oF (average) on the unexposed surface of the assembly.
- Inability to carry the applied design load, that is, structural collapse.
For walls, specimens must additionally be subjected to a hose stream test that has long been a source of controversy. The purpose of the hose stream test is to provide a measure of the ruggedness or survivability of an assembly after exposure to fire. In an attempt to simulate the rough usage conditions that often exist in a fire (for example, the impact due to falling debris), the standard defines a test procedure for exposing a wall assembly to the impact, erosion, and cooling effects of a hose stream test. An inconsistency is present, however, in that the procedure permits the hose stream test to either be performed on the test specimen upon completion of the fire resistance portion of the test or upon a duplicate test specimen subjected to an abbreviated fire exposure period. The fire exposure period for the duplicate specimen is one half of the desired fire resistance period of the assembly, but not more than 1 hour.
Fire-resistance rated concrete and masonry assemblies are typically subjected to the hose stream test after being exposed to fire for the full fire resistance period. Other assemblies are routinely subjected to the duplicate specimen procedure. Recognizing the importance that fire walls be able to withstand the rough usage conditions during a fire, building ordinances in New York and North Carolina now require that the ratings of qualifying walls be based on tests in which the hose stream portion of the test is applied at the end of the full fire resistance period.
It should be noted that model codes primarily focus on minimum provisions to assure life-safety, with secondary consideration given to limiting property loss. However, owners and specifiers should be aware of the advantages offered by non-combustible masonry and concrete construction systems over other systems having equivalent fire resistance ratings. The added protection provided to both life and property must not be overlooked.