GUIDELINE<strong>LRFD</strong> MANUALTO <strong>LRFD</strong>FORFORENGINEEREDSTRUCTURALWOODCOMPOSITECONSTRUCTIONLUMBER 7Fire-rated AssembliesThe previous section explained that some occupanciesrequire the use of fire-rated assemblies or membersto prevent collapse or fire spread from one compartmentof a building to another or from one building to another.Members and assemblies are rated <strong>for</strong> their abilityeither to continue to carry design loads during fire exposureor to prevent the passage of fire through them. Suchratings are arrived at either by calculation or experiment<strong>for</strong> both members and assemblies. The fire exposure isdefined as that given in ASTM E119. A one-hourfire-resistance rating <strong>for</strong> wall, floor, and floor-ceiling assembliesincorporating nominal two-inch structural lumbercan be accomplished through the use of noncombustiblesurfaces (such as gypsum wallboard). However, fasteningof these surface materials is critical <strong>for</strong> ceilingmembranes and is carefully specified. For some woodassemblies, two-hour ratings have been achieved.Experimental ratings are also obtained independentlyon assemblies and members by materials and structuralmember producers. For a given assembly type incorporatingproprietary components, the company supplying thecomponent can be contacted to obtain the fire rating ofthe assembly. Typically rated floor-ceiling assemblies <strong>for</strong>various products are provided in the product supplementsor guidelines.Analytically RatedIn lieu of experimentally rating the fire endurance ofmembers and assemblies, major building codes will acceptengineering calculations of the expected fireendurance, based upon engineering principles and materialproperties. This applies to the rating of previously1INTRODUCTIONAMERICAN FOREST & PAPER ASSOCIATION
8 INTRODUCTIONuntested members or assemblies, or in cases where it isdesired to substitute one material or component <strong>for</strong> another.Although calculation procedures may be conservative,they have the advantage of quickly rating an assembly ormember and allowing interpolation or some extrapolationof expected per<strong>for</strong>mance. Additional details regardingthe analytical approach are provided in AF&PA’s DCANo. 4 - CAM <strong>for</strong> Calculating and Demonstrating AssemblyFire Endurance.Beams and ColumnsHeavy timber construction has traditionally been recognizedto provide a fire-resistant building. This isprimarily due to the large size of the members, the connectiondetails, and the lack of concealed spaces. Such aconstruction type has often satisfied the fire-resistive requirementin all building codes by simple prescription.Although heavy timber construction has not been “rated”in the United States, Canada has assigned it a 45-minutefire-endurance rating.Using calculations, glulam timber columns and beamscan be designed <strong>for</strong> desired fire-endurance ratings. Additionaldetails regarding the analytical approach areprovided in AF&PA’s DCA No. 2 - Design of Fire-ResistiveExposed <strong>Wood</strong> Members.Fire and Draft StoppingIn all construction types, no greater emphasis can beplaced on the control of construction to reduce the firegrowth hazard than the emplacement of fire and draft stopsin concealed spaces. The spread of fire and smoke throughthese concealed openings within large rooms or betweenrooms is a continuous cause of major life and propertyloss. As a result, most building codes en<strong>for</strong>ce detailing offire blocking and draft stopping within building plans. Fireblocking considered acceptable are (1) two-inch nominallumber, (2) two thicknesses of two-inch nominal lumber,and (3) one thickness of 3/4-inch plywood, with jointsbacked with 3/4-inch plywood.Draft stopping does not require fire resistance of fireblocking. There<strong>for</strong>e, draft stopping material is not requiredto be as thick. Typical draft stop materials and their minimumthicknesses are (1) l/2-inch gypsum wallboard and(2) 3/8-inch plywood. Building codes consider an areabetween draft stops of 1,000 square feet as reasonable.Concealed spaces consisting of open-web floor truss componentsin protected floor-ceiling assemblies are animportant location to draft-stop parallel to the component.Areas of 500 square feet in single-family dwellings and1,000 square feet in other buildings are recommended,and areas between family compartments are absolutelynecessary. Critical draft stop locations are in the concealedspaces in floor-ceiling assemblies and in attics of multifamilydwellings when separation walls do not extend tothe roof sheathing above.Other important locations to fire block in wood frameconstruction are in the following concealed spaces:1. Stud walls and partitions at ceiling and floor levels.2. Intersections between concealed horizontal and verticalspaces such as soffits.3. Top and bottom of stairs between stair stringers.4. Openings around vents, pipes, ducts, chimneys (and fireplacesat ceiling and floor levels) with noncombustiblefire stops.Flame SpreadRegulation of materials used on interior building surfaces(and sometimes exterior surfaces) of other than oneandtwo-family structures is provided to minimize thedanger of rapid flame spread. ASTM E84 gives the methodused to obtain the flame-spread property <strong>for</strong> regulatorypurposes of paneling materials. Materials are classifiedas having a flame spread of more or less than that of redoak, which has an assigned flame spread of 100. A noncombustibleinorganic rein<strong>for</strong>ced cement board has anassigned flame spread of zero. A list of accreditedflame-spread ratings <strong>for</strong> various commercial woods andwood products is given in AF&PA’s DCA No. 1 - FlameSpread Per<strong>for</strong>mance of <strong>Wood</strong> Products.Fire-Retardant TreatmentsIt is possible to make wood highly resistant to thespread of fire by pressure impregnating it with an approvedchemical <strong>for</strong>mulation. <strong>Wood</strong> will char if exposed to fireor fire temperatures, even if it is treated with a fire-retardantsolution, but the rate of its destruction and the transmissionof heat can be retarded by chemicals. However, themost significant contribution of chemicals is reducing thespread of fire. <strong>Wood</strong> that has absorbed adequate amountsof a fire-retardant solution will not support combustion orcontribute fuel, and will cease to burn as soon as the sourceof ignition is removed.Two general methods of improving resistance of woodto fire are (1) impregnation with an effective chemical,and (2) coating the surface with a layer of intumescentpaint. The first method is more effective. For interiors orlocations protected from weather, impregnation treatmentscan be considered permanent and have considerable valuein preventing ignition. These surface applications offerthe principal means of increasing fire-retardant propertiesof existing structures. However, these coatings mayrequire periodic renewal if their effectiveness is to be maintained.In the past, the only effective chemicals were waterAMERICAN WOOD COUNCIL
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