Structures 2008: Crossing Borders 2008 ASCEFire and Concrete StructuresAuthors:David N. Bilow, P.E., S.E., Director, Engineered Structures, Portland Cement Association 5420Old Orchard Road, Skokie, IL 60077,Phone 847-972-9064, email: [email protected] E. Kamara, PhD., Senior Structural Engineer, Portland Cement Association 5420 OldOrchard Road, Skokie, IL 60077, Phone 847-972-9012, email: [email protected] the 9-11 attack on the World Trade Center, interest in the design of structures for firegreatly increased. Some engineers have promoted the use of advanced analytical models todetermine fire growth within a compartment and have used finite element models of structuralcomponents to determine temperatures within a component by heat transfer analysis. Followingthe calculation of temperatures, the mechanical properties at various times during the period ofthe fire must be determined. This paper provides structural engineers with a summary of thecomplex behavior of structures in fire and the simplified techniques which have been usedsuccessfully for many years to design concrete structures to resist the effects of severe fires.IntroductionOne of the advantages of concrete over other building materials is its inherent fire-resistiveproperties; however, concrete structures must still be designed for fire effects. Structuralcomponents still must be able to withstand dead and live loads without collapse even though therise in temperature causes a decrease in the strength and modulus of elasticity for concrete andsteel reinforcement. In addition, fully developed fires cause expansion of structural componentsand the resulting stresses and strains must be resisted.In the design of structures, building code requirements for fire resistance are sometimesoverlooked and this may lead to costly mistakes. It is not uncommon, to find that a concrete slabfloor system may require a smaller thickness to satisfy ACI 318 strength requirements than thethickness required by a building code for a 2-hour fire resistance. For sound and safe design, fireconsiderations must, be part of the preliminary design stage.Determining the fire rating for a structure member, can vary in complicity from extractingthe relevant rating using a simple table to a fairly complex and elaborate analysis. In the UnitedStates, structural design for fire safety is based on prescriptive approach. Attempts are beingmade to develop performance based design approach for structural design for fire. State andmunicipal building codes throughout the country regulate the fire resistance of the variouselements and assemblies comprising a building structure. The 2006 International Building Code(IBC) (1) contains prescriptive requirements for building elements in Section 720. This section isbased on ACI 216.1 “Standard Method for Determining Fire Resistance of Concrete andMasonry Construction Assemblies and contains tables describing various assemblies of buildingmaterials and finishes that meet specific fire ratings.
Structures 2008: Crossing Borders 2008 ASCEEffect of Fire on Building MaterialsA relatively new method for determining fire exposure used by fire protection engineers is tofirst calculate the fire load density in a compartment. Then, based on the ventilation conditionsand an assumed source of combustion determine the compartment temperature at various times.Another factor considered in the analysis is the effect of active fire protection systems e.g.sprinklers or fire brigades on the growth of the fire. The size and timing of the fire growthdetermined by fire analysis is sensitive to changes in the fuel load over time and changingventilation conditions during the fire. This method of fire analysis requires special software andextensive training and is used only in very large or unusual buildings.Once the temperature time relationship is determined using a standard curve or from themethod described above, the effect of the rise in temperature on the structure can be determined.The rise in temperature causes the free water in concrete to change from a liquid state to agaseous state. This change in state causes changes in the rate with which heat is transmittedfrom the surface into the interior of the concrete component.The rise in temperature causes a decrease in the strength and modulus of elasticity for bothconcrete and steel reinforcement. However, the rate at which the strength and modulus decreasedepends on the rate of increase in the temperature of the fire and the insulating properties ofconcrete. Note that concrete does not burn.ConcreteThe change in concrete properties due to high temperature depends on the type of coarseaggregate used. Aggregate used in concrete can be classified into three types: carbonate,siliceous and lightweight. Carbonate aggregates include limestone and dolomite. Siliceousaggregate include materials consisting of silica and include granite and sandstone. Lightweightaggregates are usually manufactured by heating shale, slate, or clay,.Figure 1 shows the effect of high temperature on the compressive strength of concrete. Thespecimens represented in the figure were stressed to 40% of their compressive strength duringthe heating period. After the designated test temperature was reached, the load was increasedgradually until the specimen failed. The figure shows that the strength of concrete containingsiliceous aggregate begins to drop off at about 800 F and is reduced to about 55% at 1200oF.Concrete containing lightweight aggregates and carbonate aggregates retain most of theircompressive strength up to about 1200 oF. Lightweight concrete has insulating properties, andtransmits heat at a slower rate than normal weight concrete with the same thickness, andtherefore generally provides increased fire resistance.Figure 2 shows the effect of high temperature on the modulus of elasticity of concrete. Thefigure shows that the modulus of elasticity for concretes manufactured of all three types ofaggregates is reduced with the increase in temperature. Also, at high temperatures, creep andrelaxation for concrete increase significantly.
Structures 2008: Crossing Borders 2008 ASCESteelReinforcing steel is much more sensitive to high temperatures than concrete. Figure 3 shows theeffect of high temperature on the yield strength of steel. Figure 4 shows the effect on themodulus of elasticity. As indicated in the figures, hot-rolled steels (reinforcing bars) retain muchof their yield strength up to about 800 F, while cold-drawn steels (prestressing strands) begin tolose strength at about 500 F. Fire resistance ratings therefore vary between prestressed andnonprestressed elements, as well as for different types of concrete.Fire Resistance RatingFire resistance can be defined as the ability of structural elements to withstand fire or to giveprotection from it (2). This includes the ability to confine a fire or to continue to perform a givenstructural function, or both. Fire Resistance Rating (or fire rating), is defined as the duration oftime that an assembly (roof, floor, beam, wall, or column) can endure a “standard fire” as definedin ASTM E 119 (3).Fire Endurance of StructuresFigure 5 shows the effect of fire on the resistance of a simply supported reinforced concrete slab.If the bottom side of the slab is subjected to fire, the strength of the concrete and the reinforcingsteel will decrease as the temperature increase. However, it can take up to three hours for theheat to penetrate through the concrete cover to the steel reinforcement. As the strength of thesteel reinforcement decreases, the moment capacity of the slab decreases. When the momentcapacity of the slab is reduced to the magnitude of the moment caused by the applied load,flexural collapse will occur. It is important to point out that duration of fire until the reinforcingsteel reaches the critical strength depends on the protection to the reinforcement provided by theconcrete cover.ASTM E119 Standard Fire TestThe fire-resistive properties of building components and structural assemblies are determined byfire test methods. The most widely used and nationally accepted test procedure is that developedby the American Society of Testing and Materials (ASTM). It is designated as ASTM E 119,Standard Methods of Fire Tests of Building Construction and Materials. A standard fire test isconducted by placing a full size assembly in a test furnace. Floor and roof specimens are exposedto a controlled fire from beneath, beams are exposed from the bottom and sides, walls from oneside, and columns are exposed to fire from all sides. The temperature is raised in the furnace overa given period of time in accordance with ASTM E 119 standard time-temperature curve shownin Figure 6. This specified time-temperature relationship provides for a furnace temperature of1000oF at five minutes from the beginning of the test, 1300oF at 10 minutes, 1700oF at one hour,1850oF at two hours, and 2000oF at four hours. The end of the test is reached and the fireendurance of the specimen is established when any one of the following conditions first occurs:1. For walls, floors, and roof assemblies, the temperature of the unexposed surface rises anaverage of 150oF above its initial temperature of 325oF at any location. In addition, walls
Structures 2008: Crossing Borders 2008 ASCEachieving a rating classification of one hour or greater must withstand the impact, erosionand cooling affects of a hose steam test.2. Cotton waste placed on the unexposed side of a wall, floor, or roof system is ignited throughcracks or fissures which develop in the specimen during the test.3. The test assembly fails to sustain the applied service load.4. For certain restrained and all unrestrained floors, roofs and beams, the reinforcing steeltemperature rises to 1100oF.The complete requirements of ASTM E 119 and the conditions of acceptance are muchmore detailed than summarized above. Experience shows that concrete floor/roof assemblies andwalls usually fail by heat transmission (item 1); and columns and beams by failure to sustain theapplied loads (item 3), or by beam reinforcement failing to meet the temperature criterion (item4).Advanced Analytical ModelsRecently some engineers have suggested using 3D finite element software to calculate thechange in spatial temperatures over time in structural components using as input the time,temperature, and pore pressure data from the fire analysis described in previous sections. Thesoftware has to be able to model the non-linear non-isotropic behavior of reinforcement steel andconcrete including crack development and crushing of the concrete. In addition to the externalservice loads, the model has to be able to include the following: (1) internal forces due torestraints that prevent free expansion, (2) internal forces due to pore pressure changes, (3)internal forces due to redistribution due to degradation of the mechanical properties of the steelreinforcement and concrete, (4) internal forces due to second order effects from the interaction ofexternal loads and the deformations due to the three types of internal forces mentioned above.CTLGroup performed a 3D analysis using the software DIANA for the Portland Cement Associationand was able to obtain a fair correlation to actual ASTM E119 tests on high strength concrete columns.Needless to say, this type of analysis is very complex and expensive and therefore is not suitable forgeneral structural design.ACI 216 MethodAlthough testing according to ASTM E 119 is probably the most reliable method, the time andexpense required to build and test the assemblies makes this method impractical and is actuallyunnecessary for most situations. The methods contained in ACI 216.1 (2) are based on fireresearch performed from 1958 through 2005 and are by far the most commonly used in typicaldesign situations. The fire resistance (based on the heat transmission end point) of a concretemember or assembly is found by calculating the equivalent thickness for the assembly and thenfinding the corresponding rating in the charts and tables provided. The equivalent thickness ofsolid walls and slabs with flat surfaces is the actual thickness. The equivalent thickness of wallsand slabs that have voids, undulations, ribs, or multiple layers of various materials (for example,a sandwich of concrete, insulation, and concrete) must be calculated using equations found inACI 216.1.
Structures 2008: Crossing Borders 2008 ASCEAn analytical method of calculating fire resistance for flexural members is contained in ACI216.1 (4). This method involves estimating the actual temperatures of the concrete andreinforcing steel and using the properties of the materials at those temperatures in the analysis.The method assumes that the bottom, positive moment steel will reach elevated temperatures andbegin to weaken before the top concrete and reinforcement. This allows the moment in themember to be redistributed from the weaker, positive moment region to the negative momentregion where little reduction in strength will have occurred.Once it is established that the member or the assembly has enough equivalent thickness tosatisfy the heat transmission end point, it must also be determined whether there is enough coveron the reinforcing steel to prevent excessive heat from reducing the yield strength to the pointwhere it can no longer carry the loads. The cover requirements for slabs are functions of therequired fire rating, aggregate type, restrained or unrestrained construction, and prestressed ornon-prestressed reinforcement.The Code ApproachState and municipal building codes throughout the country regulate the fire resistance of thevarious elements and assemblies comprising a building structure. Structural frames (columns andbeams), floor and roof systems, and load bearing walls must be able to withstand the stresses andstrains imposed by fully developed fires and carry t