Structural Protection common questions answered Got questions about ? Farid Alfawakhiri Check out this Christopher Hewitt Robert Solomon handyguide! tear-out

AISC engineers and a representative from the NFPA provide answers to questions about the materials, components and systems designed to provide life safety and structural integrity during .

December 2002 • Modern What is a fire load? Where do What is “thermal mass”? combustibles, the size of the room and fire loads primarily come from Thermal mass is used sometimes ventilation conditions. In larger rooms, in buildings? for effective specific heat or heat ca- the time to flashover will usually be Fire loads account for all com- pacity. Effective specific heat in longer. In well-ventilated large prem- bustible building contents, including fur- Btu/(lb°F) is the amount of energy, per ises, like open parking garages and nishings, equipment, and combustible unit mass of material, required to raise large atriums, flashover is unlikely. construction components. Most of a the temperature of the material by one Flashover is also unlikely to occur in building’s fire load results from contents temperature unit. Similarly, effective- sprinklered premises. that have been introduced after the in Btu/(ft3°F) is the construction is complete. The fire load amount of energy, per unit volume of Where can one find the thermal is usually expressed in terms of the material, required to raise the tempera- conductivity or thermal resist- “-equivalent” weight of com- ture of the material by one temperature ance values of different fire-pro- bustible building contents per unit build- unit. For most construction materials, tective materials? ing-floor area in psf. The actual weight specific heat and heat capacity values A good source is the third edition of combustible contents is adjusted to (as well as values) (2002) of the SFPE Handbook of the wood-equivalent weight based on are temperature dependent. This Fire Protection Engineering by the So- the estimated potential heat of contents means that these values change in the ciety of Fire Protection Engineers normalized to the potential heat of temperature range associated with (www.sfpe.org). For some materials, in- wood (8000 Btu/lb). Alternatively, the building fires (50°F to 1800°F), be- formation might be unavailable. fire load could be expressed in terms of cause many materials undergo physic- the potential heat of building contents ochemical changes at elevated What is the meaning of “fire re- per unit building-floor area in Btu/ft2. temperatures. These thermal proper- sistant”? Is it the same thing as ties are also sensitive to the testing “fireproof”? How fast does a conventional method used, and different sources list Fire resistance is the ability of build- fire spread? varying values of material properties for ing components and systems to per- The rate at which a fire spreads and the same material. form fire-separating and/or load- grows in a building depends on many bearing functions under fire exposure. factors: What is a heat sink? Where are Fire-resistant building components and ■ the properties of the they found? systems are those with specified fire- construction materials A heat sink refers to anything ab- resistance ratings based on fire-resist- ■ building furnishings and contents sorbing large amounts of heat through ance tests. These ratings, expressed in ■ ventilation conditions physical and/or chemical processes. minutes and hours, describe the time ■ room geometry and configuration Materials containing large amounts of duration for which given building com- ■ timely detection and effectiveness of chemically combined in their ponents and systems maintain specific fire suppression response by occu- structure, like gypsum or , can functions while exposed to simulated pants and/or form heat sinks.They absorb significant fire events. Various test protocols de- ■ availability and effectiveness of au- amounts of heat due to the energy con- scribe the procedures to evaluate the tomatic fire detection and suppres- sumed in the water-evaporation performance of doors, windows, walls, sion systems process. Materials with high thermal floors, beams and columns. ■ availability and effectiveness of fire conductivity and high effective-heat ca- The term fireproof is a misnomer— barriers pacity will also act as heat sinks. Mate- nothing is truly fireproof. All construc- rials with low thermal conductivity will tion materials, components and At what temperature does a reflect and not absorb heat. systems have limits and can be ir- typical fire burn? reparably damaged by fire. The duration and the maximum tem- What is flashover? perature of a fire in a building compart- Flashover is an abrupt transition Is steel a fire-resistant system ment depend on the amount and from the burning of a small number of or material? configuration of available combustibles, items in the room, or a small portion of Fire-resistance ratings are assigned ventilation conditions, properties of the the room, to full-room involvement in a to construction components and sys- compartment enclosure, and weather fire. Room-fire flashover is a state in tems, not materials. Materials are clas- conditions. The maximum temperature which all the combustible contents of sified for their combustion properties, of a fully developed building fire will the room experience a nearly simulta- and steel is non-combustible. Steel also rarely exceed 1800°F.The average neous ignition. Most fires never reach exhibits other valuable structural and temperature in a fully developed fire is flashover because they are extin- durability properties. It is used in many not likely to reach 1500°F. Tempera- guished before this can occur. Some fire-resistant building components and tures of fires that have not developed to fires self extinguish when the air supply systems, where load-bearing struc- post-flashover stage will not exceed is insufficient, or when the fire has a tural-steel members are insulated from 1000°F. low-enough energy source to prevent the thermal effects of a fire. ignition of multiple targets. The time to flashover depends on many factors, such as the properties of

Modern Steel Construction • December 2002 What are spray-applied fire-pro- ■ Heat-absorbing physical reactions, compared to the bond strength of the tective materials made of? like transpiration, evaporation, sub- same fire-resistive material when ap- Spray-applied fire-protective materi- limation and ; or heat- plied to a clean, unpainted/ungalva- als fall into two broad categories: mineral absorbing chemical reactions, like nized steel-plate surface. Some AHJs fiber and cementitious. These materials endothermic decomposition and enforce similar requirements for galva- are based on proprietary formulations, pyrolysis nized surfaces. Producers of fire-resis- supplied in a dry form, and must be ■ Intumescence, the formation of a tive materials usually maintain a list of mixed and applied according to the thick foam upon heating “pre-approved” paints that have passed manufacturer’s recommendations. ■ Radiation or reflection. the ASTM E736 tests. They can advise The mineral-fiber mixture combines on the applicability of a product to gal- fibers, mineral binders, air and water. It How do painting vanized surfaces. Additional tests are is usually spray-applied. The dry mix- systems work? needed if a coated-steel surface is not ture of mineral fibers and binding An intumescent chars, pre-approved. If bond strength is found agents is fed to a spraying nozzle. foams and expands when heated. The unacceptable, a mechanical bond can Water is added to the mixture in the compounds of intumescent systems be obtained by wrapping the structural nozzle as it is sprayed onto the generally can be placed into four cate- member with expanded metal lath (min. surface. In its final, cured form, the min- gories: 1.7 lb/sq. yd). eral-fiber coating is lightweight, non- 1. inorganic acid or material yielding combustible, chemically inert and a an acid at temperatures of 212°F to How reliable are sprinkler sys- poor conductor of heat (a low thermal 570°F; tems? conductivity insulator). 2. polyhydric material rich in carbon; Sprinkler systems have proven to be Cementitious incorporate 3. organic amine or amide, as a flow- effective and reliable when properly de- lightweight aggregates, like perlite or ing agent; and signed, installed and maintained. Strict vermiculite, in a heat-absorbing matrix 4. halogenated material. regulations in the United States enforce of gypsum and/or Portland cement. Various binders and additives are standard practices for sprinkler-system Some formulations also use magne- mixed in to provide specific physical inspection, test and maintenance pro- sium oxychloride, oxysul- properties of the total system. The sys- grams. The effectiveness of sprinkler- fate, calcium aluminate or ammonium tem often is made of several coats with system performance is noted in the sulfate. Various additives and foaming different functions. For example, a top- National Fire Protection Association’s agents can be added to the mixture. coat will provide a durable finish sur- (NFPA) annual report: “The NFPA has Cementitious coatings are often classi- face, while the base coat will provide a no record of a fire killing more than two fied by their (as low, medium strong bond to substrate. Extensive de- people in a completely sprinklered and high). velopment over the last decade has led building where the system was properly to improved formulations that do not operating, except in an explosion or Other than spray-applied fire- use traditional compounds. flash fire, or where industrial fire protective materials, what brigade members or employees were other materials/methods can be Can spray-applied fire-protec- killed during fire suppression opera- used to fire protect steel? tion be applied to painted or tions.” Concrete and masonry encase- galvanized steel? ments are traditional fire-protective ma- In most cases, steel that is to be fire- Are there any structural sys- terials that can be used. Gypsum-board protected should not be painted or gal- tems that should be avoided and mineral-board products, ceramic- vanized. When such steel must be when designing for an area of a wool wraps, and various intumescent painted, additional measures can be building that contains a large coatings are common alternatives. taken to ensure adhesion. It is prudent fire load? Steel itself is an effective fire-protective to consult a fire-protection contractor Building and fire codes usually re- material when used in sheet form to and an authority having jurisdiction strict excessive fire loads or impose re- provide a protective and reflective (AHJ) during early project stages if you strictions on the height and area of shield for other materials. Steel can expect your painted or galvanized steel buildings with large fire loads.These re- also be used in the form of meshes or to be fire protected by spray-applied strictions are severe for buildings of wraps to help other materials maintain materials. combustible construction. Combustible their integrity under heat exposure. Spray-applied fire-resistive materi- construction is not always permitted for Other fire-protection methods for struc- als can be applied to primed or painted high-hazard . Even build- tural steel include using rain screens— steel shapes provided they have ings that are deemed to have a minimal water sprinklers that protect steel passed bond tests in accordance with fire load, such as office buildings, are members—or filling tubular structures ASTM E736 “Standard Test Method for subject to codes that limit allowable- with concrete or water. Cohesion/Adhesion of Sprayed Fire construction types and increase fire-re- In general, structural fire-protection Resistive Materials Applied to Struc- sistance-rating requirements as is achieved through one or more of the tural Members.” These tests should in- building height and area increases. following mechanisms: dicate a minimum average bond ■ Low thermal conductivity strength of 80 percent and a minimum ■ High effective-heat capacity individual bond strength of 50 percent,

December 2002 • Modern Steel Construction What are the general rules-of- When a building or system is hours. In a severe-challenge fire, usu- thumb for interpreting model said to have a two-hour fire rat- ally the result of an extreme, unantici- building codes when it comes ing, what does that mean? pated event, performance can be less to fire ratings in mixed-use It means that the system has satis- than two hours. buildings? fied the requirements for a two-hour rat- Building codes have special require- ing specified in a relevant standard test. What is the difference between ments for buildings with more than one In the case of a building, it could mean restrained and unrestrained rat- group. If different occupan- that some of the construction elements ings? cies are separated by firewalls, fire-bar- and/or assemblies in that building have Restrained and unrestrained classi- rier walls and/or floors, each portion is achieved a two-hour rating in a standard fications pertain to ASTM E119 tests considered a separate building when ASTM E119 fire-resistance test. on beams, floors and roofs. They de- establishing allowable heights, areas pend on whether the test arrangements and fire-resistance requirements (some Does a two-hour fire rating allowed for free thermal expansion of conditions/exemptions apply). mean that a building will last the tested specimen (unrestrained test) Without this degree of fire separa- for two hours in a fire? or not (restrained test). tion, a building of mixed occupancy is No. The ratings only reflect the abil- ASTM E119 tests on unloaded limited by the most restrictive height ity of individual components and as- and composite and area requirements specified for semblies in a building to meet the steel/concrete beams can be restrained any of the occupancies in the building required performance in the standard or unrestrained, but always require the (there are several exemptions). When test. Buildings are classified by types of longitudinal expansion of the applied establishing the fire-resistance require- construction. Each has specific require- fire-protection material to be restrained ments for non-separated uses, the ments pertaining to the combustibility (this conservative requirement can re- regulations applicable to mixed occu- of construction materials and the fire- sult in earlier fall-off of the fire protec- pancies govern the respective portions resistance ratings required for building tion and faster heating of the tested of the buildings. Where the require- components (members, elements) and steel beam).This test results in a single ments conflict, those that provide assemblies (systems). In some fire Unrestrained Beam Rating, based on greater safety prevail. events, performance exceeds two the period of fire exposure where the average measured temperature at any section of the beam remains under 1000°F, and the measured temperature SPRAY-APPLIED FIRE-RESISTIVE MATERIALS, COM- at any single location of the beam re- MONLY USED TO PROTECT STRUCTURAL STEEL, mains under 1200°F.This type of test is ARE TESTED FOR: conducted only when the loading de- vice has lower capacity than the re- ■ quired test load. ASTM E 84 "Standard Test Method for Surface Burning Charac- ASTM E119 tests on loaded struc- teristics of Building Materials" tural steel and composite steel/con- ■ ASTM E 605 "Standard Test Methods for Thickness and Density crete beams are always restrained and of Sprayed Fire-Resistive Material Applied to Structural Mem- result in two ratings: ■ the Restrained Beam Rating, based bers" on the period of fire exposure where ■ ASTM E 736 "Standard Test Method for Cohesion/Adhesion of the beam sustains the applied de- Sprayed Fire-Resistive Materials Applied to Structural Mem- sign load, but NOT more than twice the corresponding Unrestrained bers" Beam Rating, AND provided the lat- ■ ASTM E 759 "Standard Test Method for Effect of Deflection on ter is one hour or more; and Sprayed Fire-Resistive Material Applied to Structural Members" ■ the Unrestrained Beam Rating, ■ ASTM E 760 "Standard Test Method for Effect of Impact on based on the period of fire exposure where the average measured tem- Bonding of Sprayed Fire-Resistive Material Applied to Structural perature at any section of the steel Members" beam remains under 1100°F, and ■ ASTM E 761 "Standard Test Method for Compressive Strength the measured temperature at any of Sprayed Fire-Resistive Material Applied to Structural Mem- single location of the steel beam re- mains under 1300°F. bers" ASTM E119 tests on floor and roof ■ ASTM E 859 "Standard Test Method for Air Erosion of Sprayed assemblies are always loaded. The as- Fire-Resistive Materials Applied to Structural Members" semblies can be tested in the unre- strained or restrained condition around ■ ASTM E 937 "Standard Test Method for Corrosion of Steel by the floor/roof perimeter. Whenever the Sprayed Fire-Resistive Material Applied to Structural Members" tested floor/roof assembly contains a structural-steel beam, both restrained

Modern Steel Construction • December 2002 and unrestrained assembly tests will re- specifications are based on extensive Can steel continue to be used sult in an Unrestrained Beam Rating experimental data from standard in a building after it has been in (based on the same temperature crite- (ASTM E119) fire-resistance tests. a fire? How can you assess the ria specified for loaded restrained beam Concrete-filled HSS columns are capacity of steel that has been tests) in addition to Assembly Ratings. another example of generic construc- exposed to fire? Are there con- For any Assembly Rating period, the tion that is not listed in the UL directory. cerns about internal or residual unexposed surface of the tested Article 5.2.3 of ASCE/SFPE 29-99 stress effects that have to be floor/roof should neither develop condi- specifies how to determine the fire re- considered? tions that will ignite cotton waste, nor sistance of concrete-filled hollow steel Steel is born in a melting process exhibit an average temperature rise in columns. The relevant background in- that is significantly hotter than any excess of 250°F. An unrestrained as- formation can be found in: building fire, and significant residual sembly test will result in an Unre- V.K. R. Kodur, and D. H. MacKinnon, stresses are present in all newly manu- strained Assembly Rating, based on “Design of Concrete-Filled Hollow factured steel members. A detailed dis- the period of fire exposure where the Structural Steel Columns for Fire En- cussion of post-fire steel assessment assembly sustains the applied design durance”, Engineering Journal, First issues is provided in: load. A restrained assembly test will re- Quarter, 2000, pp. 13-24. R. H. R. Tide, “Integrity of Structural sult in two assembly ratings: Steel After Exposure to Fire”, Engi- ■ the Restrained Assembly Rating, Does the grade of steel used af- neering Journal, First Quarter, 1998, based on the period of fire exposure fect its response to a fire? pp. 26-38. where the assembly sustains the Common structural-steel grades ex- A general rule of thumb: “If it is still applied design load, but NOT more hibit similar deterioration of mechanical straight after exposure to fire—the steel than twice the corresponding Unre- properties at elevated temperatures, is okay.” Straightening techniques are strained Assembly Rating, AND pro- and all structural grades perform es- available for steel members that have vided the latter is one hour or more; sentially the same way. There have been misaligned after fire exposure. and been efforts in several countries to in- ■ the Unrestrained Assembly Rating, troduce a “fire-resistant” steel grade What percentage of its total ca- based on the same temperature cri- into construction. This steel reportedly pacity does a steel beam retain teria specified for Unrestrained exhibits improved properties at ele- when subjected to the heat of a Beam Rating. This excludes steel vated temperatures. However, its use normal fire? At what tempera- structural members spaced 4’ or remains limited in construction be- ture does steel lose all of its ca- less on center, where the criterion cause improved mechanical properties pacity? for the average measured tempera- of steel at elevated temperatures do not The strength of steel remains es- ture of all members remaining under translate into significant increases in sentially unchanged until about 600°F. 1100°F applies. the fire resistance of building elements Steel retains about half of its strength at and systems. 1100°F, and loses all of its capacity Are structural steel systems re- when it melts at about 2700°F. For de- strained or unrestrained? How does a fire impact steel sign purposes, it is usually assumed Appendix X3 and Table X3.1 of connections? Does it affect that all capacity is lost at about 2200°F. ASTM E119 provide guidance on the connections differently than classification of beams, floor and roof the members themselves? How do concrete- or water- systems as restrained or unrestrained. Connections usually contain more filled tubular steel columns per- Structural-steel beams and floor sys- material (additional plates, bolts, etc.) form in a fire? tems within steel-framed buildings are than connected members. Often con- Water or concrete inside tubular- classified as restrained. nections also have less exposure to steel members act as heat sinks. They heat and a higher capacity for heat dis- reduce temperature rise in the steel How can one determine a fire sipation because of their proximity to and significantly enhance fire resist- rating for a system that has not other members. As a result, tempera- ance. In the case of concrete-filled tu- been prequalified, such as a tures are likely to rise faster in mem- bular columns, the concrete also concrete-encased steel col- bers than in connections, making contributes to the load-bearing capacity umn? connections less critical for fire-protec- when the outside steel shell deterio- Concrete-encased steel columns tion design. rates under heat exposure. have been “pre-qualified” by many fire tests.These columns are of generic de- Compared to regular steel fram- sign and are not listed in the Underwrit- ing, how do steel joists, chan- ers Laboratories (UL) directory. nels, tees or castellated beams However, most building codes, such as perform in a fire? Are there any IBC (Table 719.1(1) and Article special procedures required to 720.5.1.4), and ASCE/SFPE 29-99 (Ar- fire protect them effectively? ticle 5.2.4), contain specifications to de- Open-web steel joists and castel- termine the fire resistance of lated beams are proprietary system de- concrete-encased columns. These signs. For many of them, fire-resistance

December 2002 • Modern Steel Construction ratings are listed in the UL and other tions ensure the quality performance of What is performance-based de- fire-resistance directories. relevant products (See table). sign with respect to fire? Performance-based design consists Which document specifies the I have heard that the ASTM fire in part of well-defined performance fire-resistance requirements for specifications are undergoing goals, objectives and criteria for an ex- the structural system? revision. Is this because of the pected fire event. Thereafter, a corre- The of the jurisdiction, World Trade Center attacks? sponding set of systems is designed to which is usually based upon the IBC or ASTM specifications are revised achieve these performance objectives NFPA model building code, will specify routinely every few years. Like many and criteria. In many cases, perform- the requirements. codes and standards, revisions are ance-based designs include evalua- promulgated on a periodic basis to rec- tions of candidate designs against a Which ASTM Specifications re- ognize new technologies and methods series of fire-design scenarios. late to fire-protection and engi- as well as new materials. neering? How prevalent have perform- The most relevant ASTM specifica- I have heard that the 2005 AISC ance-based designs become in tions pertain to combustibility, ignition Specification is going to ad- fire protection? and flammability characteristics, heat- dress fire protection and engi- It is still a design option that is typi- and -release parameters, and neering. What kinds of issues is cally reserved for use on certain high- fire resistance. the Specification going to ad- profile projects. At this point, it is best ASTM E119 specifies standard test dress? described as a design alternative rather procedures to establish the fire-resist- The Specification will provide a gen- than a prevalent design methodology. ance (endurance) ratings for construc- eral outline of procedures for engi- tion elements (beams, columns) and neered structural fire-protection design What is AISC doing to help peo- assemblies (walls, floors, roofs). There that will include steel properties at ele- ple design effectively for fire re- are several other ASTM tests that es- vated temperatures, expected fire ex- sistance? tablish fire-resistance ratings for win- posures, heat-transfer calculations, AISC is currently developing a sin- dows, doors, fire-stops, and other load combinations for fire-limit states, gle-source design guide, for engineers components. Many ASTM specifica- and analysis of heated structures. and , on the fire resistance of structural steel. AISC is researching ways to make engineered fire protec- tion for steel structures more accessi- ble as an alternative to current WHERE CAN YOU FIND FIRE RATINGS? prescriptive solutions.

Methods to determine fire-resistance ratings for generic designs Farid Alfawakhiri is Senior Engineer, can be found in most building codes. Fire Design, and Christopher Hewitt is Staff Engineer, Structures, with AISC in ■ Chapter 7, "Fire-Resistance-Rated Construction", International Chicago. Robert Solomon is NFPA’s Building Code, International Code Council, Falls Church, VA, Assistant Vice President for Building 2000. and Life Safety Codes. ■ Chapter 8, "Fire Resistive Materials and Construction", NFPA 5000: Building Construction and Safety Code, NFPA, Quincy, MA, 2002. ■ ASCE/SFPE 29-99, Standard Calculation Methods for Structural Fire Protection, Structural Engineering Institute of the American Society of Civil Engineers, Reston, VA, 1998.

Fire resistance ratings for proprietary designs are found in directo- ries published by testing laboratories.

■ Fire Resistance Directory, Volume I, Underwriters Laboratories Inc., Northbrook, IL, 2002 (updated annually), online directory at www.ul.com. ■ Directory of Listed Products, Intertek Testing Services NA Inc., Cortland, NY, 2000, online directory at www.etlsemko.com. ■ Directory of Listed Building Products, Materials and Assemblies, Volume II, Omega Point Laboratories Inc., Elmendorf, TX, 2002 (updated annually), online directory at www.opl.com.

Modern Steel Construction • December 2002