BUILDING CONSTRUCTION - IHMC Public Cmaps (2)cmapspublic2.ihmc.us/rid=1148947649616_185333784_3503/Building... · BUILDING CONSTRUCTION 341 OBJECTIVES After completing this chapter,

BUILDING CONSTRUCTION ■ 339

BUILDING CONSTRUCTIONBUILDING CONSTRUCTIONDave Dodson, Lead Instructor, Response Solutions

CHAPTER

13

ObjectivesIntroductionBuilding Construction Termsand MechanicsStructural ElementsFire Effects on CommonBuilding ConstructionMaterials

Types of BuildingConstructionCollapse Hazards AtStructure FiresLessons LearnedKey Terms

Review QuestionsAdditional Resources

OUTLINEOUTLINE

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STREET STORYA firefighter’s worst fear is being trapped in a collapsing building and burningto death. Real-life encounters become implanted in our memory. It is one thingto read or hear about certain occurrences, it is another thing to have livedthrough one.

As a chief officer arriving at a fire scene, I encountered a large body of fireon the top floor of a three-story building. The first-due companies were initiat-ing an offensive attack. My initial reaction was that the tactics being employedwere correct. I surveyed the scene by doing a 360-degree walk-around of thebuilding. I realized that conditions were not improving. I contacted the interiorsector officer, and he informed me that interior conditions were deteriorating. Ihad noticed the following collapse indicators in my size-up: a large volume ofunabated fire despite the aggressive interior attack; heavy smoke conditionsindicating that the fire was probably attacking the building’s structural sup-ports; the ordinary constructed building contained a corbelled brick cornice,which added an eccentric load to the bearing wall supporting it; and lack ofprogress in the interior attack.

I had witnessed similar indicators in previous collapses. I ordered the unitsto withdraw from the building, set up a collapse zone, and initiated a defensiveattack. The building’s masonry wall collapsed shortly after the units’ removal.The ensuing investigation found that the collapse occurred due to the combina-tion of the collapse indicators.

Knowledge of building construction is vital before firefighters can attempt tofight a fire within a structure. Each type of construction contains inherent prob-lems or positive features that can impede or assist in our ability to control abuilding fire. Examine the building. If it contains collapse indicators, decidetheir potential impact on the building’s stability. If in doubt, it is better to erron the side of safety. Remember that once all civilians are removed from astructure, the life safety of firefighters is our utmost consideration.

—Street Story by James P. Smith, Deputy Chief, Philadelphia Fire Department,Philadelphia, Pennsylvania


OBJECTIVESAfter completing this chapter, the reader should beable to:

Describe the relationship between loads,imposition of loads, and forces.

List and define four structural elements.

Identify the effects of fire on five commonbuilding materials.

List and define the five general types ofbuilding construction.

List and define hazards associated withalternative building construction types.

List five building collapse hazardsassociated with fire suppression operations.

List five indicators of collapse or structuralfailure that might be found during firesuppression operations.

INTRODUCTIONMany fire departments pride themselves in theirability to launch aggressive interior structural fireattacks. Unfortunately, many firefighters areinjured and killed when that same structure col-lapses, Figure 13-1. Often, buildings collapsewithout a “visual” warning such as sagging floorsand roofs, leaning walls, and cracks. To keep fromgetting trapped in a collapse, firefighters mustunderstand the types of structures they enter fromthe perspective of how the buildings are assem-bled, what materials are used, and how buildingsreact to fire. Additionally, firefighters must under-stand how fire travels through a building andchoose appropriate tactics to stop the fire beforekey structural elements are attacked by the fire.Many firefighter fatality investigations concludethat fire departments need more training and edu-cation on building construction and the effects offire on buildings. This chapter introduces severalkey topics regarding building construction andhow fire affects buildings. It is important to note,however, that this chapter is merely an introduc-tion. Firefighters must bridge the information inthis chapter with a long-term commitment to studyand research building construction and, moreimportantly, to explore the buildings within theirjurisdiction.

Streetsmart Tip Firefighters must realizethat most of the buildings they will work in arealready in place, and it is very difficult to deter-mine the type of construction and fire-resistiverating by driving by or standing in front of astructure. Conducting in-service inspections,and securing the building owner’s permission towalk through a building and get an “insideview” are ways of learning more about thestructures in a particular jurisdiction.

If new construction is being conducted in afirefighter’s response area or jurisdiction, contactthe building department and secure a set of theplans. The fire department should be involved inthe plan review process; however, this is notalways the case. Also, take photographs of howthe building is built and what materials are usedfor future training references. Many new materi-als and construction methods are introducedregularly. Firefighters should find out what mate-rials are in the buildings in their jurisdictions.

Figure 13-1 This collapse happened seconds afterfirefighters were repositioned.

342 ■ CHAPTER 13

This chapter begins by exploring some basicterms and mechanics of building construction, andthen examines structural hierarchy and fire effectson materials. That information is then applied toclassic and new construction types. The chapterconcludes with a look at collapse hazards associatedwith structural fires.

BUILDING CONSTRUCTIONTERMS AND MECHANICS

Firefighters need a basic understanding of certainterms and concepts associated with building con-struction. Obviously, buildings are constructed toprovide a protected space to shield occupants fromelements. The building must be built to resist wind,snow, rain, and still resist the force of gravity. Addi-tionally, the intended use of the building can add atremendous amount of weight, placing more stresson the building’s ability to resist gravity. In buildingterms, these elements create building loading.Loads are then imposed on building materials. Thisimposition causes stress on the materials, calledforce. Forces must be delivered to the earth in orderfor the building to be structurally sound. With thisbasic understanding, we can start to define termsand mechanics.

Types of LoadsLoads can be divided into two broad categories as itrelates to building construction: dead loads and liveloads. Dead loads include the weight of all materi-als and equipment that are permanently attached tothe building. Live loads include equipment, people,movement, and materials not attached to the struc-ture. Dead loads and live loads can be more specifi-cally described using the following terms:

Concentrated load: A concentrated loadis a load that is applied to a small area,Figure 13-2. An example of a concen-trated load is a heating, ventilation, andair-conditioning (HVAC) unit on a roof.

Distributed load: A distributed load is aload applied equally over a broad area,Figure 13-3. Examples of this includesnow on a roof or a hoist attached tonumerous roof supports.

Impact load: Impact load is a loadthat is in motion when applied,Figure 13-4. Crowds of people, firestreams, and wind gusts are examplesof impact loads.

Figure 13-2 The steel stairs and air-conditioning unitapply a concentrated load on this roof structure. Alsonote the potential instability of the air-conditioningunit placed on cement blocks.

Snow

Parapet

Distributed Load

Roof Line

Figure 13-3 The weight of snow is a distributedload on this roof structure.

Figure 13-4 This ladder pipe operation is applyingan impact load to the wall of this structure. As the wallweakens, it will eventually collapse. (Photo courtesy ofWilliam H. Schmitt, Jr.)


Many firefighters have been killed as a result ofbuilding collapse during firefighting operations.With each of these tragic losses, lessons can belearned. The following is a brief look at some ofthe more significant collapses. Each of theseevents should be researched to find all the con-tributing factors that led to the event. The itali-cized lessons are perceptions that are shared inthe spirit of preventing firefighter injuries anddeath.

New York City, 1966

Firefighters responded to a commercial structurefire only to discover that the building shared abasement with another building. The concealedfire advanced rapidly and undetected. The ensu-ing firefight trapped and killed twelve firefight-ers. It is vitally important to preplan buildingsprior to a fire event. Older buildings may haveaccess ways sealed from adjoining buildings.Shared utilities and other hidden voids can facili-tate fire spread.

The Boston Vendome Hotel Fire, 1972

Nine firefighters died during overhaul of a firein an old, remodeled hotel. The investigationrevealed that a masonry wall had beenbreached to make way for an air duct. Justabove the breach, a column carried the loadfrom floors above. A corner of the five-storybuilding collapsed, trapping the firefighters.There were no obvious signs of impending col-lapse. The Vendome building was brought backfrom disrepair in 1971, and many alterationswere made that were unknown to the firedepartment. Firefighters should take an interestin the construction activities in and aroundbuildings. Remodeling and restoration can com-promise structural elements.

Detroit, Abandoned Building, 1980

A fire was reported in a large, abandoned build-ing that was scheduled for demolition.Responding firefighters found “light smoke”showing. The fire escalated rapidly due to thepoor interior conditions and wide-open spaces.One firefighter died while trying to escape. Twoother firefighters died when a firewall collapsed.Abandoned buildings are much like a buildingunder construction. Firefighters cannot take

anything for granted. Rapid fire spread and sus-pect integrity should be the order of the day.Defensive operations must respect collapsezones.

Hackensack, New Jersey, Ford Dealer, 1988

A fire was discovered in the attic space above anautomobile repair garage. Responding firefight-ers launched an aggressive attack. The bowstringtruss roof space was being used as a parts stor-age area, placing additional load on the struc-ture. During firefighting operations, the roofcollapsed, trapping and killing five firefighters.Fighting fires in truss spaces is like playingRussian roulette. Trusses help form a wide, openspace beneath. Clear spans are a warning sign ofquick collapse should the truss space beinvolved in fire. Where there are no occupants torescue, firefighters should reduce their risk andfight fire from safe attack points.

Orange County, Florida, 1989

Firefighters responded to a fire in a single-storycommercial structure. Interior conditions weredescribed as light smoke and no heat. The firehad gained headway in a truss space above theceiling. The tile-covered roof collapsed twelveminutes after firefighters arrived and killed twofirefighters. A fire can be roaring over firefighters’heads without them being aware. Firefightersshould routinely inspect the ceiling space abovetheir heads for fire. Once fire or heavy, darksmoke conditions are found in truss spaces, tac-tics should change. Tile roof coverings are quiteattractive but show very little signs that the roofsupporting them is about to collapse.

Brackenridge, Pennsylvania, 1991

Firefighters were attempting to attack a fire inthe basement of a large commercial buildingwith concrete floors supported by steel columns.During the attack, the floor collapsed, trappingand killing four firefighters. Basement fires pres-ent many difficult challenges to firefighters. Lim-ited access, trapped heat and smoke, and thestorage nature of basements must be factored infire attack. Unfinished basements allow the fire toattack the floor above and floor supports ratherquickly. Unprotected steel exposed to fire willsoften quickly, leading to collapse.

HISTORICALLY SIGNIFICANT BUILDING COLLAPSES

Contributed by Dave Dodson

(continued)

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One Meridian Plaza, Philadelphia,Pennsylvania, 1991

Three firefighters died when they became disori-ented and ran out of air while fighting fire in thehigh-rise building. The fire started on the twenti-eth floor and ran up to the thirtieth floor where asprinkler system extinguished the fire. Althoughthe firefighters did not die from a collapse, thisevent is significant in that fire officers feared a cat-astrophic collapse due to stress cracks found inthe concrete stair towers and withdrew their fire-fighters. There are many lessons learned from thisevent. The fatality and fire investigation detailsmany building construction issues associated withhigh-rise firefighting. It is available from the U.S.Fire Administration (http://www.fema.gov/usfa).

Mary Pang Fire, Seattle, Washington, 1995

An arson fire in a multiuse commercial buildingcaused the deaths of four firefighters when aportion of the floor collapsed. The building hadbeen altered several times, and a lightweight“pony wall” had been used to replace a portionof a load-bearing wall. The building had a con-fusing layout including entry points at two differ-ent elevations (like a walkout basement). Theadvanced fire was not apparent from the “front”side of the building. An aggressive interior attackwas under way when the floor collapsed. Build-ings that have gone through several owners andoccupancy changes should always be suspect.Prefire planning helps uncover hazards that couldchange firefighting tactics. Firefighters shouldmake a habit of reporting fire and building con-ditions and observations.

Stockton, California, 1997

Two firefighters died when a home addition col-lapsed during interior firefighting operations.The homeowners had built a large, two-storyclear-span addition on the back of the home.Firefighters entered the front of the building andfound a heavy fire and dense smoke conditions.Homeowners do not necessarily follow estab-lished building practices and codes when makingadditions. A “360” prior to fire attack canuncover significant hazards when “reading” thebuilding. Firefighters cannot preplan everyhome, so they must rely on their ability to readbuildings and read smoke conditions.

World Trade Center, New York City, 2001

Terrorists hijacked two large airliners and hit thetwin towers of the World Trade Center. The FireDepartment of New York (FDNY) responded andbegan the biggest rescue effort in fire servicehistory. The high-rise towers collapsed, killing343 of FDNY’s bravest. Thousands of civiliansalso died in the collapse. Steel high-rise con-struction relies on fire-resistive coatings to pro-tect the steel. The combination of burning jetfuel and the trauma of the aircraft strikes ren-dered the steel unprotected. Failure came muchquicker than the four-hour time limit prescribedfor Type I fire-resistive construction found inhigh-rises. Firefighters should never rely on fireprotection time ratings when making decisionsfor fire attack. History will always remember thatthe FDNY firefighters died trying to rescuetrapped civilians. They have the undying respectof people throughout the world.

HISTORICALLY SIGNIFICANT BUILDING COLLAPSES (CONTINUED)

Design load: Design loads are loads thatan engineer has planned for oranticipated in the structural design.

Undesigned load: Undesigned load is aload that was not planned for oranticipated. Buildings that are altered orare being used for occupancy other thanoriginal intent create an undesignedload. One common example is aresidential structure that is converted toa print shop or legal office. Thesebuildings were not designed to hold theadditional live loads caused by thechange in occupancy.

Fire load: A fire load is the number ofBritish thermal units (Btus) generatedwhen the building and its contentsburn. It is important to note that theconstruction industry does notrecognize fire load in its vocabulary—it is a fire engineering term.

Imposition of LoadsLoads must be transmitted to structural elements.This is called imposition of loads. Terms associatedwith imposition include axial load, eccentric load,and torsional load, Figure 13-5.


An axial load is a load that is transmittedthrough the center of an element and runs perpen-dicular to the element. Eccentric load is appliedperpendicular to an element and, subsequently, doesnot pass through the center of the element. Torsionload is a load that is applied offset to an element,causing a twisting stress to the material.

ForcesLoads imposed on materials create stress and strainon the materials used to make the element. Stressand strain are defined as forces applied to materials.These forces are defined as compression, tension,and shear, Figure 13-6. In compression, forcestend to push materials together. Tension occurswhen forces tend to pull a material apart. Shearoccurs when a force tends to “tear” a materialapart—the molecules of the material are sliding pasteach other.

All loads—and the forces they create—musteventually pass through the structure and be deliv-ered to the earth through the foundation of thebuilding. Under normal conditions, structures willresist failure. Under fire conditions, the materialsused to resist forces start breaking down. Eventu-ally, gravity takes over and pushes the building tothe earth.

AxialLoad

EccentricLoad

TorsionLoad

Load

Load Load

Application of Loads

Figure 13-5 There are three types of loads that can be transmittedthrough a structural member: axial, eccentric, and torsional.

Streetsmart Tip As a building burns, thestructural elements decompose and lose theirstrength. This causes a change in the forces andthe way the design loads are applied, leading tostructural failure and collapse.

Shear

Types of Loads

Note: Shear canoccur in both directions.

Compression Tension

Load Load

Figure 13-6 Loads are applied to a structural mem-ber as compression, tension, and shear forces.

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It stands to reason that beams are used to create acovered space. In doing so, the beam is subjectedto an eccentric load. This load causes the beam todeflect. The top of the beam is subjected to a com-pressive force whereas the bottom of the beam issubjected to tension, Figure 13-7. The distancebetween the top of the beam and the bottom of thebeam dictates the amount of load the beam cancarry. I beams are very typical and usually refer tothe use of steel to form a beam. The top of the I isknown as the top chord; the bottom of the I iscalled the bottom chord. The material in betweenis known as the web. There are numerous types ofbeams although the principal method of load trans-fer remains the same. A few types of beamsinclude:

Simple beam: A beam supported at the twopoints near its ends.Continuous beam: a beam that is supported inthree or more places.Cantilever beam: A beam that is supported atonly one end—or a beam that extends over asupport in such a way that the unsupportedoverhang places the top of the beam in tensionand the bottom in compression.Lintel: A beam that spans an opening in a load-bearing masonry wall—such as over a garagedoor opening. In wood construction, the samebeam is often called a header.Girder: A beam that supports other beams.Joist: A wood framing member used to supportfloors or roof decking. A rafter is a joist that isattached to a ridge board to help form a peak.Truss: A series of triangles used to form a struc-tural element that in many ways is really a“fake” beam. That is, a truss uses geometricshapes, lightweight materials, and connectionsto transfer loads just like a beam. Trusses willbe covered in detail later in this chapter.Purlin: A series of wood beams placed perpen-dicular to steel trusses to help support roofdecking.

ColumnsA column is any structural component that transmitsa compressive force parallel through its center.Columns typically support beams and othercolumns, Figure 13-8. Columns are typicallyviewed as the vertical supports of a building; how-ever, columns can be diagonal or even horizontal.The guiding principle is that a column is totally incompression.

Streetsmart Tip Surface-to-Mass Ratio:Surface-to-mass ratio is defined as the exposedexterior surface area of a material divided by itsweight. In simple terms, smaller, lighter struc-tural members will have a large surface withsmall mass when compared to larger structuralmembers capable of carrying an equal load. A3- � 14-inch solid wood beam may carry thesame design load as six 2- � 4-inch parallelchord trusses. The trusses have much morewood surface exposed and are more likely toignite and burn rapidly.

The larger the surface area, the smaller themass, the quicker it will burn or fail. Also, incombustible construction, the large surface areaprovides more fuel for the fire. Surface-to-massratio may also be applied to lightweight steel orpre-engineered buildings. These lightweightstructural steel members will absorb heatquickly, and the steel elements will lose theirstrength and fail, causing collapse.

The time it takes for gravity to overcome the struc-ture during a fire is not predictable. A number of vari-ables determine the amount of time a material canresist gravity and fire degradation. These include:

Material massSurface-to-mass ratioOverall load being imposedBtu development (fire load)Type of construction (assembly method)Alterations (undesigned loading)Age deterioration/care and maintenance of thestructureFirefighting impact loadsCondition of fire-resistive barriers

STRUCTURAL ELEMENTSBuildings are an assembly of structural elementsdesigned to transfer loads to the earth. Structuralelements can be defined simply as beams, columns,and walls. Each of these elements must be con-nected in some fashion in order to effectively makethe load transfers to the building foundation, whichdelivers the building live and dead loads to earth.

BeamsA beam is a structural element that delivers loadsperpendicular to its length. Obviously, somethingmust support the beam—usually a wall or column.


Tensile Force

Load

Top Chord

Bottom Chord

Neutral Plain

CompressiveForce

CompressiveForce

CompressiveForce

Compressive ForceCompressive Force

Beam

Support Support

Figure 13-7 A beam transfers a load perpendicular to the load—creating compressive and tensile forces withinthe beam.

WallsA wall is also a component that transmits compres-sive force through its center. Simply put, a wall is areally long, but slender, column. Walls are subdi-vided into two categories: load-bearing and non-load-bearing. A load-bearing wall carries the weightof beams, other walls, floors, roofs, or other struc-tural elements as well as the weight of the wallitself. A non-load-bearing wall need only support itsown weight. A partition wall is an example of anon-load-bearing wall.

ConnectionsAs mentioned previously, beams, columns, andwalls must be connected in some fashion in order toeffectively transfer loads. Often, the connection isthe weak link as it relates to structural failure dur-

ing fires. The connection point is often a small,low-mass material that lacks the capacity to absorbmuch heat, thereby failing quicker than an elementthat has more mass such as a column or wall. Con-nections fall into three categories: pinned, rigid, andgravity. Pinned connections use bolts, screws, nails,rivets, and similar devices to transfer load. Rigidconnections refer to a system where the elementsare bonded together such that all the columns (orload-bearing walls) are bonded to all the beams.Typically, failure of one element will cause theloads to be transferred to other elements. Gravityconnections are just that—the load from an elementis held in place by gravity alone.

Together, structural elements defy gravity andmake a building sound. A series of columns andbeams used to hold up a building are often referredto as the skeletal frame. Post and beam describes thesame concept. Beams resting on walls are simplycalled wall-bearing buildings. One factor that hasnot been discussed is the suitability of the materialsused to form structural elements. The next sectioncovers these materials and how they act during fires.

FIRE EFFECTS ONCOMMON BUILDINGCONSTRUCTIONMATERIALS

Many factors determine which material is used toform structural elements. Quality, cost, applica-tion, engineering capabilities, and adaptability allplay in the suitability of a material. In some cases,the material chosen for a structural application

Figure 13-8 This column is supporting a beam,flooring, and another column. Columns are subjectedto compressive forces.

needs to meet fire resilience criteria. Regardless,the firefighter needs to understand how thesematerials react to fire. In the past, the fire servicelooked at the characteristics of four basic materialtypes: wood, steel, concrete, and masonry. Each ofthese materials can be found together or sepa-rately. Each material reacts to fire in a differentway, Table 13-1. Now, advanced material technol-ogy has found its way into structural elements.Buildings are being assembled using plastics,graphites, wood derivatives, and other composites.This section covers the four basic building materi-als as well as some of the new composites.

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WoodWood is perhaps the most common building material.It is used in millions of residential and commercialbuildings. Wood is relatively inexpensive, easy tomanipulate, and a replenishable natural resource(although that can be argued). Wood has marginalresistance to forces compared to its weight, but it doesthe job for most residential and small commercialbuildings. Wood also burns—and in doing so givesaway its mass. The more mass a section of wood has,the more material it must burn away before strength islost. This is true of native wood—that is, wood that

Francis L. Brannigan is a true friend to the fireservice. For over thirty-five years Mr. Branniganhas shared his knowledge of fire and, morespecifically, effects of fire on buildings. His bookBuilding Construction for the Fire Service (seethe Additional Resources section at the end ofthis chapter) is a must-read for any firefighter andcritical reading for anyone wanting to promoteinto fireground decision-making positions.

The fire service has affectionately calledMr. Brannigan the “Ol’ Professor.” His teachingshave saved untold numbers of firefighters. I willnever forget my first exposure to Mr. Brannigan.It was at a national conference in Cincinnati. TheOl’ Professor was teaching a daylong class onsteel buildings. As a young and inexperiencedfirefighter, I was all ears. The class taught me twothings. First, I had much to learn about fireeffects on building construction. I was waybehind in my knowledge, and the Ol’ Professormotivated me to make a never-ending knowl-edge quest to understand buildings. Second, Irealized that reading buildings and readingsmoke were the keys to rescuing people andputting the “wet stuff on the red stuff.”

Over the years, Mr. Brannigan has coinedmany powerful—and lifesaving—phrases. Thesebits of advice have remained part of the teachingsof many fire instructors. Among my favorites:

Trusses

“BEWARE the TRUSS!”

“A truss is a truss, is a truss . . .”

—in reply to the notion that a bowstring trussis more dangerous than other trusses.

“The bottom chord of a truss is under tension—It’s like you hanging on a rope. If the rope gets

cut, you will fall. So it is with a truss.”

“Failure of one element of a truss may cause theentire truss to fail—failure of one truss can cause

other trusses to fail.”

Columns

“The failure of a column is likely to be moresudden than failure of a beam.”

“The slightest indication of column failure shouldcause the building to be cleared immediately.”

On Collapses

“There is a tendency among those concernedabout building stability to make light of partialcollapse . . . a partial collapse is very important

to at least two groups—those under it and thoseon top of it!”

“From an engineering point of view, (lightweight,trussed) buildings are made to be disposable . . .

we don’t make disposable firefighters!”

In response to those who claim a buildingcollapsed without warning during a fire:

“The warning is the brain—in your ability tounderstand buildings and anticipate how they

will react to a fire.”

The fire service is indebted to the Ol’ Profes-sor. Thanks for all you have done!

TRIBUTE TO THE OL’ PROFESSOR

Contributed by Dave Dodson


has been cut from a tree. Engineered wood can reactdifferently when exposed to heat from a fire. Engi-neered wood includes a host of products that takemany pieces of native wood and glue them together tomake a sheet, longer beam (trees only grow so tall!),or stronger column. Plywood delaminates whenexposed to fire. Some newer wood products such ascomposites, which are discussed later in this chapter,present safety concerns for all firefighters.

SteelSteel is a mixture of carbon and iron ore heated androlled into structural shapes to form elements for abuilding. Steel has excellent tensile, shear, and com-pressive strength. For this reason, steel is a popularchoice for girders, lintels, cantilevered beams, andcolumns. Additionally, steel has high factory control.It is easy to change its shape, increase its strength,and otherwise manipulate it during production.

As it relates to fires, steel loses strength as tem-peratures increase. The specific range of tempera-tures depends on how the steel was manufactured.Cold drawn steel, like cables, bolts, rebar, andlightweight fasteners, loses 55 percent of itsstrength at 800°F. Extruded structural steel usedfor beams and columns loses 50 percent of itsstrength at 1,100°F. Structural steel will also elon-gate or expand as temperatures rise. At 1,000°F, a

100-foot-long beam will elongate 10 inches. Imag-ine what that could do to a building. If a beam isfixed at two ends, it will try to expand—and likelydeform, buckle, and collapse. If the beam sits in apocket of a masonry wall, it will stretch outwardand place a shear force on the wall—which wasdesigned only for a compressive force. This couldknock down the whole wall!

Because steel is an excellent conductor of heat, itwill carry heat of a fire to other combustibles. Thiscan cause additional fire spread, sometimes a con-siderable distance from the original fire.

Performance of Common Building Materialsunder Stress and Fire

MATERIAL COMPRESSION TENSION SHEAR FIRE EXPOSURE

Brick Good Poor Poor Fractures, spalls,crumbles

Masonry block Good Poor Poor Fractures, spalls

Concrete Good Poor Poor Spalls

Reinforced Good Fair Fair Spallsconcrete

Stone Good Poor Fair Fractures, spalls

Wood Good w/grain; Marginal Poor Burns, loss of materialpoor across grain

Structural steel Good Good Good Softens, bends, loses strength

Cast iron* Good Poor Poor Fractures

*Some cast iron may be ornamental in nature and not part of the structure or load bearing.

TABLE 13-1

Caution Steel softens, elongates, and sagswhen heated, leading to collapse. Coolingstructural steel with fire streams is just as impor-tant as attacking the fire.

ConcreteConcrete is a mixture of portland cement, sand,gravel, and water. It has excellent compressivestrength when cured. The curing process creates achemical reaction that bonds the mixture to achievestrength. The final strength of concrete depends onthe ratio of these materials, especially the ratio ofwater to portland cement. Because concrete haspoor tensile and shear strength, steel is added as

350 ■ CHAPTER 13

Streetsmart Tip Masonry Walls Collapse:Masonry has very little lateral stability, and inmany cases the roof or floor structure of a build-ing holds the walls in place. Steel beams orjoists will expand during a fire, creating lateralloads that the walls were not designed to with-stand. In addition, wood roof structures willburn away or collapse during a fire, leaving littlelateral support. The effects of the fire, pullingforces of the collapsing wood structure, or theforce of a hose stream may cause the wall tocollapse. Remember, the designer and contrac-tor did not plan for the building to burn down.

Figure 13-9 Prior to a fire, the effects of age willtake their toll on masonry walls. How stable is this wallwith joint deterioration and lack of full mortar bond?

reinforcement. Steel can be added to concrete inmany ways. Concrete can be poured over steel rebarand become part of the concrete mass when cured.Cables can be placed through the plane of concreteand be tensioned, compressing the concrete to giveit required strength. Cables can be pretensioned (ata factory) or posttensioned (at the job site). Precastconcrete refers to slabs of concrete that are pouredat a factory and then shipped to a job site. Precastslabs are “tilted up” to form load-bearing walls—thus the term tilt-up construction.

All concrete contains some moisture and continuesto absorb moisture as it ages. When heated, this mois-ture content will expand, causing the concrete to crackor spall. Spalling refers to a large pocket of concretethat has basically crumbled into fine particles, takingaway the mass of the concrete. Reinforcing steel thatbecomes exposed to a fire can transmit heat within theconcrete, causing catastrophic spalling and failure ofthe structure. Unlike steel, concrete is a heat sink andtends to absorb and retain heat rather than conduct it.This heat is not easily reduced. Concrete can stay hotlong after the fire is out, causing additional thermalstress to firefighters performing overhaul.

MasonryMasonry is a common term that refers to brick, con-crete block, and stone. Masonry is used to formload-bearing walls because of its compressivestrength. Masonry can also be used to build aveneer wall. A veneer wall supports only its ownweight and is most commonly used as a decorativefinish. Masonry units (blocks, bricks, and stone) areheld together using mortar. Mortar mixes are var-ied but usually contain a mixture of lime, portlandcement, water, and sand. These mixes have little tono tensile or shear strength. They rely on compres-sive forces to give a masonry wall strength. A lateralforce that exceeds the compressive forces within amasonry wall will cause quick collapse of the wall.

Brick, concrete block, and stone have excellentfire-resistive qualities when taken individually.Many masonry walls are typically still standingafter a fire has ravaged the interior of the building.Unfortunately, the mortar used to bond the masonryis subject to spalling, age deterioration, andwashout. Whether from age, water, or fire, the lossof bond will cause a masonry wall to be very unsta-ble, Figure 13-9.

CompositesNew material technologies have introduced someinteresting challenges for the firefighting commu-nity. Composites are a combination of the four basicmaterials listed above as well as various plastics,glues, and assembly techniques. Of particular inter-est are the many wood products that are widely usedfor structural elements.

Lightweight wooden I beams (joists) are nothingmore than wood chips that are press-glued togetherinto the shape of an I beam, Figure 13-10. Whilestructurally strong (stronger than a comparable solidwood joist), the wooden I beam fails quickly whenheated. Actually, no fire contact is required. Ambi-ent heating causes the binding glue to fail, leading toa quick collapse. The bottom of a beam is under ten-sile forces. If the bottom of the beam falls off, due toglue failure, the beam will immediately snap andcollapse.

New products, known as FiRP (fiber-reinforcedproducts) are becoming common in the constructionindustry. FiRP can be plastic fibers mixed withwood to give the wood increased tensile strength.As with most plastics, fire exposure can cause quickfailure as the plastic melts.

The mixture of steel and wood as a structural ele-ment can cause rapid collapse because steel expands


Figure 13-10 To save on materials and cost, the useof composites or engineered wood structural membersis becoming popular. Shown here is a typical wood Ibeam with 2- � 3-in. flanges and a 3⁄8-in. structure boardweb. In addition to providing a large surface-to-massratio, the flanges and web are fastened with glue, whichmay deteriorate quickly under fire conditions.

Figure 13-11 A composite truss. Rapid heating willcause the steel to separate from the wood chords.

faster than wood. This causes stress at the intersec-tion of the two materials, Figure 13-11.

Structural insulated panels (SIP) are another inter-esting composite. This technique is characterized bylarge wall panels made of expanded polystyrene sand-

wiched between two sheets of oriented strand board(OSB). OSB is a sheet of wood chips bonded by glue,Figure 13-12A and B. The OSB is covered with atypical wall finish. It is anticipated that a fire willcause rapid deterioration of the load-bearing panel.

(A)

(B)

Figure 13-12 These are wall panels that are load-bearing (A). Expanded polystyrene is sandwichedbetween OSB sheets. Fire can easily enter the wallspace. (B) Failure of the wall panel will cause instabil-ity in the roof structure.

352 ■ CHAPTER 13

To review, this chapter has so far explored thebasic terminology, mechanics, elements, andmaterials used in the construction of buildings. Ithas also discussed a bit about how fire attacksmaterials and causes failure. The next sectionexplores the various methods used to assemblebuildings.

TYPES OF BUILDINGCONSTRUCTION

Over time, five broad categories of building con-struction types have been developed to help classifystructures. These categories give firefighters a basic

Types of Construction from NFPA 220TYPE I TYPE II TYPE III TYPE IV TYPE V

443 332 222 111 000 211 200 2HH 111 000

Exterior Bearing Walls—

Supporting more than one floor,

columns, or other bearing walls 4 3 2 1 0 2 2 2 1 0

Supporting one floor only 4 3 2 1 0 2 2 2 1 0

Supporting a roof only 4 3 1 1 0 2 2 2 1 0

Interior Bearing Walls—


columns, or other bearing walls 4 3 2 1 0 1 0 2 1 0

Supporting one floor only 3 2 2 1 0 1 0 1 1 0

Supporting roofs only 3 2 1 1 0 1 0 1 1 0

Columns—


columns, or other bearing walls 4 3 2 1 0 1 0 H1 1 0

Supporting one floor only 3 2 2 1 0 1 0 H1 1 0

Supporting roofs only 3 2 1 1 0 1 0 H1 1 0

Beams, Girders, Trusses

& Arches—


columns, or other bearing walls 4 3 2 1 0 1 0 H1 1 0

Supporting one floor only 3 2 2 1 0 1 0 H1 1 0

Supporting roofs only 3 2 1 1 0 1 0 H1 1 0

Floor Construction 3 2 2 1 0 1 0 H1 1 0

Roof Construction 2 11⁄2 1 1 0 1 0 H1 1 0

Exterior Nonbearing Walls 0 0 0 0 0 0 0 0 0 0

Those members that shall be permitted to be of approved combustible material.1“H” indicates heavy timber members; see text for requirements.Source: Reprinted with permission from NFPA 220, Types of Building Construction, copyright © 1995, National Fire ProtectionAssociation, Quincy, MA 02269. This reprinted material is not the complete and official position of the National Fire ProtectionAssociation on the referenced subject which is represented only by the standard in its entirety.

TABLE 13-2


understanding of the arrangement of structural ele-ments and the materials used to construct the build-ing. Unfortunately, these broad classifications aredangerously incomplete for firefighters and maylead to deadly assumptions about the makeup of abuilding. As stated before, firefighters need toexplore the buildings within their jurisdiction todetermine how buildings are assembled.

It is important to note that buildings are built tomeet certain codes. These codes are designed togive occupants time to escape during a fire. Con-crete is fire resistive—meaning it has some capac-ity to withstand the effects of fire. Other materials,like steel and wood, need fire-resistive assistance togive occupants a chance to escape. Building codesoutline fire-resistive ratings, occupancy classifica-tions, and means of egress based on five generaltypes of buildings. The features of each type of con-struction will be discussed shortly, but first it isimportant to understand fire resistance for structuralelements. Table 13-2 outlines the number of hoursthat a structural element needs to be protected forthe five types of construction. Simply put, firefight-ing time is not part of the fire-resistive and buildingconstruction equation. Fire-resistive ratings areestablished in a laboratory. In the real world, fireresistance ratings could “underperform” due tomany factors. For example, a structural elementwith a two-hour fire rating may fail in thirty min-utes if it was not assembled correctly or if improp-erly inspected. Fire resistance for structuralmembers can be achieved by various methodsincluding drywall (gypsum wallboard), spray-oncoatings, and concrete, Figure 13-13. Aging, alter-

ations, and wear can damage fire-resistive methodsto the point that structural elements have no fireresistance protection.

The following paragraphs outline the basic defi-nition of each building type, its general configura-tion, and some historical fire spread problemsassociated with each. Also included are some con-struction methods that do not fit into the five com-mon types.

Type I: Fire-ResistiveType I fire-resistive construction is a type inwhich structural elements are of an approved non-combustible or limited combustible material withsufficient fire-resistive rating to withstand theeffects of fire and prevent its spread from story tostory. Concrete-encased steel, Figure 13-14,monolithic-poured cement, and steel with spray-onfire protection coatings are typical of Type I, Fig-ure 13-15. Generally, the fire-resistive rating mustbe three to four hours depending on the specificstructural element. Fire-resistive construction isused for high-rises, large sporting arenas, and otherbuildings where a high volume of people areexpected to occupy the building.

Most Type I buildings are typically large, multi-storied structures with multiple exit points. Fires aredifficult to fight due to the large size of the buildingand the subsequent high fire load. Type I buildingsrely on protective systems to rapidly detect andextinguish fires. If these systems do not contain thefire, a difficult firefight will be required. Fire canspread from floor to floor on high-rises as windowsbreak and the next floor windows fail, allowing thefire to jump. Fire can also make vertical runsthrough utility and elevator shafts. Regardless, fire-fighters are relying on the fire-resistive methods toprotect the structure from collapsing. The collapse

Figure 13-13 This parking garage is of Type II con-struction. The protective coating applied to thestructural steel may increase the fire-resistive rating,but note that the unprotected corrugated metalflooring and interior steel structure are not pro-tected. These unprotected structural members mayfail early in a fire. (Photo courtesy of William H. Schmitt, Jr.)

Steel StructuralMember

Concrete for Fire Protection

Figure 13-14 To achieve a Type I fire-resistive rat-ing, structural steel members are encased with con-crete to prevent failure from the effects of a fire.

of fire-resistive structures can be massive, as we arereminded from the World Trade Center collapse inNew York City.

Type II: NoncombustibleType II noncombustible construction is a type inwhich structural elements do not qualify for Type Iconstruction and are of an approved noncombustibleor limited combustible material with sufficient fire-resistive rating to withstand the effects of fire andprevent its spread from story to story. More oftenthan not, Type II buildings are steel, Figure 13-16.Modern warehouses, small arenas, and newerchurches and schools are built as noncombustible.Because the steel is not required to have significantfire-resistive coatings, Type II buildings are suscep-tible to steel deformation and resulting collapse.Fire spread in Type II buildings is influenced by thecontents. While the structure itself will not burn,rapid collapse is possible from the content Bturelease stressing the steel.

354 ■ CHAPTER 13

Suburban strip malls with concrete block load-bearing walls and steel roof structures can be classi-fied as Type II. Fires can spread from store to storethrough wall openings and shared ceiling and roofsupport spaces. The roof structure is often of light-weight steel that fails rapidly. More often than not,the fire-resistive device used to protect the roofstructure is a dropped-in ceiling. Missing ceilingtiles, damaged drywall, and utility penetrations canrender the steel unprotected. These buildings mayhave combustible attachments such as facades andsigns as well as significant content fire loading.

Type III: OrdinaryThe term Type III ordinary construction is oftenmisapplied to wood frame buildings. By defini-tion, ordinary construction includes buildingswhere the load-bearing walls are noncombustible,and the roof and floor assemblies are wood. Mostcommonly, this is load-bearing brick or concreteblock with wood roofs and floors. Ordinary con-

Figure 13-15 A typical Type I building, with struc-tural members designed to resist the effects of fire forthree to four hours. This building is of reinforced con-crete construction.

Figure 13-16 Buildings of Type II construction willhave structural elements with little or no protectionfrom the effects of fire. Remember, in the event of afire, these unprotected steel structural members mayfail and collapse with little warning.


struction is prevalent in most downtown or “mainstreet” areas of established towns and villages,Figure 13-17. Firefighters have long called ordi-nary construction “taxpayers.” This slang isderived from landlords who built buildings withshops and/or restaurants on the first floor withapartments above in order to maximize income tohelp pay property taxes. Newer Type III buildingsinclude strip malls with block walls and woodtruss roofs, Figure 13-18.

Ordinary construction presents many challengesto firefighters. In older buildings, numerous remod-els, restorations, and repairs have created suspectwall stability and hidden dangers.

Figure 13-17 Buildings of Type III, ordinary con-struction, are common throughout North America.These typical “Downtown USA” buildings providemany challenges to firefighters, such as void spacesand common walls allowing rapid fire extension andlittle structural protection, with early collapse duringfirefighting operations.

Figure 13-18 One of the most common uses of Type III, ordinary construction,is the “strip mall” with masonry walls and lightweight steel or wood trusses.Common problems associated with this type of construction are void spacesallowing for rapid fire extension and collapse of lightweight structural elements.

Firefighter Fact Sagging or bowing load-bearing walls are often pulled back in alignmentby tightening a steel rod that runs through thebuilding from wall to wall. A small interior firecan elongate this steel and cause catastrophicwall failure. These buildings can be spotted bydecorative stars or ornaments (called spreaders)on the outside brick wall.

Ordinary construction has many void spaceswhere fire can spread undetected. Common hall-ways, utilities, and attic spaces can communicatefire rapidly. Masonry walls hold heat inside, makingfor difficult firefighting. Wood floors and roofbeams are often gravity fit within the masonrywalls. These can release quickly and cause a generalcollapse, leaving an unsupported masonry wall.Older Type III buildings have structural mass; there-fore, they burn for a long time.

Type IV: Heavy TimberType IV heavy timber construction can be definedas those buildings that have block or brick exteriorload-bearing walls and interior structural members,roofs, floors, and arches of solid or laminated woodwithout concealed spaces. The minimum dimen-sions for structural wood must meet the criteria inTable 13-3. Heavy timber buildings, as the namesuggests, are quite stout and are used for ware-houses, manufacturing buildings, and some olderchurches, Figure 13-19. In many ways, a Type IVbuilding is like a Type III—just larger dimensionlumber instead of common wood beams and trusses.

356 ■ CHAPTER 13

Some firefighters mistakenly call Type IV buildings“mill construction.” Mill construction is a muchmore stout, collapse-resistive building that may ormay not have block walls. A new Type IV buildingis hard to find. The cost of large-dimension lumberand laminated wood beams makes this type of con-struction rare.

Fire spread in a heavy timber building can be fastdue to wide-open areas and content exposure. Theexposed timbers contribute Btus to the fire. Becauseof the mass and large quantity of exposed structuralwood, fires burn a long time. If the building housedmachinery at one time, oil-soaked floors will addmore heat to the fire and accelerate collapse. Oncefloors and roofs start to sag, heavy timber beamsmay release from the walls. This is accomplished bymaking a fire-cut on the beam, and the beam isgravity fit into a pocket within the exterior load-bearing masonry wall, Figure 13-20. As the floorsags, it loses its contact point with the wall and sim-ply slides out of its pocket without damage to thewall. It is important to recall that a free-standingmasonry wall has little lateral support and requirescompressive weight from floors and roofs to make itsound.

Type V: Wood FrameType V wood frame construction is perhaps themost common construction type. Homes, newersmall businesses, and even chain hotels are builtprimarily with wood, Figure 13-21. Older woodframe buildings were built as balloon frame—

Heavy Timber DimensionsTYPE OF ELEMENT USE SIZE

Column Supporting floor load 8- � 8-in. minimum any dimension

Column Supporting roof load 6-in. smallest dimension, 8-in. depthminimum

Beams and girders Supporting floor load 6-in. width and 10-in. depth minimum

Beams, girders, Supporting roof loads 4-in. width minimum, 6-in. depthand roof framing only minimum

Framed or laminated arches As designed 8-in. minimum dimension

Tongued and grooved planks Floor systems 3-in. minimum thickness withadditional 1-in. boards at right angles

Tongued and grooved planks Roof decking 2-in. minimum thickness

TABLE 13-3

Figure 13-19 Type IV buildings, heavy timber con-struction, have large wood structural elements withgreat mass. The mass of these structural membersrequires a long burn time for failure. The connec-tions, usually steel, are the weak points in this type ofconstruction.


that is, wood studs ran from the foundation to theroof and floors were “hung” on the studs. As canbe envisioned, fire could enter the wall space andrun straight to the attic. In the early 1950s,builders started using a platform framingarrangement where one floor was built as a plat-form for the next floor. This created fire stoppingto help minimize fire spread. Newer wood framebuildings utilize lightweight wood trusses forroofs and floors. This is akin to a “horizontal bal-loon frame” that can allow quicker lateral firespread. Coupled with high surface-to-mass wood

exposure, collapse becomes a real possibility, Fig-ure 13-22. Some codes require truss spaces tohave fire stopping every 500 square feet. Evenwith this fire stopping, it remains dangerous tostep onto the 500 square feet where the fire is.Wood frame structures may appear more like aType III ordinary building because of a brick-wallappearance. Remember, brickwork may be a sim-ple veneer to add aesthetics.

To protect structural members from a fire, woodframe construction typically uses gypsum board(drywall or the brand name Sheetrock). Once fin-ished, wood frame buildings typically have manyrooms that can help compartmentalize contentfires. Fire that penetrates wall, floor, or attic spacesbecomes a significant collapse threat, especially innewer buildings. Often, the only warning that firehas penetrated these spaces is the issuance ofsmoke from crawl space vents, gable end vents,and eaves.

Other Construction TypesAs mentioned earlier, the five broad building typescan actually lead to dangerous assumptions. Newerconstruction and alternative building methods maynot fit cleanly into one of the above types. Somebuildings are actually two types of construction.For example, a particular restaurant located in Col-orado is built as a Type II noncombustible yet istopped with a large wood frame structure to hiderooftop HVACs and cooking vent hoods,Figure 13-23A and B. The square feet space of thefalse dormers and wood frame structure exceedsthat of most homes.

New lightweight steel homes resemble woodframe homes. These buildings are actually a “postand beam” steel building with lightweight steelstuds to help partition the home. OSB is added to thestuds to help make the house more “stiff” andincrease wind-load strength, Figure 13-24. Anotherinteresting construction type uses foam blocks tomake a form for a lightweight concrete mud mix-ture. The concrete is not contiguous—there aremany voids, utility runs, and foam block spacers(made of plastic or galvanized steel), Figure 13-25.These structures are called “ICF” or insulated con-crete formed.” However, it is important not to befooled by any claims that these buildings are con-crete or are less combustible. In reality, these com-posite buildings are assembled with plastics,polystyrene, lightweight steel, and lightweight con-crete. When finished, these buildings may resemblewood frame or even ordinary construction.

FloorBeam

Floor Covering

MasonryWall

Fire-Cut

Wall Pocket

Gravity-Held

Figure 13-20 Wood and heavy timber beams wereoften “fire-cut” so that a fire-damaged, sagging floorwould simply slide out of the wall pocket in order topreserve the wall.

Figure 13-21 The wood frame structure, Type Vconstruction, is the most common type of construc-tion in North America.

358 ■ CHAPTER 13

Extended window and door jambs are clues thatindicate the wall is thicker than that of typical woodor masonry built buildings.

The fire service has very little research infor-mation on the stability of these new types ofbuildings during fires. One thing is certain:Firefighters should expect rapid collapse due tothe low-mass, high surface-to-mass exposure ofstructural elements.

Manufactured buildings can be defined as thosestructures that are built at a factory and thentrucked to a job site. These building are quite lightwith little mass. Where a stick-built home uses 2 � 4 or 2 � 6 lumber, the manufactured homeuses 1 � 2 and 2 � 2 lumber. These buildings usegalvanized strapping to give required strength. Inany case, these buildings burn quickly and col-lapse equally fast.

Figure 13-22 (A, B) Note the void spaces at the first and second floor levels and in the attic area that are cre-ated by the use of truss systems. (C) The building after it was completed. Firefighters should survey the buildingsin their area before they are completed.

(A) (B)

(C)


Relationship of ConstructionType to Occupancy UseBefore considering the basic types of construc-tion, many officials and builders first look at theanticipated use of the building—its occupancytype. Occupancy classifications are called manydifferent names around the country, but they areusually broken down into five basic arenas: resi-dential, commercial, business, industrial, and edu-cational. Each of these general occupancies has anumber of hazards that firefighters must under-stand, Table 13-4. Remember, a building mayhave been built for one type of occupancy only tobe sold and converted to another occupancy typefor which it may not have been designed. Fire-fighters should go out and explore the buildings intheir community.

Figure 13-23 (A) The decorative roof assembly is aType V wood frame structure while the occupancyspace is Type II noncombustible. (B) This buildinguses two types of construction.

(A)

(B)

Figure 13-24 This lightweight steel home is builtsimilar to a Type V. OSB sheeting gives the steel rigid-ity to torsional loads like wind.

COLLAPSE HAZARDS ATSTRUCTURE FIRES

It cannot be overstated that firefighters have tounderstand the buildings in their jurisdiction.Constant reading, study, and site visits will help

360 ■ CHAPTER 13

them “read” buildings. Reading buildings isessential to anticipating collapse proactively. Thissection addresses some specific collapse threatsthat the fire service has experienced throughouthistory and the importance of understandingbuildings and how they react at structure fires.

(A) (B)

Figure 13-25 (A) This wall is a load-bearing foam block unit filled with a lightweight concrete mud mix. (B) Notethe black plastic spacers that will fail early in a fire.

Typical Hazards Associated with OccupanciesOCCUPANCY TYPE OF CONSTRUCTION HAZARDS

Residential Type V, most common Fire loading, truss construction, owneralterations, rapid fire extension in void spaces

Commercial Type III, most common Fire loading, truss construction, rapid fireextension in void spaces, unknownoccupancy change

Educational Type II, most common Unprotected structural steel, collapse, highfire load in some areas

Business Types II and III, most common Unknown change in occupancy, high fireload, difficult to ventilate

Industrial Types I and II, most common Hazardous materials, difficult to ventilate

TABLE 13-4


TrussesTruss roof collapses have killed many firefighters.As stated previously, a truss is actually a fake beam.A truss uses geometric shapes (the triangle) to cre-ate a structural element similar to a beam. A woodtruss can actually be stronger than a like-sized solidwood beam, and does so with less material,Figure 13-26. It is this loss of material and subse-quent increase in exposed surface area that makethem so vulnerable during fires. Trusses rely oneach and every part of the truss to carry a portion ofthe imposed load. Like a beam, the top of the truss(called the top chord) is typically under a compres-sive force. The bottom chord is under tension. Inbetween the two chords, connecting members (theweb) transfer the two forces creating stress andstrain. Failure of one part of the truss will likelycause the whole truss to fail. This distributes theweight of the failed truss to other trusses—whichmay not have the capacity to take that weight—thereby starting a domino effect collapse. Trussescome in many styles and shapes. Bowstring truss,parallel chord truss, and open web joists are some ofthe more common names. Trusses are also classifiedby the type of material used in assembly.

Wood TrussesWood trusses are an assembly of many pieces ofwood. Some may even be press-glued particles.These pieces are connected using gusset plates. Agusset plate is a simple galvanized steel plate (verythin) with perforations punched into the plate. Theperforations are used to pierce into wood fibers tohold pieces together. These perforations only pene-trate the wood a fraction of an inch (3/8 inch is typi-cal), Figure 13-27. During fires, the steel gusset

heats up and transfers heat into the very wood fibersthat are being held. If the heating is slow (like asmoke-filled attic), the wood decomposes, allowingthe gusset plate to fall out. If the heating is fast, likesudden exposure to flame, the steel expands tooquickly for the wood and the gusset simply popsout. Either way, truss failure is imminent. Some-times the truss gingerly stays together because ofthe weight of roofing or flooring materials, yet asudden force, like a walking firefighter, will causethe truss to disassemble and suddenly collapse.

Wood trusses are mass-produced at a factorywhere quality control may not be adequate. Further,the truss gusset plates may vibrate or be damagedwhile being delivered to a job site. Once on the jobsite, contractors may use shortcuts to lift the trussinto position, furthering the damage to gusset plates.

Steel TrussesSteel trusses are no less susceptible to collapse thanwood trusses. Like wood, steel trusses are an assem-bly of pieces—typically angle iron for the chords andcold-drawn round stock for the web. The pieces aretack-welded together to form the truss unit. While nota true joist by definition, many call the common steeltruss an open web steel joist, Figure 13-28. The termbar joist is also used to describe an open web steeljoist. These trusses expose a large surface area to heatduring fires. Given the lack of mass, the truss heatsquickly and will soften and expand. The expansioncan cause wall movement. (Remember, masonrywalls must be loaded axial with compressive force.)Lateral movement can cause wall collapse. If thewall does not move, the steel truss will twist andbuckle to allow expansion. It is very important tokeep steel trusses cool.

Figure 13-26 Wood trusses provide a large surface-to-mass ratio, fuel load, and void spaces—three ofthe worst conditions a firefighter will encounter duringstructural firefighting operations.

Figure 13-27 A typical parallel chord truss. The gus-set plates on this truss are pressed into the wood. Inaddition to the decomposing of the wood element,the light-gauge steel plates will deform and pull awayfrom the wood under fire conditions.

362 ■ CHAPTER 13

Void SpacesTrusses create large void spaces. The area betweenthe chords of trusses will allow fires to spread hori-zontally, Figure 13-29. Some codes require firestopping in floor truss spaces but may allow wide-open attic spaces. Fires can start in void spaces dueto electrical and other utility problems. In Type IIIordinary construction, voids are numerous. Somevoids may pass through masonry walls, causing firespread from one store to the next in a row of build-ings. The obvious collapse danger with void spaces

is that the fire may be undetected with simultaneousdestruction of structural elements.

Roof StructuresThe roof of a building can be flat, pitched, orinverted. Many factors help determine how andwhy the roof is built the way it is. Sometimes theroof is designed just to hide rooftop HVACs. Othertimes the roof shape is designed to shed snow,accommodate a vaulted ceiling, or merely give thebuilding character, Figure 13-30. As it relates to

Figure 13-29 Lightweight floor truss systems havemany void spaces. This could be called “horizontalballoon frame.”

Gable Roof Hip Roof Intersecting Roof

Gambrel Roof Mansard Roof Butterfly Roof

Shed Roof

Figure 13-30 Some common roof framing styles used in wood frame orordinary construction.

Figure 13-28 Unprotected open web steel joistspresent a large surface area to absorb the heat of afire, expand, and collapse. Structural steel will lose50 percent of its strength at temperatures of 1,000°F.


structural collapse, the roof style may allow a largevolume of fire to develop. Other roofs, like themansard, have many concealed spaces. Dormersare protrusions from a roof structure. Dormers canbe used to introduce daylight into a roof space thatis converted into a living space, Figure 13-31.Other dormers are actually aesthetic (false) and canfool ventilation crews attempting to relieve heatfrom a roof space.

StairsFirst arriving firefighting crews rely on internalstairways to help gain access for rescue and fireattack. For years, firefighters have found stair-ways to be durable and a bit stronger than otherinterior components. This is a dangerous assump-tion in newer wood frame buildings. Stairs arenow being built offsite and simply hung in placeusing light metal strapping, Figure 13-32. Addi-tionally, stairs are being made using lightweightengineered wood products that fail quickly whenheated. Remember, press-glued wood chip prod-ucts can fail from the heat of smoke—no flame isrequired.

Figure 13-31 An internal view of dormers in amansard roof. Fire can run through the many voidsbetween the exterior and the roof.

Figure 13-32 This prefab stair assembly is hung inplace by thin metal strapping. Note the staples andplastic shims that can quickly fail under fire conditions.

Parapet WallsA parapet wall is the extension of a wall past thetop of the roof. Parapets are used to help hideunsightly roof equipment and HVACs and give abuilding a finished look. Typically masonry, thesewalls are free-standing with little stability. Collapsemay be caused by the failure of the roof structure,Figure 13-33. Business owners hang signs, utilityconnections, and other loads on the parapet. Duringa fire, the steel cables and bolts holding these willweaken and subsequently pull down the parapet,Figure 13-34.

Figure 13-33 This is the scene of a typical “parapet”wall failure common in Type III ordinary construction.

364 ■ CHAPTER 13

Collapse SignsFirefighters must rely on building material knowl-edge, building construction principles, and anunderstanding of fire effects on buildings in order topredict or anticipate collapse. Waiting for a visualsign that a building will collapse is dangerous, espe-cially in newer buildings. There are, however, somefactors and observations that can be used to helpanticipate collapse. These include:

Overall age and condition of the buildingDeterioration of mortar joints and masonryCracks, in anythingSigns of building repair including reinforcingcables and tie-rodsLarge open spansBulges and bowing of wallsSagging floorsAbandoned buildingsLarge volume of fireLong firefighting operations—remembergravity?Smoke coming from cracks in wallsDark smoke coming from truss roof or floorspaces (Brown smoke indicates that wood is being heated significantly; black smokemeans combustibles have ignited or are nearignition.)Multiple fires in the same building or damagefrom previous fires

Buildings under ConstructionBuildings are especially unsafe during construction,remodeling, and restoration. The word unsafe appliesnot only to fire operations but also to rescues, odorinvestigations, and on-site inspections. Buildingsneed only meet fire and life safety codes when theyare completed. During construction, many of the pro-tective features and fire-resistive components areincomplete. Additionally, stacked construction mate-rial may overload other structural components. Thisis not to say contractors are using unsafe practices,but to underscore that exposed structural elements,incomplete assemblies, and material stacks will con-tribute to a rapid collapse if a fire were to develop.

Figure 13-34 This electrical service entrance andweather head may be the eccentric load causing anearly failure of this parapet wall.

Streetsmart Tip Beware of Buildings underConstruction: A building as a complete unit hasa number of interdependent parts. During con-struction, these parts may not be fully con-nected (steel), may not be at their full designstrength (concrete), and may lack any type offire protection (gypsum board or concrete).Also, many structural elements may be held inplace by scaffoldings, false work, or forms.Because of this, a building under construction isexposed to early collapse due to the effects offire or other elements such as high winds.

Historical building restoration and generalremodel projects in buildings are similar to build-ings under construction. Firefighters may find tem-porary shoring up of walls, floors, and roofs whileother structural components are being updated,replaced, or strengthened. Contractors may use sim-ple 2 � 4s to temporarily shore up heavy timber,leading to disastrous results during fire conditions.The best approach for firefighters to take whenresponding to fires in buildings under constructionis to be defensive. They should make sure everyoneis out and accounted for and then attack the firefrom a safe location. A building under constructioncan be replaced—a firefighter’s life cannot.

Preparing for CollapseThere are no time limits for firefighting operationswithin a building. Tests have shown that the age-old“twenty-minute rule” used by previous fire officersis no longer accurate. Roofs and walls can collapsewithin minutes of fire involvement given certainconditions. An overloaded (due to improper storageor other factors) truss can collapse immediatelywhen heated.


H

11/2 H

CollapseZone

Grade

H = Building Height

Figure 13-35 A minimum collapse zone should be 11⁄2 times the height ofthe building.

Streetsmart Tip Collapse: Every firefightermust understand two rules about structural col-lapse during fire operations. The first is that thepotential for structural failure during a fire alwaysexists. Do not set artificial time limits based onexperience. The second rule is to establish a col-lapse zone, as shown in Figure 13-35, which isan area around and away from a building wheredebris will land if the building fails. As anabsolute minimum this distance must be at least11⁄2 times the height of the building. The wallsmay crumble into a pile or they may tip out thefull height of the building. Also you need to pro-vide extra room for cascading debris.

Figure 13-36 These photos show the effects of fire on a masonry wall. Note the debris and distance the bricksfell away from the building. Firefighters should always establish a collapse zone, as shown in Figure 13-35.

Once a building has been searched for occupants,the risks firefighters take to control the fire shouldbe reduced—after all, it is now a property issue.Many firefighters have been killed fighting interiorfires, only to have the building torn down after theinvestigation. Outside (defensive) firefighting oper-ations can be equally dangerous if firefighters wan-der into the collapse zone, Figure 13-36.

366 ■ CHAPTER 13

Many firefighters have been killed as a result ofbuilding collapse from structural fires. To preventfuture deaths, firefighters must understand thebuildings in which they fight fires. This under-standing comes from a long-term commitment toread and study building construction information.Additionally, firefighters must get into buildingswithin their jurisdictions to survey and explore theway buildings are assembled, remodeled, andused in the real world. Knowledge of buildingconstruction starts with an understanding ofloads, forces, and materials found in the structuralmakeup of buildings. Firefighters also study the

effects of fires on materials and constructiontypes. The five classic types of construction arebeing challenged by new construction methods.Trusses are used in virtually all new buildings.Trusses have high surface-to-mass characteristicsthat rapidly absorb heat and subsequently failquickly. Failure of one truss can cause failure ofother trusses. There are no rules for how long abuilding will last while on fire. Many factors deter-mine when materials and construction design failand gravity pushes down the building. Buildingsunder construction are losers from a firefightingpoint of view—they collapse quickly.

KEY TERMSAxial Load A load passing through the center of

the mass of the supporting element, perpendicu-lar to its cross section.

Balloon Frame A style of wood frame construc-tion in which studs are continuous the fullheight of a building.

Beam A structural member subjected to loads per-pendicular to its length.

Cantilever Beam A beam that is supported atonly one end.

Chord The top and bottom components of a beamor truss. The top chord is subjected to compres-sive force; the bottom chord is subjected to ten-sile force.

Collapse Zone The area around a building wheredebris will land when it falls. As an absoluteminimum this distance must be at least 11⁄2 timesthe height of the building.

Column A structural element that is subjected tocompressive forces—typically a vertical member.

Compression A force that tends to push materialstogether.

Concentrated Load A load applied to a small area.Continuous Beam A beam that is supported in

three or more places.Dead Load The weight of the building materials

and any part of the building permanentlyattached or built in.

Design Load A load the engineer planned for oranticipated in the structural design.

Distributed Load A load applied equally over abroad area.

Eccentric Load A load perpendicular to the crosssection of the supporting element that does notpass through the center of mass.

Fire Load The amount of heat energy released whencombustibles burn in a given area or building—expressed in British thermal units (Btus).

Fire Resistive The capacity of a material to with-stand the effects of fire.

Fire-Resistive Rating The time in hours that amaterial or assembly can withstand fire expo-sure. Fire-resistive ratings are usually providedfor testing organizations. The ratings areexpressed in a time frame, usually hours or por-tions thereof.

Fire Stopping Pieces of material, usually wood ormasonry, placed in stud or joist channels to slowthe extension of fire.

Girder A large structural member used to sup-port beams or joists—that is, a beam that sup-ports beams.

Gusset Plate A connecting plate used in trussconstruction. In steel trusses, these plates areflat steel stock. In wood trusses, the plates areeither light-gauge metal or plywood.


HVAC Acronym for heating, ventilation, and air-conditioning unit. HVACs are typically arooftop unit on commercial buildings. Buildingsmay have one or dozens of these units.

Impact Load A load that is in motion when it isapplied.

Joist A wood framing member that supports flooror roof decking.

Lintel A beam that spans an opening in a load-bearing masonry wall.

Live Load The weight of all materials and peopleassociated with but not part of a structure.

Load-Bearing Wall Any wall that supports otherwalls, floors, or roofs.

Loading The weight of building materials orobjects in a building.

Mortar Mixture of sand, lime, and portlandcement used as a bonding material in masonryconstruction.

Occupancy Classifications The use for which abuilding or structure is designed.

Parapet The projection of a wall above theroofline of a building.

Platform Framing A style of wood frame con-struction in which each story is built on a plat-form, providing fire stopping at each level.

Purlins A series of wood beams placed perpendi-cular to steel trusses to help support roofdecking.

Rafter A wood joist that is attached to a ridgeboard to help form a peak.

Shear A force that tends to tear a material bycausing its molecules to slide past each other.

Simple Beam A beam supported at the two pointsnear its end.

Spalling Deterioration of concrete by the loss ofsurface material due to the expansion of mois-ture when exposed to heat.

Surface-to-Mass Ratio Exposed exterior surfacearea of a material divided by its weight.

Tension A force that pulls materials apart.Torsion Load A load parallel to the cross section

of the supporting member that does not passthrough the long axis. A torsion load tries to“twist” a structural element.

Truss A rigid framework using the triangle as itsbasic shape to emulate a beam.

Type I, Fire-Resistive Construction Type inwhich the structural members, including walls,columns, beams, girders, trusses, arches,floors, and roofs, are of approved noncom-bustible or limited combustible materials withsufficient fire-resistive rating to withstand theeffects of fire and prevent its spread from storyto story.

Type II, Noncombustible Construction Typenot qualifying as Type I construction, in whichthe structural members, including walls,columns, beams, girders, trusses, arches,floors, and roofs, are of approved noncom-bustible or limited combustible materials withsufficient fire-resistive rating to withstand theeffects of fire and prevent its spread from storyto story.

Type III, Ordinary Construction Type in whichthe exterior walls and structural members thatare portions of exterior walls are of approvednoncombustible or limited combustible materi-als, and interior structural members, includingwall, columns, beams, girders, trusses, arches,floors, and roofs, are entirely or partially ofwood of smaller dimension than required forType IV construction or of approved noncom-bustible or limited combustible materials.

Type IV, Heavy Timber Construction Type inwhich exterior and interior walls and structuralmembers that are portions of such walls are ofapproved noncombustible or limited com-bustible materials. Other interior structuralmembers, including columns, beams, girders,trusses, arches, floors, and roofs, shall be ofsolid or laminated wood without concealedspaces.

Type V, Wood Frame Construction Type inwhich the exterior walls, bearing walls,columns, beams, girders, trusses, arches,floors, and roofs are entirely or partially ofwood or other approved combustible materialsmaller than the material required for Type IVconstruction.

Undesigned Load A load not planned for oranticipated.

Veneer A covering or facing, not a load-bearingwall, usually with brick or stone.

Web The portion of a truss or I beam that con-nects the top chord with the bottom chord.

368 ■ CHAPTER 13

Brannigan, Francis, Building Constructionfor the Fire Service, 3rd ed. NationalFire Protection Association, Quincy,MA, 1992.

“High Rise Office Building Fire, OneMeridian Plaza, Philadelphia, PA,”Technical Report Series, Report #049,U.S. Fire Administration,Washington, DC.

“Without Warning, A Report on the HotelVendome Fire,” Boston SparksAssociation.

Building Construction for Fire SuppressionForces—Wood & Ordinary. NationalFire Academy, National EmergencyTraining Center, Emmittsburg, MD,1986.

Angle, James; Harlow, David; Gala,Michael; Maciuba, Craig; andLombardo, Williams; FirefightingStrategies and Tactics. Delmar Learning,a part of the Thomson Corporation,Clifton Park, NY, 2001.

Diamantes, David, Fire Prevention:Inspection & Code Enforcement, 2nd ed.Delmar Learning, a part of the ThomsonCorporation, Clifton Park, NY, 2003.

Additional Resources

1. What are three ways loads are imposed onmaterials?

2. List the three types of forces created when loadsare imposed on materials.

3. Name three kinds of beams.4. Explain the effects of fire on steel structural

elements.5. How does a masonry wall achieve strength?6. List and define the five common types of build-

ing construction. Give an example of each typethat is located in your district or response area.

7. List the three parts of a truss and explain whatforces are being applied to each.

8. List three buildings in your district or responsearea that have truss construction.

9. List how fire affects the four more commonbuilding materials in use today.

10. Diagram and label four different roof shapes.11. List and describe eight conditions or observa-

tions that might indicate potential structuralcollapse.

REVIEW QUESTIONS

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