Brannigan's Building Construction-CH.2_split_1.pdf

CHAPTER2 Concepts of Construction OBJECTIVES At the conclusion of this chapter, you will be able to: Identify the variety of forces to which buildings are subjected. Identify the different types of loads, and explain how they are applie...

CHAPTER2 Concepts of Construction OBJECTIVES At the conclusion of this chapter, you will be able to: Identify the variety of forces to which buildings are subjected. Identify the different types of loads, and explain how they are applied to buildings. Define safety factor, and explain how it affects the choice of materials used in a building assembly. Identify composite materials, and explain how the combination of two different materials affects structural elements. Identify and define key types of structural members, such as beams, trusses, columns, walls, roofs, and arches. Identify the various types of structural connections, and explain how they affect transmission of loads and how they behave in a fire. Case Study Roof collapse is a constant danger for fire fighters. On March 10, 1941, in Brockton, Massachusetts, fire fighters responded to a report of a fire at the Strand Theatre (FIGURE 2-1). This structure, of ordinary construction, with brick load-bearing walls and wood joists, had seating for more than 1600 people, including a balcony that spanned the width of the building. To cover this large distance between the exterior walls (and to avoid columns in the balcony itself), the wood roof joists were supported by long-span unprotected steel trusses. A plaster and wire lath ceiling was installed under the trusses. FIGURE 2-1 The Strand Theatre collapse in Brockton, Massachusetts, March 10, 1941. Courtesy of the Stanley A. Bauman Photograph Collection at Stonehill College. The fire began in the basement and quickly ran up the wall void spaces and vertical ductwork to the roof. Fire fighters ascended to the balcony to attack the roof fire; even though fire was breaking through the roof on the outside, little fire was visible on the inside. The roof suddenly collapsed onto the balcony, trapping fire fighters between the roof and the balcony seats. A portion of the balcony itself then collapsed. A total of 13 fire fighters died. Subsequent investigations revealed that the steel trusses, heated by the burning wood joists, had twisted and likely elongated, dropping their load. A moderate snow load on the roof was discounted as causing the collapse. This incident raised serious questions about the relationship between construction and firefighting operations, which continue to plague modern- day fire fighters: 1. Is it possible that fire conditions can appear different on the outside of a building than they appear inside a building? 2. How does the span of a truss relate to load-carrying capacity? 3. When a truss fails, how much area is involved? Introduction Building construction has evolved over time. Although the overall rationale for a structure—to provide shelter—has remained the same, the way in which buildings are built has changed as new technologies and techniques emerge. From a firefighting perspective, it is important to realize that the buildings you respond to in your community are an aging collection of different types of buildings, incorporating myriad construction methods, materials, and design concepts, along with the hazards associated with each one. In essence, you must become the local “historian of building construction” to understand how the buildings you protect—and the people in them—will behave under fire conditions. The laws of physics have not changed, however. As a fire fighter, you must have a working knowledge of how the forces of nature and humans cause buildings to collapse. Anticipating the reaction of a particular building to these forces is essential to your safety. An understanding of the basic concepts of construction in addition to your fire-ground experience is critical. From experience, some fire officers have learned to make useful, but limited, judgments regarding the loss of structural stability in a fire. Many of the commonly taught indicators, such as sagging floors and strange noises, may be cases of “too little, too late.” In addition, experience with one type of building element, such as a solid sawn wood joist, is not valid for a truss or wooden I-beam. Relying on experience alone is not sufficient for today’s fire fighters; they must be aware of the theories and principles involved in building construction. The greatest enemy of all fire fighters is gravity. Gravity The force of gravity is the eternal enemy of every building every hour of every day. A building may appear to be only sitting on its foundation, but, in fact, it is under great stresses, although these may not be apparent as you look at the structure. By one means or another, in assembling a structure, the builder has defied gravity. The gravity resistance system in a building consists of all the structural elements and the connections that support and transfer the loads—that is, the forces attributable to gravity and other sources of stress on the structure. Fire weakens and destroys the structural elements and/or the connections in a building and places loads on structural elements that cannot handle them, which can cause a collapse. Once the collapse starts, the results are unpredictable. Some buildings are designed to be fire resistive. This means that to some degree they will resist fire-caused collapse. The majority of buildings are non-fire-resistive, and thus may easily collapse as a result of assault by a significant fire on the gravity resistance system. Compression, Tension, Torsion, and Shear Forces Generally, four types of forces can be applied to a structural member: compression, tension, torsion, and shear (FIGURE 2- 2). In essence, a compressive force occurs when the force squeezes a structural member, such as a concrete column supporting a floor. Tensile forces stretch a member such as a steel cable that is supporting a suspended walkway. A torsional force is a twisting force such as that created by turning a screwdriver on a screw; it is the result of torque (the measurable turning force applied to a structural member). Shear forces occur within a building member when opposing forces pull the member in opposite directions and are often the result of wind load (FIGURE 2-3). FIGURE 2-2 The four types of forces. Description FIGURE 2-3 The design of a suspension bridge anticipates three separate forces: compression, torsion, and tension. © Brandon/Shutterstock. Stress and Strain Stress and strain are the result of forces applied to a structural member. In the common vernacular, stress and strain are considered synonyms; however, a clear understanding of the technical definitions of these terms is critical for fire fighters. An external force that acts on a structure is called a load (discussed next). The internal forces that resist the load are called stress and strain. Stress is the force per unit area that produces a deformation. It is usually measured in pounds per square inch (psi), although occasional references to pounds per square foot (psf) are also found. The unit area measurement is at the discretion of the person making the calculations. Always be careful to note the unit area when examining any calculations. KIP, a term meaning 1000 pounds, is used in engineering calculations where the number would be so large as to be unwieldy. Strain, in contrast, is the actual percentage of elongation (deformation) that occurs when a material is stressed. It is measured in fractions of an inch of deformation per inch of original length of the material. Loads A load in a building works to destroy the gravity resistance system of the structure. According to NFPA 5000: Building Construction and Safety Code, loads are “forces or other actions that result from the weight of all building materials, occupants and their possessions, environmental effects, differential movement, and restrained dimensional changes.” Specific terms are used to describe the various types of loads and the way they are placed on a building. It is important to understand and use these terms correctly. Dictionary definitions are not always identical to the definitions used in the construction field. In addition, the various terms are not always mutually exclusive. For example, a load may be a live load and an impact load at the same time. Dead Loads Dead load is the weight of the building itself and any equipment permanently attached to it or built in. A more accurate term is self- weight. Years ago, dead load was often piled on the building without regard for the consequences. The modern designer clearly understands that dead weight breeds dead weight. Consequently, efforts are now made to lighten all parts of a building. Removing a pound of dead weight at the top of a building enables the builder to reduce ounces at many points in the supporting structure. As proof of the criticality of this consideration, advertisements for building materials such as gypsum shaft liners now emphasize their weight saving over the traditional masonry enclosures. Today, buildings can be considered as being bought “by the pound.” Fire resistance is closely related to mass. All other things being equal, a heavier steel beam will take longer to fail due to heating than a lighter beam. A 4 by 10 in. (102 by 254 mm) wooden beam will support its load longer while burning than will a 2 by 10 in. (51 by 254 mm) beam. This point highlights a valid general principle: Any substitute structural element that is of less mass than the element previously used to carry an equivalent load is inherently less fire resistant. Added Dead Loads A structure’s dead load is often increased during alteration of a building. For example, the addition of air conditioners to the roof of a building previously without air conditioning may mean that its dead load is increased without any strengthening of the structure (FIGURE 2-4). At one fire in a large city, fire fighters were overhauling a fire in a restaurant, seeking out hidden pockets of fire in the overhead. They attacked the fire-weakened supports of the building. Above them, added-on air conditioning units fell, causing several fatalities. FIGURE 2-4 What will be the result of a fire burning underneath this roof with its dead loads? © Donald King/AP Images. Even when a structure is strengthened to cope with the added load, the modifications may not prove adequate to avoid collapse in a fire (FIGURE 2-5). For instance, if a steel lightweight truss (also known as a bar joist) is strengthened by additional trusses, resistance from fire collapse does not improve because all the trusses will burn and lose strength at the same time. FIGURE 2-5 Note the close spacing of the lightweight steel trusses on either side of an air conditioning unit, supporting the unit on the roof.  Courtesy of Glenn Corbett. In one case, a supermarket was converted to a Japanese restaurant with many grills. Each grill required a heavy fume hood. These hoods were hung from the roof. When the building was a supermarket, the roof did not have to support such a dead load. Now the same roof must carry the hoods—and safety has been greatly reduced. Dead loads can be deadly—especially dead loads for which the building has not been designed. It is critical that your local building department be notified and that a structural engineer assess the situation when you suspect a dead load is a catastrophe waiting to happen or you see dead loads on a building. Live Loads Live loads are any loads other than dead loads. An elevated water tank, for example, is a dead load (the tank itself is a dead load). The water in the tank may be there for years—but it is still a live load. A concrete vault is a dead load. A movable safe, no matter how heavy, is a live load. The differences between these types of loads are not just picky distinctions; they have the following practical implications for firefighting operations: Dead loads can be accurately calculated. Live loads are indeterminate. The live load must be estimated based on the projected use of the building and such variables as snow, wind, or rain. All buildings are designed with a given use in mind. The building code, in turn, specifies the minimum live-floor load design for specific types of buildings. Typical building code minimum design load requirements for modern buildings are spelled out in NFPA 5000: Building Construction and Safety Code (TABLE 2-1). The uniform live load column refers to loads distributed over the entire area (in psf), while the concentrated load applies to specific large objects. An office building floor, for example, must be able to sustain a load of at least 50 psf (2.4 kN/m2) over the entire floor but also have the ability to sustain a load of 2000 lb (8.9 kN) at a specific location on the floor for individual or collected heavy objects like a heavy safe. Note that these are minimum design loads—if you were to find loading in areas of actual buildings that exceed these limits, it is best to contact your buildings department and advise it of the potentially overloaded conditions. TABLE 2-1 Live Loads Concentrate Uniform d Occupancy or Use psf (kN/m2) lb (kN) Apartments (see residential) Access floor systems Office use 50 (2.4) 2000 (8.9) Computer use 100 (4.79) 2000 (8.9) Armories and drill rooms 150 (7.18)a Assembly areas and theaters Fixed seats (fastened to floors) 60 (2.87)a Lobbies 100 (4.79)a Movable seats Platforms (assembly) 100 (4.79)a Stage floors 100 (4.79)a 150 (7.18)a Balconies and decks 1.5 times the live load for the occupancy served. Not  Concentrate Uniform d Occupancy or Use psf (kN/m2) lb (kN) required to exceed 100 psf (4.79 kN/m2) Catwalks for maintenance access 40 (1.92) 300 (1.33) Corridors 100 (4.79) First floor Other floors, same as occupancy served, except as indicated Dining rooms and restaurants 100 (4.79)a Dwellings (see residential) Elevator machine room grating (on 300 (1.33) area of 2 by 2 in. [50 by 50 mm]) Finish light floor plate construction 200 (0.89) (on area of 1 by 1 in. [25 by 25 mm]) Fire escapes 100 (4.79)a On single-family dwellings only 40 (1.92) Fixed ladders See Section 4.5 [of ASCE/SEI 7] Garages 40 (1.92)a, b, c Passenger vehicles only Trucks and buses Handrails, guardrails, and grab bars See Section 4.5 [of ASCE/SEI 7] Helipads e, f, g 60 (2.87)d, e Nonreducible  Concentrate Uniform d Occupancy or Use psf (kN/m2) lb (kN) Hospitals Operating rooms, laboratories 60 (2.87) 1000 (4.45) Patient rooms 40 (1.92) 1000 (4.45) Corridors above first floor 80 (3.83) 1000 (4.45) Hotels (see residential) Libraries Reading rooms 60 (2.87) 1000 (4.45) Stack rooms 1000 (4.45) 150 (7.18)a, h Corridors above first floor 1000 (4.45) 80 (3.83) Manufacturing Light 2000 (8.90) 125 (6.00)a Heavy 3000 (13.40) 250 (11.97)a Office buildings File and computer rooms shall be designed for heavier loads based on anticipated occupancy Lobbies and first floor corridors Offices Corridors above first floor 100 (4.79) 2000 (8.90) 50 (2.40) 2000 (8.90) 80 (3.83) 2000 (8.90) Penal institutions Cell blocks 40 (1.92) Corridors 100 (4.79)  Concentrate Uniform d Occupancy or Use psf (kN/m2) lb (kN) Recreational uses 75 (3.59)a Bowling alleys, poolrooms, and 100 (4.79)a similar uses Dance halls and ballrooms 100 (4.79)a Gymnasiums 100 (4.79)a, i Reviewing stands, grandstands, 60 (2.87)a, i and bleachers Stadiums and arenas with fixed seats (fastened to the floor) Residential One- and two-family dwellings Uninhabitable attics without storage 10 (0.48) Uninhabitable attics with 20 (0.96) storage 30 (1.44) Habitable attics and sleeping 40 (1.92) areas All other areas, except stairs 40 (1.92) All other residential 100 (4.79) occupancies Private rooms and corridors serving them Public roomsa and corridors serving them  Concentrate Uniform d Occupancy or Use psf (kN/m2) lb (kN) Roofs 20 (0.96) Ordinary flat, pitched, and 100 (4.79) curved roofs Roofs used for roof gardens Same as occupancy Roofs used for assembly served 300 (1.33) purposes applied to Roofs used for other skeleton 5 (0.24) structure occupanciesm nonreducible 200 (0.89) Awnings and canopies 5 (0.24) applied to Fabric construction supporting roof supported by a skeleton nonreducible and applied to frame members structure only the roof frame Screen enclosure support frame members only, not the screen 20 (0.96) All other construction 2000 (8.9) Primary roof members, exposed to a work floor Single panel point of lower chord of roof trusses or any point along primary 300 (1.33) structural members 300 (1.33) supporting roofs over manufacturing, storage warehouses, and repair garages All other primary roof members All roof surfaces subject to maintenance workers  Concentrate Uniform d Occupancy or Use psf (kN/m2) lb (kN) Schools 40 (1.92) 1000 (4.45) Classrooms 80 (3.83) 1000 (4.45) Corridors above first floor 100 (4.79) 1000 (4.45) First-floor corridors Scuttles, skylight ribs, accessible 200 (0.89) ceilings Sidewalks, vehicular driveways, and 250 (11.97)a, n 8000 (35.60)o yards subject to trucking Stairs and exit ways 100 (4.79) 300p One- and two-family dwellings 40 (1.92) 300p only Storage areas above ceilings 20 (0.96) Storage warehouses (shall be designed for heavier loads if required for anticipated storage) 125 (6.00)a Light 250 (11.97)a Heavy Stores 1000 (4.45) Retail 100 (4.79) 1000 (4.45) First floor 75 (3.59) 1000 (4.45) Upper floors 125 (6.00)a Wholesale, all floors Vehicle barriers See Section 4.5 [of ASCE/SEI 7] Walkways and elevated platforms 60 (2.87) (other than exit ways) Yards and terraces, pedestrian 100 (4.79)a Reprinted from: NFPA 5000, Building Construction and Safety Code, 2018. Quincy, MA: National Fire Protection Agency, Table A.35.6.1.2: Minimum Uniformly Distributed Live Loads, LL, and Minimum Concentrated Live Loads. Reprinted with permission from the American Society of Civil Engineers/Structural Engineering Institute (ASCE/SEI) standard 7. a Live load reduction for this use is not permitted by Section 4.7 [of ASCE SEI 7] unless specific exceptions apply. b Floors in garages or portions of buildings used for the storage of motor vehicles shall be designed for the uniformly distributed live loads of [Table A.35.6.1.2] or the following concentrated load: (1) for garages restricted to passenger vehicles accommodating not more than nine passengers, 3000 lb (13.35 kN) acting on an area of 4.5 by 4.5 in. (114 by 114 mm); and (2) for mechanical parking structures without slab or deck that are used for storing passenger vehicles only, 2250 lb (10 kN) per wheel. c Design for trucks and buses shall be per AASHTO LRFD Bridge Design Specifications; however, provisions for fatigue and dynamic load allowance are not required to be applied. d Uniform load shall be 40 psf (1.9 kN/m2) where the design basis helicopter has a maximum take-off weight of 3000 lb (13.6 kN) or less. This load shall not be reduced. e Labeling of helicopter capacity shall be as required by the authority having jurisdiction. f Two single concentrated loads, 8 ft (2.4 m) apart, shall be applied on the landing area (representing the helicopter’s two main landing gear, whether skid type or wheeled type), each having a magnitude of 0.75 times the maximum take-off weight of the helicopter and located to produce the maximum load effect on the structural elements under consideration. The concentrated loads shall be applied over an area of 8 by 8 in. (200 by 200 mm) and shall not be concurrent with other uniform or concentrated live loads. g A single concentrated load of 3000 lb (13.35 kN) shall be applied over an area 4.5 by 4.5 in. (114 mm by 114 mm), located so as to produce the maximum load effects on the structural elements under consideration. The concentrated load need not be assumed to act concurrently with other uniform or concentrated live loads. h The loading applies to stack room floors that support nonmobile, double- faced library book stacks, subject to the following limitations: (1) the  nominal book stack unit height shall not exceed 90 in. (2290 mm); (2) the nominal shelf depth shall not exceed 12 in. (305 mm) for each face; and (3) parallel rows of double-faced book stacks shall be separated by aisles not less than 36 in. (914 mm) wide. i In addition to the vertical live loads, the design shall include horizontal swaying forces applied to each row of the seats as follows: 24 lb per linear ft of seat applied in a direction parallel to each row of seats and 10 lb per linear ft of seat applied in a direction perpendicular to each row of seats. The parallel and perpendicular horizontal swaying forces need not be applied simultaneously. j Uninhabitable attic areas without storage are those where the maximum clear height between the joist and rafter is less than 42 in. (1067 mm), or where there are not two or more adjacent trusses with web configurations capable of accommodating an assumed rectangle 42 in. (1067 mm) in height by 24 in. (610 mm) in width, or greater, within the plane of the trusses. This live load need not be assumed to act concurrently with any other live load requirement. k Uninhabitable attic areas without storage are those where the maximum clear height between the joist and rafter is 42 in. (1067 mm) or greater, or where there are two or more adjacent trusses with web configurations capable of accommodating an assumed rectangle 42 in. (1067 mm) in height by 24 in. (610 mm) in width, or greater, within the plane of the trusses. At the trusses, the live load need only be applied to those portions of the bottom chords where both of the following conditions are met: i. The attic area is accessible from an opening not less than 20 in. (508 mm) in width by 30 in. (762 mm) in length that is located where the clear height in the attic is a minimum of 30 in. (762 mm); and ii. The slope of the truss bottom chord is no greater than 2 units vertical to 12 units horizontal (9.5 percent slope). The remaining portions of the bottom chords shall be designed for a uniformly distributed nonconcurrent live load of not less than 10 lb/ft2 (0.48 kN/m3). l Where uniform roof live loads are reduced to less than 20 lb/ft2 (0.96 kN/m2) in accordance with Section 4.8.1 [of ASCE/SEI 7] and are applied to the design of structural members arranged so as to create continuity, the reduced roof live load shall be applied to adjacent spans or to alternate spans, whichever produces the greatest unfavorable load effect. m Roofs used for other occupancies shall be designed for appropriate loads as approved by the authority having jurisdiction. n Other uniform loads in accordance with an approved method, which contains provisions for truck loadings, shall also be considered where appropriate. o The concentrated wheel load shall be applied on an area of 4.5 by 4.5 in. (114 by 114 mm). p Minimum concentrated load on stair treads [on an area of 2 by 2 in. (50 by 50 mm)] is to be applied noncurrent with the uniform load. Added Live Loads The water trapped in a building or on a roof can be a significant added live load. Flat-roof buildings, for example, often are built with little reserve strength, and the added weight of water—such as that delivered to the roof either from natural events, like rain or snow storms, or as part of a firefighting operation—could lead to their collapse (FIGURE 2-6). Similarly, an elevated water tank is designed for a certain live load—the water in the tank. If the tank overflows during freezing weather, the escaping water will form an ice coating on the structure—and the added load could overload it to the point of collapse.  FIGURE 2-6 Snow pushed by plows to the corner of this parking garage caused the deck to collapse. Courtesy of Glenn Corbett. In the past, both rainwater and snow have been the cause of many roof collapses. One of the most notable early collapses due to snow involved the Washington, D.C., Knickerbocker Theater in 1922; its roof caved in after a 28-in. (71-cm) snowfall (FIGURE 2-7). A total of 98 people were killed during a movie at the theater, including a congressman, in what is still known today as the Knickerbocker storm. Subsequent investigations pointed to an underdesigned steel truss and wall anchorage problems. FIGURE 2-7 The Knickerbocker Theater roof collapsed in 1922 after a 28-in. (71-cm) snowfall. Courtesy of the Library of Congress. In 1978, the spaceframe roof (characterized by three- dimensional pyramid-like trusses) of the Hartford Civic Center in Hartford, Connecticut, collapsed under a heavy snow load. This collapse was blamed on a variety of factors, including an underestimation of the dead load of the truss frame itself and inadequate bracing within the spaceframe. As both the Knickerbocker Theater and Hartford Civic Center collapses illustrate, structural frame design flaws may not be immediately obvious; indeed, they may not become apparent until a catastrophic failure occurs. Be wary when you see heavy rainwater and snow loads on roofs—the structures may be moments away from collapse. When the use of a building changes (i.e., in “recycled buildings”), the design of the building should be reviewed to determine whether the structure should be structurally strengthened for its new use and whether its fire safety features should be upgraded. (This topic is discussed in detail in Chapter 4, Building and Fire Codes.) The need to update the building’s infrastructure is often ignored, however, at times owing to pressure to eliminate bureaucratic obstructions or to save a historic building from wreckers. As mentioned earlier, live loads can also be added to a building by firefighting operations at the scene of a fire. In one example, water thrown on a theater marquee in freezing weather turned to ice and collapsed the marquee, trapping several fire fighters. Fortunately, one fire fighter had access to a construction crane and the marquee was lifted. The use of heavy-caliber streams adds a tremendous amount of weight to a building. Water weighs 8.34 lb (3.8 kg) per gallon, so a 1000-gallon-per-minute (gpm; 3785 lpm) master stream will potentially add over 4 tons (3632 kg) of weight to the building in just 1 minute (FIGURE 2-8). Be aware of the amount of water runoff: How much water is remaining and adding to an already fire- compromised structure? FIGURE 2-8 A lower-level townhouse floor collapsed into a crawl space under the building after water from ladder stream collected and overloaded the floor. The arrow on the wall indicates the level of water collecting on the floor prior to collapse, about 2 feet deep. Courtesy of Glenn Corbett. Impact Loads Impact loads are loads that are delivered in a short time. A load that the structure might resist, if delivered as a static load over time, may cause collapse if delivered as an impact load. The jetliner attacks on the twin towers of New York’s World Trade Center on September 11, 2001, are examples of a tremendous impact load. Buildings have a habit of standing as long as they are not disturbed. However, fire can release different kinds of impact loads on a building. In the case of a fire, many types of interrelationships among building elements, which have mutually supported one another and provided strength not even calculated by the designer, are disturbed. A fire can cause innumerable undesigned changes in loading, many of which can cause collapse. An explosion; the overturning of heavy live loads, such as a big safe; the collapse of heavy, nonstructural, ornamental masonry; or even the weight of a fire fighter jumping onto a roof may be enough of an impact load to cause collapse. There is no such thing as a no impact load. No matter how gingerly personnel and equipment are placed on a structure, there is a substantial increase in the stress on that structure, at least momentarily. In any event, assaults on a structure already under attack by fire are dangerous. Buildings may also experience impact loads unrelated to fires. Buildings have been hit by vehicles as well as by falling trees, towers, and other buildings (FIGURE 2-9). Most buildings are not designed for these kinds of assaults, and structural failure may occur as a result. Fire fighters must be kept clear of such a building in anticipation of a possible collapse as they extinguish the fire. FIGURE 2-9 A tractor trailer ran off a highway, hit an office building (with an exposed structural frame in a parking area on the first floor), and burst into flames. Courtesy of Glenn Corbett. Lateral Impact Loads Lateral impact loads can produce disastrously high stresses. A lateral impact load, such as from an explosion, can be delivered from a direction that has little or no stress resistance. Some walls are designed as load bearing—to accept vertical compressive loads. However, even impressive masonry walls are not necessarily resistant to undesigned lateral loads. The lateral thrust of collapsing floors has brought down many brick walls. The sudden ignition of trapped carbon monoxide or other fire- generated combustible gases has sometimes caused detonations that have destroyed substantial masonry walls. In 1968, for example, a cooking-gas explosion caused the collapse of a 22-story concrete-panel apartment house in England—the so-called Ronan Point collapse. In this incident, the undesigned lateral impact load from the explosion blew out one bearing wall of the structure. The floor it was supporting then fell and became an impact load on the floor below, and the successive collapse of the underlying floors followed. In the wake of this event, gas service in the Ronan Point building was replaced with electricity, and repairs were made to other units. In 1984, however, tenants were moved out of eight other buildings in the same group because of a fear of collapse. The repairs after the first collapse were apparently inadequate. The safety factor built into ordinary buildings is rarely large enough to assure that there will not be progressive collapse in the wake of the first excessive impact load. Progressive collapse is a particular hazard in the construction of concrete-frame buildings. Backdraft explosions can occur when the combination of products of combustion, including a mixture of carbon monoxide and oxygen, is exactly right, and they can blow the building apart. Impact Loads Due to Terrorism While terrorist attacks using explosives have occurred for many years, the past few decades have seen an increase in both their frequency and public awareness. This increase has led to research efforts in an attempt to better fortify structures from such attacks. Locating buildings farther away from the street (i.e., increasing the setback) and placing bollards (posts) and barriers in front of terrorist targets are two types of modifications that use distance as a means to minimize the effects of a bomb exploded near a building (FIGURE 2-10). Structural engineers are now routinely hired to design blast resistance into high-profile buildings. FIGURE 2-10 The new Freedom Tower at the World Trade Center site in New York City uses setback distance and a reinforced concrete enclosed core for  protection against a terrorist bomb attack. © Jay Lazarin/iStockphoto. Broken and flying glazing (such as window glass and decorative glass) from an exploding bomb may kill and injure many people. Specially designed blast-resistant security windows were in the process of being installed in the Pentagon on September 11, 2001; the area of the building hit by the terrorist-hijacked plane had been retrofitted with these features. The windows in the immediate area of the impact remained intact—a factor that likely saved many lives. Certain security-type windows present significant challenges for forcible entry and escape in a fire incident. Glazing and window materials are covered in greater depth in Chapter 3, Methods and Materials of Construction, Renovation, and Demolition. Static and Repeated Loads Static loads are loads that are applied slowly and remain constant. A heavy safe is an example of a live, static load; it is not a dead load. A repeated load is a load that is applied intermittently. A rolling bridge crane in an industrial plant applies repeated loads to the columns as it passes over them. Wind Loads Wind load is the force applied to a building by the wind. The designer can counter this force in a variety of ways. A shear wall is installed in a building to resist lateral loads from wind and earthquakes. In a small wood-frame structure, the plywood sheathing over the studs provides the needed shear strength. Larger structures also use shear walls constructed of steel, concrete, or reinforced concrete. The goal is to provide a stiff and strong enclosure. Two closely related structural frames, the braced frame and the moment frame, are also used in buildings to resist lateral wind and earthquake loads (and, more recently, the lateral blast loads from a terrorist’s bomb). A braced frame uses diagonal members for bracing purposes (FIGURE 2-11), while moment frames use special moment connections between columns and beams that resist rotation. FIGURE 2-11 Diagonal bracing in a steel-frame high-rise. Courtesy of Glenn Corbett.  The term moment refers to a force causing rotation of a structural member around a fixed point of connection, where the force is applied some distance from the fixed point. An analogous example would be the force you apply to the edge of a book to actually open the book—the force you apply causes the book’s cover to rotate at the book’s spine and open to the first page. The force you applied multiplied by the distance between the location of the force to the point of rotation (cover edge to spine) gives the moment. Wood-frame homes built in coastal areas subject to hurricane- force winds demand more than simply wood-panel sheathing. Originally, small wood-frame buildings in these areas were built with only a gravity connection to the foundation. Today, hurricane bracing in wood-frame dwellings, using strategically placed metal straps to hold roofs in place and bolts to hold the frame to the foundation, has found great acceptance since Hurricane Andrew devastated Florida in 1992 (FIGURE 2-12).  FIGURE 2-12 The straps are designed to counter the uplift forces on roof rafters during a hurricane. Courtesy of IIBHS – The Insurance Institute for Business & Home Safety. Low-rise bearing wall buildings made of masonry usually have enough mass in the walls that special consideration of wind load is unnecessary. High-rise buildings, in contrast, must be reinforced against the substantial wind forces that can occur at greater heights, often with bracing as described previously. Some arrangements of braces between columns resemble the letter “K” and, therefore, are called K-bracing. In old buildings and railroad bridges, heavy riveting of girders to columns from top to bottom of the frame is called portal bracing.  The framing of lightweight, unprotected, noncombustible steel buildings, however, is tied together to resist wind forces. Fire-caused failure of one part may cause the collapse of other sections when unanticipated torsional loads, which the building design does not take into account, are transmitted through the ties. Most floors simply carry loads to the support system of columns and floors. A diaphragm floor is designed to stiffen the building against wind and other lateral loads such as earthquakes. Masonry walls, which are needed to enclose vertical shafts, can also perform as shear walls, resisting overturning during high winds. This method of construction is satisfactory only for low- or medium- rise buildings. If used in a megastructure, masonry walls would require shear walls, with no other purpose than wind resistance, to be erected above the floors where certain elevators terminate. Very tall high-rise buildings are built to take the wind load on the exterior walls rather than on the interior core. Sometimes diagonal columns are visible on the exterior. Externally braced buildings are known as tube construction, as contrasted with core construction (FIGURE 2-13). The unique giant Vierendeel truss (rectangular truss with very rigid corner bracing) is formed by exterior box columns and spandrels. They provided the wind bracing of New York’s World Trade Center and allowed the buildings to carry the vertical loads around the openings created by the planes after impact. FIGURE 2-13 Chicago’s Willis Tower is a high-rise of tube construction. © Medioimages/Photodisc/Getty Images.  The wind load can be particularly hazardous to fire fighters when operating in or near buildings under construction. This is because the full benefit of interconnecting all of the parts of the completed building has not yet been realized. Temporary bracing may be inadequate. In addition to its effect on structures, the wind is a force to be reckoned with in the movement of smoke in buildings. This force is often ignored by some whose solution to the high-rise fire and life safety problem rests first on sophisticated, state-of-the-art smoke- control systems. Concentrated Loads Concentrated loads are heavy loads located at one point in a building (FIGURE 2-14). A steel beam resting on a masonry wall is an example of a concentrated dead load. A safe is a concentrated live load. FIGURE 2-14 Some buildings are designed to hold the additional weight of a concentrated load. However, some objects are placed without the addition of structural supports. Courtesy of the estate of Francis L. Brannigan. In a typical concrete block wall, the building designer may insert solid block, brick, reinforced concrete, or a wall column to stiffen the wall and carry the weight of the concentrated load. If a wall is being breached and the structure is found to be stronger than normal, choose another location. You are probably right under a concentrated load. Years ago, floors that were considered fireproof (of course, there is no such thing as fireproof) were made of brick segmental arches. Many still exist. The wooden floor was leveled in some cases by supporting it on small piers (short columns). At some locations, there was a gap of several inches between the floor and the arch. When such a floor burned, a live load such as a safe would fall the few inches. The impact often proved too strong for the arch, which had carried the safe for years, and the safe wound up in the basement. To this day, the testing and rating of safes involve dropping the heated safe a considerable distance. Existing examples of almost every feature or practice of building construction can be found throughout the United States and Canada. Although you may not have every feature or problem in your area, you should be aware of them, even though you concentrate on those common to your area. Imposition of Loads Loads are also classified according to the orientation in which they are placed on the structure. They can be classified as axial or eccentric. An axial load is a force that passes through the centroid of the section under consideration. An axial load is perpendicular to the plane of the section. To simplify, an axial load is straight and true; the load is evenly applied to the bearing structure. All other conditions being equal, a structure will sustain its greatest load when the load is axial (FIGURE 2-15). FIGURE 2-15 Loads are applied evenly in the case of this axially loaded column.  Courtesy of Glenn Corbett. Structural elements are not always loaded in the most efficient manner. Other considerations may be more important. For instance, a ladder is strongest when vertical, so that a person standing on the ladder represents an axial load. But human nature and mechanics work against axial loading and require slanting the ladder. An eccentric load is a force that is perpendicular to the plane of the section but does not pass through the center of the section, thus bending the supporting members. The load is straight and true but is concentrated to one side of the center of the supporting wall or column (FIGURE 2-16). FIGURE 2-16 A wood post (column) that is eccentrically loaded. The bracket (arrow) for additional support was added because the beam had shrunk away from the post. Courtesy of Glenn Corbett.  For an example of such a load, place a stack of books on the floor. Sit on the stack squarely. Note how evenly applied weight stabilizes the stack. Now shift your weight until it bears on only part of the stack. Note the tendency of the stack to bend outward. One side of the stack is being compressed, while the other is being pulled apart. These forces are called compression and tension. Fire Loads Some engineers do not recognize the term fire load or even consider the impact of a fire on a structure. Fire load represents the potential fuel available to a fire. The research work done on fire load was directed toward the contents of buildings. When the building is combustible, however, the building itself is part of the fire load. Fire load represents the total amount of potential energy (heat) in the fuel; the term heat release rate (HRR) (also expressed as ) indicates the rate of available energy released. This distinction is important. All wood can generate approximately the same total amount of heat per pound, but the rate at which fuels burn varies dramatically—that is, heat is released more rapidly from certain materials. For example, a 5-lb (2.3-kg) solid chunk of wood will burn more slowly than 5 lb (2.3 kg) of wood chips; a pound of plywood will burn many times faster than a pound of heavy timber. For many years, fire fighters would assess the fire loading of a building solely in terms of pounds of fuel per square foot (psf). To make this calculation, three pieces of information are needed: the weight of the combustible material, the caloric value for the material, and the floor area. Each combustible material has its own caloric value. The basic measurement of caloric value is the Btu (British thermal unit), the amount of heat required to raise a pound of water one degree Fahrenheit. The metric equivalent is the kilojoule (kJ). One Btu is approximately equal to 1 kJ. Two estimates for caloric values are commonly used. Wood, paper, and similar materials are estimated at 8000 Btu/lb. For plastics and combustible liquids, 16,000 Btu/lb is a common estimate, although the caloric value for some of these fuels is much higher. To measure the fire load in Btu/ft2, the weight of the fuel is multiplied by the caloric value and divided by the floor area. The equivalent metric statement is kJ/m2. In some cases, you may find fire load expressed in psf, a practice that dates back to when there was only one basic fuel (i.e., wood or paper), estimated at 8000 Btu/lb. Under this system, plastics were converted into “equivalent pounds” on the basis that 1 lb (0.5 kg) of plastics equals 2 lb (0.9 kg) of wood. A fire load of 80,000 Btu/ft2 or 10 lb of ordinary combustibles per ft2 is approximately the equivalent of a 1-hour exposure to the standard fire endurance test, American Society for Testing and Materials (ASTM) E-119, Standard Test Methods for Fire Tests of Building Construction and Materials (discussed in greater detail in subsequent chapters); 160,000 Btu/ft2 is equivalent to a 2-hour exposure! Although this fire load seems severe, it is more important to assess a fire in terms of its total HRR, or. is usually expressed in terms of watts (W), kilowatts (KW), or megawatts (MW). For example, a typical polyurethane sofa fire has a peak HRR of approximately 3120 KW (TABLE 2-2). It is this HRR that is a primary determinant of whether a compartment will reach flashover. Fire protection engineers also use HRR to describe the size of a fire (and resultant severity). To understand this relationship, readers are encouraged to read Chapter 5, Basic Fire Science, in NFPA 921: Guide for Fire and Explosion Investigations. TABLE 2-2 Representative Peak HRRs (Unconfined Burning) Reproduced with permission from NFPA 921-2011, Guide for Fire and Explosion Investigations, Copyright © 2011, National Fire Protection Association. This reprinted material is not the complete and official position of the NFPA on the referenced subject, which is represented only by the standard in its entirety. Suspended Loads Slender tensile members can carry a load that would require a compressive member of much greater size. This feature currently is being used in the design of many buildings. In many buildings, designers are eliminating interior columns at selected locations by hanging the ends of beams from the overhead structure—thereby creating a suspended load. A slender rod can replace a much larger column. When this is done, however, several fire problems develop. The tensile member, having less mass, has less fire resistance. In addition, the load cannot be delivered to the ground in tension; it must be converted into a compressive load. This requires one or more connections in the overhead area, where fire temperatures are often greater than on the floor. These connections may be hidden in the structure. In such a case, an attic fire might cause the collapse of the first floor. In old heavy timber buildings, overloaded beams are sometimes restored by inserting a tie-rod, which goes up through the building, generally to a truss or beam extending from wall to wall in the cockloft (the space between the top floor ceiling and roof of a building of ordinary construction). This may be well hidden. Look for the telltale end of the tie-rod about midpoint of the beam. Suspended High-Rise Buildings The concept of a suspended beam has been applied in the construction of some high-rise buildings. Columns are replaced with cables suspended from beams cantilevered out from the top of the central reinforced concrete core, which provides some economy of construction, unobstructed floor space, and an open plaza at the entry level to the building. Possible fire problems of this type of structure are discussed further in Chapter 12, Fire-Resistive Construction. Safety Factor The safety factor represents the ratio of the strength of the material just before failure to the safe working stress. The term safety factor is sometimes misunderstood. It is not practical to use a material in a structure so that it will be loaded to its ultimate strength, as shown in tests. The material used in a structure may not be as good as the sample tested, the workmanship may be inferior, or the material may deteriorate over the years. For these reasons, the design load is only a fraction of the tested strength of the material. If the design load is only one-tenth of the tested strength, the safety factor is 10; if the design load is half the tested strength, the factor of safety is 2. Steel, which is made under controlled conditions, has a safety factor of 2; masonry constructed in place might have a safety factor of 10. The less that is known about the characteristics of a material and its role in a building assembly, the greater the factor of safety required. The safety factor represents a measure of what is not known about the material. Lower safety factors made on the basis of knowledge of the material and its loading are not reductions in safety. In contrast, improper reductions in the safety factor that might survive normal loading may be significant under fire conditions, creating hazards for residents of the building and fire fighters alike. The safety factor of steel used in excavation bracing is half what would be permitted in a structure because it is temporary; in other words, the permitted load is doubled while the building is under construction. If a fire occurs in such an unfinished structure, the steel will fail much sooner because the temperature at which heated steel fails decreases as the load increases. Composite Materials At times, two materials are combined to take advantage of the best characteristics of each. For instance, concrete is a relatively inexpensive material that is strong in compression but weak in tension; steel is strong in both ways but is more costly than concrete. By providing steel at the locations where tensile stresses develop, a composite material, called reinforced concrete, is developed (FIGURE 2-17). FIGURE 2-17 With the addition of steel rebar, the concrete will withstand loads under tension. © Graham R Prentice/Shutterstock. All elements of a composite material must react together if there is to be no failure. If the materials separate, the composite no longer exists, and the two materials separately may be unable to carry the load successfully. Composite Structural Elements Two different materials may be combined in a structural element. Steel and concrete are combined in composite floors. In some cases, studs are welded to steel beams and then embedded in the concrete of the floor. This is done to produce a diaphragm floor, which stiffens the structure. In one serious high-rise building fire, heat caused shear connectors, which joined the beams to columns, to fail. The beams did not fall because the embedded studs kept the beams attached to the concrete. Composite concrete-steel floors can be constructed with bar-joist trusses. The top chord of the truss is set below the tops of the web, allowing triangles of steel to project upward. These are embedded in the concrete. A Flitch plate girder is made by sandwiching a piece of steel between two wooden beams. The girder is much stronger than a piece of wood of the same dimensions, yet it can be installed using ordinary carpentry techniques. A sheet of plywood may also be sandwiched between the two wooden beams. A brick and block composite wall, in which cheaper concrete block substitutes for brick where it will not be seen, is another common example of a composite structural element (FIGURE 2- 18). Do not confuse this type of wall with a brick-veneered concrete block wall, in which the brick and block are not structurally united. In older construction, hollow tile is found in this type of composite wall instead of concrete block.  FIGURE 2-18 This photo depicts a brick and block wall. © Remi Cauzid/Shutterstock. The term composite construction is sometimes used to describe buildings in which two different materials carry structural loads. Some concrete buildings, for instance, have a top floor or penthouse of lightweight steel. Structural Elements Buildings are made up of structural elements, such as beams, trusses, columns, arches, and walls, that differ in how they carry the load and transfer it to the next element. The principles that govern each element are the same regardless of the material. Elements (or members) that are assembled together into a structure are often called a structural frame. Beams The beam is probably the oldest structural member (FIGURE 2-19). It is not hard to imagine primitive humans dropping a tree across a stream to form the first bridge. A beam transmits forces in a direction perpendicular to such forces to the reaction points (points of support). Consider a load placed on a floor beam: The beam receives the load, turns it laterally, divides it, and delivers it to the reaction points. FIGURE 2-19 Beams are used to transfer loads laterally to two supporting ends. Description  The definition of a beam does not consider its attitude—that is, its vertical or horizontal orientation. Although beams are ordinarily thought of as horizontal members, this is not always the case. A vertical or diagonal member that performs the functions of a beam, although it may have another name such as a rafter, is structurally a beam. When a beam is loaded, it deflects or bends downward. The initial load is its own dead weight. The load placed on it is a superimposed load. Some beams are built with a slight camber, or upward rise, so that when the design load is superimposed, the beam will be more nearly horizontal. A carpenter selects wood floor beams accordingly. Deflection causes the top of a beam to shorten so that the top is in compression. The bottom of the beam elongates and thus is in tension. The line along which the length of the beam does not change is termed the neutral axis or plane. It is along this line that the material in the beam is doing the least work and material can be most safely removed. The cable is an ideal beam. Fully in tension, it makes the most economical use of the available material. Cable-supported roofs are being used for some large open-area structures. Nevertheless, for a beam to be safe and economical is not always enough; it must also deflect so little that the deflection will not be noticed. This is accomplished by using additional material or by rearranging the shape of the material. In other words, stiffness, or reduced deflection, can be achieved by material mass or by geometry. The economics of using geometry (e.g., truss shapes) over mass has had a tremendous effect on structures. Carrying Capacity and Depth of Beams The load-carrying capacity of a beam increases by the square of its depth. Consider a 2 by 4 in. (51 by 102 mm) wood beam carrying a certain load. If another 2 by 4 in. beam is laid alongside the first beam, the carrying capacity is increased by a factor of two. If the same amount of wood is sawn into a 2 by 8 in. (51 by 203 mm) beam, thus multiplying the depth by two, the carrying capacity is increased by the square of two—that is, by four. The principles governing beams are the same for all beams, regardless of the material. For example, for wooden beams, standard carpentry manuals provide a table that details safe longtime loads, uniformly distributed, for 1800-lb (816-kg ) structural grade lumber. For convenience in determining loads for beams of different thicknesses, the information is arranged by size from 1 by 4 in. to 1 by 16 in. (25 by 102 mm to 25 by 406 mm) even though no beams are as thin as 1 in. (25 mm). To find the strength of a 3-in. (76-mm) thick beam, the figure is multiplied by three (FIGURE 2- 20). A 1 by 4 in. (25 by 102 mm) beam spanning 6 ft (1.8 m) can carry 533 lb (242 kg); a 1 by 8 in. (25 by 203 mm) beam spanning 6 ft (1.8 m) can carry 2133 pounds (968 kg; four times as much); and a 1 by 16 in. (25 by 406 mm) beam spanning 6 ft (1.8 m) can carry 8533 pounds (3871 kg). This latter beam is four times the depth of a 1 by 4 in. (25 by 102 mm) beam.  FIGURE 2-20 The load-carrying capabilities of a beam increase with added dimensions. Description To repeat, the load-carrying capacity or strength of a beam increases as the square of the depth but increases only in direct proportion to increases in width. To look at this calculation in another way, a 4 by 8 in. (102 by 203 mm) beam spanning 10 ft (3 m) can carry 5120 pounds (2322 kg). In standard terminology, the width is given first. Suppose the 4 by 8 in. (102 by 203 mm) beam is used as an 8 by 4 in. (203 by 102 mm) beam. The 8 by 4 in. beam can carry only half the load. The greater efficiency of a deeper beam must be balanced against other considerations, such as the desired thickness of the floor and the deflection of flooring between widely spaced beams. Floor boards are themselves beams. For a variety of reasons, then, the almost universal spacing for sawn wooden beams in ordinary construction is 16 inches (406 mm) and the depth of the beam is determined by the design load and the span. The depth of the beam required is particularly significant in the case of trusses because of the greater void in which combustible gases can be generated or accumulated. The length of a span, or the distance between supports, is a determinant of the safe load of a beam. As the length of the span increases, the safe load capacity decreases in direct proportion. TABLE 2-3 demonstrates this principle. TABLE 2-3 Safe Load of a Beam Span Distance (Feet) Load Capacity (Pounds) 6 (1.8 m) 3333 (1512 kg) 12 (3.7 m) 1666 (756 kg) 18 (5.5 m) 1111 (504 kg) 24 (7.3 m) 833 (378 kg) 30 (9.1 m) 667 (303 kg) 36 (11 m) 555 (252 kg) The figures related to load also consider the beam’s weight, which must be deducted to get the permissible superimposed or added load. The figures assume a uniformly distributed load (FIGURE 2-21). If the load is concentrated at the center of a beam, the permitted load is half the distributed load. The principles involved are the same for all beams, regardless of the material used. FIGURE 2-21 The uniformly distributed load on the beam is being transferred evenly between two structural supporting members. Description Types of Beams Different terms used for various beams are as follows: A simple beam is supported at two points near its ends. In simple beam construction, the load is delivered to the two reaction points and the rest of the structure renders no assistance in an overload. A continuous beam is supported at three or more points. Continuous construction is structurally advantageous because if the span between two supports is overloaded, the rest of the beam assists in carrying the load. A fixed beam is supported at two points and is rigidly held in position at both points. This rigidity may cause collapse of a wall if the beam collapses and the rigid connection does not yield properly. An overhanging beam projects beyond its support but not far enough to be a cantilever. A bracket is a diagonal member that supports what would otherwise be a cantilever. A joist is a beam, often made of wood. A steel joist or bar joist is a lightweight steel truss joist. A girder is any beam, of any material (not just steel), that supports other beams. A built-up girder is made of steel plates and angles riveted together, as distinguished from a girder rolled from one piece of steel. A spandrel girder is a beam that carries the load on the exterior of a framed building between the top of one window and the bottom of the window above. A lintel is a beam that spans an opening in a masonry wall (FIGURE 2-22). Stone lacks tensile strength so it can only be used for short lintels. Many precast concrete lintels have the word “TOP” cast into the top to be sure they are erected with the reinforcing rods that provide tensile strength at the bottom.  FIGURE 2-22 Lintels are used to support the downward force of the wall above them. This appears to be a window replacement in an old wall; note the dangerous lack of mortar between the bricks. © Meredith Lamb/Shutterstock. A grillage is a series of closely spaced beams designed to carry a particularly heavy load. A cantilever beam is supported at only one end, but it is rigidly held in position at that end (FIGURE 2-23). It projects out over a support point. Beyond the support point, the tension is in the top, and the compression is in the bottom. Cantilever structures are likely to be unstable in a serious fire because the fire may destroy the method by which the beam is held in place. The cantilever beam resembles a playground seesaw. Many of us can recall being lifted up in the air by a playmate who then ran off and dropped us to the ground. Failure of the interior section of the cantilever can cause collapse of the projecting section. As a cantilevered structure such as a balcony collapses, it is likely to pull down a wall. When this happened in a New Orleans incident, three fire fighters were killed. Cantilevers are used widely for both architectural and design economy considerations. Wooden cantilevered balconies are common. Temporary construction platforms, for example, on which a crane delivers material, are cantilevered out from a building. They are supported by wooden compression members delivering the load to the floor above. A fire could destroy the wood and cause the platform to fall. FIGURE 2-23 A cantilevered floor in a high-rise under construction. Courtesy of Glenn Corbett. When any change is to be made in the foundation of an existing wall, the wall must be supported. Often holes are cut through the wall, and so-called needle beams are inserted and supported on both sides. They pick up the load of the walls.  A suspended beam is a simple beam, with one or both ends suspended on a tension member such as a chain, cable, or rod. The typical theater marquee is a suspended beam. Fire may destroy the anchoring connection; the beam then becomes an undesigned cantilever. In a Washington, D.C., theater, the ends of the chains were connected to the roof trusses. When an attic fire destroyed the connections, alert junior officers, who were fire science students, noticed that the front brick wall had been pulled out almost to the point of collapse. Another old theater was converted into a furniture store. In a subsequent fire, its suspended marquee collapsed, killing six fire fighters. The collapsing marquee pulled down the masonry along the entire length of the building; in fact, one victim was about 100 ft (30.5 m) from the initial collapse. A transfer beam moves loads laterally when it is not convenient to arrange columns one above the other—the ideal arrangement. If it is necessary to change the vertical alignment, a transfer beam must be designed to receive the concentrated load of the column and deliver it laterally to supports. Improper alterations of buildings may produce unintended transfer beams that are points of weakness. In 1981, the Kansas City Hyatt Regency Hotel suffered a disastrous collapse of a walkway in which more than 100 people were killed. A construction change had created a weak connection and caused a short section of the upper walkway beam to act as an undesigned transfer beam; when placed under stress, it failed, with disastrous effects. The critical nature of beams as part of building structure becomes evident when their failure occurs. Consider the nine-story Alfred P. Murrah Federal Building in Oklahoma City, which was destroyed in a terrorist attack in 1995 (FIGURE 2-24). It used a reinforced concrete transfer girder at the third-floor level to allow larger spacings between exposed exterior columns on the first two floors. The blast from the bomb set off at the front of the building destroyed these exterior columns, resulting in a lack of support for the transfer girder and initiating a collapse of all of the upper floors. FIGURE 2-24 The Alfred P. Murrah Federal Building was destroyed in a terrorist attack in 1995. Courtesy of FEMA. Beam Loading Beam loading refers to the distribution of loads along a beam. Assume a given simple beam, which can carry eight units of distributed load. If the load is concentrated at the center, it can carry only four units. If the beam is cantilevered and the load is distributed, two units can be carried. If the load is at the unsupported end of the cantilever, only one unit can be carried. Particularly in buildings under construction or under demolition by fire or otherwise, excessive loads may be concentrated in one area. This may have been the case in New Orleans when scaffolding with stacks of bricks ready for use collapsed suddenly at a building that was being reconstructed. Do not increase the overload by adding the weight of fire fighters and equipment. Reaction and Bending of Beams The reaction of a beam is the result of force exerted by a beam on a support. The total of the reactions of all of the supports of a beam must equal the weight of the beam and its load. The bending moment of a beam can be simply described as that load that will bend or break the beam. The amount of bending moment depends not only on the weight of the load, but also on its position. The farther a load is removed from the point of support, the greater the moment. Heavy loads should be placed directly over or very close to the point of support. Trusses A truss is a type of beam, a framed structure consisting of a triangle or group of triangles arranged in a single plane in such a manner that loads applied at the points of intersections of the members will cause only direct stresses (tension or compression) in the members. Loads applied between these points cause flexural (bending) stresses. Trusses can support axial loads but are not designed to handle rotational moments.  BRANNIGANISM It’s been said people judge others by, among other things, the way they write. When it comes to building construction terms, fire fighters must traverse a spelling minefield. For example, a lintel is a beam that carries the load over an opening in a wall; a lentil is a bean in soup. The word is masonry, not masonary. Trusses have top and bottom chords, not cords. The rigidity of the truss rests in the geometric principle that only one triangle can be formed from any three lines. Thus, the triangle is inherently stable. An infinite number of quadrilaterals can be formed from four lines, so the rectangle is inherently unstable. The economy of a truss is derived from the separation of compressive and tensile stresses so that a minimum of material can be used. There are many designs of trusses, each with its own name (FIGURE 2-25).  FIGURE 2-25 Types of trusses. Description The top and bottom members of the truss are called chords. The compressive connecting members are called struts. The tensile connecting members are called ties. Connections are called panel points. As a group, the struts, ties, and panel points are called the web. The principal types of trusses can be described by their overall appearance. In a parallel-chord truss, the bottom and top chords are parallel. The steel bar joist (a parallel-chord truss) used for both roofs and floors has been around for some time. Parallel-chord roof trusses that have a very long span may have a slight upward pitch to the center to facilitate water runoff. Truss floors have been designed in which the trusses were buried in the ceilings or floors of apartments, so that the living space is within the truss. Wooden parallel-chord trusses are being used for floors and roofs in single- family homes, row houses, apartment houses, and smaller office buildings. Recently, builders have been using wooden parallel-chord trusses as roof rafters. These trusses provide a peaked roof with usable attic space. Dormer windows may be a clue to this type of roof. A roof truss may be triangular in shape to provide a peaked roof; this is known as a triangular truss. Lightweight triangular trusses must be closely spaced. As a result, they leave no clear space that might be used as an attic. A serious and continuous problem in truss design has been connectors. In recent years, tremendous strides have been made in the development of connectors capable of transmitting heavy loads. Trusses can provide huge clear spans yet have a dead weight considerably less than that of a corresponding ordinary beam. In some cases, no ordinary beam can do the job. Compression Versus Tension in Trusses The top chord of a truss is in compression. The bottom chord is in tension. When a truss is cantilevered out, the situation reverses. Beyond the point of support, the tension is in the top chord and the compression is in the bottom chord. In sketches of trusses, compression members are often shown as thick lines, while tension members are shown as thin lines. This graphic device accounts for the fact that compressive loads are best resisted by columns whose material is as far as possible from the center (e.g., a cylinder). Tensile loads need only the required strength of the material. Shape is unimportant. Truss Principles Consider a building span of 20 ft (6.1 m), in which two 10-ft (3-m) I- beams extend from the walls on opposite sides to a column support in the center. Assume that each beam can carry 1000 pounds (454 kg). If the column is removed, the beam would have to be 20 ft (6.1 m) long. According to the principle that doubling the length of a beam decreases its carrying capacity by half, a beam that is 20 ft (6.1 m) long, of the same material and cross-section area, can carry a load of only 500 lb (227 kg). Suppose the column is cut off and a stub remains at the junction of the beams. If a tension member is tied from the stub to the beam ends at the wall, a triangle (truss) will be formed, in effect restoring the load-carrying capacity of the beam, yet removing the obstruction of the column from the floor below. This type of truss is called an inverted king post truss. It is called a king post because of the single compression member and inverted because the compression member extends downward. A truss with two compression members is called a queen post truss. Problems with Trusses The truss can be designed and constructed as the minimum structure that will carry the designed load. Use of trusses, in fact, is emphasized in some engineering schools. Competitions are held in which the winner produces a truss with the highest ratio of superimposed load to weight of the truss. “Our trusses are engineered” is sometimes presented as an unquestioned virtue. The more important question is, “Engineered to what objective?” Unfortunately, the answer is often, “To meet the lowest price in a competitive market.” Trusses can fail in a variety of ways. All parts and connections of a truss are vital to its stability. The failure of one element of a truss may cause the entire truss to fail. Multiple truss failures can occur. The failure of one truss can have serious impacts on other parts of the structure, even parts far from the initial failure point. For example, steel trusses are tied together for stability to increase wind resistance. When one truss fails, unanticipated torsional stresses can be passed to other trusses by way of the ties, which may cause multiple failures. In one case, a 1400-ft (427-m) television tower collapsed as an antenna was being installed. Two diagonals had been removed to permit the antenna to be hoisted up the interior of the tower. Some trusses come with labels warning carpenters not to cut any part of the truss. Despite this, trusses are often cut to accommodate plumbing or other fixtures. A rising roof presents a serious wood and metal plate truss problem (FIGURE 2-26). For example, some attic trusses bow upward, causing ceilings to separate from walls. The Journal of Forestry blames this defect on early harvested tree species. Juvenile wood is more prone to shrinkage and warping. Another theory is that during the cold winter months, the bottom chord of the attic truss, buried in insulation, is warmer and loses more moisture. The upper chord, which is much cooler, retains moisture. The result is that the top chord expands slightly and the bottom chord shrinks slightly, bending the truss and making it rise. For fire fighters, the most serious effect of rising roofs is the weakened joints that can result from shrinking wood pulling away from gusset plates. Gas and electric lines located in such attics may be dangerously affected as well.  FIGURE 2-26 A rising roof. Courtesy of Mark Sulcov. Designers rely on standard tables of allowable design stresses in structural components when developing their building plans. A timber truss can have many defects that may lead it to collapse under nonfire conditions such as high wind or snow loads. These same defects may precipitate early collapse under fire conditions. For example, shrinkage of wood may cause stresses that the design of the structure did not take into account. Chemical action, wet and dry rot, enzyme decay, and insect infestation can destroy the timber. Excessive loads may be carried on the trusses. Defective design may lead to the truss not acting as a truss. Poor maintenance also affects trusses. For example, the building’s owners may fail to tighten loose tension rods. Lightweight gusset plate trusses (gusset plates made of small, thin pieces of galvanized sheet steel with teeth punched into one side; the teeth are then pressed into two pieces of wood to hold them together) are also sometimes poorly made or mishandled in the field. Intense competition exists in the building materials supply business, because builders have a clear economic incentive to purchase their materials from the least expensive supplier. Clearly, this factor can lead to provision of less-than-desirable (or even outright shoddy) materials, especially given that the materials will be hidden in the building—that is, trusses and any of their potential defects are unseen in a completed structure. With luck, the defects may not cause failure for some time, if ever. When the defects do become apparent in a fire situation, however, the results can be disastrous. Truss Failure in Fires The truss is efficient because it separates compression and tension forces. However, this efficiency can also cause disaster. The bottom chord of a truss is under tension. Recall that a tension member is like a chain—one break precipitates failure. Conversely, the top chord of a truss is under compression; therefore, it responds exactly like a column. Consider a truss with panel points 5 ft (1.5 m) apart. This is loaded like a column tied off every 5 feet—a series of 5-foot columns one atop the other. If a panel point or connection fails, two 5-foot segments join to become a 10-ft (3-m) segment. This segment can carry only 25 percent of the design load. Its failure can precipitate the failure of other trusses. Compare a lightweight wooden parallel-chord truss with a sawn beam. The top of an ordinary beam is in compression and the bottom is in tension. The greatest compression is at the top, gradually lessening as the neutral plane is approached. The greatest tension is in the bottom, gradually lessening as the neutral plane is approached. There is a continuum in the loading of the beam. The loss of some material, therefore, is not necessarily fatal. In a truss, all of the compressive load is carried on the top chord, typically a light member such as a 2 by 4 or 2 by 6. Loss of wood can precipitate crushing failure in compression. All of the tensile load is carried on the bottom chord. Except for quite short trusses, the bottom chord is made up of two or more pieces of wood, joined end-to-end with metal gusset plates (FIGURE 2-27). The loss of a gusset plate would cause the tensile load to pull the truss apart. FIGURE 2-27 Note the numerous gusset plates connecting the floor trusses at ceiling level in an apartment building under construction. Some of the 3/8- in. (9.5-mm) teeth in the top and bottom plate are not penetrating the wood. Courtesy of Glenn Corbett.  Web members may be connected by gusset plates or by nailing and gluing. In one design, the ties or tension members are stamped from one piece of metal and flattened on the end to make the gusset plate. The steel tie could act as a heat collector, delivering heat to the gusset plate teeth, which can pyrolytically destroy the tensioned wood fibers that had been gripping the 3/8-in. (9.5-mm) metal teeth. Tubular steel and wood trusses can burn through at panel points, where there is little wood. A steel pin that carries heat into the wood can cause pyrolytic decomposition. Large triangular trusses such as those used in apartment house roofs have bottom chords made up of two or more pieces of wood butted together and held by gusset plates. In the case of these trusses used in apartment house roofs, disaster to fire fighters operating below may be averted because room partitions support the failing trusses so that only parts of the truss fall into the top floor. The same trusses, used in a commercial building to provide a wide clear space, can and have failed catastrophically. Columns A column is a structural member that transmits a compressive force along a straight path in the direction of the member (FIGURE 2- 28). Columns are usually thought of as being vertical, but any structural member that is compressively loaded is governed by the laws of columns, despite its attitude.  FIGURE 2-28 Columns transmit compressive force along a straight path in the direction of the member. © S.Borisov/Shutterstock. Columns by themselves are often used for monuments. Nonvertical columns are often called by other names, such as struts or rakers, which are diagonal columns with brace foundation piling. A bent is a line of columns in any direction. If a line of columns is specially braced to resist wind, it is called a wind bent. A bay is the floor area between any two bents. A pillar is a freestanding masonry load-carrying column, as in a cathedral. Load-Carrying Capacity of Columns Beams decrease in load-carrying capacity proportionately—a 12 ft (3.7 m) long beam can carry half the load of a similar-sized 6 ft (1.8 m) long beam. Columns, however, lose strength by the square of the change in length. Thus, a 12-ft (3.7-m) column can carry only one- fourth the load of a 6-ft (1.8-m) column of the same material and cross section. Shapes of Columns The most efficient shape for a column is one that distributes the material equally around the axis as far as possible from the center of the cylinder. The thin wall tubing used for the legs of a child’s swing set is an example of an efficient use of steel in forming a column. In theory, column material could be paper thin, but it would be subject to local damage, as by a puncture or dent. Most codes provide for minimum wall thickness for columns to prevent local damage. To obtain a free floor space 63 by 357 ft (19.2 by 108.8 m) in a 48-story high-rise, the designer provided super-columns. They consisted of four 39-inch (99-cm) square steel box columns spaced so they form a super-column 21 ft (6.4 m) on a side. The box- column steel walls are as thick as 3 ⅛ inches (79.4 mm) at the base and get narrower farther up the height of the column. Note that the principle of handling a compressive load—place the column material as far from the center as possible—was followed. The space inside the square is used for stairs and elevators. It is more difficult to attach beams to round columns than to rectangular columns, so less efficient rectangular columns are often used. In cast-iron construction in buildings, interior columns are usually circular, while wall columns (within exterior walls) are rectangular. Rectangular columns often fit better into floor plans. Structural design is often a compromise between competing needs. When beams were discussed, we explained how a 2 by 8 in. (51 by 203 mm) joist set on the narrow edge would be a much more efficient use of wood than the same amount of wood in a 4 by 4 in. (102 by 102 mm) shape. Conversely, the 4 by 4 in. shape would be the most efficient use of materials under a compressive load in a column because it is most nearly circular in cross section. Masonry walls under construction are often braced against high winds. Available scaffold planks, usually 2 by 8 in. (51 by 203 mm) or 2 by 10 in. (51 by 254 mm), are most often used for this purpose. The load on the braces is a compressive load, so the braces should be shaped like columns, but they are not. One builder described the 2 by 8 in. planks as “snapping like matchsticks” at a construction site before the walls fell in a windstorm. If four 2 by 8 in. planks had been spiked together to make a hollow column and set diagonally against the wall, with 2 by 8 in. planks laid flat against the walls as beams, the wind load would have been much better resisted using the available material. Wooden Columns Most wooden columns are simply smoothed off tree trunks. Large wooden columns, almost always ornamental as well as structural, are hollow. The column consists of curved, usually tongue-and- grooved, sections glued together. I-Beams Versus H-Columns Whereas steel beams are “I”-shaped, steel columns are “H”-shaped, box-shaped, or cylindrical. Beams are shaped like the letter “I” because the depth determines strength. Columns are shaped like the letter “H” and of a dimension that permits a circle to be inscribed through the four points of the H. It is important for fire fighters to use the correct terminology both to demonstrate their professionalism and to ensure safety through common understanding. A builder may use an available “I”-shaped steel section as a column but it is wrong to speak of I-beam columns (FIGURE 2-29). FIGURE 2-29 In this cross-sectional image, the I-beam is located at the top, and the H-column is located on the bottom. Although similar in appearance, the structural properties of these two structural elements differ greatly. Types of Columns There are three types of columns, which can be differentiated by the manner in which they generally fail. Piers are short, squat columns, which fail by crushing. Long, slender columns fail by buckling. In buckling, the column normally assumes an “S” shape. Intermediate columns can fail either way. Understanding the characteristics of long, slender columns yields information and principles applicable to columns and other structures under compressive loads, notably the top chord of trusses. Euler’s Law Columns Very long, thin columns are known as Euler’s Law columns. A straight column, axially loaded, may suddenly collapse. Euler discovered that there is a critical load for a column and that the addition of even a single atom over the critical load can cause sudden buckling and collapse. The formula for Euler’s Law incorporates the following three vital variables in column stability: in which Pc is the critical load, the absolute maximum load. π2 is 3.1415 squared. E is the modulus of elasticity of the material in the column. (The modulus of elasticity is a measure of the ability of the material to yield and return to its original shape.)  I is the moment of inertia, a mathematical value for the geometric cross-sectional shape of the column. L2 is the length of the column squared. Once the column starts to yield, there is generally very little reserve strength left and the column is on the verge of total collapse. By contrast, in beam action, there is generally considerable reserve strength beyond initial yielding. Consider a long, slender column. The load on the column tends to cause it to buckle. If the column is braced rigidly at the midpoint, the effective length of the column is cut in half. Because the square of the length is the divisor in the Euler’s Law formula for column loading, cutting the length of the column in two increases the carrying capacity to four times what it was initially. The critical load is four times higher than it was initially. Shortening the effective length of a column by intermediate bracing pays dividends in load- carrying capacity (FIGURE 2-30).  FIGURE 2-30 The load-carrying capacity of a column can be increased by bracing it. The loss of bracing is a cause of column failure. Consider a high scaffold. Several long, narrow columns are securely braced at several points along its height, making several 8-ft (2.4-m) segments. The effective length of the column, for this purpose, can be considered to be that of one of the segments. If a bracing connection fails, the effective length of that portion of the column becomes 16 ft (4.9 m). The decrease in load-carrying capacity is geometric (FIGURE 2- 31). The critical load is reduced to 25 percent. FIGURE 2-31 As a column gets longer, its load-carrying capacity decreases. Temporary Bracing When a building is under construction, many elements are not in position or are not permanently connected. It is possible that columns might not have the full benefit of the bracing that will be provided by the completed building. Guy or temporary bracing is commonly used in such situations. This type of bracing may not be adequate to resist high winds or other unexpected loads, and it may be vulnerable to fire. Walls Walls transmit to the ground the compressive forces applied along the top or received at any point on the wall. A wall resembles a wide slender column. The wall may also be required to resist flexural or bending forces, as does a beam. Wind load is an example of flexural force. Walls are classified in two main divisions: the load-bearing wall and the non-load-bearing wall. Load-bearing walls carry a load of some part of the structure in addition to the weight of the wall itself. Non-load-bearing walls support only their own weight. A veneer wall, a panel wall (curtain wall), and a partition wall are some examples of non-load-bearing walls. A load-bearing wall is more stable than a non-load-bearing wall of identical construction because of the weight of the superimposed load. The load carried by any component in addition to its own dead weight provides stability. Stack up a wall of books or blocks. The wall is easily pushed over. Superimpose your weight on this wall; it is now much more difficult to push over. The structural consequences of the collapse of a load-bearing wall may be much more serious than those following the collapse of a non-load-bearing wall because of the collapse of building elements supported by the bearing wall. To a fire fighter caught in a collapse, however, it makes little difference whether a masonry wall is load- bearing or non-load-bearing. Within these two broad classes of walls, other descriptors can be applied to walls. A cross wall is any wall at right angles to any other wall. The walls should brace one another. Sometimes the bond is so poor, however, that there is no stabilizing benefit from the cross wall. A veneer wall is made of a single vertical thickness of masonry; it is designed simply to improve the exterior appearance of the building (FIGURE 2-32). Decorative masonry such as stone, brick, or marble may be veneered over common stone masonry, concrete block, reinforced concrete, or steel. By far, however, the greatest use of veneer walls is brick veneer on wood frame.  FIGURE 2-32 The use of veneer walls has become commonplace. Courtesy of the estate of Francis L. Brannigan. The veneer wall totally depends on the underlying wall for stability. It should be tied to the wall with metal ties embedded in the mortar. It can be catastrophically unsafe if left to support itself. For example, brick veneer was applied to the visible parapeted portion of a masonry block fire wall in an apartment development near Charlotte, North Carolina, to improve its appearance. The brick was applied without ties and was resting on the wood roof. In 1989, the brick collapsed through the roof. Fortunately, no one was in the area, so injuries were avoided. A composite wall is composed of two or more masonry materials that react together under the load. To save costly brick used in masonry walls, brick and concrete block (in the past, terra cotta tile) are used to form a composite wall. When walls were all brick, some bricks were headers, which were turned on end to cross the wall and tie it together (FIGURE 2-33). This design was unsatisfactory when concrete block was used in brick and block composite walls, however. In modern construction, a steel-wire masonry truss is now laid in the wall. As a result, the bricks are all laid lengthwise as stretchers. Thus, it is no longer possible to differentiate a brick veneer wall from a brick masonry wall at a glance because both walls now can be laid as all stretchers—a technique that is also known as a running bond. FIGURE 2-33 Load-bearing brick walls often have both header and stretcher courses. Courtesy of the estate of Francis L. Brannigan. The terms panel walls and curtain walls are often used interchangeably to describe non-load-bearing enclosing walls on framed buildings (FIGURE 2-34). Technically, panel walls are one story in height and curtain walls are more than one story.  FIGURE 2-34 A glass curtain wall. Courtesy of Glenn Corbett. A party wall is a load-bearing wall common to two structures. If structural members of both buildings are located in common sockets, fire can extend through the opening.  Fire walls should be able to contain the fire with little or no assistance from firefighting forces (FIGURE 2-35). All penetrations of the fire wall must be rated according to the building code in effect, with a rating similar to the fire wall itself. Door openings should be protected with properly rated fire doors. Fire-rated assemblies are discussed in greater detail in Chapter 6, Features of Fire Protection. FIGURE 2-35 Fire walls are used to contain fire spread, but penetrations in the wall that do not meet the fire resistance requirements will allow fire to advance through the wall. Courtesy of Achim Hering. Description  Partition walls are non-load-bearing walls that subdivide areas of a floor. They may be required to have some fire or smoke resistance and also may be required to extend to the underside of the floor above or only to the ceiling line, leaving a void above. A knee wall is a wall typically found in the top floor of a wood- frame home with a peaked roof (FIGURE 2-36). This short, 2 to 3 ft (61 to 91 cm) high wall squares off the triangular area at the edge of the room where the sloping roof meets the floor. This creates a void space behind the wall. The resulting triangular spaced area is sometimes used for storage; access to it may be provided through a door—either a full-height closet door or a short access panel or door. FIGURE 2-36 A knee wall. Courtesy of Glenn Corbett. A demising wall is a wall that bounds a tenant space. A shear wall in a framed building is designed to help resist the force of the wind. It is usually incorporated in a required enclosure such as an elevator shaft or a stair shaft. In a rubble masonry wall, the outer and inner wythes (a single vertical thickness of masonry) are laid in courses and finished. The interior consists of stones dumped in at random. Sometimes the stones are loose; in other cases, mortar is dumped in around them. Cantilever Walls When a masonry wall is under construction, it acts as a cantilever beam with respect to wind loads received on the face of the wall. When the roof is in place, the wall, with respect to wind loads, becomes a simple beam supported at both ends. High, freestanding walls are common in the construction of large one-story-high buildings, such as shopping centers, churches, and industrial buildings. Severe winds may topple a freestanding wall. In addition, stakes so short that they move when the rain turns the ground to mud are sometimes used—and sometimes fail. Eccentric loading of the wall by hanging work platforms on one side is another cause of failure. If the wall is braced at one end by another cross wall, the unbraced end will be more vulnerable to collapse. During high winds, there are often gusts that greatly exceed an already high wind speed. In several cases, failure of what had been regarded as well-braced walls has been attributed to gusts. A precast concrete tilt-slab wall acts as a vertical cantilever when being erected and is braced by tormentors or temporary bracing poles (FIGURE 2-37). The roof of the building provides the permanent bracing. If the roof fails in a fire, the walls revert to undesigned vertical cantilevers and may collapse.  FIGURE 2-37 Tilt-slab walls being erected. Note the raker shores that are holding the walls in place until the roof is completed. Courtesy of the estate of Francis L. Brannigan. Wall Bracing A wall can be compared to a column extended along a line. Most walls have a high height-to-thickness ratio that is comparable to a tall slender column. They are therefore similarly unstable. President Thomas Jefferson once tried to build a one-brick-thick garden wall. It failed. He reasoned that it was buckling like a column, so he built a buckle into it to brace it internally. The result is the famous Thomas Jefferson-designed serpentine wall at the University of Virginia. Walls can be braced or stiffened by several means as follows: A buttress is a masonry structure built on the outside surface of the wall. The flying buttresses of Gothic cathedrals, which cope with the outthrust of the roof on the high slender walls, are built away from the wall. A prominent English politician once said, “I am not a pillar of the church, but a buttress since I support it from without rather than from within.” A pilaster is a masonry column projecting from one or both faces of the wall in which they are located (FIGURE 2-38). FIGURE 2-38 A set of brick pilasters on the face of an old mill. Courtesy of Glenn Corbett.  Wall columns are columns of steel, reinforced concrete, or solid masonry (such as brick or solid block) in a block wall. Concentrated loads such as main girders are applied to the wall directly above the wall column. A cavity or hollow wall is built of two wythes separated by a space for rain drainage or insulation. Such a wall might be built of two rows of 4-in. (102-mm) brick, separated by 4 in.