Chapter 13 Building Construction
Introduction • Fire departments pride themselves in ability to launch aggressive interior structural attacks • Often, buildings collapse without a “visual” warning • Firefighters must understand how fire travels • Departments need more training on building construction 13. 2
Building Construction Terms and Mechanics • Firefighters need understanding of concepts associated with construction • Intended use of building can add tremendous weight • Elements create building loading • Imposition of loads causes stress called force • Forces delivered to earth for building to be structurally sound 13. 3
Types of Loads • Two broad categories: – Dead loads – Live loads • Specific terms for dead loads and live loads: – – – Concentrated load Distributed load Design load Undesigned load Fire load 13. 4
Figure 13 -2 The steel stairs and air-conditioning unit apply a concentrated load on this roof structure. Also note the potential instability of the air-conditioning unit placed on cement blocks. 13. 5
Imposition of Loads • Loads must be transmitted to structural elements • Terms associated with imposition: – Axial load – Eccentric load – Torsion load 13. 6
Figure 13 -5 There are three types of loads that can be transmitted through a structural member: axial, eccentric, and torsion. 13. 7
Forces • Loads imposed on materials create stress • Stress and strain: defined as forces applied to materials: – Compression – Tension – Shear • Several variables determine amount of time a material can resist gravity and fire degradation 13. 8
Figure 13 -6 Loads are applied to a structural member as compression, tension, and shear forces. 13. 9
Forces (cont’d. ) • Several variables determine amount of time a material can resist gravity and fire degradation – – – Material type and mass Surface-to-mass ratio Overall load being imposed BTU development Type of construction (assembly method) 13. 10
Forces (cont’d. ) • More variables – Alterations (undersigned loading) – Age deterioration/care and maintenance of the structure – Firefighting impact loads – Condition of fire-resistive barriers 13. 11
Structural Elements • Buildings are an assembly of structural elements designed to transfer loads to the earth • Can be defined simply as: – – Beams Columns Walls Connections 13. 12
Beams • Transfers loads perpendicular to its length • Types of beams: – – – – Simple beam Continuous beam Cantilever beam Lintel Girder Joist Truss and Purlin 13. 13
Figure 13 -7 A beam transfers a load perpendicular to the load—creating compressive and tensile forces within the beam. 13. 14
Columns • Any structural component that transmits a compressive force parallel through its center • Typically support beams and other columns • Generally vertical supports of building • Can be vertical, horizontal, or diagonal 13. 15
Walls • Really long, but slender, column • Two categories: – Load-bearing walls • Carries weight of beams, other walls, floors, roofs, other structural elements • Also carries weight of the wall itself – Non-load-bearing walls • Need only support its own weight • Example: partition wall between two stores in a strip mall 13. 16
Connections • Weak link as it relates to structural failure during fires – Often small, low-mass material that lacks capacity to absorb heat • Three categories: – Pinned – Rigid – Gravity 13. 17
Fire Effects on Common Building Construction Materials • Many factors determine which material is used to form structural elements: – – Cost Application Engineering capabilities Adaptability • Each material reacts to fire in a different way 13. 18
Table 13 -1 Performance of Common Building Materials under Stress and Fire 13. 19
Wood • Most common building material • Relatively inexpensive • Marginal resistance to forces compared to weight • Native wood with more mass takes longer to burn before strength is lost • Engineered wood – Plywood delaminates when exposed to fire – Some composites fail through exposure to heat without burning 13. 20
Steel • Mixture of carbon and iron ore • Excellent tensile, shear, and compressive strength • Popular choice for: – – Girders Lintels Cantilevered beams Columns • Loses strength as temperatures increase 13. 21
Concrete • Mixture of portland cement, sand, gravel, and water • Excellent compressive strength • All concrete contains some moisture • Under heat, moisture expands and causes concrete to crack and spall • Concrete can stay hot long after the fire is out 13. 22
Masonry • Common term that refers to brick, concrete block, and stone • Used to form load-bearing walls • Veneer wall supports its own weight • Mortar holds units together and have little or no tensile or shear strength • Excellent fire-resistive qualities 13. 23
Composites • Combination of the four basic materials • Includes plastics, glues, and assembly techniques • Example: wooden I beams composed of wood chips and veneers pressed together in I-shape – Structurally stronger but fail quickly when heated – No fire contact required • Steel expands faster than wood, causing stress at intersection between the two materials 13. 24
Figure 13 -11 A composite truss. Rapid heating will cause the stamped-steel to separate from the wood chords. 13. 25
Types of Building Construction • Five broad categories of building construction have been developed • Give firefighters basic understanding of: – Arrangement of structural elements – Materials used to construct building • Broad classifications are dangerously incomplete for firefighters • Buildings are built to meet certain codes 13. 26
Type I: Fire Resistive • Elements are of an approved noncombustible or limited combustible material • Typical of Type I: – Monolithic-poured cement – Steel with spray-on fire protection coatings • Typically large multi-storied structures with multiple exit points • Examples: arenas, high-rises, World Trade Center 13. 27
Figure 13 -15 A typical Type I building, with structural members designed to resist the effects of fire for three to four hours. This building is of reinforced concrete construction. 13. 28
Type II: Noncombustible • Not qualified for Type I construction • Are of an approved noncombustible or limited combustible material • Type II buildings are steel – Warehouses – Small arenas – Newer churches • Steel not required to have fireresistant coating – Susceptible to deformation 13. 29
Figure 13 -16 Buildings of Type II construction will have structural elements with little or no protection from the effects of fire. Remember, in the event of a fire, these unprotected steel structural members may fail and collapse quickly. 13. 30
Type III: Ordinary • Buildings where load-bearing walls are noncombustible • Roof and floor assemblies are wood • Prevalent in most older town “main street” areas • Many void spaces where fire can spread undetected – Common hallways, utilities, attics • Masonry walls hold heat inside, floors and roof beams gravity fit 13. 31
Figure 13 -17 Buildings of Type III, ordinary construction, are common throughout North America. These typical “Downtown USA” buildings provide many challenges to firefighters, such as void spaces and common walls allowing rapid fire extension and little structural protection. 13. 32
Type IV: Heavy Timber • Block or brick exterior load-bearing walls and interior structural members, roofs, floors, and arches of solid or laminated wood without concealed spaces • Buildings are quite stout – Used for warehouses, manufacturing buildings, older churches • New Type IV buildings hard to find – Cost of large-dimension lumber and laminated wood beams makes this building type rare 13. 33
Figure 13 -19 Type IV buildings, heavy timber construction, have large wood structural elements with great mass. The mass of these structural members requires a long burn time for failure. The connections, usually steel, are the weak points in this type of construction. 13. 34
Figure 13 -20 Wood and heavy timber beams were often “fire -cut” so that a fire-damaged, sagging floor would simply slide out of the wall pocket in order to preserve the wall. 13. 35
Type V: Wood Frame • Most common construction type – Homes – Newer small businesses – Chain hotels • Balloon frame versus platform framing • Platform framing creates fire stopping • Gypsum board protects wood frame members • Fires that penetrate wall, floor, attic spaces become significant collapse threat 13. 36
Figure 13 -21 The wood frame structure, Type V construction, is the most common type of construction in North America. 13. 37
Other Construction Types (Hybrids) • Methods that do not fit into one of the five types • Combination of more than one type • Other types: – Lightweight steel frame – Insulated concrete formed (ICF) – Structural insulated panel (SIP) wall 13. 38
Figure 13 -24 This lightweight steel home is built similar to a Type V. OSB sheeting gives the steel rigidity to torsional loads such as wind. 13. 39
Relationship of Construction Type to Occupancy Use • Many officials and builders first look at anticipated use of building • Occupancy classifications: – – – Basic arenas Residential Commercial Business Industrial Education 13. 40
Collapse Hazards at Structure Fires • Firefighters must understand buildings in their jurisdiction • Reading buildings is essential to anticipate collapse 13. 41
Trusses • Truss roof collapses have killed many firefighters • Come in many styles and shapes • Wood trusses commonly used for roof assemblies and floor assemblies • Steel trusses no less susceptible to collapse than wood trusses 13. 42
Figure 13 -26 Wood trusses provide a large surface-tomass ratio, fuel load, and void spaces—three of the worst structural collapse contributors a firefighter will encounter during structural firefighting operations. 13. 43
Void Spaces • Trusses create large void areas • Fires are able to spread horizontally • Fires can start in void spaces because of electrical and utility problems • In Type III ordinary construction, voids are numerous 13. 44
Roof Structures • Flat, pitched, or inverted • Many factors determine construction • Roof style may allow a large volume of fire to develop • Some dormers are actually aesthetic and fool ventilation crews 13. 45
Figure 13 -30 Some common roof framing styles used in wood frame or ordinary construction. 13. 46
Stairs • Believing stairways are durable is a dangerous assumption • Stairs are built offsite and hung in place with metal strapping • Stairs are made with lightweight engineered wood products – Fail quickly when heated 13. 47
Parapet Walls • Extension of a wall past top of roof • Used to help hide unsightly roof equipment • Free standing with little stability • Typically collapse when roof starts to sag • Historically, dozens of firefighters have been killed by collapsing parapets 13. 48
Figure 13 -34 This electrical service entrance and attached sign may be the eccentric load causing an early failure of this parapet wall. 13. 49
Collapse Warning Signs • Factors anticipating collapse: – – – – – Deterioration of mortar joints and masonry Overall age and condition of building Cracks Signs of building repair Large open spans Bulges and bowing of walls Sagging floors Large volume of fire Long firefighting operations 13. 50
Buildings under Construction • Especially unsafe during construction, remodeling, and restoration • Building only meet fire codes when completed • Stacked construction materials may overload other structural components 13. 51
Time • No time limits for firefighting operations • Truisms have emerged: – The lighter the structural element, the faster it comes down – The heavier the imposed load, the faster it comes down – Wet (cooled) steel buys time – Gravity and time are constant – There is no window of time – Brown or dark smoke from lightweight structures means time is up 13. 52
Preparing for Collapse • Incident commander needs to predict collapse PROACTIVELY • Communicated information between teams help with predicting collapse • Once occupants have been found, fire control should be reduced • Firefighters must not wander into collapse zone 13. 53
Figure 13 -35 A minimum collapse zone should be 1½ times the height of the building. 13. 54
Lessons Learned • Many firefighters have been killed as a result of building collapse • Firefighters must understand the buildings in which they fight fires • Knowledge of building construction starts with understanding of loads, forces, and materials • Five class types are being challenged by new construction methods • No rule for how long a building will last on fire 13. 55
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