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
• non-combustible 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: Non-combustible
• Not qualified for Type I construction
• Are of an approved non-combustible
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 non-combustible
• 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-to-mass
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