Chapter 13 - Delmar

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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

13.
5

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.

Imposition of Loads


Loads must be transmitted to
structural elements


Terms associated with imposition:


Axial load


Eccentric load


Torsion load

13.
6

13.
7

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

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

13.
9

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

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

13.
14

Figure 13
-
7 A beam transfers a load perpendicular to the
load

creating compressive and tensile forces within the
beam.

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

13.
19

Table 13
-
1 Performance of Common Building Materials under
Stress and Fire

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

13.
25

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

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

13.
28

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.

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 fire
-
resistant coating


Susceptible to deformation

13.
29

13.
30

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.

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

13.
32

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.

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

13.
34

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.
35

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.

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

13.
37

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

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

13.
39

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.

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

13.
43

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.

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

13.
46

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

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

13.
49

Figure 13
-
34 This electrical service entrance and attached
sign may be the eccentric load causing an early failure of
this parapet wall.

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

13.
54

Figure 13
-
35 A minimum collapse zone should be 1½ times the
height of the building.

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