# Chapter 13 - Delmar

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29 Νοε 2013 (πριν από 4 χρόνια και 7 μήνες)

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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

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2

Building Construction Terms

and Mechanics

Firefighters need understanding of
concepts associated with
construction

Intended use of building can add
tremendous weight

Imposition of loads causes stress
called force

Forces delivered to earth for building
to be structurally sound

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13.
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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.

Loads must be transmitted to
structural elements

Terms associated with imposition:

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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

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13.
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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

Material type and mass

Surface
-
to
-
mass ratio

Overall load being imposed

BTU development

Type of construction (assembly method)

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Forces (cont’d.)

More variables

Age deterioration/care and maintenance
of the structure

Condition of fire
-
resistive barriers

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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

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Beams

Transfers loads perpendicular to its
length

Types of beams:

Simple beam

Continuous beam

Cantilever beam

Lintel

Girder

Joist

Truss and Purlin

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13

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

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

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15

Walls

Really long, but slender, column

Two categories:

-
bearing walls

Carries weight of beams, other walls,
floors, roofs, other structural elements

Also carries weight of the wall itself

Non
-
-
bearing walls

Need only support its own weight

Example: partition wall between two
stores in a strip mall

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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

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17

Fire Effects on Common Building
Construction Materials

Many factors determine which
material is used to form structural
elements:

Cost

Application

Engineering capabilities

Each material reacts to fire in a
different way

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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

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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

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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

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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

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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

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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

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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
-
Center

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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

Steel not required to have fire
-
resistant coating

Susceptible to deformation

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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

-
bearing walls
are noncombustible

Roof and floor assemblies are wood

Prevalent in most older town “main
street” areas

Many void spaces where fire can

Common hallways, utilities, attics

Masonry walls hold heat inside,
floors and roof beams gravity fit

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13.
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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

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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.

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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

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

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13.
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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

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13.
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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

Industrial

Education

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40

Collapse Hazards at

Structure Fires

Firefighters must understand buildings
in their jurisdiction

Reading buildings is essential to
anticipate collapse

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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

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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

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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

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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

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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

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48

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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

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Buildings under Construction

Especially unsafe during
construction, remodeling, and
restoration

Building only meet fire codes when
completed

Stacked construction materials may
components

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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

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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

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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

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