Mechanical and Electrical Equipment for Buildings, Eleventh Edition

81 Pages·2010·1.88 MB·English

P A R T V I
FIRE
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ne of the most challenging aspects
of building design is providing pro-
tection against the effects of fire.
Designers may enjoy considering how
their buildings can be made bearably cool on
a stifling hot day or lighted and warm on a bit-
terly cold night. It is less pleasant to imagine a
building burning and considering how first its
occupants, then the building itself (along with
its contents)—can be saved.
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1084 PART VI FIRE PROTECTION
To complicate this critically important concern, there are inherent con-
flicts between some optimum features of fire resistance and some common
and useful features of design strategies such as daylighting, passive cool-
ing, or forced-air HVAC systems. There are even potential design conflicts
between systems for the safe evacuation of people and systems for the sup-
pression of fire—because both people and fire thrive on oxygen. There is also
some common ground among daylighting, passive and active climate control,
acoustics, and fire protection. The challenge is to find the optimum balance.
Design for fire protection is an intriguing challenge. Many aspects of fire
protection are rigidly governed by building codes that have evolved over the
past 100 years in response to disasters and milestone events. On the other
hand, most large buildings include some feature (such as an atrium) that is
generally unique and not clearly addressed by otherwise explicit fire codes.
Another challenge is to find an acceptable balance between respect for the
codes and ingenuity to move beyond the codes when required.
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24
C H A P T E R
Fire Protection
JUST AS BUILDINGS CAN BE DESIGNED TO
minimize heating, cooling, and lighting equipment FIRE RESISTANCE, EGRESS,
and thus to reduce energy consumption, they can AND EXTINGUISHMENT
be des igned to reduce the size of fire-fighting sys-
tems and retard the spread of smoke and fire. As
the average size of fires within buildings continues 24.1 DESIGN FOR FIRE RESISTANCE N
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to decline, the emphasis is shifting to minimizing TI
water and smoke damage. Whole-building confla- Fire is a special kind of oxidation known as combus- C
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grations, while spectacular, are relatively rare in tion. Oxidation, which has been discussed in previ- OT
large buildings in North America now that both ous chapters in terms of rust within water supply PR

automatic fire detection and fire-extinguishing sys- equipment and aerobic digestion in waste disposal E
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tems are widespread. systems, is a process in which molecules of a fuel FI

This discussion of fire protection begins with are combined with molecules of oxygen, producing
basic design considerations for fire resistance. a mixture of gases and energy. When this occurs
Smoke management (for safe evacuation and for rapidly, as in a fire, energy is released as heat and
limited smoke damage) is considered next, followed light, and some gases become visible as smoke. Table
by fire-suppression systems such as sprinklers and 24.1 lists the stages of a fire and the factors that
non-water-based approaches. Lightning protection influence its growth. Many of these factors involve
is then discussed, along with the many fire detec- decisions made by building designers.
tion and alarm systems that are keyed to the major Fire has a triangle of needs: fuel, high tempera-
stages of the typical building fire. ture, and oxygen. If deprived of any of these needs,
Throughout this chapter, the influence of building fires will be extinguished. In general, this
the National Fire Protection Association (NFPA) is triangle’s influence on building design is as follows.
apparent. The periodically updated Fire Protection The fuel is the building’s structure and contents; the
Handbook is an especially useful source of infor- designer controls the choice of structural and finish
mation for design, from which several illustrations materials but rarely the final contents. The tempera-
have been reprinted in this text. tures achieved in fires are well beyond the ability of
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1086 CHAPTER 24 FIRE PROTECTION
TABLE 24.1 Major Factors Influencing Fire Growth
Approximate Ranges Major Factors
Realm of Fire Sizes That Influence Growth
1 Overheat to ignition Amount and duration of heat flux
Preburning Surface area receiving heat
Material ignitability
2 Ignition to radiation point Fuel continuity
Initial burning (10-in. [254-mm] high flame) Material ignitability
Thickness
Surface roughness
Thermal inertia of the fuel
3 Radiation point to enclosure point Interior finish
Vigorous burning (10-in. to 5-ft high flame [254 mm Fuel continuity
to 1.5 m]) Feedback
Material ignitability
Thermal inertia of the fuel
Proximity of flames to walls
4 Enclosure point to ceiling point (5-ft Interior finish
Interactive burning [1.5-m] high flame to flame touching Fuel arrangement
ceiling) Feedback
Height of fuels
Proximity of flames to walls
Ceiling height
Room insulation
Size and location of openings
HVAC operation
5 Ceiling point to full room involvement Fuel arrangement
Remote burning Ceiling height
Length/width ratio
N Room insulation
O Size and location of openings
TI HVAC operation
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T Source: Reprinted with permission from the Fire Protection Handbook, 18th ed., © 1997, National Fire Protection Association, Quincy,
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E building cooling systems to control, so special water but not enough to lower the temperature), and the
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to deprive fire of the high temperatures it needs. Oxy- (the more insulation, the easier the attainment of
gen may be denied to a fire partly by limitations on combustion temperatures).
ventilation, but these can have serious safety conse- Electrical heat energy is most commonly sup-
quences. Another design response is to install fire- plied by resistance heating, a familiar process in
suppression systems that either cover the fuel (foam, many appliances and in space-heating equipment.
dry chemicals) or displace oxygen with another Less common are electric ignition by induction, dia-
gas—for example, carbon dioxide or “clean agents” lectic process, arcing, and static electricity. Light-
that inhibit the chemical action of the flame itself. ning is an infrequent but enormously destructive
electrical energy source.
Mechanical heat energy is produced by friction
(a) Sources of Ignition
(including sparks), by overheating of machinery,
Buildings commonly contain three basic sources and occasionally by the heat of compression.
of ignition: chemical, electrical, and mechanical.
In chemical combustion, most commonly known
(b) Products of Combustion
as spontaneous combustion, some chemicals reach
ignition at ordinary temperatures within buildings. Everyone has experienced to some degree the
Chemical combustion depends upon the rate of heat dangers of the thermal products of combustion
generation (related to the degree of saturation of (Fig. 24.1)—flame and heat. These visible and tac-
combustible products determined by the chemicals tile elements of fire can cause burns, shock, dehy-
involved), the air supply (enough to supply oxygen dration, heat exhaustion, and fluid blockage of the
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DESIGN FOR FIRE RESISTANCE 1087
equipment, and cable jackets). These types of gases
may cause eye, skin, nose, and throat irritation—
and in sufficient concentrations can be psychotro-
pic and/or toxic.
In indoor fires, oxygen commonly becomes
insufficient because the fire consumes it so rapidly.
The normal concentration of oxygen in air is about
21%. At less than 17%, muscular coordination and
judgment are diminished; at 14% down to 10%,
people remain conscious but become irrational, and
fatigue is rapid; at 10% down to 6%, collapse occurs,
but revival is possible when increased oxygen is sup-
plied. The technique of starving the fire of oxygen
Fig. 24.1 Although the thermal products from a fire—flame can therefore pose a threat to human beings, both
and heat—make strong visible and tactile impressions, fire’s
by increasing the chances of carbon monoxide pro-
nonthermal products—smoke and its constituent gases—pose
the greater threat to life. duction and by depriving people of oxygen.
respiratory tract, but they are responsible for only
(c) Objectives in Fire Safety
about one-quarter of the deaths resulting from
building fires. Most fire deaths are caused by the Many older buildings were designed at a time when
nonthermal products—smoke and its wide range building fires usually resulted in the loss of several
of constituent gases, liquids, and solids. Smoke can adjacent structures; hence, the primary objective of
usually be seen and smelled. Made up of droplets of firefighting was to limit the conflagration to as few
flammable tars and small particles of carbon sus- city blocks as possible. With the increased use of fire- N
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pended in gases, it irritates the eyes and nasal pas- resistant construction and code control of site and TI
sages, sometimes blinding and/or choking a person. building planning, fires typically were confined to EC
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Gases are especially dangerous because, without one building at a time. As fire-suppression systems O
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visible smoke, they are so often difficult to detect. came into common usage within buildings, it then P

Some gases are directly toxic, but all are danger- came to be expected that fires could be confined to E
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ous because they displace oxygen. Common gases one or two floors within a building. Now that auto- FI

released in building fires include carbon monoxide matic detection/suppression systems are techni-
and carbon dioxide. cally advanced, fires can usually be confined to one
Carbon monoxide is a deadly product of combus- room or to even smaller areas. In the United States,
tion and is often the most abundant such product. most fires in sprinklered buildings are now extin-
It is produced when insufficient oxygen is available guished with one to five sprinklers operating.
to completely oxidize the burning material. It is Four common design intents related to build-
more readily attached to hemoglobin molecules in ing fire safety, in order of usual importance, are:
red blood cells than is oxygen, thus depriving the
1 Protection of life
brain and muscles of needed oxygen. This leads to
2. Protection of building
irrational behavior and loss of consciousness, then
3. Protection of contents
to death. Carbon dioxide is also likely to result from
4. Continuity of operation
combustion but, unless intentionally introduced
into a space as a means of fire suppression, should Many of the elements of building fire safety are
not be present in concentrations with serious health covered by building codes, but it is important to
impacts (beyond potential headaches and dizziness). remember that codes specify the minimum accept-
Other dangerous and commonly encountered able protection. Designers can go much further than
building-fire gases include hydrogen sulfide, sul- the codes require in order to enhance fire safety. It is
fur dioxide, ammonia, oxides of nitrogen, cyanide, also important to realize that codes cannot cover all
phosgene, and hydrogen chloride (from burning aspects of fire safety, as there are too many variables
polyvinyl chlorides found in computers, electronic in building design.
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1088 CHAPTER 24 FIRE PROTECTION
Codes typically prescribe design strategies that In many other instances, there are potential
are passive means of limiting the spread of fire and design conflicts between building performance
protecting life: wall, floor, and ceiling construc- under ordinary conditions and building perfor-
tions; maximum open floor areas; maximum dis- mance in the extraordinary event of a fire. Esca-
tances to exits; and so on. Codes commonly allow lators invite shoppers to explore the upper levels
some relaxing of such prescriptions when active of a department store or connect hotel lobbies to
fire-suppression systems (such as sprinklers) are ballrooms; they are also efficient apertures for the
designed into a building. Alternatively, a detailed vertical spread of fire and smoke. (Special fire pro-
computer analysis of fire spread and occupant tection strategies for escalators are found in Chap-
evacuation within a proposed project may allow ter 33.) Daylighting and natural ventilation are best
for even greater distances to exits, larger open served by high ceilings and low partitions, which
floor areas, and alternative constructions. This encourage light and air from the perimeter to per-
is a performance-based, rather than a prescriptive, vade building interiors. If unchecked by sprinklers,
approach to design. It requires close cooperation however, fire and smoke can easily spread through
between designers and fire code enforcement such open floor plans. (On the other hand, smoke
authorities. and fire can build up much more rapidly in small,
enclosed rooms that retain heat.) Forced-air sys-
(d) Fire Safety and Other tems that heat, ventilate, and cool are also potential
Environmental Control Systems pathways for smoke and fire; this hazard is especially
serious when the systems penetrate floors, because
In some instances, the desirable design approaches
vertically spreading fires are harder to fight than
for fire safety will track design approaches for
horizontally spreading ones. (Then again, care-
lighting, thermal, acoustic, and water systems.
fully designed forced-air systems can aid in smoke
Although these opportunities may be uncommon,
N management.) Windowless buildings that rely on
O it would be unfortunate (because of a discipline-
TI electric lighting in place of daylight are especially
C centric design process) to miss such synergies as: dangerous in fires because firefighters cannot eas-
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tightly controlled air conditioning, must be broken
High ceilings, useful for daylight distribution and
for fire evacuation/access. The higher the window,
displacement ventilation, but also for collect-
the greater the danger from falling shards of glass.
ing a large quantity of smoke before it reaches
Many excellent insulating materials will burn
the occupants and for allowing smoke and/
readily, and some will give off toxic gases. Some of
or flames from a fire to be seen from a greater
the loveliest interior finishes are both flammable
indoor distance.
and deadly sources of gases in a fire. Table 24.2
Windows, for daylight, ventilation, and view, also
summarizes fire requirements for interior finishes.
allow access for firefighting and rescue, provide
The dichotomy between ensuring comfort
escape routes, relieve smoke accumulation with
throughout a building’s normal life and safety at
fresh air, and thus relieve some of the stress of
the moment it is threatened by fire, is the principal
trapped occupants.
reason why codes alone cannot ensure fire safety.
Solid (noncombustible) overhangs over windows not The dilemma of ordinary versus extraordinary per-
only provide sunshading, but also discourage the formance must be faced by the cooperative actions
vertical spread of fire over the building face and of the designer, owner, and occupants of a building.
can serve as emergency exterior places of refuge.
Elevated water storage tanks provide both adequate
(e) Protection of Life
water pressure for plumbing fixtures and water
for firefighting in the first few minutes of a fire The NFPA Fire Protection Handbook discusses
before firefighters arrive. human behavior in fires in great detail. Although
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TABLE 24.2 Summary of Life Safety Code Requirements for Interior Finish
Occupancy Exits Access to Exits Other Spaces
Assembly—New: >300 people A A or B A or B
≤300 people A A or B A, B, or C
Assembly—Existing: >300 people A A or B A or B
≤300 people A A or B A, B, or C
Educational—New A A or B A or B; C on low partitions
Educational—Existing A A or B A, B, or C
Day Care Centers—New A A A or B
I or II I or II NR
Day Care Centers—Existing A or B A or B A or B
Group Day-Care Homes—New A or B A or B A, B, or C
Group Day-Care Homes—Existing A or B A, B, or C A, B, or C
Family Day-Care Homes A or B A, B, or C A, B, or C
Health Care—New AS* Mandatory A or B A or B A or B
C on lower portion of corridor wall C in small individual rooms
Health Care—Existing A or B A or Ba A or Ba
Detention and Correctional—New A A A, B, or C
I I
Detention and Correctional—Existing A or B A or B A, B, or C
I or II I or II
Residential, Board and Careb
Residential, Hotels and Dormitories—New A A or B A, B, or C
I or II I or II
Residential, Hotels and Dormitories— A or B A or B A, B, or C
Existing I or II I or II
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Residential, Apartment Buildings—New A A or B A, B, or C O
I or II I or II TI
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Residential, Apartment Buildings— A or B A or B A, B, or C E
Existing T
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Residential, 1- and 2-Family, Lodging or A, B, or C A, B, or C A, B, or C
Rooming Houses RE
Mercantile—New A or B A or B A or B FI

Mercantile—Existing Class A or B A or B A or B Ceiling A or B existing on walls
A, B, or C
Mercantile—Existing Class C A, B, or C A, B, or C A, B, or C
Office—New A or B A or B A, B, or C
I or II I or II
Office—Existing A or B A or B A, B, or C
Industrial A or B A, B, or C A, B, or C
Storage A or B A, B, or C A, B, or C
Source: Reprinted with permission from the Fire Protection Handbook, 18th ed., © 1997, National Fire Protection Association, Quincy,
MA 02269.
aSee occupancy chapters of NFPA 101 for details.
bSee Source, Chapters 22 and 23.
Notes: Class A Interior Wall and Ceiling Finish—flame spread 0–25, (new) smoke developed 0–450.
Class B Interior Wall and Ceiling Finish—flame spread 26–75, (new) smoke developed 0–450.
Class C Interior Wall and Ceiling Finish—flame spread 76–200, (new) smoke developed 0–450.
Class I Interior Floor Finish—critical radiant flux, minimum 0.45 Watts/cm2.
Class II Interior Floor Finish—critical radiant flux, minimum 0.22 Watts/cm2.
*Automatic sprinklers—where a complete standard system of automatic sprinklers is installed, interior finish with flame spread rating not
over Class C may be used in any location where Class B is normally specified, and with rating of Class B in any location where Class A is
normally specified; similarly, Class II interior finish may be used in any location where Class I is normally specified, and no critical flux rating
is required where Class II is normally specified. (This does not apply to new health care facilities.)
Exposed portions of structural members complying with the requirements of heavy timber construction may be permitted.
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1090 CHAPTER 24 FIRE PROTECTION
panic behavior drives many code requirements, modate a wheelchair, a minimum clear width of 32
such behavior has been found to be rare. Design- in. (813 mm) is required. Exits can take a variety of
ers should consider how building occupants make forms. Vertical exits (Figs. 24.3 and 24.4) include
decisions in a fire. In the first phase, cues are smokeproof towers, exterior and interior stairs and
detected—the smell of smoke, sounds associated ramps, and escalators that meet specific require-
with a fire (breaking glass, sirens, alarm bells), and, ments. Vertical exits do not include elevators; they are
more rarely, the sight of flames. Open plans (with too easily stalled or, worse, opened at the floor of a
longer visible indoor distances) are more amena- fire by malfunctioning signal equipment. Exits in
ble to exposing such clues to a wider population. In the horizontal plane include doors leading directly
the second phase, the occupants define the situa- to the outside, 2-hour fire-rated enclosed hallways,
tion: Just how serious is this fire? The more numer- and moving walks. Special horizontal exits are pro-
ous the cues, the more rapid the definition phase. vided by internal firewalls penetrated by two fire
How other people are reacting is influential, and doors—one swinging open in either direction. Exit
in the absence of strong cues can actually lead to discharge is the area outside an exit that leads to a
a group refusal to evacuate in the early stages of public way and may still need protection in a fire.
a fire. In the third phase, coping behavior begins: Maximum allowable distances to exits are
fight or flight? Clear exit pathways and access to specified in Table 24.3. These are influenced by past
firefighting equipment are critical to rationally experience with people exiting through smoke, espe-
making this decision. cially when it may involve briefly traveling toward
For most low-rise buildings, a reasonable goal the perceived fire location (as in the case of a dead-
is the evacuation of all occupants in the time inter- end corridor). When automatic fire suppression
val between the detection of a fire and the arrival systems such as sprinklers are used, the allowable
of the firefighters. Designers can provide clearly distances to exits are increased. Designers should
N defined pathways to exits (exit access) that can be remember, however, that at least 30% of building fire
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Fig. 24.3 Plan views of exits. (a) An enclosed stairway allows
occupants on any floor above a fire to escape. A smokeproof
tower is better, as it opens to the air at each floor, largely
Fig. 24.2 Exit access, exit, and exit discharge on the first floor preventing accumulation of smoke in the stairway. (b) Horizontal
of a multistory building. Doors A, A, E, and E are exits, and the exit through an interior firewall provides a quick refuge and less-
1 2 1 2
path (dashed line) is the exit access. To the person emerging ens the need for a hasty flight down the stairs. Two wall open-
from the exit enclosure, doors A and A and the paths (dotted ings are needed to facilitate exit in either direction. Fire-rated
1 2
lines) are the exit discharge. Doors D and D are exit discharge doors must be arranged to be self-closing or automatic-closing
1 2
doors. Solid-line paths are within the exit. (Reprinted with by smoke detection. (Reprinted with permission from the Fire
permission from the Fire Protection Handbook, 18th ed.; © 1997, Protection Handbook, 18th ed.; © 1997, National Fire Protection
National Fire Protection Association. Quincy, MA 02269.) Association. Quincy, MA 02269.)
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DESIGN FOR FIRE RESISTANCE 1091
Fig. 24.4 Four variations of smokeproof towers. Plan
A has a vestibule opening from a corridor. Plan B
shows an entrance by way of an outside balcony.
Plan C could provide a stair tower entrance common
to two areas. In plan D, smoke and gases entering
the vestibule would be exhausted by a natural or
induced draft in the open air shaft. In each case, a
double entrance to the stair tower with at least one
side open or vented is characteristic of this type of
construction. Pressurization of the stair tower in the
event of fire provides an attractive alternative for tall
buildings and is a means of eliminating the entrance
vestibule. (Reprinted with permission from the Fire
Protection Handbook, 16th ed.; © 1986, National Fire
Protection Association. Quincy, MA 02269.)
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Minimum egress widths per floor are found The building population as estimated for fire C
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with the help of Table 24.4. First, calculate the floor safety is usually much greater than the popula- T
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area (net or gross, as specified in the table). Divide tion for which HVAC, water, or elevator service R
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the floor area by the occupant load to determine the is designed. The stairs must be designed to allow
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number of occupants for whom exits must be pro- those already within the stairwell to continue R
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vided for that floor. Then calculate the exit capacity down without interference from access doors on
based on its clear width. Stairs are sized to meet the any floor. Stairs with direct access to outdoor air
requirements of Table 24.5. at each floor—so-called smokeproof towers—are
the safest kind. A fire stair must allow firefighters
to move up while occupants are moving down.
EXAMPLE 24.1 A multistory office building is 80
Another phenomenon is reentry, in which occu-
ft (24 m) wide by 300 ft (80 m) long. What exit
pants who have exited decide to reenter despite
capacity is required per floor?
the danger. (Rescue of family members, pets, and
SOLUTION (I-P units) valuables is a common reason.) This greatly com-
The gross floor area = 80 × 300 = 2400 ft2. From plicates downward flow.
Table 24.4, “business” categories are based on one High-rise buildings present much more diffi-
person per gross 100 ft2. cult problems because firefighting equipment can
The population per floor is, therefore, (2400 ordinarily reach no higher than seven floors (about
ft2)/(100 ft2/person) = 240 people. 90 ft [27 m]) and because, typically, only two exit
Exit doors (to stairs): 240 people × 0.2 in./person stairways are provided. This difficulty is recognized
= 48 in. total. in building codes, which classify buildings with an
(One 34-in. clear door into each of two stairs = 2
occupiable floor more than 75 feet above the lowest
doors × 34 = 68 in., more than the minimum.)
fire department access as high-rise with special
Stairs: 240 people × 0.3 in./person = 72 in. total.
design considerations. Downward flow rates in
(Two stairs at 44 in. each = 88 in., more than the
stairs were formerly assumed at about 45 persons/
minimum.) ■
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