OIA 2015.227 ER1 Fire engineering.pdf

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MANUAL Fire risk management
Timber ............................................................................................................................ 6
Design of Facilities for Firefighters ..................................................................................... 6
Building Code Requirements .......................................................................................... 6
Engineering Design ........................................................................................................ 7
Fire Brigade Intervention Model ...................................................................................... 7
Further Reading ............................................................................................................. 7

Building law in New Zealand
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New Zealand's main systems for governing building work, collectively known as building
controls, are the Building Act 2004 (BA), the Building Regulations 1992, and the Building
Code (BC), which is the first schedule to the Building Regulations. All building work must
comply with the BC.
The BA mainly applies to the physical aspects of building work. Other legislation may also
apply to building proposals, the ongoing use of a building, consumer protection, and the
health and safety of workplaces in buildings.
Compliance Documents are published by the Department of Building and Housing to help
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people comply with the BC. Many Compliance Documents reference New Zealand
Standards.
An alternative solution  is a building design solution that differs from those contained in the
Compliance Documents, but is accepted by a Building Consent Authority (BCA) as meeting
the requirements of the BC.
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The consents and inspections process ensures that building work complies with the BC. In
other words it is safe, durable and does not endanger health, both for the current users of the
building and to protect those who may buy and use the property in the future.
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Components of a Fire Engineering Design
To show that the performance requirements of the BC are met, the building designer would
normally appoint a fire engineer to undertake the design work in respect of fire safety.
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The engineer uses calculations, engineering design methods, and in some cases computer
fire models to demonstrate that the proposed performance-based solution for the building
meets the requirements of the BC.
Role of Fire Engineers
The NZFS employs several Fire Engineers throughout the country. Please see the chapter
on Fire Engineers for further information.
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The Design Fire
Purpose of Design Fire Selection
To carry out an assessment of the fire safety aspects of a building design, a fire engineer will
generally have to perform some calculations based on the likely fire scenarios that could
arise in the building and assess their potential impacts. These fire scenarios are known as
design fires. A design fire may be required as an input to a computer model, or may be used
to assess the risk of structural collapse, or may be required to determine fire resistance
ratings of building features.
Design Fire Location
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The fire engineer should test his/her design against a range of different fire locations, unless
the design is sufficiently simple for there to be a single obvious ‘worst case’. The locations
selected for design purposes will, in general, be dictated by the likelihood of a fire arising in a
particular location. This will depend in turn on what there is to burn and the potential for
ignition.
Design Fire Severity
Depending on the type of calculation being carried out, the design fire may be a slow,
growing or a fast or ultra fast fire. It may also be a fully developed or flashover fire. Ideally the
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fire engineer would test his/her design against a range of different fire locations and
severities. Severity is usually expressed in terms of the heat release rate (kW or MW) of the
fire.
There are well established techniques for determining the fire severity of fully developed or
flashover fires. The heat release rate in a room that is required to bring about flashover, and
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the expected steady heat release rate following flashover, can be estimated relatively simply.
For many purposes the fire is assumed to be growing. There is a range of experimental
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evidence to draw upon when estimating how fast fires will grow, but the rate of growth is
highly dependent on what is assumed to be burning. In some designs this may be
reasonably well known; in others, much less so, particularly when it is recognised that the fire
engineering design has to be valid for the life of the building.
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Design of Means of Escape
Protection of people from illness or injury
The objective of the BC in respect of means of escape, Clause C2.1 (a), is that people are to
be safeguarded from illness or injury as they escape from a fire. There are a number of ways
in which people could be injured by a fire:
  effects of heat
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  effects of toxic gases contained in smoke
  effects of irritant components of smoke

These effects increase the longer a person is exposed to the fire. Therefore anything that
delays escape can increase the potential for injury for those escaping. Delay can be caused
by people:
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  having difficulty in locating escape routes
  encountering congestion in escape routes
  suffering loss of visibility in a smoky environment
  undertaking actions not directed towards escape

Therefore, any assessment of the potential for injury must take into account the time for
which people are likely to be exposed, as well as what they are likely to be exposed to.
Smoke Generation and Movement
Once the design fires have been established, there are a number of possible calculations  Act
that might need to be carried out. Given that most people in fire die from the toxic effects of
smoke, the fire engineer will often carry out smoke movement or smoke filling calculations.
The calculation methods available range from relatively simple hand calculations and
equations, to computer models that can take days to run.
Engineers have to exercise great care when selecting and using computer models as even
the most sophisticated are prone to errors if wrongly applied. Many fire models have only
been validated against experiments in domestic room-sized compartments and may not give
valid results for large or complex spaces.
The toxic products likely to be in smoke and the effects of these products on people are
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reasonably well understood. This has led to the concept of tenability limits – values of
exposure to toxic products, heat or lack of oxygen that are thought to be the maximum that
can be tolerated by people before being overcome by the effects of the fire.
Fire engineers may use estimates of toxic product production and tenability limits to
demonstrate that exposure to smoke in particular circumstances is not likely to cause illness
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or injury to building occupants.
Movement of People
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In assessing any design, it will be essential to know that people can escape before being
overcome by the effects of fire. The time it takes people to escape depends on two separate
sets of factors – design of escape routes and people behaviour.
Design of Escape Routes
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The distance that people will have to travel before they are in a safe place will determine how
long it takes them to get there. This is important in buildings such as airports and railway
stations where travel distances can be long.
More importantly, in situations where there are many people trying to escape at once, the
width of doors, corridors and stairways has a major influence on the crowding that occurs in
escape routes. Flows of people though means of escape has been extensively studied and
much is known about how long it will take people to get out of sports stadiums or tall
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buildings, for example, using the available exits.
People Behaviour
In designing means of escape, it is easy to fall into the trap of assuming that people will all
move to escape as soon as they become aware of the fire. In practice, there is a range of
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behaviours that people might engage in depending on how sure they are that there is a fire,
what they are doing at the time and where they are physically located in the building.
In office environments, with a fire detection and alarm system and staff familiar with the
evacuation procedures, it might be reasonable to assume that people will move quickly to
escape. In shops, people may attempt to complete transactions before leaving, or wait for
staff to confirm that the alarm is genuine. In situations where the information available is
unclear, people may choose to investigate further. Even when they are sure there is a fire,
people may choose to fight it, or indeed to watch it if they feel that there is minimal personal
threat. In domestic environments, people may decide to find family members before
evacuating or collect personal valuables.
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The times taken for people to decide to start evacuating may well exceed the time it would
normally take them to evacuate. It is important, therefore, that the possible variations in
human behaviour are taken into account when undertaking building design work.
Design for Fire Spread
Protecting Escape Routes
The first objective of the BC in respect of fire spread, Clause C3.1(a), is to safeguard people
from injury or illness whilst evacuating a building during fire. The third objective, Clause
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C3.1(c), concerns the protection of other property from the effects of fire.
Flame spread
The performance requirements of the BC require that fire spread on interior surfaces shall be
controlled. In respect of surface finishes, a fire engineer would not normally depart from the
prescriptive requirements in the approved documents.
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Separation of Firecells
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By definition, fire separations must be provided between firecells. Fire separations are
required by the BC to have a fire resistance rating. Fire resistance is defined in such a way
that it must be determined in the standard test for fire resistance, or in accordance with a
specific calculation method, verified by experimental data from standard fire resistance tests.
In other words, alternative solutions for fire resistance that do not relate to the test are not
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allowed under the BC.
It may be required to design firecells to resist burnout of the contents of the firecell (S-rating).
Alternatively, a firecell may only be required not to fail within a given time (F-rating).
Protection of other Property
Where a property adjoins another property or sleeping accommodation, fire separations are
required by the performance requirements. See note above concerning fire separations.
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Where properties are separated by distance (as opposed to being separated by a fire
separation), the potential for fire spread is by radiation from the burning building to the
adjacent one. There are fire engineering calculations that can be carried out to demonstrate
that the separation is sufficient to avoid fire spread.
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Design for Structural Stability
Introduction
Many construction materials change their properties when they become hot. Depending on
what the material is being used for, the change in the material may cause the building
structure to weaken, and even give rise to structural collapse. The first objective of the BC, in
respect of structural stability, Clause C4.1(a), is to safeguard people from injury due to loss
of structural stability in a fire.
Avoidance of Collapse
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The performance requirements of the BC require that structural elements have fire resistance
to avoid structural collapse in a fire. Fire resistance must be determined in the standard test
for fire resistance, or in accordance with a specific calculation method verified by
experimental data from standard fire resistance tests. In other words, alternative solutions for
fire resistance that do not relate to the test are not allowed under the BC.
Steel
Steel loses its strength as its temperature increases. Most structural design in steel would
have built in factors of safety. All materials weaken with increasing temperature and steel is
no exception. Strength loss for steel is generally accepted to begin at about 300ºC and
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increases rapidly after 400ºC. By 550ºC steel retains about 60% of its room temperature
yield strength.  It is known that fires can reach temperatures of over 1000°C, and  fire
protection of steel is often a necessary feature of building design.
Concrete
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When heated, concrete tends to flake off – a process known as spalling. If the spalled
concrete exposes the steel reinforcing rods within the concrete, the structure can lose its
strength much quicker. The phenomenon of spalling can in some cases be quite explosive in
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nature. Usually concrete structural elements are designed by prescriptive rules that avoid the
reinforcement being compromised.
Timber
When heated timber undergoes charring that progresses from the exposed surface. The char
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has no structural strength, but it can insulate the underlying timber. There are techniques that
may be used to calculate the fire resistance rating of timber structural elements. Calculations
may be used to show that large timber structural elements may be sufficient to provide the
necessary fire resistance rating.
Design of Facilities for Firefighters
Building Code Requirements
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Several parts of the BC mention provision of facilities for Firefighters:
  Clause C2.1(b) on means of escape provisions states that they must also facilitate
fire rescue operations
  Clause C3.1(b) on fire spread provides that there must be protection to Fire Service
personnel during firefighting operations
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  Clause C4.2(b) on structural stability states that buildings shall be constructed to
allow Fire Service personnel adequate time to undertake rescue and firefighting
operations.
Engineering Design
Fire Engineers must consider the needs of firefighters when designing buildings for fire
safety. The prescriptive requirements set out in the Approved Documents are generally
followed in respect of Fire Service access and protection of staircases and other paths that
will be used for firefighting and rescue purposes. Any variations to these requirements would
generally be referred to the district Area Manager at the building design stage.
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Fire Brigade Intervention Model
Please refer to the Fire Brigade Intervention Model chapter for further information.

Further Reading
  Fire engineering Design Guide, Spearpoint, Michael, 3rd edition, Christchurch, N.Z. ,
New Zealand Centre for Advanced Engineering, 2008

Information
  SFPE Handbook of Fire Protection Engineering, DiNenno, Philip J,  4th edition,
Quincy, MA, NFPA: Society of Fire Protection Engineers, 2008

  Department of Building and Housing (DBH) website http://www.dbh.govt.nz/
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