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The requirements
for fire safety engineering are threefold:
The first and most important objective is to protect the
lives of any people who are in the structure which is
on fire, and enable them to leave the building quickly
and safely.
Secondly, the structure must be designed to allow enough
time for fire fighters to safely carry out any search
and rescue operations, and for them to safely carry out
fire fighting operations.
Thirdly, other property must be protected. This includes
preventing the fire from spreading as well as actions
such as preventing hazardous materials at the fire site entering
waterways.
The principal change in fire design from the previous NZS
1900 Chapter 5 has been to delete requirements that reflect
owner/property protection. Essentially, the fire resistance
of various inter-occupancies has been significantly reduced.
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Where an owner and their insurer wishes to increase the levels
of total protection for their property investment against
fire after earthquake or arson for example, non-combustable
concrete construction can provide significant increases in
fire resistance, as can be seen in the tables below.
The current requirements for designing for fire resistance
are related to sections Cl - C4 of the NZ Building Code and
fire performance data covered in NZ Standards document MP9. The introduction of the Building Act 1991 and its implementation
as a Building Code in July 1992 has resulted in a number of
changes in the performance criteria to be met. One of the changes relates to the presentation of Fire Resistance
Ratings. Since the introduction of the Building Code they
have been quoted not as a single figure but as a composite
of three numbers identifying the structural stability, Integrity
and insulation characteristics of the component.
An example of this may be: FRR60/60/30.
This indicates that the component has:
a) Stability to resist structural collapse for 60
minutes.
b) Integrity to resist the passage of flame for 60
minutes.
c) Insulation characteristics to resist the transfer
of heat to a specified level in 30 minutes.
Not all components will necessarily have a rating requirement
in each category.
The data contained currently in MP9 is basically single figure
stability requirements.
The FRR required for each component of the structure in question
however, depends on many variables. These include the
intended
use of the building, occupancy level, location of the component
with respect to neighbouring buildings and so on. FRR requirements
in the Building Code have been significantly reduced since
the owner property protection has been deleted from mandatory
requirements. Part of the present requirements are also based
on using automatic fire control systems.
The graphic results of a fire following an earthquake have
recently been highlighted in Kobe, Japan, where water supplies
were disrupted. Essentially, the assumption that a fire can
be put out within the fire resistance containment times is
erroneous in earthquake circumstances. To stop the fire spreading
in these circumstances the fire stop walls need to be designed
to contain the total combustion for the given fuel load affecting
the fire wall.
Likewise, in the case of arson, especially in buildings such
as schools, the intelligent use of concrete fire walls will
prevent the fire from spreading, meaning that it will cause
a minimum of damage, rather than taking out a whole row of
classrooms.
While it is not viable for us to provide the FRR values for
all situations, listed below are some of the ratings that
can easily be achieved through the use of reinforced and prestressed
concrete components. Some of this data is from NZS 3101 and
some from MP9.
| Fire
Resistance rating (minutes) |
Effective
thickness (mm) for different aggregate types
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Type A aggregate
|
Type
B aggregate
|
Type C aggregate
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|
30 |
50 |
45 |
40 |
|
60 |
75 |
70 |
55 |
|
90 |
95 |
90 |
70 |
|
120 |
110 |
105 |
80 |
|
180 |
140 |
135 |
105 |
|
240 |
165 |
160 |
120 |
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Table 1. Minimum Effective Slab and Wall Thickness for
Fire-Resistance Ratings for Insulation.
Note: Aggregate types:-
A - quartz, greywacke, basalt and all others not listed
B - dacite, phonolite, andesite, rhyolite, limestone
C - pumice and selected lightweight aggregates
| Fire
Resistance rating (minutes) |
Simply
Supported Slabs |
Continuous
Slabs |
| |
Minimum width of rib
(mm) |
Cover (mm)
|
Minimum width of rib
(mm) |
Cover (mm)
|
|
30 |
80 |
15 |
70 |
15 |
|
60 |
110 |
25 |
75 |
20 |
|
90 |
135 |
35 |
110 |
25 |
|
120 |
150 |
45 |
125 |
35 |
|
180 |
175 |
55 |
150 |
45 |
|
240 |
200 |
65 |
175 |
55 |
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Table 2: Fire Resistance Requirements for Stability of
Ribs.
Note: Cover is measured to the longitudinal bottom
reinforcement, or is the average cover to a group of prestressing
tendons.
| Fire
Resistance rating (minutes) |
Cover
to bottom reinforcement or tendons , c (mm)
|
| Simply
Supported Slabs |
Continuous
Slabs |
| Reinforcement |
Tendons |
Reinforcement |
Tendons |
|
30 |
10 |
10 |
10 |
10 |
|
60 |
10 |
20 |
10 |
20 |
|
90 |
20 |
30 |
15 |
25 |
|
120 |
30 |
40 |
15 |
25 |
|
180 |
45 |
60 |
25 |
35 |
|
240 |
55 |
70 |
35 |
45 |
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Table 3: Fire Resistance Requirements for Stability of
Slabs.
Note: For simply supported two-way slabs, the
values for cover may be reduced by 5mm.
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