Fire Resistance

 

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.

 

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

Type A aggregate

Type B aggregate

Type C aggregate

30
50
45
40
60
75
70
55
90
95
90
70
120
110
105
80
180
140
135
105
240
165
160
120

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

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

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