Fire Theory

Properties

There are two categories of fire-technical properties that are essential in design.
• Fire resistance of building components
• Properties of materials under fire exposure

Fire-technical documentation of constructions can be carried out in the following ways:
• Recognized and tested building constructions (NBI-sheets)
• Classified/certified building constructions (Building Code series)
• Building constructions documented through testing (from building material suppliers)
• Building constructions documented through calculation

Additionally, approved structures may be presented in handbooks or producers’ technical documentation.

Standards and guidelines
In Europe the structural design standards are called the Eurocodes. The standards are divided between the structural materials and usually Part 1-2 deals with fire. EN 1995-1-2 Eurocode 5 Part 1-2 is the design standard for structural fire design of timber structures. The current standard is published in 2004 and there are no special rules given for the I-joists or for the other cavity insulations than stone wool or glass wool.

There is revision process ongoing for all the Eurocodes. The new generation of Eurocodes will be presumably published on 2025. That will also consist the fire design rules for floors and walls made of I-joists and made with all kind of insulation materials.

The model of the structural system adopted for the design must reflect the performance of the structure in a fire situation. The structural system may be different in a fire situation, e.g. where a structural member is braced at ambient temperature and the bracing fails in the fire situation, the member must be regarded as unbraced in the structural fire design. Elements that are used for the stabilisation of the building, e.g. wood-based panels or gypsum plasterboard in wall or floor diaphragms, often lose their racking resistance in a fire situation unless they are protected from the fire. This effect on the global structural system must therefore be taken into account. In redundant structural systems it may be advantageous to allow for premature failure if an alternative load path is possible, e.g. a column in a fire compartment.

For calculation of the resistance of timber members, the original cross-section is reduced by the charring depth and the zero-strength layer.

Materials/surfaces

Risk classification

Determined on the basis of the activities for which the building is planned and the prerequisites for personnel in the building to reach safety.

Fire classification

Determined on the basis of the consequences a fire may have for life, health, social interests and the environment. This consequence will depend on the use of the building (risk classification), the number of floors and its layout. Requirements for the fire resistance of load-bearing building components are determined based on the fire classification in which the building is to be constructed.

Fire resistance

Is the ability of a structure to fulfil required functions (load-bearing and/or fire-separating function) under a given fire load for a given time period.

Requirements for fire resistance of load-bearing building components

R

Load-bearing structures must have fire resistance R to achieve sufficient load-bearing properties and stability in the event of fire, thus ensuring the necessary time for evacuation and rescue. In fire classification 3 buildings, load-bearing structures must also maintain their load-bearing capacity throughout a fire.
The design capacity must be checked against the design value of the load effect in limit state fire, determined in accordance with NS-EN 1991-1-2- and load combinations thereof in accordance with NS-EN 1990 and NS-EN

EI

Fire cell-limiting building components must have fire resistance EI to prevent the spread of fire and fire gases for the time necessary for evacuation and rescue.

Most often, there will also be requirements for the tightness and insulating capacity of structures (e.g. REI 60).
The purpose of dividing buildings into fire cells is to prevent the spread of fire and smoke to other parts of the building, for the time considered necessary for evacuation. Constructions enclosing a fire cell must have satisfactory fire-prevention properties to prevent the spread of fire and flue gases from one fire cell to another (min. EI30).

Fire resistance of separating structures

1) Limited flammable materials or better
2) If the fire cell limiting wall is also to be load-bearing, the requirement is REI30, REI60, etc.
The purpose of dividing buildings into fire cells is to prevent the spread of fire and smoke to other parts of the building that are considered necessary for evacuation. Constructions enclosing a fire cell must have satisfactory fire performance properties to prevent the spread of fire and flue gases from one fire cell to another
(min. EI30).

Some designations used in fire classification

R Load-bearing capacity
E Integrity/tightness
I Insulation capacity
M Mechanical resistance

Any use of non-combustible materials must be specifically indicated.

Each load-bearing (R) or separating (EI) structure has its own fire resistance properties, regardless of the reaction to fire classification.

Once a contained building fire has reached the point of flashover you need to rely on building elements, including insulation or insulated panels, with high fire resistance properties to protect the rest of the building from fire spread and/or to carry the loads in fire situation.

Fire resistance is classified in the EN 13501-2.

• Integrity (E) is the ability to withstand fire exposure on one side whilst stopping the passage of flames and hot gases through to the unexposed side for a period of time.
• Insulation (I) is the ability of the building element to stop the passage of heat through to the unexposed side. The period of time certified for insulation is the time it takes to produce an average increase in temperature of 140°C above the initial temperature or an increase in temperature at one point of 180°C above the initial temperature on the unexposed side of the system.
• Load-bearing ability (R), is the ability of the building element to carry the loads required in the fire situation throughout the declared period of time.
  Different factors, such as jurisdiction and application, determine the level of fire resistance required by different building regulations in different countries.
  
  Assessment of fire resistance
  Fire resistance of structures can be assessed by fire testing or by calculations. Calculation methods give always more conservative results compared to fire testing. For calculation methods the design parameters for wood and protective materials are needed. If there is no necessary parameters available, the fire testing will be the only option for verification of fire resistance. European standard for fire design is EN 1995-1-2. European fire test standards are given in Table 1.

Fire tests

Testing is done in full scale. For example, wall assemblies have to be tested with dimensions at least 3 x 3 meters and floor assemblies with dimensions 3 x 4 meters, while 4 m is the span. See the test standards in Table 1. In the fire tests the temperature and pressure are followed in the furnace. Temperature must follow the standard time-temperature curve and have to stay within the allowed limits. There shall be small overpressure in the furnace to allow the gases and smoke to come out.

Fire testing is done according to many different standards depending on the type of the structure.

Additionally, approved structures may be presented in handbooks or producers’ technical documentation.

Standards and guidelines

In Europe the structural design standards are called the Eurocodes. The standards are divided between the structural materials and usually Part 1-2 deals with fire. EN 1995-1-2 Eurocode 5 Part 1-2 is the design standard for structural fire design of timber structures. The current standard is published in 2004 and there are no special rules given for the I-joists or for the other cavity insulations than stone wool or glass wool.

There is revision process ongoing for all the Eurocodes. The new generation of Eurocodes will be presumably published on 2025. That will also consist the fire design rules for floors and walls made of I-joists and made with all kind of insulation materials.

The model of the structural system adopted for the design must reflect the performance of the structure in a fire situation. The structural system may be different in a fire situation, e.g. where a structural member is braced at ambient temperature and the bracing fails in the fire situation, the member must be regarded as unbraced in the structural fire design. Elements that are used for the stabilisation of the building, e.g. wood-based panels or gypsum plasterboard in wall or floor diaphragms, often lose their racking resistance in a fire situation unless they are protected from the fire. This effect on the global structural system must therefore be taken into account. In redundant structural systems it may be advantageous to allow for premature failure if an alternative load path is possible, e.g. a column in a fire compartment.

For calculation of the resistance of timber members, the original cross-section is reduced by the charring depth and the zero-strength layer.