1. Eurocode 1: Actions on structures –
EN :2002
Eurocode 1: Actions on structures –
Part 1–2: General actions –
Actions on structures exposed to fire
EN provides guidance on mechanical actions (loads) and thermal actions resulting from a defined fire scenario.
The Code is only applicable for the design of buildings.
Describes the thermal & mechanical actions for the structural design of buildings exposed to fire

2. National Annex for EN1991-1-2
Covers National Determined Parameters (NDP’s) & Informative Annexes
NDP’s in EN are introduced into clauses:
2.4(4), 3.1(10), (1), (1), (2), 3.3.2(1), 3.3.1(2), 4.2.2(2), 4.3.1(2)
Annexes A to G are informative.
As with all Eurocodes, EN must be used with the National Annex (NA), which may vary for different Nations.
The NA will provide values for the National Determined Parameters (NDPs) and also guidance on the use of Informative Annexes. In EC1-1-2 there are 9 NDP’s and all Annexes (A to G) are informative. The NA can also provide non-contradictory complementary information.
The guidance given in the NA supersedes any guidance given in the Code.
The guidance given in the National Annex supersedes any guidance given in the main code or informative annex

3. Basic Outline
EN describes the thermal and mechanical actions for the structural design of buildings exposed to fire
General objective is risk limitation relating to:
The Individual
Society
Property
Environment
The general objectives of fire design are to limit risks with respect to the Individual and Society, Property and the Environment.

4. Objectives (Risk Limitation)
Load-bearing properties maintained for a specified period of time
Fire and smoke generation limited
Fire spread limited
Safe occupant egress
Fire-fighter safety
The building must be designed, and built, such that the load-bearing capacity is maintained for a assumed period of time, the generation and spread of fire and smoke within a building is limited, the occupants can leave the building safety or can be rescued and the safety of rescue teams (Firemen) is considered.

5. Design Procedures
The Eurocodes defines a prescriptive approach as a design method that uses nominal (standard) fires to generate thermal actions. A performance based approach, using fire safety engineering, is defined as a method that derives thermal actions based on physical and chemical parameters.

6. Introduction
Code considers nominal (standard) fires relating to fire resistance rating and ‘natural’ (parametric) fire scenarios.
Prescriptive approach uses nominal (standard) fires
Performance-based approach (using fire safety engineering) refers to thermal actions based on physical and chemical parameters.
EN provides guidance on the use of nominal (standard) fires, relating to fire resistance periods, and natural (parametric) fire scenarios.

7. Scope Only applicable to buildings
Should be used in conjunction with the fire parts of EN1992 to EN1996 & EN1999.
Does not cover assessment of damage to the structure following a fire.
EN is only applicable to buildings and should not be used on other forms of structure. It is to be used in conjunction with the fire parts of EN1992 (Concrete), EN1993 (Steel), EN1994 (Steel/Concrete Composite), EN1995 (Timber), EN1996 (Masonry) EN1999 (Aluminium).
The Code does not provide any guidance on the assessment of structures following a fire.

8. Structural Fire Design Procedure
A structural fire design should take into account:
The selection of relevant design fire scenarios
Determination of corresponding design fires
Calculation of temperature within the structural members
Calculation of mechanical behaviour of the structure exposed to fire
Based on a risk assessment the relevant design fire scenarios should be defined, leading to the determination of the design fire. Based on the design fire, the temperature distribution through the structure is calculated. Once the temperature distribution is defined, the mechanical behaviour of the structure can be estimated.

9. Design Fire Scenario
A structural fire design should take into account:
The selection of relevant design fire scenarios
Determination of corresponding design fires
Calculation of temperature within the structural members
Calculation of mechanical behaviour of the structure exposed to fire
Design fire scenarios are determined on the basis of a fire risk assessment. In cases where other accidental actions may lead to a fire then both accidental actions (i.e. gas explosion followed by a fire) should be considered when determining the overall safety concept of the building.

10. Design Fire
For each design fire scenario a design fire should be estimated following the guidance given in the Code
The design fire should only be applied to one fire compartment at a time, unless otherwise specified in the fire design scenario (based on a fire risk assessment)
For structures designed to fire resistance requirements, it may be assumed that the design fire is given by the standard fire
For each defined fire scenario a design fire is estimated. The design fire should only be assumed to occur in one defined fire compartment at a time, unless the risk assessment shows that simultaneous compartment fires should be considered. However, if the risk assessment shows simultaneous fires are probably it is advisable to revise the design such that the probability of simultaneous fires is reduced.
For structures designed to fire-resistance the design fire is given by the standard fire.

11. Temperature Analysis
The position of the design fire in relation to the considered member should be considered.
For external members the fire exposure through openings in the building’s envelope should be considered.
For nominal fires the cooling phase of the fire is not considered.
For ‘natural’ fires the cooling phase of the fire should be considered.
The main difference between nominal (standard) fires and ‘natural’ fires is that for nominal fires the cooling phase of the fire is not considered, whereas for natural fires the cooling phase is considered.

12. Mechanical Analysis
For nominal fires the mechanical analysis is conducted up to the specified fire-resistance period.
For ‘natural’ fires the mechanical analysis is conducted over the duration of the design fire.
Verification can be carried out in the time, strength or temperature domain.
For natural fires the structural response must be considered for the duration of the fire, including the cooling phase.
Verification of the strength can be carried out in the time domain (i.e. the strength must remain for a given time), strength domain (i.e. the resistance exceeds the applied actions) or the temperature domain (i.e. the temperatures must remain below a stated level).

13. Thermal actions for temperature analysis
Thermal actions are given by the net heat flux:
Both the convective and radiative flux are taken into consideration
Thermal actions are given by the net heat flux to the surface of the member. On the fire exposed surfaces the net heat flux should be determined by adding the convective and radiative heat flux as shown.

14. Thermal actions for temperature analysis
The net convective heat flux component can be expanded as:
Surface temperature of the member
The net convective heat flux can be determined from the equation shown. For nominal fire curves the following values are given for the coefficient of heat transfer by convection.
c = 25W/m2K for standard and external fire curves and c = 50W/m2K for the hydrocarbon fire curve.Fdlgd
Coefficient of heat transfer by convection – values of which are discussed later
Gas temperature in vicinity of fire exposed member

15. Thermal actions for temperature analysis
The net radiative heat flux component per unit surface area can be expanded as:
Stephan Boltzmann constant
Surface temperature of the member
The net radiative heat flux component per unit surface area is determined from the equation shown.
The Stephan Boltzmann constant = 5,67  10-8 W/m2K4).
The service emissivity of the member may be taken as 0,8 UNLESS a different value is given in the fire parts of EN1992 to EN1996 and EN1999.
Configuration factor
Emissivity of fire
Effective radiation temperature of fire environment
Surface emissivity of member

16. Thermal actions for temperature analysis
The configuration factor should be taken as 1.0 unless EN specifies otherwise. Alternative values may be calculated using Annex G.
For fully engulfed members Qr may be approximated by Qg
The values for the gas temperature may be taken from the nominal temp-time curves or the natural fire models.
When calculating the net radiative heat flux the configuration factor can be taken as 1.0, provided no alternative guidance in given in EN1992 to EN1996 and EN A calculation method is given in Annex G, which can be used.
For fully engulfed members the effective radiation temperature of the fire environment may be represented by the gas temperature around the member.

17. Section 3 Nominal temperature-time curves
Standard temperature-time curve:
1049°C
1006°C
945°C
842°C
Temperature(°C)
Nominal temperature-time curves comprise the standard temperature-time curve, the external fire curve and the hydrocarbon curve.
The standard fire curve is shown. It should be noted that the temperatures continue to increase with increase in time.
Time (mins)

18. Section 3 Nominal temperature-time curves
External fire temperature-time curve:
680°C
680°C
Temperature(°C)
The external fire is shown. After 22 minutes the temperature remains constant at 680°C.
Time (mins)

19. Section 3 Nominal temperature-time curves
Hydrocarbon fire temperature-time curve:
1100°C
1100°C
Temperature(°C)
The hydrocarbon fire is shown.
Time (mins)

20. Section 3 Nominal temperature-time curves
Three fire curves used in the Code:
1098°C
1100°C
1100°C
1100°C
1049°C
842°C
1006°C
945°C
Temperature(°C)
680°C
680°C
680°C
680°C
The standard, external and hydrocarbon fire curves are shown for comparison. It can be seen that the hydrocarbon fire is the most severe followed by the standard fire, with the external fire curve being the least severe fire.
Time (mins)

21. Natural fire models
Based on specific physical parameters with a limited field of application
For compartment fires a uniform temperature distribution, as a function of time, is assumed.
For localised fires a non-uniform temperature distribution, as a function of time, is assumed.
Simplified fire models can be used, but their limit of applicability must be considered. For compartment fires a uniform temperature, as a function of time, is assumed. For localised fires a non-uniform temperature, as a function of time, is assumed.

22. Natural fire models Simplified fire models - Compartment fires
Atmosphere temperature determined based on physical parameters considering at least the fire load density and ventilation conditions
For fires within building compartments a method is given in Annex A (parametric fires) to define the time-atmosphere temperature. The method is based on the fire load density, ventilation and thermal characteristics of the compartment boundaries.
When simple fire models are used the coefficient of heat transfer by convection should be taken as 35 W/m2K.
Annex A provides a method for calculating atmosphere compartment temperatures.

23. Natural fire models Simplified fire models - External members
For external members the radiative heat flux should be taken as the sum of the contributions of the fire compartment and of the flames emerging from the openings.
Annex B provides a method for calculating the thermal action of external members exposed to a fire through openings in the building’s envelope.
For external members Annex B provides a method for estimating the thermal action. The radiative heat flux component, when considering external members, should be calculated as the sum of the contribution of the fire compartment and the flames emerging from the openings.

24. Natural fire models Simplified fire models - Localised fires
Where flash-over is unlikely to occur, thermal actions from a localised fire should be taken into account.
Annex C provides a method for calculating the thermal actions from localised fires.
When flashover is unlikely to occur, the actions from localised fires should be considered. Annex C provides a method for calculating the actions from localised fires.

25. Section 3 Natural fire models - Annexes methodology
Parametric temperature-time curves – method of determining compartment fire temperatures
Annex A
Method of calculating the heating conditions and thermal actions for external members exposed through façade
Annex B
Annex C
Thermal actions of localised fires – heating conditions etc.
Annex D
Advanced fire models – one-zone, two-zone and field models
Calculation of fire load densities and heat release rates based on building occupancy, size and type
Annex E
It is worth summarising the content of the annexes. It can be seen that the annexes deal with natural fire models.
Equivalent time of fire exposure – method of determining equivalent time and then compared with design value of standard fire resistance
Annex F
Annex G
Calculation of configuration factor including position and shadow effects

26. Section 4 Mechanical actions for structural analysis
If they are likely to occur during a fire the same actions assumed for normal design should be considered.
Indirect actions can occur due to constrained expansion and deformation caused by temperature changes within the structure caused by the fire.
The same actions (loads) considered in normal design should also be considered in the fire design, provided they are likely to occur. Compared to normal design, different partial load and material safety factors are used in the fire design.
Indirect actions can arise out of restrained thermal expansion.

27. Section 4 Mechanical actions for structural analysis
INDIRECT thermal actions should be considered. EXCEPT where the resulting actions are:
recognized a priori to be negligible or favourable.
accounted for by conservatively chosen models and boundary conditions or implicitly considered by conservatively specified fire safety requirements.
Indirect actions can be ignored if they are considered to be negligible or favourable. Further guidance is given in the fire parts of EN1992 to EN1996 and EN1999.

28. Section 4 Mechanical actions for structural analysis
For an assessment of indirect thermal actions the following should be considered:
Constrained thermal expansion of the heated members (i.e. columns in multi-storey frames)
Differing thermal expansion within statically indeterminate members.
Thermal gradients within the cross-section inducing internal stresses.
Indirect actions arising from restrained thermal expansion, thermal curvature in indeterminate members, thermal gradients should be considered.

29. Section 4 Mechanical actions for structural analysis
For an assessment of indirect thermal actions the following should be considered:
Thermal expansion of adjacent members (i.e. lateral displacement of a column head due to expanding beams/slabs.
Thermal expansion of heated members affecting other ‘cold’ members outside the fire compartment.
Indirect actions arising from thermal expansion of adjacent members and the effect on cold members outside the fire compartment should be considered.

30. Section 4 Mechanical actions for structural analysis
The indirect actions Aind,d should be determined using the thermal and mechanical properties given in the fire parts of EN1992 to EN1996 and EN1999.
For member design subjected to the standard fire only indirect actions arising from the thermal distribution through the cross-section needs to be considered.
Guidance on indirect actions is given in the fire parts of EN1992 to EN1996 and EN1999.

31. Section 4 Mechanical actions for structural analysis
Actions considered for ‘normal’ design should also be considered for fire design if they are likely to act at the time of a possible fire.
Variable actions should be defined for the accidental design situation, with associated partial load factors, as given in EN1990.
Decrease of imposed loads due to combustion should not be taken into account.
Snow loads need not be considered due if it assessed that the resulting fire will lead to melting of the snow.
Actions from industrial operations can be ignored for the fire design.
Actions considered in normal design should also be considered in the fire design. The effect of combustion on the imposed load should not be considered.
Snow loads should only be ignored if it can be shown that the load is removed due to fire melting the snow.

32. Section 4 Mechanical actions for structural analysis
Simultaneous action with other independent accidental actions does not need to be considered
Additional actions (i.e partial collapse) may need to be considered during the fire exposure
Fire walls may be required to resist horizontal impact loading according to EN1363-2
Simultaneous action of accidental load does not need to be considered. However, accidental loads that possible follow each other (i.e gas explosion followed by a fire) should be considered.

33. Section 4 Mechanical actions for structural analysis
The combination rules given in EN1990, for accidental loads should be followed for fire design.
Characteristic value of the leading variable action
Indirect thermal actions
Characteristic value of the leading variable action
Characteristic value of permanent action j
The overall action (load) is calculated using the combination rules for the accidental situation given in EN1990.
Factor for frequent value of variable action
Factor for quasi-permanent value of a variable action
Factor for quasi-permanent value of a variable action
Prestressing action

34. Section 4 Mechanical actions for structural analysis
When indirect actions do not need to be considered, and there is no prestressing force, the total design action (load) considering permanent and the leading variable action is given by;
Ignoring prestressing and indirect actions and assuming the applied actions are permanent (dead) and leading variable (live), then the equation reduces to that shown. Guidance on the use of the factor for either frequent or quasi-permanent value is given in the National Annex.
The use of 1,1 or 2,1 is defined in the National Annex

35. Section 4 Mechanical actions for structural analysis
The values of 1,1 and 2,1 are given in Annex A of EN1990:2002
The values of the factors for the frequent and quasi-permanent value are shown. It is of interest to note that the live load for offices is factored by 0.5 for the frequent value and 0.3 for the quasi-permanent value.

36. Section 4 Mechanical actions for structural analysis
As a simplification, the effect of actions in the fire condition can be determined from those used in normal temperature design.
Design value for normal temperature design
Design values of relevant actions in the fire situation at time t
A simple approach is to define a constant action for fire design based on a reduction of the action used in normal design. Guidance on reduction factors is given the fire parts of EN1992 to EN1996 and EN1999.
Constant design values of relevant actions in the fire situation
Reduction factor for design load level in the fire situation defined in EN1992-EN1996 & EN1999

37. Section 4 Mechanical actions for structural analysis
Design values of relevant actions in the fire situation at time t
Load Level:
Load levels are used in the design tables given in the fire parts of EN1992 to EN1996 and EN The load level is defined as the applied actions (load), for the fire design, divided by the resistance of the member at normal temperature.
Load level
Design value of resistance of member at normal temperature

38. End