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EN1991-1-2:2002. Eurocode 1: Actions on structures –. Part 1–2: General actions – Actions on structures exposed to fire. Describes the thermal & mechanical actions for the structural design of buildings exposed to fire. National Annex for EN1991-1-2.

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eurocode 1 actions on structures

EN1991-1-2:2002

Eurocode 1: Actions on structures –

Part 1–2: General actions –

Actions on structures exposed to fire

Describes the thermal & mechanical actions for the structural design of buildings exposed to fire

national annex for en1991 1 2

National Annex for EN1991-1-2

NDP’s in EN1991-1-2 are introduced into clauses:

2.4(4), 3.1(10), 3.3.1.1(1), 3.3.1.2(1), 3.3.1.2(2), 3.3.2(1), 3.3.1(2), 4.2.2(2), 4.3.1(2)

Annexes A to G are informative.

Covers National Determined Parameters (NDP’s) & Informative Annexes

The guidance given in the National Annex supersedes any guidance given in the main code or informative annex

basic outline
Basic Outline
  • EN 1991-1-2 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
objectives risk limitation
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
introduction
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.
scope
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.
structural fire design procedure
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
design fire scenario
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
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
temperature analysis
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.
mechanical analysis
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.
thermal actions for temperature analysis
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 for temperature analysis14
Thermal actions for temperature analysis
  • The net convective heat flux component can be expanded as:

Surface temperature of the member

Coefficient of heat transfer by convection – values of which are discussed later

Gas temperature in vicinity of fire exposed member

thermal actions for temperature analysis15
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

Configuration factor

Emissivity of fire

Effective radiation temperature of fire environment

Surface emissivity of member

thermal actions for temperature analysis16
Thermal actions for temperature analysis
  • The configuration factor should be taken as 1.0 unless EN 1992-1999 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.
section 3 nominal temperature time curves
Section 3Nominal temperature-time curves

Standard temperature-time curve:

1049°C

1006°C

945°C

842°C

Temperature(°C)

Time (mins)

section 3 nominal temperature time curves18
Section 3Nominal temperature-time curves

External fire temperature-time curve:

680°C

680°C

Temperature(°C)

Time (mins)

section 3 nominal temperature time curves19
Section 3Nominal temperature-time curves

Hydrocarbon fire temperature-time curve:

1100°C

1100°C

Temperature(°C)

Time (mins)

section 3 nominal temperature time curves20
Section 3Nominal temperature-time curves

Three fire curves used in the Code:

1100°C

1100°C

1100°C

1098°C

1049°C

1006°C

842°C

945°C

Temperature(°C)

680°C

680°C

680°C

680°C

Time (mins)

natural fire models
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.
natural fire models simplified fire models compartment fires
Natural fire modelsSimplified fire models - Compartment fires
  • Atmosphere temperature determined based on physical parameters considering at least the fire load density and ventilation conditions

Annex A provides a method for calculating atmosphere compartment temperatures.

natural fire models simplified fire models external members
Natural fire modelsSimplified 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.

natural fire models simplified fire models localised fires
Natural fire modelsSimplified 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.

section 3 natural fire models annexes methodology
Section 3Natural 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

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

section 4 mechanical actions for structural analysis
Section 4Mechanical 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.
section 4 mechanical actions for structural analysis27
Section 4Mechanical 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.
section 4 mechanical actions for structural analysis28
Section 4Mechanical 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.
section 4 mechanical actions for structural analysis29
Section 4Mechanical 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.
section 4 mechanical actions for structural analysis30
Section 4Mechanical 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.

section 4 mechanical actions for structural analysis31
Section 4Mechanical 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.
section 4 mechanical actions for structural analysis32
Section 4Mechanical 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
section 4 mechanical actions for structural analysis33

Section 4Mechanical 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

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

section 4 mechanical actions for structural analysis34

Section 4Mechanical 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;

The use of 1,1or 2,1is defined in the National Annex

section 4 mechanical actions for structural analysis35

Section 4Mechanical actions for structural analysis

The values of 1,1 and 2,1 aregiven in Annex A of EN1990:2002

section 4 mechanical actions for structural analysis36

Section 4Mechanical 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

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

section 4 mechanical actions for structural analysis37

Section 4Mechanical actions for structural analysis

Design values of relevant actions in the fire situation at time t

Load Level:

Load level

Design value of resistance of member at normal temperature