Performance-based structural fire safety design Performance-based fire safety design is an accepted methodology in both Finnish and European building regulations for the verification of structural resistance in fire conditions. A calculation procedure that takes into account the individual characteristics of the building and passive and active fire protection methods has been developed in a joint European research project. A realistic understanding of the behaviour of structures in fire can be achieved and the overall safety of the building can be verified by using performance-based fire safety design. Through the more profound understanding of phenomena and a more precise analysis of structures in fire, an equal to or higher safety level than with prescriptive fire design can be obtained.

Ruukki on metalliosaaja, johon voit tukeutua alusta loppuun, kun tarvitset metalleihin pohjautuvia materiaaleja, komponentteja, järjestelmiä ja ratkaisukokonaisuuksia. Kehitämme jatkuvasti toimintaamme ja tuotevalikoimaamme vastaamaan tarpeitasi. 1

CFI 00.000 FI/9/2008 Lönnberg Print

Performance-based structural fire safety design

Contents 1. Introduction .............................................................................................................................. 3 2. What is performance-based fire safety design? ...................................................................... 4 2.1. General ........................................................................................................................ 4 2.2. Objectives and Tools ................................................................................................... 5 2.2.1. Safety Level ............................................................................................................. 5 2.2.2. Choice of Design Fire .............................................................................................. 5 2.2.3. Calculation of Fire Development.............................................................................. 5 2.2.4. Behaviour of Structrures in Fire ............................................................................... 6 2.3. The Tasks and Deliverables of Structural Fire Safety Design..................................... 7 2.4. Design Tools ............................................................................................................. 11 3. Acceptability of Performance-Based Fire Design .................................................................. 11 3.1. Building Legislation Requirements............................................................................ 11 3.2. Required Documentation .......................................................................................... 11 4. Summary and Conclusions ................................................................................................... 11 References ................................................................................................................................. 12

2

Performance-based structural fire safety design

1. Introduction

A method has been developed, where ● the individual characteristics of a building, such as the fire scenario, fire load, pyrolysis rate, compartment type and venting, are taken into account. ● the risk of ignition in the building can be taken into account and the influence of active fire protection methods and building type can be evaluated. This risk analysis is based on probabilities that have been evaluated on the basis of real fire statistics. After the project, the method has been developed further so that the fire scenarios are based on a large number of Monte Carlo –simulations that take into account the same factors as above. ● design values for the main parameters (e.g. the fire load) are evaluated. ● the temperature development is determined as a function of the fire load. Fire risks and the extinguishing system are taken into account indirectly. - the temperature development should be taken into account during the complete duration of the fire, i.e. also during the decrease phase, which is structurally often problematic. ● the behaviour of the structure in fire is simulated on the basis of temperature development and static loading. ● the fire resistance time is determined. The fire resistance time can be infinite, which means that the structure will withstand the loads it is subjected to throughout the fire. ● the safety of the structure is secured by comparing the calculated fire resistance time to the required time that depends on evacuation times, the possibilities for required actions by fire fighters and the results of a possible collapse.

Present day structural fire resistance regulations are largely based on the so-called standard fire curve, which has led to very different practices in different European countries. The fire resistance time for a similar building can vary between 60 minutes in the Netherlands and 120 minutes in Finland, as is the case with a medium-height office building with sprinklers. The effects of sprinklers on structural fire resistance are not sufficiently taken into account in general either. Due to the different uses and other individual characteristics of buildings, fire resistance requirements should be based on factors that actually have an influence on the growth and the development of fires and the safety of persons. Such factors include: ● Fire compartment: - type of fire compartment - size of fire compartment - geometry of fire compartment - possible changes in the above during the life cycle of the building ● Fire: - different fire scenarios - probabilities of the occurrence of different fire scenarios - fire spread - duration and development of the fire; also the decrease phase should be taken into account - amount and distribution of fire load and changes in these during the life cycle of the building - heat release rate of fire ● Air conditioning and venting ● Structural system ● Evacuation possibilities ● Safety of fire fighters ● Danger of ignition of the neighbouring buildings ● Active fire resistance methods

This paper presents the basics of natural fire design, the contents of a structural fire safety plan and the acceptability of the methods for the fire safety design of buildings. In Finland, also the previously published guide book RIL 2212003 Paloturvallisuussuunnittelu (Fire Safety Design) [3] is available. It is clear that regardless what fire design method is chosen for a project, it is important to hold a fire engineering briefing among the contractor, the building administration authorities and the designers. This is of primary importance when performance based fire safety design is used.

In 1994, a European project called “Natural Fire Safety Concept” (NFSC) [1], [2] was started in order to carry out a systematic examination of the above factors. The project group included 10 research centres from different countries and the work was supervised by a guidance group that consisted of fire fighters, developers of fire regulations and designers from 11 countries. As a result of the project, a more realistic and reliable approach was developed, taking into account the effects of active fire protection and the characteristics of real fires. Natural fire development can be determined separately for different fire compartments in a building on the basis of each compartment’s individual characteristics. The method can be used for all building materials and all types of buildings.

3

Performance-based structural fire safety design

2. What is performance-based fire safety design?

ard fire curve. This leads to the simple equation t finat,d ≥ t fi,requ, which nevertheless can be used to take into account all the aforementioned factors. However, it should be noted that the required fire resistance time in this equation is not the same value as in standard fire design, but is also determined using a performance-based approach.

2.1. General Performance-based fire safety design (or natural fire safety design) is generally carried out as shown in Figure 1. Fire safety can be checked by comparing the required fire resistance time (t finatrequ) to the calculated fire resistance time (t fi d) in the same way as in fire design based on the stand-

Static actions - Permanent actions, - Variable actions, - Wind, - Snow - etc.

Characteristics of the fire compartment - Fire load - Venting - Geometry - Thermal characteristics of surfaces

On the other hand, a similar comparison can be carried out also in the strength domain.

Risk analysis - Fire activation risk - Effects of active fire fighting methods

Safety of persons - Evacuation time - Safety of fire fighters - Possible collapse of structures or required fire resistance time

Fire modelling => fire design curves - Monte Carlo –method - FDS –modelling - Other verified calculation softwares

Actions Combinations of actions in the fire situation.

Verification of design equation Comparison between the calculated and required fire resistance times. Requirement:

Structural design Rakenteiden kestävyys valituissa mitoituspaloissa.

t finat,d ≥ t fi,requ

Figure 1. Progress of performance-based fire safety design in a simplified form.

A comparison between different natural fire curves and the so-called standard fire curve (ISO fire curve) [4] is shown in Figure 2. The fire compartment size, the fire load, wall coverings, ignition properties etc. vary among the different natural fire curves. It can be seen how much the natural fires differ from the standard fire, which was developed mainly for standard testing and classification purposes.

●

●

A real or natural fire has properties that were not taken into account when the standard fire curve was developed (cf. Figure 3). In a standard fire, the temperature increases quickly to very high values in the whole compartment and continues to increase at a slower pace until the end of the test or analysis. The standard fire does not take into account for instance the geometry of the fire compartment, the type, amount and location of the fire load, the venting conditions (amount of oxygen available to the fire), the extinction phase of the fire etc. A real fire, on the other hand, consists of the following different phases: ● The smouldering phase, which includes the ignition and the smouldering phase at low temperatures. The duration of the smouldering phase is difficult to evaluate. ● The growing phase, or the pre-flashover phase (local fire or spreading fire). The duration of the growing

●

phase depends mainly on the properties of the fire compartment. The flashover, which marks the beginning of the general burning phase. The flashover is usually a relatively short event unlike the other phases of a natural fire. However, flashover does no always occur (e.g. in large spaces). The post-flashover phase, whose duration depends on the amount of fire load and the availability of oxygen. At this stage, the fire is at its strongest and the temperatures at their highest. The extinction phase, at which time the fire loses its strength and the temperatures decrease, until all burning material has been exhausted.

2.2. Objectives and Tools 2.2.1. Safety Level The objective of performance-based fire design is to try to reach a better understanding of what happens during a fire and to design buildings that are safe against fire by taking into account the effects of different factors on the safety of persons and structures. 4

Performance-based structural fire safety design

The objective is not to lower the safety level prescribed in fire safety regulations, but instead to determine more realistic individual values for factors affecting the fire safety of a given building. Through the more profound understanding of phenomena and a more precise analysis of structures in fire, an equal to or higher safety level than with prescriptive fire design can be obtained. The primary objective is to secure the safety of persons in the building and of emergency personnel and fire fighters. The secondary objective is to prevent or reduce the economic, material and structural damages caused by fires.

can then be used to determine a fractile corresponding to the desired safety level. A suitable combination of active (e.g. sprinklers, fire fighting measures) and passive (e.g. structural protection) fire protection methods can then be used to obtain the desired fire safety level, when performance-based design is used. It is the duty of building administration authorities to determine what risk scenarios should be taken into account in the design of a given building. The fire safety designer’s job is then to design the structure so that these requirements are met. The designer should present all analyses in an accurate and justifiable way and include all necessary sensitivity analyses in the report.

It has long been accepted that buildings are designed against statically determined actions such as self-weight, variable actions, snow and wind loads. The fire load of a building can also be determined as a similar type of statistical distribution. However, it is recommended nowadays to carry out a so-called Monte Carlo –computer simulation based on thousands of stochastically determined fire scenarios in order to determine the design values of fire loads and temperature curves. The large amount of fire curves

As in all other design work, also in performance-based design some things have to be taken into account in a simplified way. The designer has to make sure that all simplifications lead to results that are on the safe side. The building administration authorities may also use a third party for the verification of design plans.

1400

Uniform Gas Temperature

ISO - Curve compared to 50 Fire Tests in Laboratory (Fire Loads from 10 to 45 kg of wood / m²)

1200

REALISTIC FIRE DEVELOPMENT

1200 °C 1000 °C

1000

ISO- CURVE

ISO-CURVE

800 °C

Realistic fire curve

800

600 °C 400 °C

600

FLASHOVER

200 °C 400

Time [min]

0 °C

0

30

Pre-Flashover

200

θ

60

90

120

180

Fully Developed Fire

0 0

20

40

60

80

100

120

140

160

180

Figure 2. Time-temperatures curves for natural fires and the standard fire (EN 1636-1)

Figure 3. The different phases of natural fires and a comparison with the standard fire curve [5].

2.2.2. Choice of Design Fire

Calculation of Fire Development

The choice of the critical design fires for the designed building is an important phase in performance-based fire design. The number of possible fire scenarios is of course very large, but only a part of them can be considered critical and require further analysis. The characteristics and number of design fires depend on e.g. the geometry of the compartment, the use of the building, the fire load etc. The degree of criticality and probability of the occurrence of different fire scenarios should be determined. It is also important to remember to carry out sensitivity analyses on different factors. Fundamentally, the choice of design fires is the job of the building administration authorities and they should be discussed in the Fire Engineering Briefing at the start of the project.

There are different types of methods for the calculation of fire developments available: ● Simple models are mainly based on so-called parametric fire curves and are mainly used during the pre-design phase. Parametric fire curves take into account e.g. the amount of fire load, the size of the fire compartment and the size of openings. ● Zone models can be used to take into account all fundamental factors affecting the fire, in spite of their relative simplicity. ● Field models offer the only method for the calculation of buildings with complicated or unusual geometries. Field models include for instance the use of numerical fluid dynamics, or CFD 5

Performance-based structural fire safety design

Equations for parametric curves can be found for instance in the Eurocode (EN 1991-1-2 [6]. However, their results are often too inexact for use in performance-based fire design. Nevertheless, zone and field models can be used for sufficiently precise analysis of the temperatures and heat fluxes in the fire compartment.

zone model and local fire model) makes it possible to define the temperature fields and the temperatures of structures near the flame and further away from it. After flashover, the nature of the fire changes. The whole compartment is burning and it is assumed that it is sufficient to model the whole compartment using a single temperature curve. A one-zone model can thus be utilised.

The main parameter of fire development is the rate of heat release, RHR, which is a function of time and the size and use of the compartment. The rate of heat release is at its highest when a stable state is reached by the fire as determined by the amount of burning fire load (fuel) and the availability of oxygen. One factor to be determined is how the rate of heat release develops: whether a flashover will occur or will the fire remain localised.

Several different computer programmes have been developed for the use of zone modelling. However, their use should be restricted to compartments confined within the limits given in Table 1. Table 1. Limits for the use of zone modelling. L = length of compartment; W = width of compartment; H = height of compartment [7].

A two-zone model as shown in Figure 4 can be used before flashover. It is assumed that the burning of a local fire will produce two different types of zones in the compartment: a hot zone at the top part of the compartment and a cold (room temperature) zone at the bottom part of the compartment. In some cases, the use of two-zone models can lead to temperatures of the ceiling structures that are on the unsafe side (i.e. too low). To prevent this, it is necessary to also calculate the design case using local fire models, which have been developed within different research projects. The combined use of both models (two-

Acceptable (L/W) MAX (L/H) max (W/H) min

L/W < 3 L/H < 3 W/H > 0,4

Requires further analyses 3 < L/W < 5 3 < L/H < 6 0,2 < W/H < 0,4

Not acceptable L/W > 5 L/H >6 W/H < 0,2

At the present time, however, it is more common to model the fire using field modelling, i.e. CFD-analysis. The most commonly used CFD-software is FDS (Fire Dynamics Simulator) [8].

θ= Temperature of air at ceiling level

Temperature calculated using local fire model Temperature calculated using two-zone model

x z

Intermediate floor

Hot zone

θg Cold zone

20°C

θ

(hot zone)

θ

Figure 4. Combination of the uses of two-zone model and local fire model before flashover [4]. 2.2.3.

Behaviour of Structures in Fire

lated using different types of methods on the basis of a given temperature-time curve and the actions present during the fire situation.

When designing structures, it is essential to know their temperature development as a function of time and space. The heat transfer from the fire to structures can be calculated using methods of different accuracies. Also the resistance of structures in the fire situation can be calcu-

In a simplified model, the calculation is based on the socalled prescribed critical temperature. If the temperature of the structure stays below the critical temperature, the 6

Performance-based structural fire safety design

structure is acceptable. In other terms, if the time the structure needs for the attainment of the critical temperature is longer than the prescribed fire resistance time, the design objective is met (cf. also Figure 1).

2.3. The Tasks and Deliverables of Structural Fire Safety Design The characteristics of the building have to be known compartment by compartment before performance-based fire design can be used. In the following pages, the main tasks included in structural fire safety design and the corresponding deliverables are listed. Compulsory deliverables are marked with a dark filled square. The deliverables marked with an unfilled square are not mandatory for all design cases.

Also more advanced models, such as numerical methods based on finite element analysis, can be used. The results obtained usually include the deflections and deformations of the structure over the duration of the whole fire. The understanding of structural behaviour in fire makes it possible to set the fire safety criteria case by case on the basis of limited deformations and structural integrity and resistance. The performance-based design criteria depend also on the consequences of a possible structural collapse and the use of the building. For instance, in the case of high-rise buildings, this can mean that no kind of structural damage can be acceptable during fire.

The VTT Technical Research Centre of Finland publication “Toiminnallinen palotekninen suunnittelu ja suunnitelmien tarkastaminen: Näkökulmia ja ohjeita” [9] (Performance-based fire design and verification of designs: Points of view and instructions) has been used in the preparation of the list.

Table 2. The tasks and deliverables of structural fire safety design. Tasks

Deliverables

1. Determination of the use ja design data of the building Determination of factors affecting fire engineering design.

●

●

● ●

Assumptions made on the uses of the building during its whole life cycle. Explanations of the assumptions. Assumptions made on the possibilities for action of fire fighters and emergency personnel. Explanations of the assumptions. Failure analysis with explanations. Required service and maintenance during the use of the building.

Choice of methods and determination of their applicability. Regulations prescribe the use of only verified methods. Additionally, the methods have to be used by experts.

●

Description of the methods used. The description should state the calculation and testing methods used and their limitations, as well as initial data and made assumptions with explanations. All source references must be clearly stated.

Determination of acceptability criteria. Acceptability criteria set the limits for the safety of the design solutions. For the time being, these are agreed upon case by case with local authorities.

●

Structural acceptability criteria with explanations.

Choice of design standard and modelling methods. The same design standard system has to be used throughout the design process.

●

Determination of design standards to be used in design. If the design standard system does not include all necessary methods for the analysis of all different factors and other methods are used, the methods have to be validated. The application of all methods in the design has to be described with sufficient accuracy for it to make possible the repetition of the calculations by another party.

The inclusion of all necessary information regarding fire safety in the service and maintenance documentation of the building.

●

All necessary information on the use and modifications of use regarding fire safety are included in the service and maintenance documentation of the building. The documentation is updated during the life cycle of the building so that the latest information is always available to the owner and occupier of the building. 7

Performance-based structural fire safety design

Tasks

Deliverables

2. Fire modelling Determination of the geometry of the fire compartment.

●

The length, width, height and other necessary dimensions of the fire compartment. Small irregularities (e.g. consoles, columns and beams) do not usually have an effect on fire modelling using computer software and can often be ignored during this phase.

Determination of the surface characteristics confining the fire compartment.

●

The thermal characteristics (thermal conductivity, specific heat, emissivity, density etc.) of the walls, ceiling and floor of the fire compartment as functions of temperature,

Openings in the fire compartment.

●

Locations and dimensions of the openings, such as doors, windows and smoke vents. The opening and closing of openings during the fire. Evaluation of the durability of window panes during the fire. Assumption made on the breakage of windows. It should be noted that the breakage of windows is often difficult to determine accurately, for which reason a sensitivity analysis is necessary. Determination of the opening factor of the fire compartment.

● ●

❍

Determination of different possible fire scenarios. Performance-based fire design is based on chosen risk scenarios and the corresponding design fires that are set in cooperation with fire authorities before the start of the design project. A certain risk scenario is a description of how, where and when a fire takes place and what are the factors under fire threat.

●

Determination and description of different fire scenarios. This requires sufficient expertise from the designer and experience in the determination of different fire characteristics. Various design fires are presented on the website “Paloturvallisuussuunnittelijan oppimisympäristö” [10] (Learning environment for fire safety designers). A design fire describes how the strength of the fire or the amount of heat energy released by the fire changes over fire duration.

Choice of fire scenarios for closer analysis.

●

Critical evaluation of different fire scenarios and the choice of critical fire scenarios for closer inspection. The description of critical fire scenarios and the background for the choices. Explanation on the sufficiency of the chosen scenarios.

●

Closer determination of the critical fire scenarios.

●

The type, size and location of the fire load are determined for all different fire scenarios. The fire load density is not an accurately defined variable, but instead varies statistically according to the purpose of use of the building. A large part of fire load densities given in different reference documents are based on expired data and should be considered with caution. In certain cases it may be necessary to carry out additional verification calculations in order to define the fire loads.

Consideration of active fire fighting methods.

●

Effects of active fire fighting methods on structural fire resistance. Active fire fighting methods include appliances and instruments used to extinguish a fire or to prevent its spread by active means, such as fire detectors, first-aid extinguishing equipment and sprinklers. Also fire fighters are counted among active methods, but also the possible displacement of the fire department during the life cycle of the building should be taken into account. The sufficiently effective use of active fire fighting methods and appliances may make it possible to prevent all structural problems during the duration of the fire.

Consideration of venting.

●

Location and operation of venting and air-conditioning devices. Can the system be turned off completely or does it turn itself off automatically when signalled by fire detectors? The venting 8

Performance-based structural fire safety design

Tasks

Deliverables system may have a considerable influence on the availability of oxygen and fire spread.

Modelling of fire scenarios.

●

The modelling of each fire scenario that has been deemed critical is carried out with the chosen accuracy using appropriate methods and tools. At least the temperatures and heat fluxes of the gas in the fire compartment have to be given as functions of time and location during the whole duration of the fire (also during the decreasing phase). The deliverables shall also include the thickness of the smoke layer at different times and the realised fire heat release rate so that the verification and evaluation of the calculation results is made possible.

Sensitivity analyses.

●

Description and explanation of the sensitivity analyses carried out on different affecting factors. The sufficiency of the analyses shall be shown and conclusions made.

Reporting.

●

Detailed report on fire modelling, calculations and sensitivity analyses. Conclusions on the complete fire modelling and analysis.

3. Calculation of heat transfer from fire to structures Determination of structural geometries.

●

Dimensions and locations of individual load-carrying structural members.

Determination of heat transfer characteristics of the structural materials.

●

The heat transfer properties of different structural building materials as functions of temperature. Commonly necessary properties include heat conductivity, specific heat, density, surface emissivity and convection factor for the surface.

Definition of passive fire protection.

❍

Definition of all passive fire protection methods and products, if they are used. Examples of passive fire protection products for steel structures include fire boards, fire coatings, sprayed masses and other building materials, such as concrete. The protection provided by the fire protection product shall be determined as function of temperature and/or time on the basis of the thickness and type of the protective layer. Depending on the design case, the resulting passive fire protection is determined only after the basis of the complete fire design process.

●

Critical structural members are determined in the case of each different critical fire scenario. Depending on the design case, these can be situated close to the fire or at a distance from it. The designer may have to analyse several structural members in order to define the critical case.

●

Presentation on the accuracy level of the heat transfer analyses: 1D, 2D or 3D. In some cases, or for some building parts, several analyses of different accuracy levels may have to be carried out.

●

Determination of the influence of shadowing effects on the heat transfer to different parts of structural members. Some calculation softwares can carry this out automatically. Detailed report on the calculations taking into account passive and active fire protection methods. Separate report on each critical fire scenario.

Determination of critical structural members with regard to different fire scenarios.

Choice of accuracy level of analyses.

Heat transfer analyses in different fire scenarios.

●

●

9

Performance-based structural fire safety design

Tasks

Deliverables

Sensitivity analyses.

●

Description and explanation on the sensitivity analyses carried out on different affecting factors. The sufficiency of the analyses shall be shown and conclusions made.

Reporting.

●

Detailed report on heat transfer, calculations and sensitivity analyses. Conclusions on the complete fire modelling and analysis. Results of the temperature development calculations at the chosen accuracy for different structural members during the whole duration of the fire.

●

4. Structural analysis Determination of fire resistance requirements.

● ●

Required structural fire resistance time. Limits to the use of the structure.

Determination of the properties of structural building materials.

●

Strength and heat expansion properties as functions of temperature for the load-carrying structural members according to the applicable EN- (or national) standard.

Specification of design standard system used for the structural analysis.

●

Accurate and unambiguous determination of design standards and methods used during structural analyses. If the chosen standards system does not include instructions for the consideration of all necessary factors, and a different method is used, the reliability and applicability of this other method has to be established. Design standards belonging to different standardization systems are generally not allowed to be used in combination, i.e. if for instance actions are determined according to EN-standards, also the structural resistance shall be determined according to EN-standards.

Determination of structural analysis model.

●

The sketching of a simplified structural model. This can be done in a similar way as in normal temperature design. The connections between members and other boundary conditions that may cause forced actions due to heat expansion shall be shown in the structural analysis model. The designer shall decide if the structural members are considered as individual members or if the complete structure is considered. This will also have an effect on the choice of analysis software, and vice versa.

●

Determination of actions and combinations of actions on structures during the fire situation according to the regulations and instructions given in the applicable design standard.

●

Stability analysis. The verification of the functioning of the complete structural frame. Design analysis in the accidental situation (fire situation). Calculation of deflections and deformations. Local stability analysis. Design of connections.

Determination of actions on structures during the fire situation. Structural analysis.

● ● ● ●

Sensitivity analyses.

●

Description and explanation on the sensitivity analyses carried out on different affecting factors. The sufficiency of the analyses shall be shown and conclusions made.

Reporting.

●

Detailed report on analyses of structural behaviour, calculations and sensitivity analyses. Conclusions on the complete structural analysis. 10

Performance-based structural fire safety design

Tasks Reporting.

Deliverables ●

Documentation of the behaviour and degrees of utilization of all different structural members at the chosen accuracy.

2.4. Design Tools

– one-zone models assuming a uniform, time dependent temperature distribution in the compartment; – two-zone models assuming an upper layer with time dependent thickness and with time dependent uniform temperature, as well as a lower layer with a time dependent uniform and lower temperature; – Computational Fluid Dynamic models giving the temperature evolution in the compartment in a completely time dependent and space dependent manner.”

Different types of special computer softwares are needed for performance-based design of buildings due to the difficulty of the task. In the future, the objective will be to include all fire design related data and calculation methods in a product data model for the whole building, but for the time being, the use of several different computer programmes at different phases of the design process is necessary.

In the foreword to the European standard on the design of steel structures, EN 1993-1-2: 2005 Eurocode 3: Part 1.2 [13], the essential requirements for the limitation of fire risks (the resistance of structures, the development of fire and smoke and the general requirements for the safety of persons) given in Construction Products Directive 89/106/ EEC are given, and also the following is stated:

Depending on the design case, fire modelling can be carried out using zone models (e.g. OZone-software [11]) or computational fluid dynamics (e.g. FDS [8]). After the fire modelling has been done, the heat transfer to the structures has to be modelled. In a simple case, this can be carried out using spreadsheet calculations and basic heat transfer equations, but a more advanced analysis using e.g. finite element analysis (FEA) is often needed. Finally, the resistance of structures has to be modelled using different types of structural analysis and/or FEA-softwares.

“ Required functions and levels of performance can be specified either in terms of nominal (standard) fire resistance rating, generally given in national fire regulations or by referring to fire safety engineering for assessing passive and active measures.”

3. Acceptability of Performance-Based Fire Design 3.2. Required Documentation 3.1. Building Legislation Requirements Section 1.3.2 of the Finnish National Building Code Part E1 [12] lists the requirements set for the contents of the documentation on fire design. These requirements have been included also in the list of tasks and deliverables given in Table 2.

The use of performance-based fire design is accepted in both Finnish and European building regulations. The Finnish Building Code Part E1 “Rakennusten paloturvallisuus, Määräykset ja ohjeet, 2002” (Fire Safety of Buildings, Requirements and guidelines, 2002) [12] section 1.3.2 states that

4. Summary and Conclusions

”The fire safety requirement is deemed to be satisfied also when the building is design and built on the basis of performance-based fire design that covers all likely events taking place in the building in question. The satisfaction of the requirements is verified case by case taking into account the characteristics and use of the building.”

Performance-based fire safety design is an accepted methodology in both Finnish and European building regulations for the verification of structural resistance in fire conditions. A calculation procedure that takes into account the individual characteristics of the building and passive and active fire protection methods has been developed in a joint European research project.

In the European design standard EN 1991-1-2 [6] dealing with actions on structures, it is prescribed that “[a]dvanced fire models should take into account the following: – gas properties; – mass exchange; – energy exchange.”

A realistic understanding of the behaviour of structures in fire can be achieved and the overall safety of the building can be verified by using performance-based fire safety design. Through the more profound understanding of phenomena and a more precise analysis of structures in fire, an equal to or higher safety level than with prescriptive fire design can be obtained. The primary objective is to secure the safety of persons in the building and of emergency personnel and fire fighters. The secondary objective is to prevent or reduce the economic, material and structural damages caused by fires.

These factors are taken into account in both one-zone and two-zone models and in the more advanced fluid dynamic models. In the same standard, it is furthermore stated that “[o]ne of the following models should be used: 11

Performance-based structural fire safety design

This paper has been drafted by D.Sc. (Tech.) Olli Kaitila of the Finnish Constructional Steelwork Association on the basis of the referenced documents. The text has been commented upon by D.Sc. (Tech.) Jyri Outinen, Rautaruukki Ltd.; D.Sc. (Tech.) Jukka Hietaniemi, VTT Technical Research Centre of Finland; Professor, D.Sc. (Tech.) Markku Heinisuo, Tampere University of Technology; and Jussi Rahikainen, Director of Risk Management, Keski-Uusimaa Rescue Department.

A fire engineering briefing, where the contractor, building administration authorities and the designers agree on the fire safety design process, should be held at the start of the project. It is the duty of the authorities to set the necessary requirements used in the design of the building.

References [1]

Schleich J-B., Cajot L-G., et al.: ”Competitive steel buildings through natural fire safety concept.” ECSC Research 7210-SA/125,126,213,214,323,423,522, 623,839,937, B-D-E-F-I-L-NL-UK & ECCS, 1994-98, Draft Final Report July 2000 - Parts 1 to 5.

[2]

Schleich J-B., Cajot L-G., et al.: ”Valorisation project – Natural fire safety concept.” Final report. European Commission EUR 20349 EN, European Communities, 2002.

[3]

RIL 221-2003 Paloturvallisuussuunnittelu. Suomen rakennusinsinöörien liitto RIL r.y. 2003. (Fire Safety Design, The Finnish Association of Civil Engineers, RIL, 2003.) In Finnish.

[4]

EN 1363-1 Fire resistance tests. Part 1: General requirements. CEN European Committee for Standardization, Brussels 2000.

[5]

DIFISEK Dissemination of Structural Fire Safety Engineering Knowledge, Draft Final Report, Research Programme of the Research Fund for Coal and Steel, 2005.

[6]

[7]

[8]

Fire Dynamics Simulator (Version 5). User’s Guide. Kevin McGrattan & Bryan Klein (NIST), Simo Hostikka (VTT), Jason Floyd (Hughes Associates, Inc.), NIST National Institute of Standards and Technology in cooperation with VTT Technical Research Centre of Finland. NIST Special Publication 1019-5, January 8, 2008. www.fire.nist.gov/fds

[9]

Toiminnallinen palotekninen suunnittelu ja suunnitelmien tarkastaminen: Näkökulmia ja ohjeita. 2. versio, päivitetty 10.9.2007. (Performance-based fire design and the verification of design plans. Points of view and guidelines. 2. version, updated 10.9.2007.) Jukka Hietaniemi, VTT, 2007. In Finnish.

[10] Paloturvallisuussuunnittelijan oppimisympäristö. (Learning environment for fire safety designers.) http://proxnet.vtt.fi/fise/simon/Fise/opetusmateriaali/fise_ etusivu.html Login name: fise-reader. Password: June 1906. Mostly in Finnish. [11] OZone V2.0 – Theoretical Description and Validation on Experimental Fire Tests. JF Cadorin, D. Pintea, JM Franssen, University of Liege, Belgium. 1st Draft, June 11, 2001. http://www.argenco.ulg.ac.be/logiciel.php

EN 1991-1-2: 2003 Actions on structures – Part 1.2 : General Actions – Actions on structures exposed to fire. CEN European Committee for Standardization, Brussels. 2003.

[12] E1 Suomen Rakentamismääräyskokoelma. Rakennusten paloturvallisuus. Määräykset ja ohjeet 2002. Ympäristöministeriö 2002. (E1 Finnish National Building Code. Fire Safety of Buildings. Requirements and Guidelines 2002. Ministry of Environment 2002.) In Finnish.

Palotekninen erityissuunnittelu vyöhykemalleja käyttäen. Lehtimäki et al., Helsinki. Suomen Pelastusalan Keskusjärjestö. 58 s. + liitt. 16 s. (Tekniikka opastaa 12.) (Advanced Fire Engineering Design using Zone Modelling. Lehtimäki et al., Helsinki. Suomen Pelastusalan Keskusjärjestö SPEK (The Finnish National Rescue Association).

[13] EN 1993-1-2: 2005 Eurocode 3: Design of steel structures – Part 1.2: General rules – Structural fire design. CEN European Committee for Standardization, Brussels 2005.

• Ota yhteyttä asiakaspalveluumme Myynti ja tekninen tuki

puh. 020 59 11

Rautaruukki Oyj

www.ruukki.com

Tämä ohjelehti on tarkistettu mahdollisimman huolellisesti. Emme kuitenkaan vastaa mahdollisista virheistä tai tietojen väärästä soveltamisesta aiheutuneista välittömistä tai välillisistä vahingoista. Oikeudet muutoksiin pidätetään. Copyright © 2008 Rautaruukki Oyj. Kaikki oikeudet pidätetään. Ruukki, More With Metals, Rautaruukki ja Ruukin tuotenimet ovat Rautaruukki Oyj:n tavaramerkkejä tai rekisteröityjä tavaramerkkejä.

12