ISO TC 92/SC 4 Reference number ISO/TS 24679 2011(E) © ISO 2011 TECHNICAL SPECIFICATION ISO/TS 24679 First edition 2011 03 15 Fire safety engineering — Performance of structures in fire Ingénierie de[.]
Design process for fire safety of structures
Many countries continue to rely on prescriptive requirements and standardized tests for fire safety design, but there is a growing trend towards using calculation methods to assess structural performance in fires This shift is driven by a deeper understanding of how structures behave in fire conditions and advancements in knowledge regarding their thermal and mechanical responses at high temperatures Such insights facilitate more accurate simulations of real fire scenarios in built environments However, most calculation methods are still in development, primarily serving as alternatives to traditional fire tests to address their limitations, and they often consist of simple models focused on isolated elements and assemblies.
⎯ load-bearing fire performance for common construction materials such as steel, concrete and timber;
⎯ heat transfer, by conduction, through non-load-bearing separating elements, when the thermal properties of the component materials are known
Simple calculation methods and standard tests can rank elements based on their resistance to conventional fire, but they fall short in assessing performance during real-fire scenarios, such as localized or fully developed fires and the cooling phase Consequently, the existing design approach for fire safety relies on basic assumptions, potentially resulting in inadequate safety measures.
3 limited flexibility in design as well as very little or no opportunity for accurate optimization of fire safety measures in a built environment
However, it is being made increasingly possible to either use advanced calculation or develop simplified calculation to deal with the behaviour of structure in real-fire situations
This Technical Specification outlines a methodology for evaluating the fire performance of structures during actual fire incidents It emphasizes an engineering approach to designing fire safety in structures.
⎯ defining the built-environment characteristics, including geometry, actions, materials, etc.;
⎯ identifying clear objectives for the fire safety of structures;
⎯ identifying performance criteria for elements of construction in the context of the objectives for fire safety of structures;
⎯ considering design fire scenarios that could develop in the built environment and challenge the structure and the enclosure boundaries;
⎯ assessing the fire performance of the built environment (load-bearing and non-load-bearing) elements and the structure as a whole system;
⎯ examining the fire performance of the structure against the identified objectives and established performance criteria by taking into account realistic design fire scenarios
Figure 1 is a flow chart showing the overall design process for the fire safety of structures More details are provided in Clause 5 on quantification
Scope of the project for structural fire safety (See 5.2 for details about the scope)
Identify structural fire safety objectives, functional requirements, and performance criteria
Specify a trial design plan for fire safety of structures
Determine design fire scenarios and design fires
Evaluate the thermal response of the structure
Evaluate the mechanical response of the structure
Are objectives and performance criteria satisfied ?
Reconsider the scope of the project
Reconsider non-mandatory objectives, functional requirements and performance criteria
Figure 1 — Fire safety of structures — Design process
Objectives and functional requirements for fire safety of structures
Conducting a rational fire safety design of structures requires the establishment of fire safety objectives and functional requirements
The fire safety objectives usually address life safety, conservation of property, continuity of operations, preservation of heritage, and protection of the environment (singly or in combination)
The functional requirements for ensuring fire safety in structures focus on compartmentation, integrity, and stability Compartmentation is essential for preventing or limiting the spread of fire within a building.
To prevent or limit the spread of fire within buildings, it is essential to understand fire dynamics and the effects of pressure, thermomechanical deformation, and heat transfer Fire, including flames and smoke, can jeopardize life safety and diminish the value of the structure and its contents Implementing the concept of compartmentation, which involves dividing the built environment into fire enclosures with barriers such as floors or walls, is crucial for containing fires to their origin.
To prevent or limit the spread of fire to adjacent built environments and the natural surroundings, it is crucial to ensure that enclosure boundary walls, floors, and roofs are designed to resist secondary ignition and contain interior fires These structures can act as secondary fuel sources or fail, allowing fires to vent outside and pose risks to nearby areas, especially when hazardous materials are involved Therefore, maintaining adequate fire performance in these enclosures is essential, and strategically positioning buildings at a safe distance from potential fire hazards can further mitigate the risk of fire spread.
To ensure safety in the built environment, it is crucial to maintain the integrity of separating elements This approach enhances escape time, safeguards escape routes, and improves firefighter access during rescue operations Additionally, it limits potential loss areas, mitigates fire impact on structures and contents, separates different occupancies, isolates hazards, and contains hazardous material releases during and after a fire Furthermore, preserving the integrity and stability of the structure is essential for preventing or minimizing structural failure.
To ensure life safety and protect property, it is crucial to prevent or limit structural failure caused by thermal deformation and the reduction of strength in heated components Collapse can occur in heated or non-heated portions of a structure, creating dangerous situations, especially if occupants are present Even without collapse, deformation can compromise exit paths and lead to significant property damage Therefore, structural elements must possess adequate fire performance in terms of integrity and stability to delay or prevent failure This is essential for load-bearing members and barriers, which also provide containment, necessitating that both primary and secondary structural elements are designed with appropriate fire performance standards.
2) Maintain the integrity and/or limit the deformation of the structural elements of the built environment
To meet the fire safety objectives and functional requirements for structures, the duration required to achieve these goals can be determined by the stakeholders as the time needed for complete burnout.
The time required for complete evacuation and the response time for the fire department to control a fire are critical factors Interested and affected parties may also identify additional timeframes relevant to their specific situations.
In satisfying the functional requirements, consideration should be given to the existence of active and passive fire control systems and their effectiveness.
Performance criteria for fire safety of structures
Performance criteria to limit fire spread (compartmentation)
The existing performance criteria relate to those found in ISO 834-1 and are as follows:
⎯ Insulation criteria: in the form of a limited temperature rise of 140 °C on average, reaching a maximum of
180 °C, on the unexposed side of separating (load-bearing and non-load-bearing) elements These limiting values are generally a very conservative means of assessing the risk of fire spread
Integrity criteria are evaluated by igniting a cotton pad or examining gaps between load-bearing and non-load-bearing elements However, both the cotton pad test and the gap test lack the ability to provide adequate quantitative data.
The new (relevant) performance criteria are concerned with setting limit values so that enclosure boundaries meet the objectives and functional requirements for the fire safety of structures
To prevent the ignition of combustible materials on the unexposed side of separating elements, it is essential to establish criteria that limit heat transfer and thermal radiation from both load-bearing and non-load-bearing components This consideration must account for the relative positioning of materials, including penetrating and lining materials, as well as any combustible substances in adjacent enclosures Additionally, the types of materials involved and the potential risk of injury to occupants should be evaluated These criteria can be quantified through measurements of heat flux or the temperature on the unexposed side.
To limit the spread of hot fire gases, it is essential to implement a criterion that separates load-bearing and non-load-bearing elements This approach aims to prevent the ignition of combustible materials on the opposite side and protect occupants from injury The effectiveness of this criterion can be quantified by measuring the leakage rate.
Performance criteria to limit structural damage (structural stability)
The existing performance criteria relate to those found in ISO 834-1 and are as follows:
Load-bearing criteria are determined by limiting deflection and elongation, as well as the rate of deformation To ensure the stability of load-bearing elements, it is essential to realistically consider their interaction with boundary and other structural elements under appropriate service load conditions.
The new performance criteria focus on establishing limit values to ensure that load-bearing elements and the overall structure fulfill fire safety objectives and functional requirements When defining these criteria, it is essential to consider the limits for structural collapse and the permissible deflections, elongations, and contractions of structural elements, as well as the effects of additional mechanical actions on adjacent load-bearing and non-load-bearing elements, which may lead to cracks and openings.
The levels of structural stability that should be considered are as follows:
⎯ A criterion for providing sufficient structural stability of the load-bearing element for safe evacuation from the built environment
⎯ A criterion for providing sufficient structural stability of the load-bearing element for safe internal firefighting rescue and extinguishment activities in the built environment
⎯ A criterion for providing sufficient structural stability to critical elements of the structure (local failure of non-critical structural members is permitted)
⎯ A criterion for providing sufficient structural stability to avoid any progressive or sudden global failure of the structure
To ensure reliability and address uncertainties, it is essential to incorporate safety margins in calculating the annual probability of structural failure and associated risks when limit states are exceeded Typically, the values for reliability and uncertainty are derived from historical data collected over time Additionally, risk assessment results offer a logical foundation for establishing target reliability levels.
The new performance standards for protecting individuals should be established based on the ASET (Available Safe Escape Time) and RSET (Required Safe Escape Time) framework, considering the potential damage to structures and the environment, as well as the maximum allowable downtime before reoccupying the space It is essential for all interested and affected parties to participate in determining these performance criteria during the initial design report and whenever the criteria are updated.
5 Quantification of the performance of structures in fire
Fire performance of structures — Design process
Table 1 outlines the key steps and parameters essential for evaluating the behavior of structures exposed to fire Further details regarding these steps are provided in the subsequent sections of the article.
Figure 2 presents a flow chart that outlines the methodology encompassing three key steps: "Determine design fire scenarios and design fires," "Evaluate the thermal performance of the structure," and "Evaluate the mechanical response of the structure," as referenced in Figure 1 and Steps 4 to 6 of Table 1 This chart offers a comprehensive understanding of a rational approach to fire safety for structures subjected to real fire conditions As depicted in Figure 2, the inputs are derived from Steps 1, 2, and 3 of Table 1, leading to the outputs necessary for the assessment in Step 7 of Table 1.
Table 1 — Design and quantification process
No To consider To determine or identify Input Factors of influence
1 Scope of the project for fire safety of structures
⎯ Context and purpose of the design and/or the different parts
⎯ Mechanical actions, including existing structural loads on the elements of the structure or loads induced by the fire such as pressures
⎯ Structural systems to be analysed
2 Identifying objectives, functional requirements and performance criteria for fire safety of structures
⎯ Limiting or preventing fire spread
⎯ Limiting or preventing structural failure
⎯ Performance criteria to fulfil the objectives and requirements
⎯ Statements in codes, standards and guidance documents
⎯ Type of occupancy of built environment to be designed
⎯ Interested and affected parties including code officials, owners, and fire safety professionals
⎯ Existence of active and passive fire systems and effectiveness of these systems
3 Trial design plan for fire safety of structures
⎯ Strategy for fire safety of structures
⎯ Design elements and functions to be considered for the fire safety of structures include structural stability, integrity, containment and compartmentation
⎯ Objectives, functional requirements and performance criteria
⎯ Type and method of analysis
⎯ Type of occupancy of built environment to be designed
⎯ Fuel loads and distribution in compartments
⎯ Pressure in the fire enclosures
4 Design fire scenarios and design fires (fire development)
⎯ Thermal actions on the elements of the structure
⎯ Reliability and response time of suppression systems
⎯ Fire safety management plan and procedures
⎯ Fire department response and intervention time
No To consider To determine or identify Input Factors of influence
⎯ Ignition by flames and/or smoke
⎯ Paths of fire spread (openings and/or breaching of boundaries)
⎯ Temperatures and pressures in enclosures
⎯ Method of analysis chosen (e.g deterministic fire analysis or fire risk assessment)
5 Thermal response of the structure ⎯ Temperatures in elements of the structure ⎯ Temperatures in every enclosure
⎯ Heat transfer data for thermal response of the elements of the structure
⎯ Thermal properties of the elements of the structure
⎯ Paths of fire spread (openings and/or breaching of boundaries)
⎯ Effects of temperatures and pressures in enclosures
6 Mechanical response of the structure ⎯ Structural analysis (stability and deformation of separating elements and structural elements including connections)
⎯ Failure and time to failure of the different elements of the structure
⎯ Failure and time to failure of the whole structure
⎯ Temperatures in elements of the structure
⎯ Mechanical properties of the elements of the structure
⎯ Characteristics of structural elements and connections
⎯ Effects of connections on load redistribution and continuity
7 Assessment against the fire safety objectives
⎯ Are the objectives defined in step 2 satisfied?
⎯ No, make changes in Steps 1, 2 or 3 (depending on reconsiderations) and repeat the process from the appropriate step
⎯ Results of the analysis ⎯ Interested and affected parties
8 Documentation of the design for fire safety of structures
⎯ A document containing all the assumptions and calculations ⎯ Results of the analysis ⎯ Interested and affected parties
I nputs from Steps 1, 2 and 3 in Table 1
Start of a fire in a compartment or outside the structure
New fire compartment size or fire spread to another built environment
Evolution of thermal fire and pressure conditions (temperature or heat flux history) exposing the separating and structural elements
Evolution of temperatures within the separating and structural elements (Thermal response )
Fire spread outside the fire compartment or inside the built environment
Degradation, deformation and loss of strength of materials used for separating and structural elements (Structural response )
SEPARATING ELEMENTS Cracks in elements or openings between elements leading to integrity failure
STRUCTURAL ELEMENTS Thermal deformations, interaction with other elements, including connections, loss of strength
- Temperature rise on unexposed side
- Increase in radiation to combustibles in adjacent compartments and built environments
Loss of stability of the whole structure
Outputs to Step 7 in Table 1
Figure 2 — Overview of a rational design process for Steps 4 to 6 in the fire safety of structures
The subsequent subclauses elaborate on the steps outlined in Table 1, enabling readers to better comprehend how structures respond to fire and evaluate their fire performance.
Scope of the project for fire safety of structures
Built-environment characteristics
The project designer generally has knowledge of the general characteristics of the built environment and of the enclosure of fire origin More specific characteristics are then usually developed (see 5.4).
Fuel loads
To evaluate the appropriate design fire for a structure, it is essential to assess fuel loads or fuel load densities, which can be obtained from existing databases or surveys of the built environment Fuel load densities are typically measured in megajoules per unit area and are influenced by the types of combustible materials present, their quantities, and their locations within the structure.
Mechanical actions
When evaluating the mechanical effects of applied or weathering loads, the likelihood of a fire coinciding with extreme mechanical loads in a built environment is deemed low, as fire is classified as an accidental action Consequently, the loads considered for assessing the fire behavior of a structure or its components are less than those utilized in standard structural design.
The load ratio is a crucial concept in fire design for structures, representing the ratio of anticipated loads during a fire to the load-bearing capacity under normal conditions A lower load ratio indicates improved structural fire performance.
In regions with significant seismic risk, fire risk assessments should include potential structural damage and impacts on both load-bearing and non-load-bearing elements, as well as threats to fire suppression systems and water supplies during an earthquake.
Fire can trigger mechanical actions through various assessment methods These actions include: a) gas pressure from the fire; b) impacts from falling elements on structural components; c) hose stream impacts from firefighters on the unexposed sides of structural elements; d) forces from thermal expansion or contraction at structural boundaries; and e) deformations in elements like beams or floors that apply loads on non-load-bearing elements or cause deflections affecting the integrity of both load-bearing and non-load-bearing components.
Identifying objectives, functional requirements and performance criteria for fire safety of
Fire safety objectives are usually addressed in terms of life safety, conservation of property, continuity of operations, preservation of heritage, and protection of the environment The functional requirements and
Performance criteria for ensuring fire safety in structures focus on compartmentation, integrity, and stability Detailed information on this topic can be found in Clause 4.
Trial design plan for fire safety of structures
The fire safety trial design plan for structures outlines a comprehensive strategy that includes essential design elements, focusing on stability and compartmentation to ensure effective fire safety.
The fire design report must detail the plan to evaluate if the fire safety objectives and performance criteria for structures are achieved when analyzed against design fire scenarios This design plan outlines the functions of the built environment in alignment with the fire safety strategy, considering the interactions among all components of the fire safety design.
ISO 23932 provides some useful information on the functions and design elements for consideration in a fire safety design.
Design fire scenarios and design fires
General
Design fire scenarios and design fires play a crucial role in evaluating the fire performance of structures A design fire scenario provides a qualitative outline of how a fire may develop, while a design fire offers a quantitative analysis of the expected fire characteristics within that scenario.
See ISO 16733 for more information on the selection of design fire scenarios and design fires.
Design fire scenarios
Selecting appropriate fire scenarios is essential for effective fire safety design, as these scenarios significantly impact all design elements and serve as the foundation for various quantification processes.
In any built environment, countless fire scenarios can occur, making it impractical to analyze every potential situation, even with advanced computing resources Therefore, it is essential to narrow down these possibilities to a manageable set of design fire scenarios that can be effectively analyzed.
Characterizing a design fire scenario for analysis involves detailing the initiation, growth, and extinction of fire, as well as potential fire spread routes under specific conditions This characterization must consider the impacts of fire on people, property, structures, and the environment, ensuring that these consequences align with the defined fire safety objectives.
For the design of structures in fire, a fire scenario represents a particular combination of events and circumstances associated with factors such as:
⎯ type of fire (e.g location with respect to the load-bearing elements, size of fire);
⎯ distribution and type of combustible materials;
⎯ status of the active systems and passive fire safety measures, and their performance and reliability
A localized compartment fire may occur in a corner near a column, characterized by an open door and the absence of sprinkler protection In this scenario, there is no manual intervention from either occupants or fire services to extinguish the fire.
See ISO 16733 for more information on the selection of design fire scenarios and design fires.
Design fires (thermal actions)
When evaluating a structure's behavior in a fire, it is essential to consider thermal actions or design fires based on realistic fire scenarios These thermal actions are typically represented as time-temperature or time-heat flux relationships To accurately assess the impact of temperature or heat flux on both load-bearing and non-load-bearing elements, it is crucial to account for both convective and radiative heat effects.
Temperature in the volume affected by a fire is a function of time and space The parameters to consider when determining the design fire (fire development) in a built environment include:
⎯ built-environment geometry (surface area, height of storeys, enclosures, wall and floor types, sizes and positions of openings, types of glazing, etc.); and
⎯ fire characteristics (fire load based on statistics or actual evaluation, ventilation conditions, glass breakage, and heat release rate determined from existing literature or tests)
The design fire is affected by various factors, including human behavior, active fire protection measures like sprinkler systems, and firefighting operations Considering these elements is essential for accurately assessing the heat flux-time and temperature-time relationships.
The design fire used in calculations must accurately reflect real fire scenarios It is essential to define the fire scenario for design considerations, distinguishing between various fire types A nominal fire is characterized by a specific temperature-time curve, such as the standard ISO fire or hydrocarbon fire; however, their correlation to actual fires is not well established, making them unsuitable for assessing structural safety An "analytical" fire considers key parameters influencing gas temperature but remains a rough approximation of real fires, including parametric fires and localized plume fires Additionally, a fire can be modeled based on its development within enclosures, factoring in the built environment's geometry, ventilation, and fuel characteristics, as well as external fire exposures Lastly, fires resulting from tests should be evaluated for their relevance to real situations.
In fully developed fires, the combustion rate is constrained by either the available fuel or ventilation The burning rate in a compartment is primarily influenced by the airflow entering it While fuel-bed-controlled fires are less common, they can occur in specific scenarios, such as in storage-type environments or external fires with ample ventilation.
To evaluate various design fire types for structural fire safety, several calculation methods or models can be employed These include simple analytical formulas, which rely on assumptions and approximations for fire development in a single enclosure, as well as numerical calculations that utilize advanced models to analyze fire development in one or more enclosures.
1) one-zone models, which are generally applied in post-flash over conditions;
NOTE Homogeneous properties of the gas are assumed in the enclosure
2) two-zone models, which are based on the assumption that combustion products accumulate in a layer beneath the ceiling, with a horizontal interface with the lower cold layer;
3) field models that solve numerically the differential equations governing combustion and give calculated quantities for all points of the enclosure
See ISO 16733 for more information on the selection of design fire scenarios and design fires.
Thermal response of the structure
Effective structural design for fire safety involves calculating temperature profiles in elements exposed to thermal actions By understanding the thermal conditions, such as heat flux and temperatures, on both load-bearing and non-load-bearing elements, it is possible to determine the temperature field over time.
⎯ heat transfer from flames and smoke to the structural elements through radiation and convection;
⎯ heat transfer within the element (mainly by conduction in the case of a solid element, but also by convection and radiation when there are cavities within the element); and
⎯ heat loss to adjacent elements, adjacent spaces or materials
Temperature variations in structural elements can occur due to localized fire effects or heat transfer to cooler areas To determine temperature profiles, several assumptions can be applied: a) for fully engulfed elements made of high thermal conductivity materials like steel or aluminum alloys, a uniform temperature is assumed across the cross-section; b) for flat elements heated on one side, such as concrete slabs, or fully engulfed axisymmetric elements like circular columns, one-dimensional heat transfer is considered; c) to analyze the temperature distribution within a cross-section, two-dimensional heat transfer is utilized; and d) for elements with non-uniform temperature distributions along their axis or surface, three-dimensional heat transfer analysis is necessary.
Understanding the reference time-temperature relationship of surrounding gas and boundaries, as well as the time-heat flux, is crucial It is essential to consider the thermal properties of the materials involved, which include thermal conductivity, specific heat, density (especially when it varies with temperature), melting points, and other phase-changing points Additionally, the conditions under which these thermal properties were determined must be taken into account, as properties suitable for one fire severity may not be applicable to another, as highlighted in ISO 834-1:1999, Figure 7.
To achieve precise temperature fields, it is essential to consider mass transfer resulting from moisture content in various structural elements, or alternatively, to select the specific heat accurately when direct mass transfer analysis is challenging Furthermore, the temperature distribution can be significantly affected by phenomena such as spalling, melting, or cracking of materials.
Mechanical response of the structure
Heating structural elements can lead to expansion in materials like aluminum, steel, and concrete, or contraction in wood, resulting in thermal gradients and a decrease in mechanical properties such as stiffness and strength These changes, combined with mechanical actions, cause deformations The mechanical analysis of a heated structure aims to evaluate either the load-bearing capacity after a specific duration of fire exposure or the deformation of the structure or its components.
The load-bearing capacity of materials diminishes as temperature rises, while deformation tends to increase with temperature Understanding these changes necessitates knowledge of the mechanical properties at varying temperatures The assessment of a structure's fire performance can be conducted through two distinct representations.
A comprehensive global structural analysis must consider critical failure modes during fire exposure, the temperature-dependent properties and stiffness of materials, and the impacts of thermal expansion or contraction that can lead to interactions between structural elements.
The analysis of structural components assumes that the loading and restraint at the boundaries remain constant during fire exposure, disregarding the effects of thermal expansion or contraction at these boundaries This approach introduces uncertainties in the calculations, which designers must consider when evaluating the structural integrity under fire conditions.
This Technical Specification does not cover the analysis of elements related to standard fire-resistance requirements, such as ISO fire or other nominal fire standards, as these topics are discussed in separate ISO documents Typically, factors like thermal expansion or contraction, continuity, and load redistribution are not considered in this context.
Assessment against the fire safety objectives
To evaluate the fire safety of a structure in a specific fire scenario, it is essential to compare the relevant performance criteria with the outcomes of analysis, testing, or judgment This involves assessing the maximum deformation of structural components or, when applicable, the time until collapse.
The evaluation of structural design for fire safety is based on performance criteria aligned with the selected strategy Key load-bearing functions include: a) maintaining load-bearing capability throughout the fire duration (ultimate limit state); b) controlling deflection, contraction, or elongation to ensure the integrity of load-bearing elements (deflection limit state); and c) assessing the extent of structural damage, such as spalling, corrosion, or deformation, that allows for post-fire repairs (re-serviceability or re-usability limit state).
For non-load-bearing separating functions, they could be:
⎯ the limit of the unexposed face temperature;
⎯ the limit of the radiation level from the unexposed face of the element; and
⎯ the limit of cracks and boundary deformation in order to reduce leakage (e.g flames and smoke) through the element
For floors and load-bearing walls, both functions shall be satisfied.
Documentation of the design for fire safety of structures
Documentation on fire safety assessment for structures is made available to all stakeholders in the design process to enhance understanding of the assessment's scope, methodology, assumptions, and outcomes This documentation outlines key aspects, including the roles of interested and affected parties involved in the assessment and the project's overall scope.
1) description of the built environment, including type of occupancy, dimensions of the built environment, compartmentation, and openings;
2) aspects relevant to fire safety performance, including type of structural material, building content (amounts and type of combustible material), occupant load, design structural loads, fire management and maintenance schedules;
4) scope of the assessment, which should also include the limitations and boundaries considered in the assessment
The article outlines essential components for assessing fire safety in structures, including objectives, functional requirements, and performance criteria, which are crucial for evaluating fire safety It emphasizes the importance of a trial design plan that details the strategy for assessing fire safety Additionally, it discusses the necessity of representative design fire scenarios and fires used in evaluations The assessment methods for fire development, thermal analysis, and structural analysis are highlighted, along with their appropriateness and limitations Furthermore, it documents the data sources utilized in the assessment and their rationale for suitability Finally, the evaluation of assessment results is crucial, comparing them against established performance criteria to ensure the design's adequacy for fire safety, culminating in a summary and conclusions.
Factors and influences to be considered in the quantification process
Material properties
Heat transfer calculations should consider relevant data for thermal properties as a function of temperature for each material involved, including:
The relevant materials are those used for construction, including any protective material, lining or acoustical products that may influence the temperature of the structural elements
In general, materials within a built environment lose strength and stiffness at elevated temperatures, leading to a decrease in their load-bearing capacity and an increase in deformation
Mechanical behaviour calculations should consider relevant data for mechanical properties as a function of temperature for each material involved, including:
⎯ stress-strain relationships at elevated temperatures;
⎯ reduction factors for strength and stiffness;
⎯ expansion or contraction due to elevated temperatures; and
⎯ when necessary, the degradation of sections (by charring, spalling, etc.) due to the effect of temperature
The materials considered are mainly those used for separating and structural elements, as well as any other materials that may influence deformation and stability
The thermal and mechanical behavior of structural and separating elements is influenced by the variability in material properties, which may arise from the source of the materials, the manufacturing processes, or the construction methods used on-site.
Mechanical properties are typically based on nominal values from relevant standards; however, actual strength levels of structural materials often differ from these values, especially at elevated temperatures For instance, steel generally exhibits yield and ultimate tensile strengths that exceed the guaranteed standard values, a trend that continues under high temperatures This variability also applies to pre-cast products like masonry and concrete planks delivered to construction sites In contrast, materials produced on-site (in situ) tend to have less predictable properties Timber, classified as either softwood or hardwood, presents significant variability in characteristic properties due to the broad range of wood species included Consequently, the nominal properties provided often carry a degree of uncertainty and conservatism.
The thermal properties of structural materials are significantly affected by parameters like moisture content and phase changes In concrete, the free-moisture content greatly influences specific heat and the duration at approximately 100 °C, where free moisture transforms into steam, resulting in heat loss due to latent heat of vaporization It is crucial to assign minimal moisture levels in heat transfer models to prevent underestimating heat transfer Phase changes in materials often lead to variations in heat content, which are well-documented and impact specific heat and thermal conductivity Additionally, heat transfer is highly reliant on the thermal emissivity of a material's surface, which can vary considerably during a fire; for instance, the emissivity of steel increases from around 0.8 at ambient temperature to as high as 1.0 as the surface oxidizes While thermal models typically account for changes in specific heat and thermal conductivity, they often overlook variations in emissivity, relying instead on a fixed value.
Effect of continuity and restraint (interaction between elements and materials)
When evaluating a structure's fire behavior, it is crucial to consider the risk of fire spread through both load-bearing and non-load-bearing elements, as well as the potential for collapse of the load-bearing structure The interaction between materials within a single element or between different elements subjected to varying degrees of heating can significantly impact the performance of composite elements and the overall system.
Fire behaviors observed from tests on structural elements, such as those following the ISO 834-1:1999 time-temperature curve, cannot be readily applied to different thermal actions without a precise calculation method.
When evaluating the fire performance of a structure, it is crucial to consider the interactions between materials within elements and the behavior of different elements in the overall system, such as composite concrete-steel components connected by shear connectors Additionally, the interaction between elements, like the elongation of a beam or floor at the top of a column or wall, must be taken into account Testing that focuses on these interactions, rather than isolated elements, is essential for accurately determining the fire performance of load-bearing structures The heating of beams and floors can lead to elongation, which generates additional shear forces and increased bending moments at the top of columns due to constraints from surrounding elements This adverse effect can result in premature column failure, highlighting the importance of incorporating these factors into the fire performance design of the structure.
Full-scale tests conducted on realistic structures indicate that computer models, when validated against basic laboratory tests like simply supported beams, may not accurately reflect the true fire behavior of an entire structure.
Use of test results
Tests conducted in accordance with ISO 834-1 primarily evaluate separating and structural elements However, the fire performance results related to insulating and integrity failures from these tests may not accurately reflect the actual performance of these elements in real fire scenarios.
To ensure accurate fire safety assessments, it is essential to conduct tests using a relevant time-temperature curve that reflects the design fire Alternatively, validated calculation models can be employed to translate results from one fire curve to the expected outcomes under a different fire scenario.
Fire spread routes
Fire can spread to nearby enclosures or spaces through various direct and indirect routes, posing risks of secondary ignition and fire growth These routes have been outlined in ISO/TR 13387-6, with Figure 4 illustrating the mechanisms and pathways identified in ISO/TR 13387-6:1999, Figure 2.
The figures depict common pathways for potential fire spread Designers must not only identify direct routes but also consider the risk of fire spreading between adjacent enclosures through independent spaces These pathways often combine direct spread routes and should be regarded as distinct mechanisms of direct fire spread.
To effectively manage fire risks, it is essential to quantify all potential routes for fire spread from the enclosure and determine the time required for the fire to reach critical conditions Any discrepancies between this time and the necessary fire spread time should be mitigated by improving the fire and smoke containment capabilities of the relevant elements In some cases, design efforts can be streamlined if expert judgment identifies the routes most vulnerable to rapid fire and smoke spread The likelihood of fire spread is influenced by the surrounding environment, both within the fire enclosure and adjacent areas, as well as their susceptibility to secondary ignition.
Fire can spread through walls and floors via openings or edges, utilizing conduction and convection as its primary mechanisms Additionally, direct pyrolysis can occur, leading to collapse or ignition.
UNDAMPERED HORIZONTAL DUCTWORK (1) UNDAMPERED HORIZONTAL DUCTWORK (2) a Spread route: Along or through horizontal duct. b Spread mechanism: Conduction, convection. a Spread route: Along or through horizontal duct. b Spread mechanism: Conduction, convection.
DAMPERED DUCTWORK PROTECTED OPENINGS a Spread route: Through damper or openings created.
NOTE Spread mechanism: Convection, conduction a Spread route: Through door, glazing, etc and openings created in them or around edges. b Spread mechanism: Conduction, radiation, direct pyrolysis (collapse or ignition).
SERVICES (PIPES/CABLES AND SUPPORTS) SUSPENDED CEILING VOIDS a Spread route: Fire transferred through service or via penetration to accommodate service
NOTE Spread mechanism: Complex, including radiation, mass transfer, conduction.
Spread route: a Enclosure to ceiling void b Ceiling void to adjacent enclosure
NOTE Spread mechanism: Complex, including pyrolysis, mass transfer and conduction
Spread route: a Enclosure to roof b Roof to adjacent enclosure
NOTE Spread mechanism: Pyrolysis, radiation.
Spread route: a Enclosure to floor void b Void to adjacent enclosure
NOTE Spread mechanism: Complex, including pyrolysis, mass transfer, conduction and convection.
RAISED FLOOR VOIDS (2) EXTERNAL WALLS/WINDOWS
1 void a Spread route: Void to enclosure via floor
NOTE Spread mechanism: Conduction and convection
Spread route: a Enclosure to faỗade surface b Faỗade to adjacent enclosure
NOTE Spread mechanism: Complex, including pyrolysis of surface
EXTERNAL WINDOWS UPPER PARTS OF BUILDINGS
Spread route: a Enclosure to outside b Outside to adjacent enclosure
NOTE 1 Spread mechanism: Collapse, convection, radiation
NOTE 2 Can occur horizontally between windows in certain conditions.
Spread route: a Enclosure to roof outside b Roof flames through external envelope, e.g window
NOTE Spread mechanism: Pyrolysis (collapse/ignition), convection, radiation
3 exposed building a Spread route: Enclosure to space adjacent to enclosure
NOTE Spread mechanism: Complex, including radiation, mass transfer
Spread route: a Enclosure to lift shaft b Lift shaft to enclosure
NOTE Spread mechanism: Convection, conduction
Figure 4 — Routes of fire spread
6 Guidance on use of engineering methods
Assessment of the behaviour of a structure, in real-fire situations, uses the following approaches:
The expected behavior of a structure under fire conditions involves a combination of three approaches, as no single method currently offers a complete solution.
Using calculation methods
Calculation methods are essential for comprehending fire-related phenomena and evaluating structural behavior in various fire scenarios Numerical simulation tools can effectively calculate heat transfer, deformation, and the load-bearing capacity of heated structural members However, users must recognize the limitations of these tools.
⎯ the limited number of validated calculation methods;
⎯ the current impossibility of accurately modelling some of the physical phenomena such as spalling;
⎯ the lack of information on the thermal and mechanical properties at elevated temperature of the materials used in elements constituting the structure
To ensure accurate assessment of fire performance in structures, it is essential to first validate the model and input data by comparing them against tested representative specimens This verification process involves confirming that the design and thermal exposure closely align with the intended final target.
Predicting the fire behavior of non-load-bearing elements like partitions and doors through calculations is challenging While heat transfer calculations for multi-layer elements are feasible, accurately forecasting thermal distribution at boundaries and assessing integrity performance remains difficult Current calculation methods primarily enhance understanding of the physical phenomena associated with design or size changes in tested elements Ultimately, reference test results and expert judgment are essential for evaluating these partition elements.
Using experimental methods
Testing for fire safety in structures is costly, necessitating limitations on the number, size, and complexity of specimens Three approaches can be considered: a) Testing isolated elements or assemblies under a defined thermal action (design fire) yields results closely aligned with real fire behavior, particularly when the design fire is well-defined and boundary conditions can be accurately replicated This method is applicable to various structural elements, such as beams, columns, and walls, allowing for direct use of test results with minimal interpretation b) Testing isolated elements against standard fires (e.g., ISO 834-1) is essential for compliance with prescriptive requirements While these results are valuable for assessment, they come with more limitations compared to the first method.
1) Mainly isolated elements are tested and test results are only seldom made available for assemblies
When assessing the behavior of the built environment in relation to design fire scenarios, it is crucial to consider factors such as thermal deformation, strength, rigidity as a function of temperature, and shrinkage These factors depend on the relative fire development between the nominal fire used for testing and the actual fire phenomena.
Establishing guidelines for utilizing test results in fire engineering is essential These rules can connect standard fire testing outcomes with real-fire testing results, ensuring that existing standard data remains relevant One effective approach is to create a framework that facilitates this relationship.
The evaluation method should be compared with the test results to assess the same element for the relevant fire design Subsequently, it can be utilized to evaluate the element under different boundary conditions.
Improving the instrumentation of routine fire tests is essential to gather comprehensive data on the fire behavior of structural elements Key information such as initial stress conditions, distortion, deflection, temperature profiles, restraining forces, heat flux from unexposed surfaces, and flame or smoke leakage can be obtained Full-scale testing, while costly and time-consuming, is crucial for understanding real structural behavior and validating numerical models This type of testing should be reserved for rare instances where it directly informs specific construction projects.
Using engineering judgment
The data gathered through testing and calculations will assist the fire expert in thoroughly assessing the anticipated fire behavior of a structure A key focus for the expert will be to analyze the potential effects of various thermal actions, including heating rate, peak temperature, and cooling phase, in relation to the established failure criteria.
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[2] SFPE Handbook of Fire Protection Engineering, Fourth Edition, National Fire Protection Association,
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[4] Structural Fire Protection, ASCE Manuals and Reports on Engineering Practice No 78, T.T Lie, editor,
America Society of Civil Engineers, New York, 1992
[5] EN 1991-1-2:2002, Eurocode 1: Actions on structures — Part 1-2: General actions — Actions on structures exposed to fire
[6] EN 1992-1-2:2004, Eurocode 2: Design of concrete structures — Part 1-2: General rules — Structural fire design
[7] EN 1993-1-2:2005, Eurocode 3: Design of steel structures — Part 1-2: General rules — Structural fire design
[8] EN 1994-1-2:2005, Eurocode 4: Design of composite steel and concrete structures — Part 1-2:
General rules — Structural fire design
[9] EN 1995-1-2:2004, Eurocode 5: Design of timber structures — Part 1-2: General rules — Structural fire design
[10] EN 1996-1-2:2005, Eurocode 6: Design of masonry structures — Part 1-2: General rules — Structural fire design
[11] EN 1999-1-2:2007, Eurocode 9: Design of aluminum structures — Part 1-2: Structural fire design
[12] Fire Engineering Guidelines, First Edition, March 1996, Fire Code Reform Centre Ltd, Sydney NSW,
[13] DRYSDALE, D., An Introduction to Fire Dynamics, Second Edition, John Wiley and Sons, New York,
[14] CIB W14 – N269 – Rational fire safety engineering approach to fire resistance of buildings, 2001
[15] ISO/TR 12470, Fire resistance tests — Guidance on the application and extension of results
[16] ISO/TR 12471, Computational structural fire design — Review of calculation models, fire tests for determining input material data and needs for further development
[17] ISO/TR 22898, Review of outputs for fire containment tests for buildings in the context of fire safety engineering
[18] ISO/TR 13387-1, Fire safety engineering — Part 1: Application of fire performance concepts to design objectives
[19] ISO/TR 13387-6:1999, Fire safety engineering — Part 6: Structural response and fire spread beyond the enclosure of origin
[20] ISO 16730, Fire safety engineering — Assessment, verification and validation of calculation methods
[21] ISO 16733, Fire safety engineering — Selection of design fire scenarios and design fires
[22] ISO 13824, Bases for design of structures — General principles on risk assessment of systems involving structures
[23] BS 7974:2001, Application of fire safety engineering principles to the design of buildings Code of practice