Magdalena Prus of HKA Global discusses fire protection to structural steelwork.
Fire protection to structural steelwork is crucial to the safety of buildings and their occupants. Whilst structural steelwork has some inherent fire resistance, steel generally loses its strength and stiffness when subjected to high temperatures. Tests have demonstrated that a fully loaded steel beam exposed on all four sides fails at 500°C and a fully loaded steel beam exposed on three sides (where it supports a concrete slab) fails at 620°C. Fire resistance of construction elements is determined in standardised fire test conditions, based on the test method and criteria described in the BS 476 suite of British Standards¹ and BS EN 1363-1². Where the steel does not have sufficient (inherent) fire resistance to satisfy the requirements of the Building Regulations, then additional fire protection is required to protect structural steelwork from the effects of fire. Note that the inherent fire resistance of structural steel is dependent on several factors, including the size of the section and loading, and should only be determined by a structural and/or fire engineer.
This article discusses effective fire protection methods for structural steelwork, by reference to the applicable requirements of the Building Regulations. In particular, it focuses on the passive fire protection solutions in the scenario where the structural steelwork interfaces with the compartment wall lines.
Steelwork protection – requirements
Requirement B3 in Schedule 1 of the Building Regulations concerns internal fire spread in buildings and states:
“(1) The building shall be designed and constructed so that, in the event of fire, its stability will be maintained for a reasonable period…
(3) Where reasonably necessary to inhibit the spread of fire within the building, measures shall be taken, to an extent appropriate to the size and intended use of the building, comprising either or both of the following –
(a) sub-division of the building with fire resisting construction;
(b) installation of suitable automatic fire suppression systems…”.³
On a typical project, the design and construction needs to comply with both Requirement B3(1), concerning the stability of the building via fire protection to the elements of structure (including, columns, beams, frame, and loadbearing internal/external walls and floors), and Requirement B3(3), necessitating the sub-division of the building via compartments of certain fire resistance.4
Approved Document B ‘Volume 2: Buildings other than dwellings’, 2019 edition with 2020 and 2022 amendments for use in England (ADB) defines a term ‘fire resisting’ or ‘fire-resistance’ as “the ability of a component or a building to satisfy, for a stated period of time, some or all of the appropriate criteria given in the relevant standard”.5 Fire resistance is measured in minutes6 and by one or more of the following conditions7:
- “Resistance to collapse (loadbearing capacity)”8, that is, the ability of the element to main loadbearing capacity (‘R’ in the European classification).
- “Resistance to fire penetration (integrity)”?, that is, the ability of the element to stop the passage of flames and/or hot gases from one side to another (‘E’ in the European classification).
- “Resistance to the transfer of excessive heat (insulation)”¹0, that is, the ability of the element to limit the increase of temperature on the unexposed/non-fire side (‘I’ in the European classification).
Table B3 of ADB¹¹ provides that structural frame, beam, or column should achieve the minimum required fire resistance for loadbearing capacity only¹². Put another way, those elements are not required to provide resistance to fire penetration (integrity) or transfer of excessive heat (insulation); the only requirement is to ensure that the loadbearing function of structural steel elements can be maintained for the period of fire resistance required by ADB.
By contrast, compartment walls and compartment floors are required to form “a complete barrier to fire between the compartments they separate”,¹³ using construction of the appropriate fire resistance for all three measures, that is, loadbearing capacity, integrity, and insulation.
ADB, and other available guidance such as BS 9999: 2017 ‘Fire safety in the design, management and use of buildings – Code of practice’ and ‘Steel Construction Fire Protection’ published by the British Constructional Steelwork Association (BCSA) in 2013, recognises that where elements of structure (with loadbearing function) interface with fire compartment lines (for example, walls separating flats in a residential block or walls forming enclosures to ‘protected shafts’, such as staircases or services risers), such elements “should be constructed to the standard of the greater of the relevant provisions”.¹4 In other words, where the structural element forms part of compartment floor/wall construction, then such an element needs to be designed and constructed that not only its loadbearing function is maintained during a fire event, but also the requirements for the insulation and integrity of compartment lines (including the incorporated member) is satisfied.
Fire protection methods
The consideration of appropriate fire protection is key where ‘as-designed’/‘as-constructed’ structural steel members need to satisfy both the requirements for fire protection to maintain the stability of the building and the criteria for fire separation to maintain the sub-division of the building.
Fire protection to structural steelwork can be provided through Passive Fire Protection (PFP) and/or Active Fire Protection (AFP). PFP consists of components or systems that form part of the building fabric, for example, fire resistant coatings or fire-resistant materials. Examples of AFP include sprinkler systems or fire alarms. In the scenarios, where the building is not sprinklered,¹5 the fire protection to the structural steelwork needs to be provided by PFP measures,¹6 which can be divided into Reactive PFP and Non-Reactive PFP. The purpose of the PFP materials in structural fire protection is to provide resistance against high temperatures by insulating steel members for the required period of time, to prevent the temperature of the steel reaching a critical/failure point.
The Reactive PFP measure commonly used to provide fire protection to steelwork is thin film intumescent coatings applied to steel members on-site or off-site. The intumescent coatings remain inert at low temperatures but expand or swell when subjected to high temperatures, typically at 200-250°C, providing an insulating ‘resistance’ layer to steel sections during a fire event. The ‘reactive’ characteristic of intumescent coating can satisfy the requirements for maintaining the loadbearing capacity of the steelwork for the required period of fire resistance, but the additional fire safety requirements for the insulation and integrity of compartment lines are not met. When structural steelwork interfaces with a compartment wall line, the steelwork needs to be protected by a Non-Reactive PFP system, for example, by encasing the steelwork using inherently fire resistant boards of certain thicknesses. Such a solution offers not only the protection of loadbearing capacity of the steelwork but also insulation and integrity required for compartmentation.
Conclusion
Structural steel designed and constructed in parallel with the fire compartment lines cannot be protected by a ‘reactive’ intumescent coating system alone, as this solution would only offer the fire protection to the loadbearing function of the steelwork. In the proposals where structural steelwork interfaces with the compartment floor or wall, the steel members need to be encased in a ‘non-reactive’ board system to provide the fire protection to insulation and integrity of the compartment line.
One of the key issues to consider for the designer involved in the preparation of structural steel protection proposals would be, in the first instance, to determine the required fire resistance period for the relevant construction elements by reference to relevant guidance, and to identify whether or not any steel members are required to meet all three fire-resistance criteria. Input from a structural engineer, a fire engineer, and/or the manufacturer of fire protection systems should be sought during the development of the design proposals.
Footnotes
- BS 476-20:1987 ‘Fire tests on building materials and structures. Method for determination of the fire resistance of elements of construction (general principles)’, and BS 476-21:1987 ‘Fire tests on building materials and structures. Methods for determination of the fire resistance of load bearing elements of construction’ and BS 476-22:1987 ‘Fire tests on building materials and structures. Methods for determination of the fire resistance of non-load bearing elements of construction’.
- BS EN 1363-1: 2018 ‘Fire classification of construction products and building elements. Classification using data from reaction to fire tests’.
- https://www.legislation.gov.uk/uksi/2010/2214/schedule/1/made.
- Subject to the provision of a sprinkler system – see (b) under Requirement B3(3).
- ADB, Appendix A: Key terms, pg.132.
- The period of fire resistance relates to time elapsed in a fire test and not a real time.
- ADB, Appendix B, para. B19, pg.140.
- ADB, Appendix B, para. B19a., pg.140.
- ADB, Appendix B, para. B19b., pg.140.
- ADB, Appendix B, para. B19c., pg.140.
- ADB, Appendix B, pg.141 (Appendix A, Table A1 in previous editions of ADB).
- The minimum periods of fire resistance are provided in ADB, Appendix B, Table B4, pg.146-147, and would depend on the height and the purpose group of building.
- ADB, Section 8: Compartmentation/sprinklers, para 8.15a, pg.69.
- ADB, Appendix B, para. B26b., pg.148.
- For example, in a building with a top storey less than 30m above ground level where the sprinkler system is not required by ADB.
- Also in sprinkler protected buildings PFP is usually required but the fire resistance requirements can be lower.
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Magdalena Prus, BA(Hons), DipArch, RIBA, MSc (Construction Law), FCIArb, is a Chartered Architect.