Fire Resistance: Structural Design for Fire Safety

In practice, and with some notable exceptions, the fire resistance (and fire protection) requirements for elements of structure in the construction industry are commonly determined through the 'unthinking' application of prescriptive guidance. Prescriptive guidance and approaches obviously have their place, but the 'blind' adoption of prescriptive solutions to fire resistance is hard to defend and may result in the following:

l               Apathy : There is a false belief that the responsible application of prescriptive documents requires less ability, relative to 'engineering' alternatives. This is rarely the case as lethargy in the application of such documents often results in ignorance regarding their scope of application, and thus their limitations.

l               Risk of Failure : Prescriptive guidance inherently assumes that the effects of real building fires can be sufficiently and conservatively represented by a standard fire curve, and that the response of real structures can be sufficiently and conservatively represented by isolated test elements. Neither of these criteria is fulfilled in some cases.

l               Failure to Meet Client Goals: The objective of prescriptive guidance is to provide an adequate standard for compliance with regulation that controls the standard of health and safety of those in and around completed buildings. There is no explicit recognition of other safety goals such as conservation, property protection, business continuity, or sustainability.

l               Uneconomic Design Solutions: Notwithstanding the above, there are many instances where prescriptive solutions are not the most economic. In these situations, the overall cost of a building can be reduced by alternative solutions that ensure fire resistance (and protection) is provided where it is most needed, and eliminated where it is not required.

It could be argued that the prescriptive approach has served the industry well: loss of life as a result of structural failure is extremely rare; government is deregulating, and the insurance industry typically assumes that prescriptive guidance results in an acceptable risk of loss. However, prescriptive guidance does not guarantee absence of damage or failure, resilience against loss, or delivery against a client's other fire safety objectives.

In the modern world, unthinking prescription both constrains innovation and ignores potential weaknesses in fire performance. Good design requires that fire safety solutions be engineered to meet the project goals. This requires explicit definition of the goals and an engineering approach that uses an appropriate balance of prescription, analysis, and judgment throughout the building life to ensure that the goals are met within the multiple, often competing constraints.

All of this requires knowledge and technical competence. The IFE has recognised this, and hence the SIG in Fire Resistance has been convened to promote the application of structural fire engineering in practice and, more generally, rational approaches to fire resistance design. The purpose of this article is to introduce the new IFE SIG, the motivations in engaging fire resistance specialists, and the SIG's initial objectives, with the hope that the broader fire safety engineering community will engage with the SIG and help to advance its goals.

Initially the Fire Resistance SIG steering panel will comprise ten members, who will be in place by the end of 2014. Representation in the SIG steering panel will come from various stakeholder groups, including: (1) consultancies, (2) fire and rescue services, (3) academia, (4) testing laboratories, (5) building control bodies, and (6) insurer/loss experts.

'Fire Resistance' and the Fire Engineer

Within the structural and fire safety engineering communities, 'fire resistance' is conventionally referenced in the context of the performance of an isolated member (or construction element), relative to specific performance criteria (integrity, insulation and load-bearing), under defined furnace heating (eg ISO 834i) and, where appropriate, loading conditions. Performance is typically measured in terms of time taken to breach any one or all of the given performance criteria, depending upon the nature of the construction element tested. It is effectively a means of bench-marking and thus has little correlation with how an element might perform in a real fire. For this reason, fire resistance is rarely a measure of damage to or survivability of, a structure in a real fire.

For the majority in the fire engineering industry, the term 'fire resistance' is synonymous with standard furnace testing and associated prescribed time periods, as advocated in life safety guidance, such as Approved Document B (AD B)ii and BS 9999iii. Unlike many disciplines in the wider construction industry, fire engineering professionals are equipped to make the distinction between life safety fire resistance periods and how real buildings might perform in a fire. Despite this, performance based fire resistance design is not given the necessary attention by fire engineers (or authorities having jurisdiction). As a result, the fire engineering industry often does not adequately address a number of fundamental challenges. Most notably:

l               If meeting prescriptive recommendations as part of strategy development is sufficient to arrive at a building that fulfils life safety objectives. In this regard, it is necessary to challenge the scope and basis of guidance documents such as AD B.

l               Whether delivering a building that meets only the life safety expectations of the Building Regulations is good enough for the end-user/client.

l               If the consultancy service being offered has positive (or possibly negative) influences on other aspirations that might be valued by any particular client.

Given the above, it is apparent that specialist fire engineering input is necessary where:

l               Life safety performance is to be demonstrated for buildings that fall outside of the definition of 'common'; ie (a) those where the fire and its  impact cannot be adequately/conservatively represented by the standard fire curve, and (b) where the mechanics of the structural stability system are not consistent with those of isolated elements; and

l               The design goal extends beyond simply life safety and into other motivations, such as sustainability, conservation, business continuity, and asset protection.

Fig 1. Structural fire engineering input is helping the 4 Pancras Square design team and client realize their aspiration for an expressed weathering steel exoskeleton (picture courtesy of Argent)

Fig 2. Heron Tower is a positive example of where life safety goals have been achieved in the most cost effective manner possible, whilst also interrogating the structural frame to explicitly assure a robust full-frame response to fire (picture published with thanks to Dr Graeme Flint – Arup Fire)

Fig 3. Wenlock Road, London (Regal Homes): An example where structural fire engineering input has facilitated the integration of cross laminated timber (CLT) and conventional steel frame construction

Motivations

The initial focus of the group will be to promote the importance of adopting a holistic approach to fire resistance design, through consideration of the goals of all stakeholders and the full life cycle of an asset/building. A core component of this is the interaction of the fire engineering and structural engineering disciplines, broadly referred to as Structural Fire Engineering (SFE). The application of SFE is increasingly important in delivering structures that meet life safety objectives; it can also yield other significant project benefits:

1.            Facilitating innovative architectural and structural design – from exoskeletal construction to blobitecture, architects are frequently challenged to conceive buildings that are both striking and functional, whilst also commercially viable. These challenges subsume the wider design team, most notably structural engineers, who are frequently challenged to defy gravity. This leads to novel structural systems, often making use of traditional materials in innovative ways. Recent examples include mega-frame construction (such as that adopted on 122 Leadenhall Street), bi-linear inclined columns (52-54 Lime Street) and the expression of the structure as part of the architectural concept (4 Pancras Square). In such examples, it is reasonable and necessary to question the applicability of prescriptive design guides, or even some performance based codes, and whether their implementation will lead to a building that ultimately fulfils its life safety objectives. Such buildings certainly fall outside of the definition of 'common' and, for this reason, the input of a structural fire engineer can be regarded as the only viable way of demonstrating compliance with life safety legislation.

2.            Optimisation of fire protection solutions – a major advantage of structural fire engineering design is that, by carefully interrogating the structural response of a building to fire, the amount and location of fire protection applied to the structure can be rationalised and optimised. This is done so as to ensure that excessive fire protection, which may be required by prescriptive rules, is needed and whether additional fire protection above the prescriptive requirements may be necessary in order to assure an acceptably robust response in a real fire. In such cases it is typical to perform a full structural analysis of the building's structural frame (or a significant portion thereof) under exposure to a range of design fire scenarios dictated by the acceptable level of risk. These design fires might include the standard fire as part of the rationalisation of an otherwise prescriptively designed building, or they may be credible representations of real fires that are proportionate to the 'risk profile' of the building. The design intention is to arrive at a robust structural/passive protection proposal informed by the outcomes of a rational structural fire analysis.

3.            Achieving sustainability goals – as the influence of climate change becomes more visible, the movement towards more energy efficient buildings is gathering momentum. To date, the fire engineering discipline, in the main, has been slow to explicitly contribute to sustainable design activities or to capitalise on the positive impact fire engineering can have on the sustainability of a building. However, this will be unavoidable in the future as the sustainability credentials of buildings take greater prominence during design. In an environment where stigma is still attached to the fire performance of inherently sustainable framing materials, such as timber, the role of the structural fire engineer as an enabler of safe and sustainable construction will become more important, in addition to supporting the on-going optimisation of buildings, with clear knock-on sustainability benefits.

4.            Meeting stakeholder robustness objectives – in recent years there has been increasing interest in the structural safety of tall buildings, with an emphasis on how safety can be achieved without compromising aesthetics or greatly increasing costs. Indeed, in some jurisdictions the building approvals process is changing, with requests for global structural response to fire to be quantified and justified, even for buildings that comply with the available prescriptive guidance. In this respect, for high profile buildings, robustness in fire is on a par with disproportionate collapse considerations. Structural fire engineering provides a means of understanding the actual structural response to heating, by determining credible fire scenarios and then calculating its thermal and mechanical response. Design weaknesses are highlighted, and remedial design measures are taken. In some cases such an approach may also lead to redeployment of fire protection, a change in structural design, or provision of increased fire protection to ensure a robust response. The building's likely serviceability post fire may also be considered and communicated to the end-user to ensure consistency with their expectations.

5.            Improving resilience of insured assets and enhancing business continuity goals – by understanding their clients' activities and operations better, the building design team can ensure that the business continuity goals are supported, if not improved, and the physical assets that are critical to the client organization can be better protected. By identifying fire related disruptions and potential direct and indirect consequences at the design stage, it is possible to incorporate design features to help reduce property loss, ensure business continuity, and provide resilience against the effects of fire. For the structural fire engineer, access to information derived from a Business Impact Analysis process will identify the activities critical to the end-user client's organisation; the resources needed to support the activities; and the fire safety objectives necessary to protect the resources. Using this information to augment the mandated life safety objectives, the fire safety objectives for consideration could contain activity-specific requirements such as: the compartment of fire origin or equipment that must be recovered, or that the business stream must be operational, within a given timeframe.

6.            Preserving heritage – with over 1,000 buildings of historical significance registered as 'at risk' by English Heritage, developers are seizing the opportunity to restore, refurbish and change the use of existing buildings. In many instances these changes are considered material under the Building Regulations, which in many instances means that such projects are either expected to comply with contemporary guidance or to demonstrate compliance via alternative means. Regarding the former, there is the obvious trial of demonstrating that a historical building can meet the expectations of documents such as BS 9999, whilst preserving heritage and satisfying conservation officers. Herein lies a considerable challenge (and opportunity) for the structural fire engineer, where many dated buildings are constructed with little, if any, passive protection measures but with serious constraints regarding possible remedial works.

Goals of the SIG

As the aspirations of building and infrastructure design advance ever further, it is essential for safe and sustainable development that rational and holistic, science-based fire resistance design (encompassing structural fire engineering) is established as an integrated core design discipline. The Fire Resistance SIG intends to lead the way in achieving this primary objective by engaging and educating key stakeholders and wider design team members of the above (and other) merits of a co-ordinated approach to structural fire resistance design. The group remit also extends to the identification of areas requiring guidance and, where appropriate, facilitating necessary research and the development of new guidance in relevant subject areas.

The initial aspiration of the special interest group is to undertake a fundamental review of structural fire resistance design leading to a best-practice white paper entitled 'Whole Life Fire Resistance.' It is the intention of the SIG to review, at a high level:

l               the aspirations of clients, architects and structural engineers

l               the goals of end users

l               the expectations of the fire and rescue service with regards firefighting operations

l               the differences in societal fire resistance expectations between nations and regions

l               the impact that base build strategy decisions have on fit out flexibility and later life alterations.

l               The impact that advanced fire engineering processes can have on:

l               the constructability of a structure

l               the ongoing fire safety management and maintenance of an asset

l               the insurability of an asset

l               the sustainability of a building's fire protection package

l               a building's robustness, both in terms of losses and operational continuity

l               the extent to which remedial works are necessary should a fire occur, and

l               how the intentions of designers are interpreted, varied and applied in practice by contractors and specialist sub-contractors.

A Dedicated Stakeholder Working Group

The steering panel members are not equipped to address the above points without supporting expertise from co-opted members. Therefore, it is the intention of the SIG to form a dedicated working group to support its activities. The SIG steering panel would be very pleased to hear from interested participants (via email to the SIG Chair at Consultations@ife.org.uk). In addition to the SIG steering panel membership, the working group will comprise (as a minimum):

l               a representative client, ie investor/developer

l               design team members, ie architect and structural engineer

l               a project manager from a main contractor and specialist subcontractor

l               a representative from the insurance industry, and

l               a specialist passive supplier and/or installer.

The Fire Resistance SIG steering panel also intends to seek input/support from like-minded working groups, including relevant complimentary IFE SIGs, RIBA committees and the IStructE Fire Engineering Study Group (FESG).

Stakeholder Workshop

The working group members will come together in a workshop environment during early 2015 to undertake a preliminary review of:

l               client's, architect's and structural engineer's understanding of what meeting the requirements of the Building Regulations (with respect to fire) means in the context of a building, and if this is consistent with their goals, aspirations and obligations

 

l               what impact engineering alternatives to prescriptive recommendations may have on the insurability of an asset and associated premiums? If the impact is positive or negative depending upon the appointment motivations (ie robustness vs value engineering)

l               how the main contractor and passive installer interpret, develop and apply fire engineering proposals during construction and determining the motives for making changes, and

l               the practicalities of implementing fire engineering proposals on site, eg can complex protection arrangements lead to installation problems and errors on site?

Steering Group Membership

The current steering group membership comprises: l Danny Hopkin: Chair (Trenton Fire) l Luke Bisby (University of Edinburgh) l Panos Kotsovinos (Arup Fire) l Eoin O'Loughlin (AECOM) l Peter Wilkinson (Pyrology) l Matt Ryan/Guy Foster (LFEPA) l Russell Kirby (FM Global) l Jon White (DS Office) The steering group is actively seeking further representatives with appropriate backgrounds in fire resistance testing. Expressions of interest can be made to the SIG chair via the email address noted previously.

References

I               International Organization For Standardization, 1999. Fire-resistance tests – Elements of building construction – Part 1: General requirements. ISO 834-1:1999. Geneva: ISO.

ii              Communities and Local Government, 2007. Approved Document B (Fire Safety) – Buildings other than dwelling houses. The Building Regulations 2000. London: CLG. iii BRITISH STANDARDS INSTITUTION, 2008. Code of practice for fire safety in the design, management and use of buildings, BS 9999:2008. London: BSI.