In the wake of the Grenfell tragedy, fire safety is under greater scrutiny than ever before. We invited questions from architects on steel and fire performance and put them to expert David Moore, director of engineering at the British Constructional Steelwork Association
Q What options are available for protective fire coatings for structural, exposed external steel in relation to appearance and performance?
Tom Osborne, Knight Architects
A Unprotected steel has an inherent fire resistance period of up to 15 minutes according to fire protection calculations, meaning that structural steelwork maintains 60% of its strength at room temperature. Fire protection calculations are based on limiting temperatures of 550°C where steelwork is exposed on all sides and 620°C where it is exposed on three sides.
To increase fire resistance using passive protection the options are reactive (intumescent film coatings) and non-reactive (boards, sprays or flexible blankets).
There are two main types or intumescent protection – acrylic based and epoxy based. These generally consist of a primer, basecoat and sealer coat and can be applied either on or off-site.
Acrylic based intumescent performs very well in wet (outside) environments while acrylic performs well in dry (internal) environments. Because of the thinness of the coatings, they are particularly suitable for applications on complex shapes.
Intumescent coatings are the most widely used passive protection, accounting for around 70-75% of applications. They typically meet requirements for fire resistance of 30, 60 and 90 minutes, and sometimes as much as 120 minutes.
Encasing structural steelwork with fire protection boards is another option that gives a regular, boxed, appearance. Although mainly used internally, some board products can be used in limited external conditions. There are two main types – lightweight and heavyweight.
Lightweight boards tend to be cheaper and are more suitable for non-visible applications. Heavyweight boards are more suitable for achieving a good decorative finish since they are better able to accept renders and decorative cladding.
Sprays can be the cheapest approach for large buildings that require high periods of fire resistance. But as the finish can be quite crude, they are unsuitable for aesthetic applications.
Whatever the method of fire protection, a fire safety engineering approach can produce cost savings by enabling a more targeted strategy that puts the fire protection exactly where it is needed. This integrated approach involves the use of codes to design fire protection for individual elements of construction.
Q Is the performance of intumescent coatings on structural steelwork affected when the steelwork is behind additional linings such as walls and ceiling that slow the rate of temperature change?
Brian Heron, Ian Ritchie Architects
A Intumescent coatings intumesce at 200°C, forming a protective char that expands to form a far thicker layer that insulates the steel from the fire. If the steelwork is shielded by other linings, they will still be able to intumesce to give protection but this process will take longer as the steel remains cooler for longer before reaching the point of intumescence.
It is important that a sufficient gap around the structure is left to enable this expansion to take place, depending on the fire resistance period and the type of coating.
Q How predictably do steel structures react in the case of a fire?
Tszwai So, Spheron Architects
There is this myth that steel is unpredictable in fire –a properly designed and protected steel-framed building will perform extremely well in a fire situation. A frequent misconception is that steel melts in fire. In fact, it softens and only melts at 1500°C; temperatures rarely get above 1000°C in building fires. A lot of research has been done in relation to steel and fire so we know a great deal about the performance of steel, which is actually very predictable. I only know of one case where a steel-framed building has entirely collapsed because of fire and that was the World Trade Center Building 7, which was a very unusual situation where there were mitigating circumstances. I was involved in full-scale fire tests at BRE’s Cardington site some years ago, where we set fire to an eight-storey steel-framed building and temperatures reached 1000°C. Some of the steel had no fire protection at all and still survived. The building was still standing after the fire.
Fire resistance is another concept that is not well understood. The fire resistance periods required for different building types and heights (see table) is not the length of time that a structure will survive in a real fire. Instead, it refers to how long it would take an element to collapse in a standard fire test within a furnace where the temperature rises quickly and increases indefinitely. This test is different from a real fire situation, where the fire may decay and/or move once the combustible material has been consumed.
Q Following Grenfell, do you believe that more comprehensive and wide-ranging legislation is necessary to coordinate all elements of a design (integrating cladding and structure) to ensure that buildings are safe?
Peter Jenkins, BDP
A building should have a reasonable provision of fire-resistance – the issue is not so much how we go about meeting that demand in the design and specification but how to ensure that the right fire protection specification for the building gets through into the construction on site, and that this is supervised and checked. As I understand it, the Hackitt review is proposing an independent authority to approve designs for certain residential buildings, which will have to demonstrate that they can satisfy fire safety requirements before they are built. It will also put some legal responsibility on ‘duty-holders’ such as the client, principle designer, contractor and some of the subcontractors. I think this would be a good thing.
From the steel perspective, BCSA has the opportunity to promote use of the National Structural Steelwork Specification as best practice for fire protection. We’re planning to update clauses regarding the use of intumescent paint to make sure that these are specified correctly and can be competently delivered on site.
Fire Resistance Legislation
Building Regulations set out the functional requirements for ensuring that buildings are safe and healthy but not, for example, how the need to maintain stability for a ‘reasonable’ period in the event of a fire can be achieved. As regulations are devolved, these requirements differ between England, Wales, Northern Ireland and Scotland.
Approved Document B is guidance issued by each devolved UK government setting out structural fire resistance requirements to meet designers’ obligations on structural stability. Fire protection requirements vary from 30-120 minutes according to building type and height (see table below).
BS 9999 – Fire Safety in the design, management and use of buildings is a code of practice aimed at providing a more flexible approach to fire safety design in order to safeguard the lives of building occupants and firefighters. Unlike the more prescriptive Approved Document B approach, this can be tailored according to an understanding of the causes of risk to life and how these can be mitigated. This can result in reduced fire resistance periods.