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MAKING THE MOST OF FIRE RESISTANCE

FOCUS ON: FIRE ENGINEERING

Maximising the value of the fire resistance of steel from the stage of conceptual design to the application of fire protection on site is vital.

For most practitioners, the term 'fire resistance' refers to the period of time for which an element of construction (for example, a column, a beam or a wall) must survive in a fire scenario. However, the period of fire resistance is derived from a standard fire test, carried out in an approved furnace under specified conditions of temperature, imposed load and restraint measured against time.

While building regulations are only intended to ensure reasonable standards of health and safety for persons in or about the building, including firefighting personnel, they are not designed to prevent structural damage or minimise fi nancial losses which arise from a fire. The most important requirement in the regulations reads: 'The building shall be designed and constructed, so that, in the event of a fire, its stability will be maintained for a reasonable period.' It is important to note that it is the requirement itself which is mandatory and the regulations provide only recommendations on how this can be achieved.

This allows for alternative methods to meet requirements, with the use of fire-engineered solutions that can be developed in collaboration with the approval authorities. The purpose of implementing alternative approaches is to meet the needs of the client when the requirements of the regulations are not sufficient.

The fire-resistance periods in the building regulations are based on a standard fire curve and are tabulated in a clearly defined way to provide a simple and user-friendly method for designers. However, the standard temperature-time curve bears little resemblance to a real fire's temperaturetime history. This is because a standard furnace does not reflect the behaviour of a compartment in a building on fire. As a result, the prescriptive guidance recommends overly conservative periods of fire resistance.

The alternative approach is to carry out fire-engineered analyses such as the Time Equivalence calculation 1, which works out the exact period of fire resistance, based on the behaviour of a real fire, and often allows for a considerable reduction of fire resistance from the periods advised in the codes.

This assessment takes into account the compartment geometry, the fire load, the heat loss through the facade and the resultant heat absorbed by the structure. Criteria required by the prescriptive guidance look only at factors such as building height and type of occupancy.

Most importantly, fireengineered solutions can be carried out to assess structural behaviour when subject to elevated temperatures.

Advanced numerical modelling and finite-element analyses can be carried out to obtain the optimum and most costeffective solution for the client.

Currently, there is a considerable amount of ongoing research to understand the true behaviour of structures in fires and to further the development of the technology of fire-protection products.

Fine-tuning structural fireengineering solutions with the performance of fire-protection products allows for a more accurate understanding of the structure's fire-resistance capability. However, designers tend to lean towards more performance-based engineering solutions, without taking the type of fire-protection product into consideration and, similarly, fire-protection manufacturers may not be aware of the structural fireengineering design approach.

This interface is particularly crucial when designing steel structures that do not have a high inherent fire resistance and will start to weaken at temperatures of more than 400infinityC. During the design process, the structural engineer tends to overlook the inherent fire resistance of the steel structure: the design process analyses the structure's performance at ambient temperature. In most cases, structural elements will have a percentage of inherent fire resistance which is not often taken into account. In fact, fire protection to steel elements is normally specified using the ASFP Yellow Book 2, which does not consider the performance of the structure in a fire scenario.

This results in over-provision of fi re resistance.

The best solution can be achieved if the designer is aware, at an early stage of the design process, of the potential benefits in having a structure that can perform sufficiently when subjected to elevated temperatures. Increasing the steel tonnage to achieve an inherent fire resistance may not necessarily result in higher overall costs when compared to the price of intumescent paints.

The added benefit of a good inherent fire resistance is often an indirect consequence of a design that requires larger structural elements to satisfy other criteria. For example, should the structure be governed by deflection or designed to withstand seismic loading? The adoption of larger elements for fire-resistance purposes is rarely considered.

SAFE (a fire and risk consultancy) encourages liaison of all parties involved, including the structural engineer, architect, fire engineer and fire-protection manufacturer, in order to maximise commercial and aesthetic value from the conceptual stages of the design process to the fire-protection application on site.

THE DESIGN PROCESS Fire protection manufacturers and designers have begun to produce software 3 that determines the amount of paint required, in addition to the inherent fire resistance of steel elements that are subject to a defined load. This combination undoubtedly allows for a more cost-effective solution to be determined as the quantity of intumescent paint is minimised and the combined fire resistance of the intumescent paint and of the structure is determined.

In addition to analysing single elements, larger fl oor plates can be fire-engineered to add further value. Following a series of fire tests on a steel composite structure at Cardington 4, a new design method 5, devised by Professor Colin Bailey at the University of Manchester, justifies the omission of fire protection to secondary composite-steel beams and identifies the structural elements where fire protection is necessary.

This process can also be carried out by implementing advanced finite element modelling to achieve more accurate and less conservative results, such as that of Vulcan, a bespoke Finite Element software developed at the University of Sheffield.

FIRE PROTECTION

Fire-protection manufacturers may interpret this design process as a conflict of interest and perceive structural fire engineering as a tool to minimise fire protection. This is a misconception as, in reality, the prescriptive guidance, from which the quantities of fire protection are derived, over-specifies fire protection, as it does not take into account the true performance of the structure. Performance-based solutions aim to identify where fire protection is truly required, as well as where it becomes redundant, therefore justifying its reduction or omission.

If you follow the fireengineering process to rationalise fire protection on steel structures, the focus shifts to the identification of the most advantageous choice of product to satisfy architectural and site constraints.

Intumescent paints are currently replacing the more traditional and conventional types of fi re protection.

The main advantages that intumescent products have over the more conventional type of fire protection methods can be summarised as follows:

reduced overall thicknesses, that lead to reduced building heights;

improved aesthetic appearance;

potential for integrating services within the structure, allowing for increased floorto-ceiling heights.

Once the fire-engineering process has been finalised, the choice of intumescent paint specification is usually driven by the structural engineer or the steel-beam manufacturer.

However, the architectural criteria and requirements may be a priority, notably for the protection of internal and external exposed steelwork.

Depending on the type of exposure, speed of drying, aesthetics and environmental criteria, intumescent coatings vary in type and composition.

Three main types are available:

solvent-based intumescents, commonly adopted for high productivity rates and harsh environments, and applied in-shop or directly on-site;

water-based intumescents, used where a minimum impact on other trades on-site is required and where an environmentally friendly product is needed to meet health and safety criteria; and - Epoxy-based paints, used in extreme environments or where long-term durability and robustness are required, mainly for external use. They can protect steel for up to three hours and last for 30 years.

Where the architectural criteria require a high-quality, aesthetically pleasing product, for exposed internal and external hollow-steel sections, epoxy-based intumescents are the preferred option.

The coating can be cast and moulded into shape, as well as applied off- or on-site, for each structural element to achieve a high-quality textured finish and colour, allowing for additional cladding to be omitted.

The most common requirement for the design team is to provide a minimal overall thickness of fire protection.

Historically, intumescent paints have not lent themselves to high fire-resistant periods, and more conventional materials like boarding and sprays have been adopted. However, recent technology has developed thin-filmed intumescent coatings that are resistant to fire ratings of up to two hours.

Off-site application of solvent-based intumescent paints allows for speedy and accurate application that maintains high quality control with minimum disruption to other trades on-site. Top coats on these intumescent types can achieve any desired colour.

Water-based intumescents are applied directly on-site, and are chosen as economical, fast and environmentally-friendly solutions. As they are solventfree, water-based intumescents have little odour, and are mainly used to fire-protect internal steel structures which are poorly ventilated, where health and safety requirements are a priority. Water-based solutions can also provide thin-filmed products, with up to two hours of fire resistance, and tend to dry faster than solventbased intumescents. Thin-filmed products can achieve up to two hours of fire resistance and prove to be particularly effective for criteria, such as increasing clear floor-to-ceiling heights, and maximising the service zones between the structure and ceilings and within the structure, if cellular steel construction is adopted.

Due to new technology, and new intumescent paints with higher performances and lower thicknesses, concerns have been raised by professionals within the fire-protection industry about the reliability and the performance of these value-engineered intumescent products. As a result of such concerns as these, a forum of intumescent manufacturers and suppliers has been established 6, in order to achieve and raise the standard of intumescent fire protection in buildings.

It will ensure that, by 2010, all intumescent coatings for the fire protection of steelwork adhere to the following set of conditions:

products tested and assessed, conforming to European standards and certified by an independent third party;

products installed by thirdparty applicators; and - products subject to independent inspection of completed works.

By the end of this year, members of the Intumescent Coatings Forum (ICF), will undertake the commitment of only supplying products that have been third-party certified.

In the near future, a code of practice will also be available with regard to the testing and assessment of intumescent coatings for structural steel fire protection.

From early stages of design through to delivery on site, research and development is extensive for both the performance of steel structures subjected to fire and for new fire-protection products that allow lower thicknesses for higher fire-resistance periods.

New structural fireengineering solutions, which include design methods and finite element modelling, are currently being developed in parallel with new fireprotection products so that intumescent paints are now replacing all conventional fireprotection materials. It is clear that, as a result of this research, which is continuously being validated and adopted by designers and suppliers, new prescriptive guidance will soon be applicable, which promises to allow more economical, engineered solutions for fire-protecting steel structures in both the engineering and the manufacturing sectors.

Nick BernabÚ is an associate director of SAFE Fire and Risk Engineering

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