A Steel for Life CPD, supported by Architecture Today, explores the performance of steel-framed structures in fire
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Structural fire resistance is a key factor in the design of most buildings, in terms of not only building and occupant safety, but also cost. Fire protection accounts for around 10-15 per cent of the price of a steel frame on a typical multi-storey commercial building. This Steel for Life CPD, supported by AT, provides an overview of the performance of steel in fire and covers the fire resistance of steel sections, building legislation and standards, fire-resistance periods, and fire protection options.
Understanding fire resistance
In the late 1990s, a series of full-scale fire tests were carried out on an eight-storey steel-framed building with composite steel deck floors at the BRE’s Cardington facility in Bedfordshire. Observations from the tests and other large building fires showed that the behaviour of the composite steel deck floor plays a crucial role in providing inherent fire resistance when compared to isolated construction elements. The tests demonstrated that the slab acts as a membrane supported by the perimeter beams and protected columns.
All hot-rolled structural steel sections have some inherent fire resistance. This is a function of the size of the section, the load it carries, and the degree of fire exposure. Fire resistance is usually measured in relation to the ability of the section to survive in a standard fire test as described in BS 476-20, BS ISO 834 and BS EN 1363-1.
Figure 1. Graph showing the effect of temperature on steel strengh. Fire protection must maintain the structural steelwork at a minimum of 60 per cent of its strength at room temperature. Fire protection calculations for steel are usually based on limiting temperatures of 550ºC, where steelwork is exposed on all four sides, and 620ºC, where a fully loaded beam is supporting a concrete floor slab.
While the strength of steel decreases with temperature, that strength reduction is quantified through standard fire tests, shown in figure 1 above. For very large hot-rolled sections, lightly loaded and with some partial protection from concrete floor slabs on the upper flange, inherent fire resistance with no additional protection can be as high as 50 minutes.
Where the inherent fire resistance of the steel is less than that necessary to meet the regulations for structural stability of the building, additional precautions must be taken. This usually takes the form of applied fire protection, which insulates the steel from the increasing temperatures, and/or a fire engineered solution.
Regulations and standards
In England, the relevant Building Regulations document for fire is Approved Document B. It contains details of the structural fire-resistance requirements to meet designers’ obligations on structural stability. For example, an office building higher than 30 metres requires 120 minutes fire resistance plus a sprinkler system. An unsprinklered assembly building 18-30-metres high requires 90 minutes fire resistance.
Table 1. Comparison of structural fire resistance requirements in selected buildings in Approved Document B and BS9999. Notes:
a Height is measured from ground to the height of the floor on the top storey
b Most steel members can achieve 15 minutes fire-resistance without added protection
c It is unlikely that planning permission would be given without sprinklers
BS 9999: Code of practice for fire safety in the design, management and use of buildings is intended to provide a more transparent and flexible approach to fire safe design through the use of a structured approach to risk. In many cases, the use of BS 9999 will lead to safer solutions for fire than are possible using the government publications. One of the most obvious changes in BS 9999 is to structural fire- resistance requirements, illustrated in table 1 above.
Fire resistance periods
The fire resistance requirement for a building and, therefore its frame, are defined in terms of the fire-resistance period and is stated in minutes (15, 30, 45, 60, 75, 90 or 120). It is important to note that fire resistance is not the length of time that a structure is likely to survive in a real fire, but a standard measure for comparing the performance of different designs in a consistent manner. In a real fire, once the combustible material or fire load has been consumed, the fire will decay and/or move. In a standard fire test, the temperature rises quickly and increases indefinitely, so conditions are far more severe.
Fire-resistance test results are expressed in terms of time to failure against one or more of three criteria for the product/element under consideration. These are:
• Loadbearing capacity – ability to support the applied load and to resist collapse.
• Integrity – ability to resist the passage of flames and hot gases.
• Insulation – ability to restrict the temperature rise on the unexposed face.
In England, the minimum period of fire resistance varies depending on the occupation and height of the building. For open-sided car parks the minimum period of fire resistance is 15 minutes; for offices over 30-metres high with sprinklers it is 120 minutes.
Office building with a 90-minute fire-resistance period provided by dark grey intumescent paint (ph: Bourne Steel Ltd)
Fire protection options
Unprotected steelwork is usually deemed to have 15 minutes inherent fire-resistance. For higher fire resistance periods, fire protection is usually required. Passive fire protection materials insulate steel structures from the effects of high temperatures and can be divided into two types: reactive, of which thin-film intumescent coatings are the most common example, and non-reactive, most frequently boards and sprays. Detailed guidance on the installation of coatings, boards and spray protection systems is available from the Association for Specialist Fire Protection.
Off-site application of intumescent coatings (ph: Sherwin Williams)
Thin-film intumescent coatings are mainly used in buildings where the fire resistance requirements are 30, 60 and 90 minutes, although some products can provide 120 minutes. They generally comprise three components: a primer, basecoat and sealer coat. The basecoat is the part that reacts in a fire and usually comprises the following ingredients:
• A catalyst which decomposes to produce a mineral acid
• A carbonific which combines with the mineral acid to form a carbonaceous char
• A binder or resin which softens at a predetermined temperature
• A spumific agent which liberates large volumes of non-flammable gases and causes the carbonaceous char to expand and provide an insulating layer many times the thickness of the original coating.
Coatings can be water or solvent-based, and applied either on or off-site. They can achieve attractive surface finishes and easily cover complex shapes. Post-protection service installation is relatively simple.
A fire engineered solution, with boards and unprotected secondary steelwork (ph: Arup)
Widely used in the UK, boards can be employed on unpainted steelwork and provide a clean, boxed appearance. Application is a dry trade and so less likely to disrupt other trades on-site. There are two types of protection: lightweight and heavyweight. Lightweight boards are typically 150-250kg/m³ and are not usually suitable for decorative finishes. They are cheaper than heavyweight equivalents and typically used where aesthetics are not important. Heavyweight boards are usually 700-950kg/m³ and will generally accept decorative finishes. Both types of board may be used in limited external conditions, but the advice of the manufacturer should be sought.
Less common is spray protection, which can be used to cover complex shapes and details. Costs do not rise significantly with increases in thickness, because much of the cost of application is in the labour and equipment. Some materials can also be used in external and hydrocarbon fire applications. Sprays are not suitable for aesthetic purposes, and as application is a wet trade this may impact other site operations. A cost allowance may have to be made for prevention of overspray.
Flexible blanket systems
Flexible blanket systems have been developed in response to the need for an easily applied fire-protection material that can be used on complex shapes and details, but where application is a dry trade.
Rogers Stirk Harbour + Partners’ Leadenhall Building where epoxy intumescent coatings were employed in external areas for durability
Concrete encasement tends to be used where resistance to impact damage, abrasion and weather exposure are important, for example in warehouses and underground car parks. It is generally more expansive than lightweight systems and has a higher space utilisation rate. Information on the thickness of concrete encasement for specific periods of fire resistance is published by BRE and can also be found in BS EN 1994-1-2.
Fire safety engineering
The simplest form of structural fire engineering is the use of codes to design individual construction elements. Techniques for structural fire engineering on assemblies of elements of construction using a simplified method have also been developed. These can be used in the design of simplified sub-assembly models and make use of the understanding of the behaviour of composite construction in fire. The models enable designers to leave large numbers of secondary beams unprotected in buildings requiring 30-120 minutes fire resistance.
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