developed by Law and O’Brien (1989) and are incorporated into the Eurocodes. Flame sizes, temperatures and heat transfer coefficients are given in Eurocode 1 Part 1.2 (CEN, 2002b) and methods for calculating the steel temperatures are given in Eurocode 3 Part 1.2 (CEN, 2005b).
The design method allows for conditions with or without wind creating a forced draught to influence the shape of the idealized flame.
Typical flame shapes and radiation geometries are shown in Figure 6.8 for two conditions of forced draught and no forced draught, which produce different flame shapes. The design documents show many additional shapes for conditions with cross winds, flame deflectors and other variations. Figure 6.8 shows three possible column locations which require different designs. Columns at locations A and C are exposed to radiation from the flame itself and also from the window opening behind the flame, but column C has less severe exposure. The column at location B is engulfed in the flame. The design documents mentioned above give equations for all of these situations.
‘stickability’ to remain in place for the duration of the expected fire, with realistic deflections of the steel member. The toughness of passive fire protection often depends on the quality of the building materials and workmanship.
6.3.1 Concrete Encasement
A traditional method for fire protection of steelwork is encasement in poured concrete.
An advantage of this system is excellent durability in corrosive environments. The required thickness of concrete to achieve standard fire resistance ratings is given in prescriptive building codes. The reinforcing in the concrete may be nominal reinforcing simply to hold the concrete in place in the event of a fire, or it may be substantial in which case the member will be designed for composite behaviour of all three materials. This form of construction is very common in Japan where it is called steel‐reinforced‐concrete. Elsewhere, concrete encase- ment is not widely used because it is expensive, bulky, and time‐consuming, requiring the combined cost of a steel frame plus boxing for all of the concrete.
6.3.2 Board Systems
There are many proprietary board systems for protecting structural steelwork, such as shown in Figure 6.9, Figure 6.10 and Figure 6.11. Most fire protective boards are manufactured from calcium silicate or gypsum plaster. Calcium silicate board is more expensive than gypsum board in many places because it is imported from manufacturers in only a few countries.
Calcium silicate boards are made of an inert material that is designed to remain in place with little damage for the duration of the fire, protecting the steel by its insulating properties.
Gypsum board also has good insulating properties, and its behaviour is enhanced by the water of crystallization which is driven off as the board is heated. This dehydration process gives an additional time delay at about 100 °C, but it reduces the strength of the residual board after fire exposure, as described in Chapter 10.
Figure 6.9 Steel beam and column protected with board materials
Protective board
Corner
beading Screws Steel channel Steel beam
Figure 6.10 Detail of steel beam protected with board materials. Reproduced from Milke (2008) by permission of Society of Fire Protection Engineers
Figure 6.11 Box protection being placed on a steel column using sheet material
6.3.3 Spray‐on Systems
Spray‐on proprietary protection is usually the cheapest form of passive fire protection for steel members. Spray‐on materials are usually cement based with some form of glass or cellulosic fibrous reinforcing to hold the material together. Earlier spray‐on materials used asbestos fibres which are no longer used for health reasons. Disadvantages of spray‐on protection are that the process is wet and messy, and the resulting finish is not suitable for decorative finishes.
The spray‐on material is often rather soft, so that it has to be protected from damage if it is in a vulnerable location. For these reasons, spray‐on materials are more often used for beams than for columns (Figure 6.12). Spray‐on protection is easy to apply to complicated details such as bolted connections or steel brackets.
Approved spray‐on systems must have proof that they have sufficient ‘stickability’ to remain in place during fire exposure. Test methods are available for testing the cohesion and adhesion of spray‐on fire protection (ICC, 2015). The required thickness of proprietary spray‐
on fire protection to achieve fire resistance ratings can be found in individual manufacturers’
literature or trade publications (e.g. ASFP, 2014). Some generic ratings are available for spray‐
on systems (e.g. NBCC, 2010) but proprietary ratings from individual manufacturers are more likely to be used.
6.3.4 Intumescent Paint
Intumescent paint is a special paint material that swells up into a thick charred mass when it is heated. The intumescent material provides insulation to the steel member beneath. Several coats of intumescent paint may have to be applied to obtain the necessary thickness. Intumescent paints have the advantages that they do not take up much space, they can be applied quickly, and they allow the structural steel members to be seen directly, without any covering other than the paint. A disadvantage is the high cost compared with‐board and spray‐on materials, espe- cially for longer duration fire resistance ratings. Many intumescent paints are not suitable for external use because of unknown durability. All intumescent paints are proprietary products, and many are under continual development. A minor disadvantage of intumescent coatings is that the protection is not obvious to casual observers, and it can be difficult to verify at a later date. Some specialist intumescent products incorporating multiple layers of fibre‐glass rein- forcing have been developed for high level protection of structural steel in the offshore oil industry. Structural elements coated with intumescent paints should be given enough clearance to expand on heating, especially when they abut other construction materials.
6.3.5 Protection with Timber
It is possible to provide fire protection to steel beams and columns with timber boards. Twilt and Witteveen (1974) describe fire tests and fixing details for fire‐exposed steel columns.
Using a conservative critical steel temperature of 200 °C they show that 35 mm thick softwood boards can provide fire resistance for 60 min to a steel column with F/V 100 m–1. It is essential that the timber completely encloses the steel member, and be firmly fixed in place with a ther- mosetting adhesive such as resorcinol. The wood must be well seasoned to prevent shrinkage cracks.
6.3.6 Concrete Filling
Hollow steel sections can be filled with concrete to improve the fire performance. A major advantage is the lack of bulky external protection, and the steel can be finished with normal paint. There are several structural possibilities. The filling concrete can either be considered simply as a heat sink to reduce the temperature increase, or as a structural material which can carry an increasing proportion of the load as the steel temperatures increase. The filling concrete can be plain concrete, or it can be reinforced with conventional bars or with steel fibres. The steel tube can provide excellent structural confinement to the concrete under non‐fire conditions, for example during seismic loading. It is essential to provide vent holes to prevent
Figure 6.12 Sprayed‐on fire protection to steel beams supporting precast concrete floor slabs
Additives may be necessary to prevent corrosion, and to prevent freezing in cold climates.
This method of protection is expensive and is only used for special structures. Design information is given by Bond (1975).
6.3.8 Flame Shields
In some situations, it is possible to use flame shields to protect external structural steelwork from radiation or direct impingement by flames coming out of window openings. In these cases, the temperatures of the steel exposed to flame contact or radiation can be calculated using the methods referred to above for external steelwork. An example of a flame shield protecting the flanges of a deep steel beam in a 54‐storey building in New York is shown in Figure 6.13 (Seigel, 1970).