2737

Calculation results

Divide the composite cross-section into steel tube, three concrete rings of equal thickness of 20 mm, and a concrete core of 70 mm radius. Table 37.9 gives, for each zone of the composite cross-section, temperature (in °C, calculated using Equations 37.35-37.37), area (mm2), second moment of area about a principle axis of the entire cross-section (cm4), reduced design strength (in N/mm2), and Young's modulus (in N/mm2, from Table 37.5 for steel and Table 37.6 for concrete) at elevated temperatures, compression resistance (in kN) and rigidity (EI, in kNm2).

Equation 37.29 gives Pufi = 2509.6 kN. Equation 37.34 gives the Euler buckling load in fire Pcr,fi = 5226.1 kN, giving a column slenderness in fire Ifi = 0.7885. Equation 37.32 gives wfi = 0.563 and Equation 37.31 gives the column compression resistance in fire Pc,fi = 1413 kN.

37.4.4.3.3 High Strength Concrete Filled Columns

With the introduction of high strength concrete, the load carrying capacity of a concrete filled column can be further enhanced. However, the increase in fire resistance is relatively small because high strength concrete loses its strength at a much lower temperature than normal strength concrete. By adding a small amount of steel fibre to the concrete, the elevated temperature performance of high strength concrete can be much improved and the performance of fibre reinforced high strength concrete filled steel columns is similar to that with normal strength concrete filling (Kodur and Wang 2001). Provided the strength and stiffness retention factors are available, Equations 37.29 and 37.30 can also be used.

37.5 Design for Unprotected Steelwork

The quest for knowledge is one of the main drivers of research to investigate the behavior of steel and composite structures under fire conditions. However, it should be recognized that the desire to reduce or

eliminate fire protection to steelwork is an equally strong incentive to carry out these studies. Fire protection to steelwork can represent a significant part of the total steel structural cost and the elimination of fire protection to steelwork represents a significant saving in construction cost to the client. But more importantly, by reducing the use of fire protection, steel becomes more competitive and the steel industry can benefit from an increased market share. After the September 11 event, it is also appropriate to consider using unprotected steelwork in fire situations for safety. Without fire protection, there would be no problem related to possible unreliable use of fire protection materials.

There are a number of ways of designing for unprotected steelwork, including risk assessment to reduce the requirement of fire resistance, using the so-called fire resistant steel (Sakumoto 1998) to increase the strength of steel at elevated temperatures, over-design steel elements at ambient temperature so as to increase their reserve of strength in fire, integrating the functions of fire protection and structural load bearing of concrete, and utilizing advanced structural behavior. Wang and Kodur (2000) provide a summary of these techniques. This section will give a brief introduction to the last two methods because they can be readily implemented in practice.

37.5.1 Integration of Structural Load Bearing and Fire Protection Functions of Concrete

It should be appreciated that it is very rare for steelwork to be used alone. Steel is usually used in combination with other materials, in particular with concrete. Concrete is not only a structural material, it also has good thermal insulation properties. Therefore, by combining these two functions of concrete, composite structures may be constructed to give inherently high fire resistance. The systems that will be described below have made special considerations of fire resistance in their design and construction. The following paragraphs will give a short description of their main features and the inherent standard fire resistance that they can achieve. This should enable the designer to determine quickly a possible structural load bearing system where the main design concern is to use unprotected steelwork. More detailed information may be found in Bailey and Newman (1998).

37.5.1.1 Beams

Three types of construction may be used:

1. Slim floor/asymmetric beam, shown in Figures 37.18a and b. In the slim floor construction, a wide plate is welded to the bottom flange of a universal column section and composite floor slabs are supported on the wide plate. In an asymmetric steel beam, the bottom flange is rolled wider than the top flange. Both systems use the same principle to achieve unprotected steelwork: the

(c)

FIGURE 37.18 Steel/composite beams of high fire resistance: (a) slim floor beam, (b) asymmetrical beam, (c) shelf angle beam, and (d) partially encased steel beam.

web of the steel section is protected by the concrete and provides the majority of the bending resistance of the steel beam at elevated temperatures. Only the steel section is assumed to have load carrying capacity, but lateral torsional buckling is prevented by the concrete slabs.

2. Shelf angle beams, shown in Figure 37.18c. In this system, steel angles are welded to the web of a steel beam and these angles are used to support precast concrete floor units. This system is mainly used to reduce the structural depth of the floor. Since the angles, the upper flange, and the upper portion of the web of the steel section are shielded from fire exposure, 60 min of fire resistance can be achieved using this system without fire protection.

3. Partially encased beams, shown in Figure 37.18d. By casting concrete in between the flanges of a regular universal beam section, only the downward side of the lower flange will be exposed to fire. Both the web and the upper flange are shielded from fire exposure and can provide high structural resistance. Composite floor slabs may be connected to the top of the partially encased steel beam via shear connectors to obtain composite action. Since concrete is cast between flanges of the steel section, no temporary formwork is necessary. By using reinforcement, standard fire resistance of up to 3 h can be obtained without fire protection to the steelwork.

Table 37.10 summarizes the standard fire resistance rating that can be achieved by different types of unprotected steel beams.

37.5.1.2 Columns

Three types of unprotected columns may be used:

1. Columns with blocked-in webs as shown in Figure 37.19a. In this construction, lightweight aerated concrete blocks are placed between the flanges of a universal steel section. The aerated concrete blocks not only provide good insulation to the column web, they also reduce the average column flange temperature compared to a bare steel column. A standard fire resistance rating of 30 min can be achieved without additional fire protection.

2. Partially encased steel columns with unreinforced and reinforced concrete as shown in Figure 37.19b. In a column with blocked-in web, the lightweight aerated concrete only provides insulation to the steel section and the system cannot provide 60 min fire resistance. If normal

TABLE 37.10 Standard Fire Resistance Rating of Unprotected Steel Beams

Standard fire resistance

Type of construction time (min)

Bare universal beams 15

Slim floor/asymmetrical beams 60

Shelf angle beams 60

Partially encased beams >60

Source: Bailey, C.G. and Newman, G.N., 1998, The design of steel framed building without applied fire protection, The Structural Engineer, 76(5), 77-81.

FIGURE 37.19 Steel/composite columns of high fire resistance: (a) blocked-in web, (b) partially encased, and (c) concrete filled.

FIGURE 37.19 Steel/composite columns of high fire resistance: (a) blocked-in web, (b) partially encased, and (c) concrete filled.

Handbook of Structural Engineering TABLE 37.11 Standard Fire Resistance Rating of Unprotected Steel Columns

Type of column

Standard fire resistance time (min)

Universal column Blocked-in column

Partially encased with unreinforced concrete Partially encased with reinforced concrete Concrete filled hollow section without reinforcement Concrete filled hollow section with reinforcement

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