4

Concrete slab I 130

Concrete slab I 130

Units in mm

FIGURE 37.15 Composite cross-section dimensions for Example 37.6. Copyright 2005 by CRC Press

Taking moment about the top surface of the slab, the residual sagging bending moment capacity of the composite cross-section is

Mf = [-208.7 x 5.22/2 + 67.3 x(130 + 15/2) + 87.9 x(130 + 400/2) + 53.5 x(130 + 400 - 15/2)]/1000 = 65.7kNm

37.4.4.3 Composite Columns

37.4.4.3.1 Resistance to Axial Load According to Eurocode 4

The general equation for calculating the squash load of a composite column is

where f is the design strength of the ith layer and subscripts ''c," ''s," and ''r'' represent concrete, steel, and reinforcement, respectively. Due to nonuniform temperature distribution, each component of the composite cross-section is divided into a number of layers of approximately the same temperature. Similarly, the rigidity (EI) of the composite cross-section is calculated using the following equation:

(E1)fi = E(EI )i,s + E 0.8(EI )i,c + E(EI)i,r (37.30)

where symbols E and I are the initial modulus of elasticity and second moment of area of the appropriate component material about the relevant axis of buckling of the entire composite cross-section, respectively. The composite column compression resistance in fire is given by

where the compression strength reduction factor in fire is calculated by

Zfi =-1 with f fi = 0.5 [1 + a(!fi - 0.2) + ifil (37.32)

ffi + \lfl - lfi in which the initial imperfection factor a has a value of 0.49. The column slenderness factor Ifi in fire is defined by

where the Euler buckling load in fire is calculated using p2(EI)

in which Le is the column effective length.

37.4.4.3.2 Simplified Temperature Calculation Method for Unprotected Concrete Filled Columns

The temperature calculation method is based on the method of Lawson and Newman (1996) with modification by Wang (2000). This method assumes that the composite column is unprotected. In this method, the steel shell temperature is calculated by

where Tfi is the standard fire temperature and C2 is a multiplication factor depending on the fire resistance time. C2 is given by

Depth from bottom of the slab x, mm

FIGURE 37.16 Temperatures of a concrete slab exposed to the standard fire from underneath (from European Committee for Standardisation (CEN), 2001, prEN 1994-1-2, Eurocode 4: Design of Composite steel and Concrete Structures, Part 1.2: Structural Fire Design (London: British Standards Institution)).

Depth from bottom of the slab x, mm

FIGURE 37.16 Temperatures of a concrete slab exposed to the standard fire from underneath (from European Committee for Standardisation (CEN), 2001, prEN 1994-1-2, Eurocode 4: Design of Composite steel and Concrete Structures, Part 1.2: Structural Fire Design (London: British Standards Institution)).

TABLE 37.8 Multiplication Factor C1

Diameter or size of square section (mm)

0 0

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