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Steel Designers' Manual - 6th Edition (2003)
20.6.2 Fire engineering method
The phrase ‘fire engineering method’ (FEM) is the term given to the means of calculating
the amount of emergency reinforcement needed for a certain fire resistance
period. This is described fully in a Steel Construction Institute publication. 9 In
principle, the moment of resistance of the section is calculated taking into account
the temperatures in the section and the reduced strength of the various elements at
elevated temperatures.Temperatures are determined from tables or indicative tests.
The plastic capacity of continuous members can be evaluated and compared to the
applied moment with load factors of unity. In some areas, BS 5950: Part 8 10 permits
a further 20% reduction in imposed load in a fire. Typically, therefore, the applied
moment in a fire would be 50–60% of the ultimate design moment.
The critical element determining the moment of resistance of the section is the
reinforcement. Mesh reinforcement is normally well-insulated from the effects of
the fire and contributes significantly to the fire resistance of the slab. Where fire
resistance periods greater than 90 minutes are required (see below) then additional
reinforcing bars or heavier mesh can be introduced. In such cases it may be economic
to design the slab as a reinforced concrete ribbed slab to BS 8110, 11 and treat
the decking as permanent formwork. Bars of diameter 10 mm or 12 mm are normally
placed singly in the deck troughs at the appropriate cover.
20.6.3 Design recommendations
Simple design tables covering common design cases are given in Reference 12. A
minimum slab depth and mesh size are given for different profile types. Data for
only two spans (i.e. 3 m and 3.6 m) and one imposed load (6.7 kN/m 2 ) are presented,
but these may be converted to other cases by using the same equivalent moment as
the tabulated cases. The data are reproduced in Table 20.2.
20.7 Diaphragm action
Diaphragm action 591
The decking serves to transfer lateral loads to vertical bracing of concrete walls. It is
normally attached on all four sides to support beams at spacings of not less than 600
mm. However, deck–deck seam fasteners are not normally installed. The shear stiffness
of the ‘diaphragm’ is very high and the strength is normally determined by the
capacity of the fixings.Typical ultimate strengths of the standard screws or shot-fired
pins are 6 kN/mm sheet thickness. Where through-deck welding of shear-connectors
is used the ultimate shear strength of the decking is considerably enhanced.
The steel beams are laterally supported in simple bending by the decking, provided
the decking crosses the beams and is attached to them by shear-connectors
or shot-fired pins at spacings not exceeding 600 mm. However, beams running