Composite Deck Slab

Steel beam

(a) Typical sub-girder system

Concrete slab

Deck Slab Resistance

Steel beam

Profiled sheeting /

(b) Composite profiled slim deck system

(b) Composite profiled slim deck system

Concrete slab

Steel beam

Profiled sheeting /

The Slimdec system shown in Figure 12.3 is manufactured by Corus. The special steel beams have a patterned tread on the top flange that provides an enhanced bond with the concrete slab so that a composite action can be developed without the use of shear studs. Deep ribbed profiled sheeting is used to support the slab with the deep ribs resting on the bottom flange of the beam. With this arrangement the steel beam is partially encased by the concrete which provides it with better fire resistance. Openings for services can be cut in the web of the beam between the concrete ribs.

The concrete slab itself can also be constructed as a composite member using the profiled steel decking on the soffit of the slab as shown in figure 12.4. The steel decking acts as the tension reinforcement for the slab and also as permanent shuttering capable of supporting the weight of the wet concrete. It is fabricated with ribs and slots to form a key and bond with the concrete. Properties of the steel decking and safe load tables for the decking and the composite floors are obtainable from the manufacturing companies.

Many composite beams are designed as simply supported non-continuous beams. Beams that are continuous require moment resisting connections at the columns and additional reinforcing bars in the slab over the support.

The method of construction may be either:

With propped construction temporary props are placed under the steel beam during construction of the floor and the props carry all the construction loads. After the concrete has hardened the props are removed and then the loads are supported by the composite beam. The use of temporary props has the disadvantage of the lack of clear space under the floor during construction and the extra cost of longer construction times.

Unpropped construction requires that the steel beam itself must support the construction loads and the steel beam has to be designed for this condition, which may govern the size of beam required. The beam can only act as a composite section when the concrete in the slab has hardened. This also means that the deflection at service is greater than that of a propped beam as the final deflection is the sum of the deflection of the steel beam during construction plus the deflection of the composite section due to the additional loading that takes place after construction. The calculations for this are shown in example 12.4 which sets out the serviceability checks for an unpropped beam.

As there are differences in the design procedures for these two types of construction it is important that the construction method should be established at the outset.

Shear studs

Profile steel decking

Profile steel decking

Shear studs

Composite Slab
Steel beam

Figure 12.4

Composite slab with steel decking

12.1 The design procedure

The design procedure for composite beams follows ihe requirements of:

(a) EC2. (EN 1992-1-1) for the design of concrete structures,

(b) EC3 (EN 1993-1-1) for the design of steel structures, and

(c) EC4 (EN 1994-1 -1) for the design of composite steel and concrete structures.

At the time of writing this chapter the UK National Annex for EC3 and EC4, and the Concise Eurocodes are not available. Parts of these codes are quite complex; for example the list of symbols for the three codes extends to 21 pages. It is intended in this chapter to try and simplify many of the complications and enable the reader to gain a grasp of the basic principles of the design of composite beams.

12.1.1 Effective width of the concrete flange (EC4, cl 5.4.1.2)

An early step in the design of the composite beam section is to determine the effective breadth beff of the concrete flange.

For building structures at mid-span or an internal support

- Y, bci where bn is the effective width of the concrete llangc on each side of the steel web and is taken as /,c/8. but not greater than half the distance to the centre of the adjacent beam. The length l.e is the approximate distance between points of zero bending moment which can be taken as L/2 for the mid-span of a continuous beam, or L for a one-span simply supported beam. The length L is the span of Ihe beam being considered.

For example, for a continuous beam with a span of L 16m and the adjacent beams being at 5 m centre to centre the effective breadth. /;cn\ of the concrete llange is

If the beam was a one-span simply supported beam the effective breadth, />eft, would be 4.0 m.

12.1.2 The principal stages in the design

These stages are listed with brief descriptions as follows:

(1) Preliminary sizing

The depth of a universal steel beam may be taken as approximately the span/20 for a simply supported span and the span/24 for a continuous beam. The yield strength, fy, and the section classification of the steel beam should be determined.

(2) During construction (for unpropped construction only)

The loading is taken as the self-weight of the steel beam with any shuttering or steel decking, the weight of the wet concrete and an imposed construction load of at least 0.75 kN/m2. The following design checks are required:

(a) At the ultimate limit state

Check the strength of the steel section in bending and shear.

(b) At the serviceability limit state Check the deflection of the steel beam.

(3) Bending and shear of the composite section at the ultimate limit state

Check the ultimate moment of resistance of the composite section and compare it with the ultimate design moment. Check the shear strength of the steel beam.

(4) Design of the shear connectors and the transverse steel at the ultimate limit state

The shear connecters are required to resist the horizontal shear at the interface of the steel and the concrete so that the steel beam and the concrete flange act as a composite unit. The shear connectors can be either a full shear connection or a partial shear connection depending on the design and detailing requirements.

Transverse reinforcement is required to resist the longitudinal shear in the concrete flange and to prevent cracking of the concrete in the region of the shear connectors.

(5) Bending and deflection at the serviceability limit state for the composite beam

The deflection of the beam is checked to ensure it is not excessive and so causing cracking of the architectural finishes.

12.2 Design of the steel beam for conditions during construction (for unpropped beams only)

The steel beam must be designed to support a dead load of its estimated self-weight, the weight of wet concrete and the weight of the profiled steel decking or the formwork, plus a construction live load of at least 0.75 kN/m" covering the floor area.

A preliminary depth for the sizing of the steel beam can be taken as the span/20 for a one-span simply supported beam.

(a) At the ultimate limit state

(i) Bending

The plastic section modulus Wpi,y, for the steel beam may be calculated from

Mrci is the ultimate design moment fy is the design strength of the steel as obtained from EC3. table 3.1

This assumes that the compression flange of the steel beam is adequately restrained against buckling by the steel decking for the slab and the steel section used can be classified as a plastic or compact section as defined in EC3. sections 5.5 and 5.6.

(ii) Shear

The shear is considered to be carried by the steel beam alone at the construction stage and also for the final composite beam. The ultimate shear strength of a rolled I-beam is based on the following shear area, of the section

,4V = A., 2bt( + (fw + 2r)t( but not less than ///tvv7w (12.2)

where Aa is the cross-sectional area of the steel beam and hw is the overall depth of the web. ■>] can be taken as 1.0. The other dimensions of the cross-section are defined in figure 12.5.

Figure 12.5

Dimensions for an l-section beam r

r - radius ol roof fillet

For class I and class 2 I-beams with a predominately uniformly distributed load the design shear stresses are seldom excessive and the shear area. Av may be conservatively taken as the web area so that

where d is the depth of the straight portion of the weh.

The design plastic shear resistance V,,i.Rd of the section is given by:

7mov3

where 7mo 1 -0 is the material partial factor of safety for the steel.

(b) At the serviceability limit state

The deflection 6 at mid-span for a uniformly distributed load on a steel beam is given by:

where w is the serviceability load per metre at construction L is the beam's span

£a is the elastic modulus of the steel = 210kN/mnr /a is the second moment of area of the steel section

The deflections at the construction stage due to the permanent loads arc locked into the beam as the concrete hardens.

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