COMPONENTE POLÍTICO-ORGANIZATIVO

ASBs will in most cases be designed to act compositely in the final stage.

Composite action is developed by the shear bond between the steel and concrete around the beam, and this is enhanced by the raised pattern rolled into the top surface of the beam. The bond is sufficient to satisfy the minimum degree of shear connection required by BS 5950-3. Only in unusual cases, for example when there is less than 30 mm concrete cover over the top flange, will a non-composite design be necessary. This will mean that section sizes may have to be increased for given span and loading requirements.

SFBs may be designed either non-compositely or compositely. Composite action can be achieved by the use of shear studs welded to the top flange. These studs are usually 19 mm diameter and 70 mm as-welded height.

RHSFB edge beams may be designed either non-compositely or compositely. As for SFBs, composite action can be achieved by the use of ‘short’ shear studs welded to the top flange. Sufficient transverse reinforcement looped around the shear connectors is required in order to transfer the shear force into the slab^[75]. For ASBs, SFBs and RHSFBs, the minimum concrete cover required for the beams depends on the beam size, exposure conditions, the concrete specification and composite interaction is required.

The plastic stress blocks assumed when calculating the moment resistance of a composite ASB section are shown in Figure 7.6. The resistance model for a composite SFB or RHSFB is basically the same as that for a ‘traditional’

composite beam (Section 5.2.1), with a requirement to consider the ability of the shear studs to transfer the envisaged longitudinal force.

For edge beams, or beams adjacent to openings in the slab, out-of-balance loading occurs during both the construction and final stages. This may be taken into account by a rigorous analysis combining the longitudinal bending effects with the torsional effects. A method is presented in Reference 79.

If possible, sufficient concrete cover should be provided to allow composite interaction between the beams and slab. This will generally allow the beam sizes to be reduced.

Slab design

The design of composite slabs using deep decking differs from that for shallow decking (Section 4.2) in the following ways:

 The ultimate load resistance of the slab is increased by placing bar reinforcement in the troughs of the decking. The benefit of these bars is considered in both the ‘normal’ and fire conditions.

 The slab depth may need to be chosen not only to satisfy the structural, durability and fire resistance requirements of the slab itself (see

Compression in concrete Effective breadth of slab B

D

Figure 7.6 Assumed stress blocks for ASB design (using BS EN 1994-1-1 notation)

Sections 4.2.3, 4.2.4 and 4.2.5), but also to provide appropriate cover over composite beam sections (Section 7.2.2).

The reinforcing bars in the troughs of the decking provide additional tensile area to that provided by the decking, and thus enhance the bending resistance of the composite slab (Figure 7.7). Diameters range from 16 mm to 32 mm, depending on the span and fire resistance requirements.

Straight bars may be used to achieve 60 minutes fire resistance (provided that shear stresses are low^[74]. In other cases, L bars (see Figure 7.8) should be used to provide sufficient end anchorage in fire conditions. Detailing rules are summarised in Table 7.1 and Figure 7.8.

Table 7.1 Detailing requirements for deep composite slabs

Fire Resistance (min) Detailing Requirement

≤60 90 120 Minimum bar dia (mm)

- unpropped 16 20 25

- propped 20 25 32

Cover to bar (mm) 70 90 120

Bar type Straight L-bar L-bar

Min fabric in topping A142 A193 A252

The minimum anchorage details depend on the level of applied shear and the diameter of the main reinforcing bars in the rib, which in turn depends on the fire resistance period and whether or not the slab is propped. For 60 minutes fire resistance, when the level of applied shear is less than 0.5 times the available shear resistance, straight bars may be used without extra anchorage bars - in accordance with BS 8110-1:2005^[30], clause 3.12.9.4. However, anchorage bars are

recommended even for low values of applied shear at 90 and 120 minutes fire resistance periods.

For more details see Reference 74.

Slip between deck and concrete

Support

Longitudinal shear bond

Concrete in compression

Tension in decking and bar reinforcement

Stress distribution Vertical

reaction

Mid-span

Bar reinforcement

Figure 7.7 Action of composite slab with reinforcement in ribs

Additional reinforcement may be required to fulfil the following roles:

 Transverse reinforcement adjacent to shear connectors.

 U-bars at composite edge beams.

 Additional crack control reinforcement (see below).

 Strengthening around openings.

 Strengthening at positions of concentrated loads.

One of the principal considerations governing the choice of slab depth is the required fire resistance period. Minimum depths are given in Table 7.2^[74] as a function of the concrete type and fire resistance required.

Table 7.2 Minimum concrete depth above decking for adequate fire insulation

Concrete Depth Above Decking (mm) Fire Resistance (mins) Normal

concrete

Lightweight concrete

60 70 60

90 80 70

120 90 80

Note: Depths given are the minimum for fire insulation purposes. Greater thicknesses may be required for spanning capability, or to achieve adequate beam cover.

The slab depth may also be governed by structural resistance requirements.

However, as for shallow decking (Section 4.2.3), the performance of a composite slab using deep decking can only be accurately determined by testing.

Detailed design procedures have been developed based on appropriate tests^[74], and should be used to determine the depth of slab needed to satisfy structural requirements.

It is normal for some cracking to occur in the slab over the beams. These cracks run parallel with the beams and are not detrimental to the structural behaviour of the slab. They may be controlled by fabric reinforcement provided across the tops of the beams. Guidance on the detailing of reinforcement to control cracking may be found in the Corus Slimdek manual ^[74].

12 _L 25

50 _L 100 mm

100 mm

_L



Figure 7.8 Detailing of bar reinforcement in slabs (need for L bars depends on level of shear stress)