BS Standard for composite slabs with profiled steel 4

Methods of developing composite action

1. General

The shear connection needed for composite action should be developed either by shear bond between the concrete and the profiled steel sheets or else by end anchorage, or by a combination of both methods (see 4.6.6).

For shear bond, the profiled steel sheets should be capable of transmitting horizontal shear at the interface between the sheet and the concrete. This should be achieved by one or more of the methods given in 4.6.3 to 4.6.5 or by any other proven method. In all cases the shear-bond capacity should be determined by testing (see section 8).

2 .Plain open profiled sheets

Plain open profiled sheets should not be used where composite action is required, unless accompanied by some means of shear connection (see 4.6.5 and 4.6.6).

3. Plain re-entrant angle profiled sheets

Plain re-entrant angle profiled sheets, as illustrated in Figure 4 a), should be designed to provide shear connection between the sheets and the concrete by means of the interlocking effect of the re-entrant shape.

4.Embossed profiled sheets

Embossed profiled sheets, as illustrated in Figure 4 b), Figure 4 c) and Figure 4 d), should be designed to develop shear connection through embossments (or embossments and indentations) in the webs and/or flanges of the sheets.

5. Small holes in profiled sheets

Holes in the webs and/or flanges of profiled steel sheets, intended to develop shear connection, should be sufficiently large for concrete to fill the hole, but sufficiently small to minimize the loss of fine material from the concrete, unless a permanent backing tape is provided on the underside which prevents this loss.

6. End anchorage

Shear connectors may be used as end anchorages to produce composite action in slabs which are designed as simply supported. Where sheets are not continuous over a support, end anchors should be provided at the ends of both sheets. Where the end anchorage provided by shear connectors is used in conjunction with the shear bond between the concrete and the profiled steel sheets, account should be taken of the influence of the deformation capacity of the shear connectors on the shear bond between the concrete and the sheets, as recommended in 6.4.3. The necessary interaction between stud shear connectors and the profiled steel sheets should normally be achieved by welding them to the structural steelwork by the site technique of through-the-sheet welding. Shear connectors directly attached to the structural steelwork prior to placing the profiled steel sheets should not be used as end anchorages unless the sheets are also attached to the steelwork as recommended in 4.8.1, by means of fixings of sufficient capacity.

NOTE If studs are welded to the beams prior to placing the profiled steel sheets, it may be found necessary to use single span sheets, in which case stop ends (see 4.8.4.3) may be needed to prevent concrete loss.

Where end anchorage is provided by types of shear connectors which connect the concrete slab directly to the profiled steel sheets, such as self-drilling self-tapping screws with enlarged washers, account should be taken of the deformation capacity of such shear connectors on the interaction between the slab and the sheets.

Where shear connectors used as end anchorages are assumed in design to act also as shear connectors in composite beams, reference should be made to 6.10.1. Where composite slabs are used in conjunction with reinforced concrete beams (see 6.10.2), any end anchorage required should normally be achieved by means of reinforcing bars.

7. Sheet edges

For profiles such as that shown in Figure 4 e), the edges of adjacent sheets should be overlapped or crimped in such a way as to provide an effective horizontal shear transfer between the sheets.

 Minimum bearing requirements

In all cases the bearing length of a composite slab should be sufficient to satisfy the recommendations of 5.2 for load carrying capacity as permanent formwork and the recommendations of BS 8110 for load carrying capacity as a composite slab. Composite slabs bearing on steel or concrete should normally have an end bearing of not less

than 50 mm [see Figure 5 a) and Figure 5 c)]. For composite slabs bearing on other materials, the end bearing should normally be not less than 70 mm [see Figure 5 b) and Figure 5 d)]. For continuous slabs the minimum bearing at intermediate supports should normally be 75 mm on steel or concrete and 100 mm on other materials

[see Figure 5 e) and Figure 5 f)]. Where smaller bearing lengths are adopted, account should be taken of all relevant factors such as tolerances, loading, span, height of support and provision of continuity reinforcement. In such cases, precautions should also be taken to ensure that fixings (see 4.8.1) can still be achieved without damage to the bearings, and that collapse cannot occur as a result of accidental displacement during erection.

 Constructional details

1. Sheet fixings

The design should incorporate provision for the profiled steel sheets to be fixed:

a) to keep them in position during construction so as to provide a subsequent safe working platform;

b) to ensure connection between the sheets and supporting beams;

c) to ensure connection between adjacent sheets where necessary;

d) to transmit horizontal forces where necessary;

e) to prevent uplift forces displacing the sheets.

2.Cantilever edges

The design should include provisions for adequate support of profiled steel sheets during construction at all cantilever edges and the like, including unsupported edges occurring at cut-outs or openings for columns.

3. Openings

a. Permanent openings

Reinforcement should be provided around permanent openings to avoid cracking of the composite slab.

b. Temporary openings

Where sheets are required to be temporarily left out (or cut out) during construction, due allowance should be made for the resulting loss of continuity in the design of the profiled steel sheeting (see section 5). Where necessary, thicker sheets or temporary supports should be used at such locations.

4 .Slab construction

a. Preparation

All extraneous grease, oil, dirt and deleterious matter should be removed from the upper surface of the sheets, but any greasiness remaining on the sheets from the forming process need not be removed.

b. Construction joints

Construction joints in composite slabs should be positioned close to the supporting beams.

c. Stop ends

Stop ends should be provided where necessary to prevent loss of grout at supports at which the sheeting is discontinuous.

d. Waterproofing

Where composite slabs are used for roofs, or other locations with impervious surface membranes, the design should incorporate provision for the free passage of water vapour.