Composite metal deck slabs

How does Composite metal deck slabs work, step by step?

1. 1

   Step 1: Design, procure and set out the deck layout

   The structural engineer sets the deck profile, the slab depth, the mesh and the shear stud arrangement to suit the grid, the loading and the fire strategy, and the deck manufacturer produces a sheeting layout showing every sheet, every cut, every edge trim and every hole. Setting out starts from the steelwork survey rather than from the drawings, because the frame as erected is what the sheets have to land on. Openings for risers, stairs and lift cores are marked and trimmed on the layout so they are formed as the deck is laid rather than cut afterwards. Long lead items are the deck itself, the edge trim and any bespoke closures, and they are ordered off the steelwork sequence so that decking follows erection floor by floor.

2. 2

   Step 2: Establish the safety system before the first sheet lands

   Decking is worked at a leading edge, so the protection has to be in place before the work starts, not alongside it. Perimeter edge protection goes up with the steel where it can, the fall-arrest or netting arrangement is installed and inspected by a competent person, and the safe access routes onto the frame are agreed and marked. Deck bundles are landed on the beams in positions the temporary works designer has accepted, never dropped in the middle of an unfixed span, and they are broken out and spread promptly because a stacked bundle is a point load on a floor that is not a floor yet. Everything above is standard practice on most projects and is confirmed in the method statement for the particular building.

3. 3

   Step 3: Lay and fix the sheets

   Sheets are lifted in bundles by the frame crane or a telehandler, distributed along the beams and then laid out by hand from the fixed edge outwards. Each sheet is butted to the last, the side laps are engaged, and the deck is fixed down to the steelwork with shot-fired pins or self-drilling fixings at the spacing the layout requires, with side lap stitching between sheets. Fixing is not a finishing operation to be caught up later - an unfixed sheet is a slip hazard and can be lifted by wind, so the working rule on most sites is that no sheet is left loose at the end of a shift. Edge trim, closures and stop ends are fitted as the deck advances so that the pour is contained.

4. 4

   Step 4: Weld the shear studs

   Studs are through-deck welded onto the beam flange with a stud welding gun, one at a time, so that the beam and the finished slab act compositely. The deck has to be clean, dry and pulled tight down onto the flange at the stud position, because a gap or a wet surface gives an unsound weld. Studs are tested as the work proceeds by bend testing an agreed sample and by visual inspection of the collar around the base of every stud. Failures are cut out and replaced. Where the deck cannot be pulled down tight, or where paint or galvanising on the flange interferes, the designer decides the alternative rather than the site.

5. 5

   Step 5: Prop where the design requires it

   Many deck spans are designed to carry the wet concrete unpropped, and that is one of the attractions of the system. Others are not, and a propped span left unpropped is a collapse waiting for a concrete wagon. The deck design states whether props are needed, where they go and what they stand on, and that is a temporary works item with a designer, a check and a permit like any other. Props are set plumb, on a sound bearing, braced where the height demands it, and they stay in place until the release is authorised in writing on the strength results. On most projects the prop layout is inspected and signed off immediately before the pour as part of the pre-pour check.

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   Step 6: Fix reinforcement and pour in a planned sequence

   Mesh is laid with the laps and cover the drawings call for, on proper spacers rather than on offcuts, with any additional bars over supports and around openings fixed as detailed. Concrete is placed by pump or skip in the sequence agreed before the day, spreading the load and working away from the pour access rather than heaping it in one bay. Overloading a fresh floor with a stationary pile of concrete is one of the common ways deck floors are damaged, so the plan controls where the wet weight sits at every stage. The surface is levelled to the agreed tolerance and finished, then cured and protected. Movement joints and any surface regularity requirement for the tenant fit-out are set by the specification.

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   Step 7: Cure, strike props and hand the floor over

   Curing starts as soon as the finish allows and continues for the period the specification sets, because a floor that dries too fast cracks and a cracked floor is an argument that runs to handover. Cube or core results, not the calendar, release the props and the floor to following trades. Once released, the slab is surveyed for level and flatness, penetrations are recorded, and the floor becomes the working platform for the frame above and the fit-out below. Fire protection to the underside, any acoustic requirement and the sealing of service penetrations are then coordinated so that the floor achieves what the designer intended rather than only what was poured.