Composite steel-concrete frame

How does Composite steel-concrete frame work, step by step?

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   Step 1: Design the floor with the frame, not after it

   The composite floor is designed alongside the steel frame because the two are one structural system. The structural engineer sets the beam sizes, the deck profile and gauge, the slab depth, the reinforcement and the shear connection together, working from the grid, the imposed loads, the fire strategy and the acoustic and vibration requirements of the finished building. Floor vibration matters more in a modern office than most people expect, particularly on long, lightly damped spans, and it often influences the design as much as strength does. The propped or unpropped decision is taken here as well, because it changes the steel sizes and it changes what happens on site.

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   Step 2: Erect the frame and land the decking

   The steel goes up first and the decking follows it closely, usually within a level or two. Bundles are craned onto the frame at planned landing points that the engineer has confirmed can take the bundle load, because a stacked bundle of deck is a heavy concentrated load on a bare beam. Sheets are laid out to the deck layout drawing, run in the direction the design requires, and lapped and seated on the supporting steel by the bearing the manufacturer and the designer call for. Working on part-laid decking is a fall risk until it is fixed, so the gang works to a method that keeps them on fixed material with edge protection and, where needed, fall arrest in place.

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   Step 3: Fix the deck down and weld the shear studs

   The deck is fixed to the steel and along its side laps so that it acts as a diaphragm and so that no sheet can lift or shift under the gang or the pour. Shear studs are then welded through the deck onto the beam flange, on the pattern the engineer has specified. Stud welding is sensitive to conditions - the steel has to be clean and dry, the deck has to be pulled tight down onto the flange, and galvanised coatings and moisture both interfere with the weld. Studs are tested by bend testing on a sample basis as the work proceeds and the results are recorded. A stud that has not welded properly is a shear connection that does not exist, and the composite action the whole design depends on quietly disappears.

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   Step 4: Set the props where the design calls for them

   On a propped design, temporary props and bearers are installed under the deck before any concrete is placed, to a scheme produced by the temporary works designer. The props take part of the construction load and reduce deflection of the bare steel, which is what allows the lighter section. The scheme sets the props out, defines what they bear on, and requires that the floor below can take the load being passed down to it - a point missed on more projects than it should be, because props transmit load down through the structure, not into thin air. The props are inspected and signed off before the pour, and they are not moved, adjusted or borrowed for anything else afterwards.

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   Step 5: Fix the reinforcement and form the penetrations

   Mesh and any additional reinforcement are placed to the drawings, at the cover the specification requires, on proper spacers rather than on offcuts. Edge trim is fixed around the slab perimeter and around every opening, and the penetrations for risers, ducts and drainage are formed now, from the coordinated drawings. Forming holes properly at this stage is far cheaper and far safer than cutting them later, and cutting a composite slab afterwards means cutting the deck, which is structure. Every opening formed in the deck is a fall risk from the moment it exists, so it is covered or protected as it is created.

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   Step 6: Place, finish and cure the concrete

   Concrete is placed to the pour sequence agreed with the engineer, usually by pump, and spread evenly rather than being heaped in one place, because a heap of wet concrete on an unpropped deck is a construction-stage overload waiting to happen. The slab is levelled and finished to the flatness the specification requires - a poor floor here is an expensive problem for the fit-out - and then cured properly. Curing is not optional. A slab that dries out too fast is a slab that cracks and a slab that has not developed the strength assumed. Cubes are taken from the pour so that strength gain can be demonstrated rather than guessed.

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   Step 7: Wait for strength before the floor is composite

   Until the concrete has gained the strength the engineer requires, the floor is not the structure shown on the drawing. Loading out a freshly poured floor with materials, plant or stacked pallets is one of the most common ways a well-designed composite floor gets damaged, and the loading limits during this period are the engineer's to set. Where props have been used, they stay in place until the engineer or the temporary works designer confirms in writing that they can come out, and they are struck in the sequence that scheme sets. Nobody on site decides that a slab looks hard enough. The evidence is the cube results and the designer's written release.

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   Step 8: Protect the floor and hand it on

   Once released, the floor becomes the working deck for every trade above and below it. The soffit is fire protected to the fire strategy, the perimeter is closed for the facade, and the slab surface is protected from the damage that follows a busy fit-out. Levels and flatness are surveyed and recorded, because raised access floors, screeds and finishes are all set out from them. Any subsequent request to cut, core or penetrate the slab goes to the structural engineer, since the deck below the concrete is a structural element and severing it in the wrong place removes capacity the design is relying on.