Concrete frame with shear walls and core

How does Concrete frame with shear walls and core work, step by step?

1. 1

   Step 1: Fix the stability strategy and the position of the core

   The structural engineer sets out where the shear walls and the core sit, and that layout then constrains the architecture for good. Cores stack vertically through the building and land on foundations sized for the loads and overturning they attract, so their position is agreed very early with the architect and the services engineer, who both want the same space for risers and lifts. The engineer also decides whether the frame is a flat slab, a slab with bands, or a beam-and-slab arrangement, since that governs the soffit, the service routes and the formwork the site will be cycling. On most projects the core and the shear wall layout are the first things fixed and the last things anybody is allowed to move.

2. 2

   Step 2: Build the core ahead of the frame

   The core is commonly taken ahead of the floors with a climbing, jumpform or slipform system so that the stability element leads the build. The system lifts itself off the concrete already cast, carrying its own working platforms, edge protection and access, which makes it a safer and faster way of working at height than repeatedly striking and re-erecting panels. It also gives the crane and the setting-out a fixed vertical reference for the whole building. Wall reinforcement, box-outs for doors and risers, and the embedded plates and couplers the floors will connect into are all placed as the core rises, from coordinated drawings - because a missing box-out in a shear wall is not something anybody cuts in later.

3. 3

   Step 3: Erect and set the formwork and falsework for the floor

   The floor deck is formed on tables, panels or a proprietary system supported by falsework designed by the temporary works designer for the wet concrete plus everything that will stand on it. It is set to line and level, sealed, and released so the finished concrete strikes cleanly, and edge protection and safe access are built into it as part of the erection rather than added afterwards. Formwork is checked for level, alignment and tightness before any steel goes on it. Falsework is inspected and signed off by a competent person against the design before it is loaded, and it is not modified afterwards to suit somebody else's access.

4. 4

   Step 4: Fix reinforcement and cast in everything the frame needs

   Reinforcement is fixed to the bending schedules and drawings, at the cover the specification demands and on proper spacers, with laps and continuity into the walls and the core as detailed. This is also the point at which every embedded item goes in: service penetrations and sleeves, drainage penetrations, cast-in channels and sockets, holding-down bolts, movement joint components and the starter bars for the level above. Coordination has to be finished by now. Anything missed is a hole to be cut in structural concrete afterwards, which needs the engineer's agreement and never comes cheap. The completed reinforcement is inspected as a hold point before the pour.

5. 5

   Step 5: Pour, compact and finish the concrete

   The pour is planned as an operation: the volume, the pour sequence, the position of construction joints, the pump or skip arrangement, the gang, the finishing crew and the contingency if a delivery fails. Concrete is placed in a controlled sequence, compacted with vibrators so that it flows around congested reinforcement without leaving voids, and finished to the flatness the specification requires. Where the soffit is to be exposed as the finish, the formwork face, the release agent and the pour discipline are all part of achieving that appearance, and the standard is agreed against a sample panel rather than argued about afterwards. Cubes are taken from the pour for strength testing.

6. 6

   Step 6: Cure the concrete properly

   Curing is the part of a concrete frame that costs nothing and gets neglected most. Keeping the concrete from drying out lets it develop the strength and the durability the design assumed, and it controls early cracking. Cold weather slows strength gain and hot or windy weather dries the surface too fast, so the curing regime and any protection are planned against the season rather than the calendar. The cycle time for the floor is governed by strength gain, not by how quickly the gang would like to move, and the site programme has to be built around that honestly from the outset.

7. 7

   Step 7: Strike and de-prop only on written authority

   Formwork is struck and props are removed only when the structural engineer or the temporary works designer confirms in writing that the concrete has reached the strength required, on the evidence of cube results or equivalent testing. The sequence for striking and back-propping is theirs to set, because a newly struck slab may still be carrying load from the floors above through the props and removing them in the wrong order transfers that load somewhere it was never designed to go. Nobody borrows a prop, nobody strikes early to release formwork for the next level, and nobody judges a slab by tapping it. Premature striking is one of the most serious failure modes on any concrete frame.

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   Step 8: Cycle the floor and hand the frame on

   Formwork, falsework and gangs move up and the cycle repeats, with the core continuing to run ahead. As the frame rises, the completed levels are surveyed for position, level and verticality and the results issued, because the facade, the lifts and the fit-out are all set out from them and the tolerances are tighter than people expect on a wet trade. Surfaces are protected, penetrations are kept sealed, and defects are recorded and repaired to an agreed method rather than made good quietly. The finished frame needs no fire protection trade behind it and already provides its acoustic separation, so the follow-on packages inherit a structure that is genuinely complete.