Formwork Table System

Overview

A formwork table is a complete section of floor form built once and reused for every level of a building. It holds the wet concrete of a suspended slab until the slab cures, then it is lowered, rolled to the edge of the building, lifted by crane, and set on the floor above to cast the next slab. The point of building the form as a single large table rather than as loose timber and props is speed: one crane lift moves a whole bay of formwork in minutes, so a tower can turn a floor every few days. The system here is built around the Decking and Facing that the concrete sits on, the Primary Beam Grid and Secondary Joist Layer that carry that deck, the Telescopic Props and Legs that stand it on the slab below, and the Drophead Mechanism, Flying Trolley, and Lifting and Fork Points gear that let it be struck and flown without taking it apart. The Edge Protection and Bracing and Alignment keep crews safe on the table and hold it square while it travels.

Deck and facing

The slab is cast on the Decking and Facing. Its working face is a layer of Phenolic Plywood Sheet panels, birch plywood faced both sides with a hard phenolic film that gives a smooth soffit and survives a great many pours. A Release Facing Coat of film and release agent lets the cured slab strip clean off the plywood without tearing the face. Round the perimeter a Edge Rail stiffens the panel and locates the side form, and where one table meets a wall or the next table a Infill Strip closes the gap so the finished soffit runs flat across the joint. The plywood is held down with countersunk Deck Screw fixings driven from above, which keeps the slab face clean and lets a damaged sheet be swapped quickly.

Beams and joists

The deck cannot span far on its own, so it sits on two layers of beam. The Secondary Joist Layer are the close-spaced upper layer: rows of H20 Timber Joist timber I-beams laid every 300 to 500 mm and held down by Joist Clip clips so they do not shift while the table is flown. A Joist Spacer Comb comb fixes the joist centres, and a Joist End Cap protects each timber end. Below the joists run the Primary Beam Grid: deep aluminium Primary Girder sections that gather the joist loads and carry them to the props. Girders are made up to length with bolted Girder Splice Plate plates, clamped to the prop heads with a Beam Clamp at each bearing, and stiffened with a Web Stiffener where the prop reaction is highest.

Props and height setting

The Telescopic Props and Legs stand the table on the slab cast below it. Each Telescopic Prop Leg is a two-tube telescopic leg: the Inner Prop Tube slides inside the Outer Prop Tube to a coarse height set by a Adjustment Pin and held by a Locking Collar. Fine height comes from a Screw Jack at the leg top, where the operator turns the Jack Nut on the Jack Spindle to trim the slab level to a few millimetres. The leg sits on a Base Plate that spreads the load onto the slab, and a Prop Head seats the girder. Setting the props is what fixes the soffit level, so a Level Indicator on the frame is read before the jacks are locked.

Striking, flying, and lifting

Once the slab has cured enough to hold itself, the table must come down. The Drophead Mechanism lets that happen without dropping the props: knocking out the Release Wedge lets the Drophead Body lower the deck by the Stroke Stop travel, around 100 to 150 mm, which is enough to break the deck free of the slab while the legs still stand. With the deck dropped, the Flying Trolley takes over. A Trolley Jack lifts the table off its props onto the Trolley Frame, which rolls on heavy Swivel Castor wheels along a Roller Track to the slab edge. There the table runs out over a Edge Roller until its Fork Pocket sections clear the building, and the Crane Lifting Fork on the crane slides in and flies the table up to the next floor. Where a fork cannot reach, a Sling Chain hooked to the Lifting Eye points through a Shackle takes the lift instead.

Materials and safety

The table mixes materials to keep weight down where it can. The girders are aluminium for a high strength-to-weight ratio, the joists are timber I-beams that are light and easy to cut on site, and the props, base plates, and lifting gear are steel for stiffness and abrasion resistance. While crews work on the table tying reinforcement, the Edge Protection guards the open edge: a Handrail Post in each Post Socket carries the Guard Rail rails, and a Toe Board stops tools falling onto the floor below. The whole table is held square by the Bracing and Alignment, where a Diagonal Brace clamped with a Brace Coupler stops it racking during the move and a Alignment Screw slides it into its exact set-out line. Reused this way for hundreds of pours, a set of these tables casts every floor of a building from one investment in formwork.