Installing a filigree slab — props and structural topping

Installing a filigree slab looks simple: the crane sets the planks down, the crew ties the reinforcement, the truck pours the topping. Most of the expensive mistakes, however, come from decisions taken earlier — when the props are set up and when the falsework comes down. Below is the erection sequence according to the manual that applies to our planks.

One rule overrides all the others: if the manual differs from the erection drawing or the unit drawing, the design documentation governs — once confirmed by the structural designer.

Before the first plank arrives

The following must be available on site: the current erection plan, unit drawings, the propping layout, drawings of the supplementary reinforcement and data on unit weights.

The geometry of the building is checked, along with the levels of the permanent supports, the location of openings, downstand beams and ring beams, and consistency with the base design. The delivery sequence is agreed so that unloading matches the erection order.

Feeding the planks straight from the vehicle into their final position is preferred. Where intermediate storage is used, the planks are stacked on level, loadbearing ground, and the packers must sit vertically one above the other, in the zones indicated by the plant.

The number of layers in a stack is set for the specific delivery, taking account of weight, geometry and the capacity of the bottom plank. A fixed number of layers must not be assumed without confirmation.

Prohibited: storage on soft or sloping ground, or with packers offset between successive layers.

Props — the most common mistake

Props, propping beams and edge props are set up and levelled before the first plank is delivered. They must carry the weight of the precast unit, the fresh concrete, the reinforcement, the workers, the equipment and the erection loads.

And now the most important point, the one most often tripped over:

Prop spacing follows solely from the erection drawing.

There is no universal rule of the "every two metres" or "every fifth of the span" kind. Spacing depends on plank thickness, span, erection loads and the structural scheme of the particular floor. Setting it on the basis of experience from another building is not acceptable — nor is levelling props beneath a slab that is already loaded.

If you are looking in this article for a number you can take to site without opening the drawings — there is no such number, and giving one would be irresponsible.

Lifting the planks

Only the points shown on the erection drawing are used for lifting. Hooks are attached at the lattice girder nodes, where the top chord meets a diagonal. Hooking onto the top chord alone, between nodes, is a mistake that can end in the reinforcement being torn out.

The number of lifting points is not determined by the length of the plank alone — it is selected on the basis of weight, geometry, the position of openings and the stiffness of the unit. Every point indicated must be used.

Unless the drawing states otherwise, the sling angle is taken as not less than 60° to the plane of the plank. A smaller angle calls for an individual check of the forces at the lifting points.

Before the full lift: a trial lift to minimum height, a check that the slings are evenly tensioned and that the unit hangs level. The crane's capacity is confirmed for the actual working radius, not for the nominal figure on the plate.

Bearing, joints and supplementary reinforcement

Bearing lengths and the position of reinforcement projecting beyond the precast unit are set by the drawings. Bearing surfaces must be clean and level.

Once the planks are laid, work covers joint reinforcement, supplementary bottom reinforcement, top reinforcement, ring beams, downstand beams and trimming around openings and penetrations. Indicative reinforcement quantities for typical spans are collected in the article on spans, thicknesses and reinforcement of filigree slabs.

The lattice girders and the reinforcement of the precast unit must not be cut without the written approval of the designer. Openings not shown in the design require their position relative to the reinforcement to be checked — regardless of diameter.

Checks before concreting

Sign-off of this stage covers eight points:

• planks laid in accordance with the erection plan and in the correct orientation,
• props complete, levelled, with no sign of buckling or settlement,
• bearing lengths in accordance with the documentation,
• joint, top and additional reinforcement executed per the drawing,
• services and openings protected,
• cover to the reinforcement assured,
• slab edges protected against falls,
• reinforcement and propping signed off before concreting.

Large openings are protected against a worker falling through; edges and penetrations are checked for tightness.

The topping

The upper surface of the precast units is cleaned. Surface preparation and wetting are adapted to the method statement and the weather.

Concrete is placed and compacted so as not to displace the reinforcement or locally overload the props. Curing begins immediately after placing is complete — not the following day.

When to strike the props

The criterion for striking is the concrete strength achieved and the load scheme, not the number of days since concreting. A calendar confuses effect with cause here: the same fourteen days at a different temperature give a different strength.

Unless the design states otherwise, partial release with back-propping may be considered once at least 70% of the required concrete strength has been reached. Full removal of the props follows only once the criterion set by the structural designer has been met.

When further storeys are built, back-propping applies: the props of the upper storey transfer concentrated loads onto the floor below. The capacity of the entire load path must be checked, down to a structure of sufficient strength — not merely the slab directly beneath.

Finishing

The soffit is prepared for finishing in accordance with the surface standard specified in the order. Joints are prepared with an anti-crack system suited to the intended finish.

Chases and openings are formed only in the zones permitted by the documentation. Reinforcement must not be cut at this stage either.

The full manual, with drawings, the back-propping scheme for successive storeys and a sign-off form, can be downloaded from the downloads section, and the product parameters are described on the filigree slab product page.