Suspended slab: formwork deductions and reinforcement laps
A 24.0 m × 18.0 m suspended slab, 250 mm thick, is supported by six internal downstand beams spanning the 18 m direction at 4.0 m centres. Each beam is 600 mm wide overall and 700 mm deep including the slab. There are two openings: one 1.20 m × 1.20 m and one 2.40 m × 1.20 m. Columns are 500 mm × 500 mm and are not deducted. The contract measurement rules state: formwork to slab soffit is measured to the overall slab area less beam soffit strips and less openings exceeding 0.50 m²; beam soffits and beam sides are measured separately; beam sides are taken from the underside of slab to the underside of beam; no deduction is made for columns. Reinforcement is measured from the bar schedule; laps, anchorage, spacers and tying wire are included in the rate and are not measured separately. The contractor’s bar schedule shows 38,450 kg net, and the contractor claims an additional 5% for laps. The contractor also claims that the 2.40 m × 1.20 m opening should not be deducted because it is needed for a lift shaft.
1. Calculate the slab soffit, beam soffit and beam side formwork areas, and the total formwork area.
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Work from the stated rules in a fixed order. Gross slab area is 24.0 × 18.0 = 432.00 m². Beam soffit strips are six beams × 0.60 m × 18.0 m = 64.80 m²; these are deducted from the slab and then measured as beam soffit. Openings are 1.20 × 1.20 = 1.44 m² and 2.40 × 1.20 = 2.88 m²; both exceed 0.50 m², so total deduction is 4.32 m².
Slab soffit = 432.00 − 64.80 − 4.32 = 362.88 m².
Beam sides: the depth below the slab is 700 − 250 = 450 mm = 0.45 m. Six beams × two sides × 18.0 m × 0.45 m = 97.20 m².
Beam soffits = 64.80 m². Total formwork = 362.88 + 97.20 + 64.80 = 524.88 m². Columns are not deducted because the rules say so, even though the slab physically passes around them.
2. Assess the contractor’s 5% lap claim and explain how the QS should value reinforcement.
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The 5% claim is a measurement claim, not a calculation dispute. The contract rule says laps, anchorage, spacers and tying wire are included in the reinforcement rate and are not measured separately. The bar schedule weight of 38,450 kg is therefore the measured net weight. The claimed addition is 38,450 × 0.05 = 1,922.5 kg, which should be rejected as a separate item.
This does not mean laps are free in a commercial sense: the contractor is expected to build their cost into the rate per kilogram. The QS should check that the bar schedule is complete and that the laps shown on the detailing are consistent with the specified lap lengths, because an unusually high lap content might justify examining the rate, but it does not create a new measured item.
If the contract had said laps are measured, the measured weight would be 38,450 + 1,922.5 = 40,372.5 kg. The QS must apply the contract rule, not a default assumption.
3. Should the 2.40 m × 1.20 m lift-shaft opening be deducted from slab soffit formwork? Explain the reasoning and identify any related item the QS should check.
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Yes. The deduction rule is based on the size of the opening, not its purpose. The opening area is 2.40 × 1.20 = 2.88 m², which exceeds the 0.50 m² threshold, so it is deducted from slab soffit formwork. The fact that it is a lift shaft does not create an exception unless the contract expressly provides one.
The QS should check whether the opening has a upstand, curb or edge form. The horizontal void is not slab soffit formwork, but the vertical faces around the opening may be measurable as beam sides, upstands or edge formwork under a separate item. The QS should also check that the 1.20 × 1.20 opening is deducted and that neither opening is deducted twice through the beam strips.