The Slab Thickness Dispute on a Tender Drawing
You are the junior engineer on a two-classroom block for a government primary school. The only drawing set the contractor has is stamped 'FOR TENDER ONLY - NOT FOR CONSTRUCTION', issue A, dated eight months ago. On the 1:100 plan, each classroom measures 7.2 m × 5.4 m clear, walls are 230 mm, and door D2 is dimensioned 1000 mm wide in the plan but listed as 900 × 2100 mm in the door schedule. The section AA shows a roof slab 125 mm thick, while a structural note on the same sheet reads 'slab 150 mm thick, M25, top and bottom reinforcement'. Between column C1 and the inner face of the external wall there is no dimension at all. The plinth level is marked +0.600, but the site pegs taken by the surveyor give existing ground at +0.450. The contractor has already set the masonry out using a 7.2 m span measured off the plan with a wooden scale.
1. What checks would you carry out on this drawing set before allowing any further work, and why is scaling off a 1:100 print unreliable?
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Checks, in a fixed order:
- Status and revision. Read the title block: the set is 'FOR TENDER ONLY', issue A, eight months old. That alone means it cannot be built from; it is a quantity document. Ask for the IFC (issued-for-construction) revision and build a revision register.
- Cross-check every element against its schedule. The door is 1000 mm on plan and 900 mm in the schedule. On a 1:100 drawing 100 mm is 1 mm on paper, exactly the thickness of a pencil line, so a plan dimension can easily be the draughtsman's slip; the schedule is more likely correct, but the answer must come from the designer, not from a guess.
- Cross-check architectural against structural. Slab 125 mm on section versus 150 mm in the structural note is a real conflict; a thin slab changes the reinforcement cover and the self-weight that the beams carry.
- Look for missing dimensions. The C1-to-wall-face dimension is absent; it controls how the wall sits on the footing and where the beam lands.
- Check levels. Plinth +0.600 against ground +0.450 gives only 150 mm of plinth height, which is far too low for a school floor and for damp-proofing; the levels are probably from different references.
Why not to scale: a 1:100 print shrinks or stretches during photocopying and is affected by paper humidity, so an error of 0.5 mm on paper becomes 50 mm at full size; you cannot read to better than about ±50 mm. Dimensions written on the drawing govern. The masonry already set out with a wooden scale must be re-checked against the IFC dimensions before it goes further.
2. The contractor wants to cast the slab at 125 mm because it is cheaper, and argues that 'thinner means less load, so it is safe'. How do you resolve the 125/150 mm conflict, and what is wrong with the contractor's reasoning?
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Resolution: the drawing itself cannot settle it, so the structural designer must. Raise a formal request for information quoting: section AA (125 mm), the structural note (150 mm, M25, top and bottom steel), the drawing issue and the date. Get the reply in writing and have the drawing reissued as a controlled revision with the superseded sheet withdrawn from site. Until then, do not cast.
Why the contractor is wrong: a slab is not a lightweight lid. Reducing 150 mm to 125 mm lowers dead load by about 0.6 kN/m², but it also reduces the effective depth, which cuts the bending capacity roughly with the square of the depth and increases deflection and crack width. Cover to the bottom steel drops, so the steel sits closer to the soffit where corrosion and fire protection are worst. Top steel over supports, the very place that governs a two-way slab, loses its lever arm. And because the slab is stiffer in the designer's model than in reality, load can redistribute onto it from elsewhere. So 'thinner is lighter and therefore safe' is a partial argument: it ignores strength, stiffness, durability and fire resistance.
Practical line: a slab that is thinner than specified is not 'close enough'. If a wrong slab is cast, the remedies are demolition or a design check with load testing, and neither is cheap. Insist on the IFC drawing, and record the RFI in the site file.
3. What does the 'FOR TENDER ONLY - NOT FOR CONSTRUCTION' stamp mean for you as site staff, and what would you do about the door width and the missing dimension in the meantime?
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Meaning of the stamp. A tender drawing exists so that bidders can measure quantities and price the job; it may contain schematic, uncoordinated or provisional information, and no liability attaches to building from it. Building to it transfers design risk to the contractor and the supervisor: a mistake traced to a tender drawing is a mistake the site chose to make. Construction may only proceed on drawings marked 'Issued for Construction' (or 'Good for Construction'), properly signed and revision-controlled.
What to do now:
- Write to the client and the designer asking for the IFC set and listing every discrepancy found, including D2, the slab and the missing dimension, so that your letter exists before the work is held up.
- Mark the site copy 'HOLD - NOT FOR CONSTRUCTION' and remove it from the mason's hands; keep it in the office as a reference only.
- Door D2: do not order frames or leave an opening until the designer confirms width. If an opening must be left to keep the programme, leave it to the larger (1000 mm) so it can be packed down later rather than rebuilt, and record the decision.
- Missing dimension: never scale it off. Ask the designer; meanwhile do not set any footing or column on that grid line.
- Levels: get the datum confirmed in writing before more excavation, because the plinth height drives the whole floor and the DPC.
- Log all of this in the site diary with dates and replies. A supervisor who documents a query is protected; one who guesses is not.