What a millimetre costs.

1. Flat slab, typical floor

   S1

   Drawn

   250 mm

   Suggested

   225 mm

   Assumed building

   8 floors of 1,000 m²

   Concrete saved

   200 m³

   Steel saved

   held equal

   London, supply

   ≈ £35,000

   New York, installed

   ≈ $210,000

   Minimum required

   EN 1992-1-2 Table 5.9: a flat slab needs 180 mm for REI 60 and 200 mm for REI 90. Nothing in EN 1992-1-1 sets a thicker minimum.

   What sets the size

   Deflection, EN 1992-1-1 7.4.2 and Table 7.4N: basic span to effective depth 17 to 24 for a flat slab, adjusted by Expressions 7.16 and 7.17 for steel stress and span.

   Why the change is possible

   At 225 mm on a 7.5 m grid the ratio is about 37 against a corrected limit that a lightly stressed slab reaches; the check is the engineer of record's, the headroom is the flag.

   A thinner slab does not cut flexural steel one for one, so reinforcement is held equal.

2. Basement retaining wall

   W3

   Drawn

   400 mm, ρ 0.6%

   Suggested

   300 mm

   Assumed building

   110 m perimeter, 3.5 m high

   Concrete saved

   38 m³

   Steel saved

   1.8 t

   London, supply

   ≈ £10,000

   New York, installed

   ≈ $54,000

   Minimum required

   ACI 318-19 Table 11.3.1.1: exterior basement and foundation walls at least 190 mm (7.5 in). EN 1992-1-1 9.6.2: vertical reinforcement at least 0.002 Ac.

   What sets the size

   Bending and shear from the retained soil, EN 1992-1-1 6.1 and 6.2 (or ACI 318-19 Chapter 22), with the earth pressure from EN 1997-1 9.5.

   Why the change is possible

   A 300 mm wall sits well above both minima; at 400 mm with ρ 0.6% the section carries three times the minimum steel and a third more concrete than the retained height asks for.

3. Columns, ground to L6

   C7

   Drawn

   500 × 500, ρ 2%

   Suggested

   450 × 450

   Assumed building

   40 columns, 6 storeys of 3.3 m

   Concrete saved

   38 m³

   Steel saved

   5.9 t

   London, supply

   ≈ £17,000

   New York, installed

   ≈ $84,000

   Minimum required

   EN 1992-1-1 9.5.2: column steel at least 0.10 NEd/fyd and not less than 0.002 Ac; ACI 318-19 10.6.1.1: 1% of Ag. Fire, EN 1992-1-2 Table 5.2a: R60 from 200 mm at low load level.

   What sets the size

   Axial load with bending, EN 1992-1-1 6.1, and slenderness, 5.8.3 (or ACI 318-19 Chapter 22 and 6.2.5).

   Why the change is possible

   The same 2% steel in a 450 mm section still meets the minima; whether the axial check passes at C35/45 is the engineer's call, and it is where the flag points.

   Dropping the class from C40/50 to C35/45 would add to this; it is not priced here.

4. Floor beams

   B12

   Drawn

   300 × 600

   Suggested

   300 × 500

   Assumed building

   60 beams of 7.5 m, 300 kg/m³

   Concrete saved

   14 m³

   Steel saved

   4.1 t

   London, supply

   ≈ £9,700

   New York, installed

   ≈ $45,000

   Minimum required

   EN 1992-1-1 9.2.1.1: tension steel at least 0.26 fctm/fyk bt d and not less than 0.0013 bt d. ACI 318-19 Table 9.3.1.1: depth at least span/18.5 for a beam continuous at one end, about 405 mm here.

   What sets the size

   Deflection, EN 1992-1-1 7.4.2 and Table 7.4N: basic span to depth 18 to 26 for an end span (K = 1.3), then bending 6.1 and shear 6.2.

   Why the change is possible

   A 500 mm deep beam over 7.5 m gives a span to effective depth ratio of about 17, inside the basic table before any correction; the 600 mm section was rationalised, not calculated.

5. Pad footings

   F2

   Drawn

   2.4 × 2.4 × 0.7 m

   Suggested

   2.2 × 2.2 × 0.7 m

   Assumed building

   30 pads, 80 kg/m³

   Concrete saved

   19 m³

   Steel saved

   1.5 t

   London, supply

   ≈ £6,000

   New York, installed

   ≈ $31,000

   Minimum required

   ACI 318-19 13.3.1.2: at least 150 mm (6 in) of depth above the bottom reinforcement for a footing on soil. EN 1992-1-1 9.8.2 and 9.3.1.1: minimum reinforcement as for slabs.

   What sets the size

   Bearing resistance from unfactored loads, EN 1997-1 6.5.2 (or ACI 318-19 13.3.1.1), then punching and bending in the pad.

   Why the change is possible

   The pad is sized by allowable bearing pressure; where the geotechnical report carries margin, the 2.2 m pad meets the same bearing check with the same 0.7 m thickness.

6. Steel floor beams, lightest code-compliant section

   SB

   Drawn

   as scheduled

   Suggested

   lightest compliant section

   Assumed building

   5,000 m² office at 90 kg/m², beams 60% of the frame

   Concrete saved

   —

   Steel saved

   72 t

   London, supply

   ≈ £250,000 of steel; ≈ £70,000 to £85,000 of frame cost

   New York, installed

   ≈ $760,000

   Minimum required

   EN 1993-1-1 6.2.5: bending resistance of the cross-section; 6.3.2: lateral-torsional buckling where the compression flange is unrestrained.

   What sets the size

   Deflection and vibration under EN 1990 A1.4 with the National Annex limits (span/360 for beams carrying brittle finishes in the UK NA to EN 1993-1-1, 7.2).

   Why the change is possible

   The lightest section that passes both checks is the code-compliant one. Across 3,500 floor beams in 30 UK buildings that section was 26.5% lighter than the one scheduled.

   Material is 30 to 40% of a steel frame cost; fabrication and erection do not fall with tonnage, so the frame figure is the honest one. The 26.5% comes from 3,500 floor beams in 30 UK buildings (Drewniok, Campbell and Orr, 2020).

Illustrative. London at Q1 2025 material rates: concrete 30 MPa £175/m³, 16 mm rebar £1,800/t, universal beams £3,500/t. New York at Q1 2025 installed rates: concrete in slab $1,046/m³, reinforcement in beams $7,500/t, steel beams $10,500/t (Turner & Townsend, Global Construction Market Intelligence 2025). Formwork, cranage, programme and the knock-on to columns and foundations are not counted. Steel at 7.85 t/m³. Do not add the six together; they are separate buildings.