SteelConstruction.info

Methodology and assumptions

Auto generate mode

The material quantities derived in the ‘Auto generate’ mode are based on a series of algorithms developed by SCI that encompass the steel frame including bracing and fittings, fire protection, cores, upper floors, roof structure and void walls.

The algorithms cover a range of common structural steel floor systems. For each system, designs for a range of typical floor grids in a rectangular building were produced, and structural steel weights estimated per m² floor area. A factor is included to increase the steel weight (beams and columns) for buildings with more complex forms.

Column sizes were calculated as a function of the floor grid and the number of storeys. In the designs, the imposed load reduction factor with number of storeys was used and the storey height was taken as 4m for buckling checks. Column sizes were reduced at higher floors, every 3 storeys, and the average size used.

Key assumptions made in the designs were:

• Imposed loading of 4 kN/m² (+1 kN/m² for partitions)
• Cladding load of 8 kN/m (used to design the edge beams)
• Services and other loads of 0.6 kN/m²
• Fire resistance of 90 minutes
• Bracing and connection weights are assumed to be 10% of the weight of steel in the columns
• Design limit of span/250 or 60 mm maximum on total deflection

Full details of the derivation of the algorithms used to estimate the structural steel quantities used in this web tool are given in SCI report RT1585.

Where concrete cores are employed for lateral stability, these are assumed to comprise 200 mm thick reinforced concrete walls. If default cores are selected, one core (7 x 6.5 m in plan) is assumed for every 800m² of floor area. Alternatively, users may input the dimensions of up to three concrete cores.

The quantities of all materials used to construct the upper floors are based on the net upper floor area, i.e. the gross upper floor area less the area of any cores and other voids in the upper floors, e.g. staircases, lifts and service cores. The area of void walls is estimated based on the perimeter length of the voids and the fixed 4 m high storey height. Void walls are either 140mm thick blockwork or proprietary dry-lining comprising plasterboard on light steel framing.

For the fire protection of beams, the quantity of intumescent paint is estimated based on the total weight of beams as follows:

• for downstand UKBs an average surface area of 22 m² per tonne
• for cellular beams an average surface area of 15 m² per tonne
• for slim floor beams an average surface area of 2.5 m² per tonne.

For the fire protection of columns, the intumescent paint quantity is based on an average surface area of 13 m² per tonne. Alternatively, for boards, 305 x 305 UKCs are assumed to be fully boxed-in.

Options for the roof level structure include the same structure as for other upper floors, for example if plant were to be placed on the roof, or alternatively, a lightweight steel structure comprising hot-rolled steel beams supporting  light gauge steel purlins. The weight of steel for the lightweight option is calculated using an algorithm based on the span and spacing of the steel beams assuming an imposed load of 1.5 kN/m² (for maintenance access), and a self-weight of 0.8 kN/m², including the weight of the steel beams. A total deflection limit of span/250 is used for roofs. Note that only the structure supporting the roof is included; the roof envelope, membranes, ballast, etc. are not included.

Embodied carbon calculations

For all materials included in the scope of the web tool (either auto-generated or manually input), site wastage rates are applied to derive gross weights of materials. Embodied carbon impacts associated with the processing, transport and disposal of construction site waste are not included within the scope.

To account for the embodied carbon impacts, the modular approach prescribed in relevant CEN standards (BS EN 15804 and BS EN 15978) is adopted in the web tool. Where relevant, the following impacts are included:

• Modules A1 to A3 – the production stage including raw material supply, transportation and manufacturing. For steel products, additional impacts are included to take account of secondary processing such as fabrication, in the case of hot-rolled structural steel, and roll-forming, in the case of decking and light gauge steel. In addition, transport impacts from the steel mill to the secondary processing facility are included.
• Module A4 – transport to site – based on average UK road trip distance and average UK HGV emissions.
• Module A5 – on-site impacts. On-site impacts are based on an SCI study which quantified the construction impacts associated with the erection of the above ground structure (frame and upper floors).
• Module C1 – deconstruction/demolition impacts.
• Module C2 – transport of materials from the deconstruction/demolition site.
• Module C3 – waste processing.
• Module C4 – disposal.
• Module D – benefits and loads beyond the system boundary including reuse and recycling potential.

Modules B1 to B7 relate to the use stage of the building. These impacts are, in general, not relevant to the structural elements of the building and therefore are not included within the scope of the tool.

The embodied carbon emissions factors for the materials and products included within the web tool are available here.