Cost comparison studies

In 2016, the BCSA and Steel for Life commissioned AECOM to provide impartial cost guidance applicable to a broad range of construction professionals. Published quarterly in Building magazine, ‘Costing Steelwork’ is a series in which AECOM provides guidance on the key elements of costing structural steelwork. These articles examine the key cost drivers for different sectors and provide up-to-date cost ranges for various frame types that can be used as a comparative cost benchmark.

The series comprises studies into office, education, residential/mixed-use, retail and industrial buildings, and a key feature is a series of building type specific cost comparisons based on actual buildings. The buildings selected were originally part of the Target Zero study conducted by a consortium of organisations including Tata Steel, AECOM, SCI, Cyril Sweet and BCSA in 2010 to provide guidance on the design and construction of sustainable, low and zero carbon buildings in the UK.

The cost comparisons presented in the ‘Costing Steelwork’ series update the cost models developed for the Target Zero project, and provide up-to-date costs for the alternative framing solutions considered for each of the five building types.

[top]Office building

One Kingdom Street

The building used for the cost model is a multi-storey office structure; One Kingdom Street, London. The project is located in the Waterside regeneration area near Paddington railway station in Central London. This Grade A office building was completed in 2008.

The building’s key features are:

10 storeys, with two levels of basement
Typical clear spans of 12m x 10.5m
Three cores - one main core with open atrium, scenic atrium bridges and lifts
Plant at roof level

[top]About the building

The building accommodates 24,490 m² of open-plan office space on ten floors and, on the eastern half of the building, two basement levels providing car parking and storage. The gross internal floor area is 33,018 m². The 40m-high building is rectilinear with approximate dimensions of 81m x 45m. The front façade faces north and comprises a reverse ellipse along the length of the building plan on podium and first floor levels.

One Kingdom Street

One Kingdom Street has three cores and is designed around two central atria on its southern elevation, which house six scenic wall chamber lifts. The western half of the building is partly constructed on a podium transfer structure enclosing future works access for Crossrail.

One Kingdom Street has a steel frame, on a typical 12m x 10.5m grid, comprising fabricated cellular steel beams supporting a lightweight concrete slab on a profiled steel deck. The larger span is dictated by the location of beams within the Crossrail podium deck on which they are supported. The steel beams are designed to act compositely with the concrete floor slabs through the use of welded shear studs.

The cellular floor system enables the services to be integrated within the structural zone, i.e. within web openings in the beams. The clear floor-to-ceiling height in the office areas is 2.8m. Upper floors support a 175mm raised floor and a perforated metal tile suspended ceiling incorporating acoustic insulation.

The foundations comprise 750mm diameter bored-piled foundations with insitu concrete pile caps. Ground beams provide lateral restraint to the pile caps. The piles are the same size as those used to support the existing Crossrail podium in order to reduce potential differential settlement arising from the use of different pile diameters. The office areas are clad with an anodised aluminium curtain walling system consisting of storey height double-glazed window units on a 1.5m module. Vertical fins at 3m centres support the external aluminium louvres for solar shading on the southern elevation and part of the east and west elevations.

[top]Cost Comparison

One Kingdom Street

Two structural options for the office building were assessed; the base case as described above, and a 350mm thick post tensioned concrete flat slab with a 650 x 1050mm perimeter beam.

The full building cost plans for each structural option have been reviewed and updated to provide current costs at Q4, 2023. The costs, which include preliminaries, overheads and profit and a contingency, are summarised in the table below.

**Table of key costs (£/m² GIFA), for City of London Office Building (Q4, 2023)**
Elements	Steel cellular composite	Post-tensioned concrete flat slab
Substructure	93	98
Frame and upper floors	556	540
Total building	3,660	3,750

The analysis shows that the cost of the steel composite solution is 3% higher than the post-tensioned concrete flat slab alternative in terms of the frame and upper floors, but 2% lower on a total building basis. The lighter frame weight and the increased speed of erection results in reduced foundations and a quicker programme. The reduced timescale is the main reason for the overall building cost to be less in the steel composite option.

[top]Embodied Carbon Comparison

The original Target Zero project also included a comparison of the embodied carbon of the two framing solutions. This was on a ‘cradle-to-cradle’ basis that included the manufacture and transport of construction materials, the construction process and the demolition and disposal of the building materials at the end-of-life. The results, which are presented in the figure right, show that the embodied carbon of the steel composite solution was 11% lower than the post-tensioned concrete flat slab alternative.

Embodied carbon emissions