Carbon Footprints - Structures

The carbon footprint of a steel frame can be conveniently split in to two parts:

• Steel manufacture and,
• Steel fabrication

The environmental impacts of these two activities are briefly explained below.

Steel manufacture

To calculate the environmental impacts of steel manufacturing, the World Steel Association adopts the ‘system expansion’ method of lifecycle assessment, which is the preferred approach of the ISO 14040 series of environmental standards. This approach considers all new steel to be part of a single global system of supply and demand. Based on this approach the carbon and energy impacts of steel products are given in the following table.

 

 

	Plate	Sections	Tubes	Hot-Dip Galvanized	Purlins & rails
CO2 (Tonnes per tonne of steel)	0.919	0.762	0.857	1.35	1.10
Energy (GJ per tonne of steel)	17.37	13.12	15.42	21.63	19.38

 

It is standard practice to express the carbon dioxide emissions associated with material production on a per tonne basis (as shown in the table). This may give the impression that steel has higher impacts than other construction products. However, steel has a higher strength-to-weight ratio than most other structural materials meaning that one tonne of steel goes much further. As a result, the CO₂ emissions associated with any steel building will be lower.

Fabrication
On the 25^th February BCSA launched its Carbon Foot-Printing Tool which allows members to calculate their own Carbon Footprint. The tool calculates the carbon footprint of a company’s manufacturing operations (e.g. the greenhouse gas emissions resulting from the factory, transport and site operations) and the carbon footprint of the company’s business activities (e.g. the greenhouse gas emissions resulting from non-manufacturing operations such as company administration, design and drawing operations, marketing and business development, etc). These two can then be summed together to give the total carbon footprint for the company. The tool can also be used to determine the carbon footprint for the finished primary and secondary structural steelwork erected on site.

The tool is a simple spreadsheet and the methodology is based on ‘The Greenhouse Gas Protocol (published by the World Business Council for Sustainable Development and the World Resource Institute) and PAS 2050 Specification for the measurement of the embodied greenhouse gas emissions in products and services (published by BSI).

 

Early indications suggest that the environmental impact of steel fabrication is about 0.3 Tonnes CO₂ per Tonne of steel. BCSA will periodically collect carbon footprints from the members of the BCSA Sustainability Charter and use this information to update the emission factor for steel fabrication and to set steel construction industry Key Performance Indicators against which members can assess the performance of their company.

The Tool will be updated as better information becomes available and BCSA members gain experience in using it.

Example

In 2004 structural and civil engineers, PBA were commissioned to design a new departmental building in the science area of Oxford. The university has a rigorous and knowledgeable approach to sustainability and encouraged PBA to investigate three structural options in terms of CO₂ emissions associated with embodied energy.

 

Structural Option One was a hybrid of the parallel beam system, utilising two levels of orthogonal steel beams, supporting a composite slab. Option Two was a 350 mm thick concrete flat slab structure. The third option was a conventional steel composite structure. However the results for this option were very similar to Option One and the results are therefore not reported here.

 

The material qualities for Options One and Two are given in the table below. This data together with the emission factors for both steel manufacture and steel fabrication have been used to re-calculate the overall CO₂ emission and embodied energy for the two structural options.

 

 

	Option 1 Steel		Option 2 Concrete
Item	Wgt (T)	CO2 (T)	Wgt (T)	CO2 (T)
Steel	740	564	50	38
Fabrication	740	222	50	15
Concrete	2,030	325	6.800	1,088
Rebar	50	45	350	315
Haulage	221 veh	88	665	168
Total		1244 T		1634 T

 

In this example the CO₂ emissions for the steel-framed option are 24% lower than the equivalent concrete option resulting in a saving of 390 T of CO₂. It should be noted that the figures given in the table do not include the environmental impacts associated with on-site concrete activities which would increase the CO₂ emission for the concrete solution.

 

In this case the steel option was chosen on the grounds that the overall structural costs was comparable, the programme faster, construction traffic reduced and the superior sustainability credentials.