Timber Solution for a Carbon Hotspot

WSP’s environmental team had been involved in life cycle analysis (LCA) of projects for ASI for some years when they were approached for advice on how to reduce the carbon footprint of a warehouse due for construction at Blackthorne Point, a logistics hub near Heathrow Airport. They undertook a whole life carbon assessment focusing on three of the building’s main structural elements as targets for alternative sustainable construction methods: the concrete ground floor slab and foundations; the warehouse steel frame spanning c.38m; and the adjoining two-storey office structure.

The building’s steel frame, typical of warehouse construction, was found to be one of the main hotspots for embedded carbon. This provided an opportunity to demonstrate how the lifecycle carbon footprint of the building could be reduced through the use of different materials.

WSP’s structural experts conducted a feasibility review of the existing structure, then proposed a complete change to the construction material whilst adhering to the geometric parameters of the building. Technical Director, Dan Hagan, explains: “Since ASI intended to maintain the 38m clear span of the warehouse, we proposed a mass timber bowstring truss over the warehouse supported by glulam columns. In addition, we proposed replacing the secondary elements such as roof purlins and cladding rails with mass timber. For the concrete elements we suggested using a modified concrete mix to reduce the cement content.”

Several design options were created, all adhering to the above principle elements, which were evaluated from the perspective of both cost and carbon reduction. “The client opted for a bowstring truss solution using glulam beams supported by the external columns, a timber-framed office and for all the secondary elements to be in timber,” continues Hagan. “We developed the concept design for this to Stage 3, passing it on to a Specialist Subcontractor (B & K Structures) for the detail design and construction stages. During the design process we worked closely with B & K Structures to inform ASI on likely cost and programme implications of our design proposals”

The environmental team modelled all the options, including the original steel design (the baseline model), and the net effect of adopting the chosen solution was a 30% cut of life carbon. From a baseline of c.1700 tonnes of carbon, the design modifications resulted in a saving of 500 tonnes.

“We could have saved even more carbon if we had been able to replace the steel cladding with a more sustainable option, but this was not possible since planning consent had already been granted and construction was imminent,” Hagan concludes. “Although our solution was more expensive than the baseline model, the client considered the carbon savings outweighed the cost. In future, with involvement from the start, we will be able to design the most efficient timber frame building; and if the warehouse were to be let before construction, we could assist the client further by offering a range of designs for tenants to select according to their different requirements.”