Breaking new ground for low-carbon construction in Hong Kong

When applying blue-sky thinking to design a structure for what could one day be Hong Kong’s most sustainable commercial office tower, there is so much scope for innovation. Just considering how to optimise a structure to reduce embodied carbon is an inspiring engineering endeavour.

For the Net Zero Building (NZB) Treehouse design for the Advanced Net Zero Ideas competition held by the Hong Kong Green Building Council in partnership with Swire Properties, we took this one step further, by looking at the interplay between all building design decisions and how each individual decision could help us lower the carbon footprint of the whole.

This kind of holistic, integrated, lifecycle thinking elevates structural engineering beyond the simple beams and members and nuts and bolts (so to speak) to take in the entire suite of functional and spatial dimensions the structure will interact with.

The design by Ronald Lu and Partners is for a 225m, 45 storey tower with extended ground plane public domain.

We were drawn to cross-laminated timber (CLT) for the floors because it is the most efficient structural material in terms of minimising embodied carbon. With a tall building, stability is a critical consideration, so we analysed different types of structural combinations with CLT, steel and low-carbon cements for the core and frame options.

This option analysis was carried out in collaboration with the architect and wider design team members, as it included looking aspects such as how the placement of the core would impact architectural design, thermal performance and the structural modelling. We have used parametric modelling and analysis tools together with our in-house developed carbon reduction script that allows us to optimise the structure both in terms of structural and carbon performance.

We examined the framing and gravity loading system and recognised it would not be practical for a building of that height to be entirely timber. In looking at the other, hybrid material approaches, we calculated embodied carbon for each iteration until we arrived at a solution that was both structurally sound, and had the lowest embodied carbon footprint, while also meeting both architectural and building performance design intents.

The wind loading in Hong Kong is very high, and there is a typhoon loading also built into the local code of practice. Getting the stability system right for a building that has an unconventional geometry given the loading constraints took ingenuity.

The final design proposes a lightweight concrete for floor support elements. We also met with concrete manufacturers in Hong Kong and Singapore to research the latest trends and options in low-carbon concrete. For the steel used in the building, we have specified steel with recycled content, which helps reduce the steel embodied carbon contribution.

The CLT floors were something the architect wanted left exposed as well as exposed CLT soffits, but to meet HK fire regulations this is problematic. CLT has not been used as a floor system yet in HK in the high-rise context and there is a general wariness around timber from the fire safety perspective. We hope that this competition can act as a pathway which showcases the advantages of the CLT timber and utilising the international experience in this field can be applied to local projects.

Another aspect of the embodied carbon modelling was benchmarking each design iteration against the embodied carbon of the existing One Tai Koo place building on the site. This is already quite a sustainable building, so it added to the intellectual challenge to ensure our Treehouse was going to have a much lower embodied carbon footprint.

I think the NZB competition is a great initiative by the HKGBC as it has resulted in an excellent marketing tool for the value and potential of CLT timber in a market where it has not been widely accepted. There are other benefits aside from the carbon aspect too, including possible reductions in overall project cost.

Engineered timber is lighter than ferro-concrete solutions which reduces the loads on beams, columns and foundations, so a project can achieve substantial cost savings, particularly on reduced foundation requirements.

I have always been intrigued by the engineered timbers, and started examining the potential for timber structures when I was with Cundall in Edinburgh and worked on some designs that used glulam and CLT.

To use engineered timber as part of a net zero approach requires a holistic approach. For example, as a structural material the CLT floor will be thicker than a ferro-concrete slab, so that affects the architecture, spatial planning and things like services design and conduits and so forth. It has a different acoustic quality, so additional treatments may be required.

The key to bringing a project like this together is having great team coordination and an excellent project lead. Someone who can bring together all the disciplines and their innovations and ensure every part of the delivery team – architects and design team, project management, contractors and developer are fully engaged in achieving a ground-breaking net zero outcome.