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CASE STUDY

TIMBER

CLT TOPS OUT AT HIGHPOINT account in the holistic design and analysis of the frame. Details where wall panel meets slab were dealt with by an innovative ‘saw tooth’ detail which mitigates the risk of cross-grain compression without the need for wet trade methods such as grouting.

2 1 Highpoint is currently one of the UK’s tallest private rented sector (PRS) developments and a flagship aspect of the wider Elephant and Castle regeneration project. The 45-storey building comprises both structural precast concrete and cross laminated timber (CLT). The scheme provides 470 new mixed-tenure apartments from PRS and shared ownership in the tower, to intermediate rent and affordable in an eight storey podium block. A number of innovative construction techniques were considered for the development. The tower employs a bespoke structural framing system of precast concrete wall panels which all work holistically with all elements contributing to the lateral stability system as well as providing vertical support. Conversely the eight storey adjacent podium is constructed from CLT providing the project with the quality control and programme advantages of timber. The ‘Newington Butts’ project began as a research-based proposition back in 2003 between client Firstbase, Architects RSH+P and AKT II as structural engineers. The brief was simple – to create a new residential typology which was lean, flexible and driven by a standardised and prefabricated approach.

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Testament to the flexibility achieved through this early design logic was then when the client and architect changed in 2011 along with the market and tenure, the structural logic remained robust, and the inherent flexibility of the system meant that the floor plates could accommodate a change of mix and tenure. The eight storey podium block was a key part of the original scheme from 2003, providing affordable accommodation at different densities. The internal logic was defined by a linear system driven by the form of the block and the mix, which lends itself to loadbearing structural walls and thus prefabrication opportunities. The question was always whether this system would be the more traditional concrete solution or structural timber or something else? The timber option was preferred given its inherent sustainability benefits, lower weight, reduced transport movements, speed and quality control. However pre-recession in 2006 CLT construction was not so established in the UK, and a traditional panelised system was concluded to not be viable. However, the post-recession supply chain and confidence meant that a conversion to CLT was achievable and the benefits described above were thus realised. CLT construction was used for the majority of structural elements with spans being pushed beyond traditional limits necessitating detailed vibration analysis. As there are no reinforced concrete (RC) elements, the timber frame performs all structural functions including strength, stiffness, fire and robustness, all of which re taken into

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All members of the design and construction team have breadth of experience (and thus data) from across the housing sector, so along the way the project has been continually tested against the more traditional product. The cycle times for the structural frame exceeded expectations and was measured as being 25% faster than an insitu RC equivalent. There were also significant advantages in terms of logistics and site management, given the accuracy of manufactured panels and that fact that the majority of walls within the development were CLT, thus reducing packages and package interface. Similarly overall costs resulted in a 12% saving, primarily due to the effect of the economy of the CLT package providing more than one function, i.e. structure, party walls, cores and facade. Further benefits which exceeded client’s expectations include simplification of thermal break issues and an improvement in both thermal performance and robust detailing for airtightness. Now complete, data collected and evaluated during the build has concluded the environmental credentials as half the embodied carbon as opposed to RC equivalent (not including sequestered carbon, which would produce a carbon deficit) with a 25% saving in terms of programme. A fifth of the transport movements as compared to an equivalent precast concrete frame. The lighter structure delivered a 30% saving on foundations and excavation plus a significant reduction of site waste, traffic, temporary works and personnel in comparison to RC frame. For more information visit: www.akt-uk.com

Images: 01-02. The CLT elements of Hightower created a highly strong and sustainable build. Courtesy AKT II

Offsite Magazine - Issue 10 (March/April)  

Offsite Magazine contains the latest news, exemplar case studies, comment, interviews and feature articles from leading lights in the indust...

Offsite Magazine - Issue 10 (March/April)  

Offsite Magazine contains the latest news, exemplar case studies, comment, interviews and feature articles from leading lights in the indust...