25 July 2018

Towards Adhesive Free Timber Buildings: an international research collaboration

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Last month TRADA’s University Engagement Manager, Tabitha Binding, had the pleasure of visiting Liverpool University’s School of Engineering where she met Dr Zhongwei Guan, Dr Dan Bradley and Dr Adeayo Sotayo, who shared their work on the international collaborative research project ‘Towards Adhesive Free Timber Buildings’ (AFTB).

 

The aim of the project is to work towards the elimination of adhesives and steel in engineered wood products by developing an alternative which incorporates both traditional techniques and advanced timber research.

 

Glued laminated (glulam) construction techniques offer significant environmental advantages compared to steel and concrete, both in terms of resource sustainability and wider environmental impacts. However, challenges remain in delivering fully sustainable engineered wood products (EWPs), given the petrochemical adhesives and steel connections inherent to glulam construction.

 

Towards Adhesive Free Timber Buildings (AFTB) is an international research collaboration funded by Interreg North West Europe, an institution of the European Commission, and led by engineers at the University of Liverpool, UK. The aim of the project is to work towards the elimination of adhesives and steel in engineered wood products by developing an alternative which incorporates both traditional techniques and advanced timber research.

 

There are two main objectives to the work. The first is to replace the adhesive in glued EWPs (e.g. glulam and cross laminated timber (CLT)) by upgrading twentieth century Brettstapel techniques with highly densified softwoods. Traditionally, Brettstapel uses fully desiccated hardwood dowelling to fix arrays of softwood laminates together to form structural elements. When the moisture content of the dowels equilibrates with the surrounding softwood, it seeks to expand creating a constant pressure at the join, maintaining a tight fit and thus a rigid EWP.

 

By heating softwood timber to c. 130°C, it can be compressed in a hydraulic press to nearly triple its original density without significant damage to its microstructure. This yields a densified wood product with greatly improved properties, for example, a 120-130% increase in strength and stiffness (based on three-point bending tests). By replacing the desiccated hardwood in Brettstapel with densified softwood dowels, the strength and stiffness of the resultant EWPs is increased. Additionally, what has long been perceived as a drawback of densified wood is an advantage here. Once in contact with moisture, an irreversible spring-back effect occurs as the densified wood seeks to relax from its compressed form. This provides a permanent tight fit in the EWP regardless of future humidity changes in the operational environment. Additional benefits of this approach are the lower cost and faster regrowth of softwood timber, compared with hardwood.

 

The second objective is to provide an alternative to steel, for semi-ductile connections, by combining the dowels with compressed wood plates that bridge connecting timber sections, for example in beam-to-column connections. Such connections exhibit ductile properties resulting from the embedment of the compressed wood dowels in the softwood sections. The research on these connections is being led by National University of Ireland Galway.

 

The three-year project will take the EWPs and construction techniques through comprehensive, multiscale structural testing, combined with a program addressing vibrational serviceability and fire performance, led by Université de Lorraine. Along with their contribution to the wider testing program, Technische Universität Dresden are seeking to deliver greater innovation by combining adhesive free techniques with advanced robotics to produce free-form space structures. The technique involves using CAD and numerical models to specify a set of cutting instructions for a robotic saw that can control precisely the angle at which timber is sawn, and can modify this angle as it moves. The resultant planks or laminates are contoured exactly, so that they may be steam formed into a ribbon of any designed 3D shape. For the AFTB project, these laminates will then be connected via densified wood before being tested or used. Assembling such EWPs allows an architect or designer huge freedom in the design of space structures and aesthetic elements, while offering a fully sustainable product.

 

The collective testing output from the project will be combined with numerical simulation methods developed by Luxembourg Institute of Science and Technology. They are also producing a user-friendly structural design tool, to allow engineers to specify and optimise adhesive-free EWP designs to fit end-use requirements.

 

In late 2019 the project will deliver three demonstrators: structures in Cheshire, UK and Epinal, France will act as test beds to prove the long term robustness of adhesive-free beams, columns, arches, connections and CLT panels, while in Dresden, Germany, a grid-space structure will be assembled. This artist-designed installation will demonstrate the feasibility of adhesive-free timber space structures while also promoting timber construction and civil engineering to students and the wider community.

 

A key requirement of the project funding is that AFTB engage with businesses, regulators and the general public in the North-West Europe area. They are very interested to hear from anyone with an interest in using the technology, or anyone with a comment or question about adhesive-free engineered wood products.

 

If you think this research could be of interest to you or your business, please follow the project on social media @AFTBbuildings or contact Tabitha Binding, University Engagement Manager, TRADA tbinding@trada.co.uk

 

Article by Dr Dan Bradley with editing and input from Caroline Chandler