Timber Split Beam Girder Research – University of Kent

Engineer
HA EY Ltd

Client/Sponsors
Ardtornish Estate; Bennerley Viaduct; Derwent Valley Cycleway;

We are currently undertaking research on the development of timber split beam girders1, a structural system championed in 1830s by the Hanoverian Court Architect/Engineer – Georg Ludwig Friedrich Laves (17 December 1788 – 30 April 1864).

In 1840 Laves’ paper “On Trussed Beams” was read before the Royal Institute of British Architects (RIBA)2, which documented his empirical experiments using 40ft long fir beams that were horizontally slit along their length but bound by iron straps at the ends to “prevent the slit from extending”. By wedging apart the upper and lower parts of the beams by various dimensions, Laves load tested the various arrangement for their increased resistance to deflection.

The structural basis of Laves’ system is the combination of the upward arc of the compression arch and the downward curve of the tension catenary. Although the lens shaped truss form had been used by George Stephenson for the 1823 Gaunless Bridge on the Stockton to Darlington Railway3, its origins date back much further (illustrations of a lens shaped truss was published in Machinae Novae by Faustus Verantius 1595). However our research reveals that Laves was most probably influenced by an illustrated article by the French engineering writer Prosper Debia published in 1829.4

Laves developed and built a number of timber and iron bridge designs using his principle between 1834 and 1840. Due to the close links between the English and Hanoverian states, Laves filed for an English patent around the same time as the RIBA paper was read (some 10 years before I K Brunel had finalised his design for the Saltash Bridge!)

In collaboration with structural engineer Joseph Eyles of HA EY ltd., we have been developing a “self-build” timber bridging system based on an updated development of Laves’ lenticular girder principle. After experimenting on numerous study models5, our first prototype was constructed during spring 2024 at the Ardtornish Estate in the Western Highlands. This 14 metre structure replaced a previous footbridge that had deteriorated beyond repair. The Achranich Bridge6 was constructed in square section larch harvested from the estate and was built by a team of architectural students from the University of Kent7 and was launched into position by the estates own workforce.8

In autumn 2024 we developed a second prototype bridge9, also located on the Ardtornish Estate, however this time it replaced a vehicle access bridge and had a shorter span of 9 metres. The structure of the Black Glen Bridge10 comprises of six lenticular girders arranged in two groups of three positioned under the wheel line. Instead of being a through structure, the decking at Black Glen sits on the top chord of the lenticular girder.

Our next project is to build prototype 3, the Erewash Footbridge at Bennerley Viaduct in Ilkeston11. We are also developing some longer cable supported prototypes for installation over the River Derwent for Derwent Valley Cycle Path in Derbyshire12.

1.    Split Beam Girder Development.

2.    Laves Paper – Civil Engineer and Architects Journal vol. 03, 1840.

3.    George Stephenson’s Gaunless Bridge drawn by Ron Yee.

4.    Prosper Débia wooden trusses – Dinglers Polytechnisches Journal.

5.    Model of Prototype 1.

6.    Achranich Bridge on the Ardtornish – Western Highlands.

7.    Construction of Prototype 1.

8.    Launching of Prototype 1.

9.   Model of Prototype 2.

10. Black Glen Bridge on the Artornish Estate – Western Highlands.

11. Model of Prototype 3

12. Concept sketches of Prototype 4 & 5.