This bespoke 7m hanging timber bench is thought to be the longest of its kind. Summum Engineering engineered the bench, which was installed in the Soldatenbos National Forest, Belgium. It hangs from two silver maple trees with a view over an adjacent field. RO&AD Architects provided the architectural design and intent, working for the municipality of Nijlen.
Swinging benches are commonly associated with North America. The longest multi-span swinging bench, located in Louisiana, USA, is 18.25m, with individual spans of around 3.6m. At 7.1m, it is thought to be the longest single-span swinging bench of its kind, installed in August 2021.
The bench was part of a larger scheme, where LAMA landscape architects, Wemake and RO&AD Architects proposed several designs specifically for the municipality of Nijlen, Belgium, ranging from websites and magazines to street furniture, such as rubbish bins, benches and signposts. The Soldatenbos is the first location where some of these objects have been placed, including the bench. The visitor numbers to Soldatenbos National Forest increased sixfold thanks to the larger project.
Structural system and materials
The bench is made up of orthogonal layers of timber beams with a cross-section of 43 mm by 65 mm, interconnected with screws, with gaps left in between. In total, the bench weighs 340 kg. Four 16 mm stainless steel threaded rods span longitudinally between the ends of the bench and are concealed within it, introduced to control deflections.
The bench is suspended with Hercules rope, which integrates a steel cable encased by synthetic fibres, giving the appearance of natural fibre rope. In general, steel cables under tension used for structures such as swings should not be openly accessible to the public.
The four suspension points are at the top and bottom of the bench for stability, with supports at the bottom allowing the bench to rest on – rather than hang from – the steel rods. The cables are in turn attached to an 8 tonne capacity Drayer TreeSave sling wrapped around the tree trunk, which was the responsibility of a specialist tree surgeon.
Thermally modified timber was sourced by the client from two different timber merchants for the project, on the assumption it would be well suited to outdoor applications. Unfortunately, thermally modified timber is ungraded for structural application, due to the relatively unknown effect the heat treatment has on its stiffness and ultimate strength. A large proportion of the engineering task was therefore determining suitable material properties from available information to enable a safe design.
Third-party test results on timber from the first manufacturer indicated it had the strength and density of a C20 section, while other sources indicated that its strength could be reduced somewhat: one cites sources suggesting that the average strength decreases by 10–50%5 and another indicates that the bending strength decreases on average by 3% and the tensile strength by 39%6.
Test data was publicly available for the timber from the second manufacturer. Eurocode 0 provides a framework to derive design values from test data. Using this method, a design bending strength of 9.1 MPa was calculated for the timber from the second manufacture, i.e. half the bending strength of a C18 section.
The governing loads were determined to be design forces for swings, taken from EN 1176-1. Based on these loads, the following procedure was used for the design: for all the timber in the bench, C18 section properties were assumed. Where the utilisation of any element was above 50%, this element was replaced with a stronger C24 section. The bench was then reanalysed. All C24 timber had a utilisation of less than 100%. Some of the thermally modified timber had a utilisation between 50% and 65%, assuming C18 properties. This higher utilisation was localised to the longitudinal members in the seating. Timber from the first manufacturer was recommended to be used here. Other transversal pieces and elements in the backrest were recommended to be made from timber by the second manufacturer. Based on this results, stock elements already procured could successfully be integrated into the project, reducing waste and expense.
More details have been published in the October 2022 issue of The Structural Engineer.