Analysing the Tree Fork Truss
As part of the elective course, ‘Rethinking Wood’ taught by Visiting Professor, Katharina Kral, my collaborator Olivier Ducharme and I did a structural analysis of The Tree Fork Truss/Wood Chip Barn in November 2017. I was going through my hard drive and wanted to resurface this project, given that I am now based in the UK with a keen interest in fabrication.
The Wood Chip Barn is a digitally fabricated barn created as part of the Architectural Association’s Design and Make Summer Course in 2016. The barn provides storage for the wood chip to fuel the Biomass Boiler House in the Hooke Park estate. The primary structure of the barn consists of 20 discrete beech forks that were 3D scanned and optimally configured for structural performance. The design consists of two vierendeel-style arching trusses that received robotically machined connection geometries that enabled the structure to sustain vertical and lateral loads. The forks followed the directional axis to give the structure dimensional stability. This project scrutinizes the inherent form and structural capacity of natural tree branches as a potential building material in architectural construction.
The students surveyed the surrounding park to create a database of optimal branches. A scaled polyline outline was generated by tracing a photograph of each fork. A more detailed 3D scan of each fork was carried out using an XBOX Kinect which features an RGB camera and depth sensor which creates a mesh scan of each tree fork. As a result of scanning and optimization through the plug-in Galapagos for Rhino-Grasshopper, the students were able to determine the connection surface geometries of either planar face-to-face surfaces between elements along the chords or mortice and tenon joints in which a distorted elliptical cone geometry was found to best satisfy the structural and assembly constraints. Such bracing methods from the forks themselves and additional straight wooden members prevent the structure from warping and twisting.
Structural Efficiency
Although the intent of the project was to showcase the potential and efficiency of found wooden forks as construction elements, the tree fork truss has a structure that is highly inefficient. The design consists of arching trusses that received robotically machined connection geometries that enable the structure to withstand lateral loads. The arching trusses are in compression and collapse under their own weight which support an additional wooden bracing to hold the structure in tension.
The first interaction of the woodchip ban optimizes the cross sections of the structura members without changing the span width, supports and materials. The cross section optimization tool in Karamba informed the cross section values of the wooden tree trunk members. Despite improvements in displacement, utilisation and axial stress values, the fundamental inefficiencies with the original design still exist. The arching trusses are still in compression and collapse under their own weight. This new, improved design will still require concrete supports and additional wooden bracing to hold the structure in tension.
The second and third iteration of the woodchip barn utilises variations of segmental dome to achieve greater structural efficiency. The vierendeel trusses of the original design had compressive forces that prodices thrusts downwards and outward. We decided to capitalise on this structural characteristic to make a circular array of the cross section to generate a dome. There are significant improvements in displacement, utilisation, and axial stress values. The new design will still require concrete supports and additional wooden bracing to hold the structure in tension. There are also additional benefits of significant spatial gain.
