The original Pont-ar-Ddyfi (Dyfi Bridge) in Machynlleth, North Wales, forms part of the A487, which is the principal strategic south to north route along the west coast of Wales. The old masonry arch structure was not designed to carry the volume of traffic currently experienced on the trunk road and frequently had to be closed during large flood events due to safety concerns, so improvements had to be made to increase the resilience of the A487.
The A487 New Dyfi Bridge scheme includes a new 725m long steel-concrete composite viaduct crossing the Afon Dyfi (Dyfi River) and adjacent flood plain. Arup engineers used Oasys GSA alongside Grasshopper, Speckle and GeometryGym for the analysis of the structure.
The new design aims to make it easier to cross the river Dyfi, allowing for easier access to employment opportunities, healthcare, and education (during closures of the existing bridge there is a 30-mile diversion for some residents to access local provisions). It also aims to make the A487 safer by helping to prevent flooding and thus, preserve historic landmarks and buildings.
How Oasys proved invaluable
The proposed structure is made up of three different structural forms: a curved ladder deck with an insitu concrete slab, a straight push launched ladder deck with precast concrete panels, and braced pairs over the river, with omnia panels and an insitu topping. Due to the complexity, Arup engineers wanted to ensure that they avoided duplication of effort and use each tool for what it was best at. To do this a parametric workflow was adopted.
The basic geometry was first defined as a stick model in Grasshopper and passed, via Speckle, to create both the BIM model and the GSA global analysis model. The links between the software were seamless and made it easier for the engineers in the analysis stage of the project to react to downstream design changes, such as updated highway alignments.
Stick Model exchanged via Speckle
Due to the structural form, hundreds of different section properties had to be defined and explicitly calculated for the structure. This process was automated using spreadsheets, which were easily loaded into the global analysis model Grasshopper script, combined with loading information and the Speckle geometry data to create a GSA model (using GeometryGym plugin). This process allowed quick updates of the GSA model, resulting in an efficient design process.
With the API and Grasshopper plugin, there is less of a need to use third party plugins, making it quicker and more seamless for engineers to extract results.
The GSA model contained meshes for abutments, 6×6 springs for foundations, joints for the different bearing articulations and elements for the different members – all created parametrically within Grasshopper.
Final GSA Model
The engineers involved in this project believe that this is an improved process that they are now adopting, and further refining, across other projects.
“For this project we stuck with GeometryGym to assemble the GSA model in Grasshopper, as we knew it well. But we’re excited that Oasys have now released the new API and their own Grasshopper Plugin as it will allow us to further refine the workflow and open new doors with quicker and easier results extraction.” – Matt Smith, Bridge Engineer at Arup.
Watch this webinar recording to discover more about the GSA Grasshopper Plugin and AdSecGH.