Multi-Purpose Centre – Mt Barker, S.A.
The ‘Big-Space’ roof structure
This Multi-Purpose School Building designed by Troppo Architects involved a 3-dimensional arrangement of rigidly connected rafters and column frames radiating from 2 hub joints. Although symmetrical about a spine, the curved shape of the roof called for rafters with varying radii of curvature along their length and some with reverse curvature.
Timberbuilt conceived of providing for the complex form of the roof and the structural requirements using a series of ply web box rafters, columns and interconnecting beams. Cadwork 3d modelling software was used to generate a virtual model of the structure and to rationalise the roof shape in a way that was visually imperceptible in comparison with the Architects vision but nevertheless assisted to reduce the complexity for the installation of the roof cladding and ceiling linings.
As of result of the 3 dimensional arrangement of the primary support members there was minimal tolerance in the assembly and consequently, a nervous pause in proceeding when there appeared to be a significant misalignment as the last member was being placed. Then it was realised that one column base was located 20 mm lower than intended; on inserting a 20 mm packer, the alignment was resolved and the last close fitting steel dowels driven home. Perfect, and with all 29 prefabricated beams and columns installed in 1 day a testament to the accuracy of the Timberbuilt planning and workmanship and the efficiency of the builders.
Curved walls and a spiral roof
It wasn’t that the remainder of the structure was easy either. Curved walls with a raking top plate and a lower roof spiralling downwards around the side of the curved wall presented considerable geometric challenges and carpentry contractors were unwilling to quote a fixed sum to build this part of the structure using a carpentry approach . Instead Timberbuilt were engaged to design and prefabricate the system.
Cadwork 3d modelling software was used to create data files for prefabrication manufacture on their Hundegger K2 joinery machine. The curved walls were divided into panels and packed complete with the wall plates, studs, noggings, all required fasteners and workshop detail drawing ready for assembly. Site assembly and installation of all 84 wall panels took just 2 days on site.
In order to define the correct shape of the spiral roof that was required to be attached to the outside of the curved walls, Timberbuilt elected to use a series of wale plates with mortices to receive matching tenons machined to the end of each rafter. The technique involved machining the mortices and the overall shape of the wale plates such that when these were bent around the outside face of the wall studs they would accurately define the required roof plane. This technique utilised some advanced modelling techniques but ultimately, and despite some understandable scepticism for the contractors undertaking the assembly, the technique proved itself and the roof was easily constructed to conform with the design expectations.
This project clearly demonstrated the many benefits of comprehensive construction pre-planning using CAD/CAM technologies.