Levitas – The PLAN Journal, 2021

Mock-up Prototype “Levitas”

Initially, a very rough full-scale prototype in fir was realized by D3Wood in order that it could be analyzed when Ian Ritchie visited their lab during a design session at the Politecnico di Milano (Lecco Campus). This was to enable the team to begin thinking about the grid and the size of the voids in it, in order to analyze its permeability to the heavy snowfalls that normally occur at Arte Sella in winter.

Due to the complexity of the curves/surfaces and their relation to the material a more precise mock-up (scale 1:3,333) was realised for the geometry form-finding. The goal of the physical prototype was basically to understand how to achieve the proposed form. Would it be enough to change the position of the three restraints on the ground to get the desired surface shape? Would the surface obtained differ considerably from the one proposed in the concept? Would the interlaced surface of the strips be strong enough to withstand wind and snow? Red oak strips measuring 20 mm x 3 mm [0.8 x 0.1 in.] in section were freely woven to re-create the grid in scale and it was closed with M4x25 bolts at the ends; the shape obtained was very similar to that of the drawing.

From the first analysis of the prototype, a difference between its edge and that of the 3D computer model was discovered. The edge output of the software model is close to a bow shape, but to get the similar shape on the physical prototype the edge becomes a 3D curve. The subsequent step was to apply a profile shape created as a 3D model to the lattice, and check what the shape of the membrane geometry would have been in an iterative path. With a small reverse engineering process, a new 3D profile was modelled in the correct scale for the final structure. The detailed design phase of fixing the different elements onto the edge, and to make the process easier and cost-efficient, resulted in the circular section of the tall 3D arches. The hypothesis of using solid wood on the edge profiles (the 3D arches) would have required well dried wood of good thickness and red oak of this quality was not available. Therefore, the arches have been realized using CNC profiled gluelam Larch, jointed with approved glues and internal metal bars. This solution, chosen as a compromise between aesthetics, economy and ease of fabrication, resulted in a first attempt at achieving the shape of the arches which were built on-site for the opening. The severe climate at Arte Sella and some unexpected torsional load revealed some weakness after one year. It was decided to replace them with tubular COR-TEN steel 3D arches as a final on-site solution, in order to offer better resistance to loads and climate.

Prototype at 1: 3

The measurements can be summarized as follows:
• Phase 1: Test of positioning to keep the distance in scale between the anchors on the ground in relation to the original concept design.
• Phase 2: Repositioning of the anchors to search for a more faithful form.
• Phase 3: Capturing the shape with the laser scanning process to create a virtual 3D model. Form-finding and reverse-engineering on to the digital model.

It was possible to use laser scanning technology, thanks to Politecnico di Milano’s equipment. 3D Laser Scanning is a non-contact, non-destructive technology that digitally captures the shape of physical objects using a line of laser light. The 3D laser scanner creates a “point cloud” of data from the surface of the object. In other words, 3D Laser Scanning captures the physical object’s exact size and shape and this is the starting point from which it is transformed into a digital three-dimensional representation. In the case of the 1: 3 prototype, the degree of fine detail and the accuracy of the sculpture’s free-form shape obtained with the real mock-up quickly generated highly accurate point clouds.
Data acquisition via the 3D Laser Scanning Process with specialized software directed the laser probe above the surface of the mock-up. The laser probe projected a line of laser light onto the surface while two sensor cameras continuously recorded the changing distance and shape of the laser line in three dimensions (XYZ coordinates) as it swept along the mock-up. The process is very fast, gathering up to 750,000 points per second and very precise (to ±.0005).

The scanned object is compared – as a sort of “Digital Twin” – to the designer’s nominal CAD data. The result of this comparison process is delivered in the form of a “color map deviation report,” in PDF format, which pictorially describes the differences between the scan data and the CAD data.
Using specialized software, the point cloud data is used to create a 3D CAD model of the real-life geometry. The CAD model enables the precise reproduction of the scanned object, and the object can be modified within the CAD model to correct eventual imperfections. Laser Design can provide a surface model or a more complex solid model, whichever results are needed for the application. An overlap in Rhino between the theoretical model and the 3D laser scanned model (Digital Twin) has been created.

A double Finite Element Analysis (FEA) assuming equivalent wooden shell strip members and edge profiles was applied to the 3D CAD model with combinations of snow and wind loads; a second analysis was done on the prototype, so that the results could then be compared using several load tests. Other load tests were done on the connections between the wooden strips.