Noahs Prism - Structural Analysis with Kangaroo2

Artists Loop.pH together with support from Format Engineers have been exploring the potential for artistic lightweight woven forms on increasingly large scales (see X and Y Archilace projects - add links). Noahs Prism, a 30m by 30m cable net structure clad with prismatic plastic film was their biggest yet.

Format worked closely with Loop from the concept stage, discussing various ways in which a private garden  in Primrose Hill, London could be encapsulated in a light and materially efficient structure with the minimum of supports and no impact  on the existing property. The installation was to provide protection to an important family event, to host a number of lectures during the day and parties at night. The surface of tent was clad in a prismatic film to refract light and bathe the inside in colourful effects.

Drawing on the work of Frei Otto the shape of the tent like enclosure is ‘form found’ as a funicular or least energy surface draped over the predetermined supports. There have been many examples of the technique being used in architecture and engineering design and there are commercially available engineering design software packages which can generate funicular forms but Noahs Prism used a different approach to collaborative modelling and structural analysis to prove the robustness of a very lightweight structure and deliver the project in an incredibly tight timescale.

Drawing on precedent lightweight architectural projects (such as those conceived by Frei Otto) the structure is minimal, utilising solid elements for compression masts only and resolving tensile loads through cables and the roof net.

This form-finding of membrane structures can be achieved through a variety of commercially-available software packages. Typically the surfaces are designed to achieve anti-clastic geometry when pre-tensioned and a project-specific membrane cutting pattern is produced.

For this project, due to a tight design stage it was only possible to use of an off-the-shelf lightweight net with 150mm square openings and a defined yield strain under a known load.

Loop.pH were already using grasshopper3d and Kangaroo physics to explore possible design options and as the structure is formed (almost) entirely from simple cable elements, it seemed possible that the K2 solver could be used for structural design as well as just defining geometry. The final workflow for both architectural geometry and analysis was built within a collaborative grasshopper definition that contained the following stages:

• Create net in plane and drape over spheres placed at each proposed mast support point. Material properties are arbitrary at this point with a high strength given to the goal that maintains the internal 150mm lengths within the net.

• Closest nodes are snapped to support points to give the un-deformed geometry.

• Material properties for the numerous cable elements are used to define the goal “strengths” within Kangaroo. Spring stiffness (a.k.a. goal strength) => k = EA/L

• Self-weight (including prismatic film) and approximated wind is applied as Unary forces at all nodes within the model.

• Following simulation convergence, strains can be calculated by comparing initial and deformed geometry.

• Reactions at base of masts and guys calculated from strains => F = kx

• Accuracy of model convergence was assessed by comparing total reactions with total load on structure

This entire workflow was hosted within a common model, allowing the entire design team to understand the impact of geometric changes and modify the form to remove load-concentrations within the net.

It is important to note that this use of Kangaroo 2 is only possible due to the stability of the new solver under high values of stiffness. Thus all geometry, stiffness and loading can be considered at full (virtual) scale.

Following this project Format are further investigating the potential of utilising Kangaroo2 for the development of off-the-shelf net-based tensile structures. The use of standard nets offers the potential for high-efficiency and low cost structures.

What: Noahs Prism

Where: London

Who: Will Pearson, James Solly, Stephen Melville, Tom Foley

Artist/Computational Design - Loop.pH, London

Structural Engineering/Computational Design - Format, Bath