De Doolhof

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De Doolhof

The temporary roof canopy for open-air theater (OLT) De Doolhof in Tegelen, ensures a dry stage all summer long. The canopy is a tensioned membrane structure that is assembled and disassembled each year for peak season from April to October. This project was built by Polyned. Summum Engineering provided form finding and structural engineering services.

Fabric roofs, or membrane structures, are lightweight, durable, and have a minimalistic design, which makes them a popular choice for covering outdoor spaces such as theaters and stadiums. In addition to providing shelter from the rain, the membrane canopy can also provide shade on sunny days, which is important for outdoor performances that may take place during the day. Overall, the membrane canopy is a great addition to the open-air theater in Tegelen, De Doolhof (lit. The Maze), that wanted to ensure the stage was protected from the elements at all times. The fabric canopy also adds to the aesthetic appeal of the space.

Design

The membrane canopy covers a podium of 12 by 8 meters, covering about 200 square meter in total. It is 8 meters high at the front and 4 meters high at the back. The membrane is tensioned via edge cables from eight corners, creating a so-called anti-clastic, or negatively curved surface which is close to a minimal surface like that of a soap film. It is supported by circular hollow section H-frames in the front and a series of steel masts and tie rods at the rear. The H-frame is designed to rotate at the base to allow for easy assembly and disassembly of the canopy, as it can be lowered and drop down to the ground. The structure is designed to avoid rigid supports and connections, resulting into a truly lightweight structural system.

Engineering

The shape of the membrane structure, based upon the preliminary design provided by the client, was determined using a process called form finding. This involves finding the shape of the membrane that distributes the loads evenly, has minimal stress concentrations and maximizes structural efficiency. The design also had to take the podium and its clearance into account. The form-found shape was then used as a basis for further analysis and design.

Owing to the large deformations in the membranes, geometrically nonlinear calculations are necessary to accurately engineer the structure. A parametric workflow coupling the form-finding and structural analysis allowed for efficient exploration of different design options and optimization of the structure. By fine tuning the structure it was possible to create a smooth distribution of prestresses in the fabric, while meeting requirements from building code. An additional challenge was to ensure that the membrane would never clash with the steel H-frame. Since the membrane structure is meant only for summer conditions, it was designed to account for seasonal load conditions and material properties, to avoid unnecessary material use.