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Fort 4 Mortsel

Tensile fabric structure over the inner courtyard 'Reduit" of a 19th century fortress

Historical Context

Fort 4 in Mortsel was initially a large military building within a defending belt of fortresses around Antwerp, built around 1861. The fortress has an internal court, called ‘The Reduit’ in military terms. A reduit is the centre of a fortified structure. It is the keep into which the defending troops can retreat when the outer defences are breached. At the outside there are moats, ramparts, ditches and fortifications, all built to protect the inner courtyard from intrusion.

This particular ‘reduit’ has a rectangular part and a wider round end.

The rectangular part of the inner court is about 12m x 55m. The circular part has a radius of about 12m and an opening angle of 140°. The surrounding brick walls are about 12m high.

The building is classified as a monument.

Project Description

The city of Mortsel asked for a convertible roof in 2001, covering about 700m² within the fortress. The incredible difficult task was to access the reduit: the widest entrance was 98cm wide into the 90cm thick walls! The massive brick structure of the fortress is invented to make the access to the reduit as difficult as possible after all. It is surrounded by several belts of fortifications, corridors and a ditch of 40 to 50 metres wide. It was not possible to access the courtyard with machines or even not accessible with a large crane from outside. But how to build a roof over this large courtyard, if anyone can access with building materials or cranes?

The answer is simple: making a flexible roof that one can carry in, like a large worm, through the inner corridors and through the narrow porches. The complete membrane roof exists of 8 parts (8 separate vaults) each small enough to be carried in. Between the different membranes, transparent foil parts made of ETFE can be zipped off or on, to make the complete roof waterproof.

The membrane roof was designed for the summer season to provide shelter for cultural open-air events (concerts, film, theatre, etc.). Since the roof will be set up and demounted regularly with a limited crew and without cranes, the setting up procedure and handling was studied to be simple and fast. Simple ropes and pulleys are used only to pull the structure upwards.

Nomad Concept - sail sculpture Fort 4 (Mortsel)


The city of Mortsel (BE)


Mortsel (Belgium)




PVC coated membrane by SergeFerrari

Project Date

May 2002

A wonderful artistic place for the muses instead of the military

That said, there was not only the functional aspect. Architect Amandus VanQuaille wanted to create a place for muses in the former military building: a place for lovely art instead of fighting and death. Secondary there was the idea of using the mass of the thick walls as counterweight for the large tensions.


The result is a cathedral-like vault, where the choir is the scene for arts

The theme is a playful contrast between convex and concave form, the contrast between the old brick walls and the ultralight textile membrane. The arches of the membrane are formed by pulling the tensioned fabric between the higher and lower parts of the rampart walls. While the first vaults between the parallel walls have a typical Romanesque form (pure arches without ribs), the choir was too large to realise the vault without ribs. The stainless steel cable ribs stabilise the complete structure in case of wind or snow.

The 'key stone'

Where typical ribs of Gothic architecture push the complete structure via concentrated lines, the ribs of this textile vault pull the structure in the opposite way. In the middle, all the ribs come together in the ‘keystone’: here the ‘keystone’ is a thick stainless-steel plate where all the ribs or cables are connected to one plate. In Gothic architecture, the keystone has a special symbolical meaning: if the keystone cracks under high point forces, the complete vault would fail and fall down. That is also the case in our textile vault.

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The Nomad Concept is a Belgium-based studio practice focused on tensile and lightweight architecture. From our inception in 2000, we have delivered exceptional private, corporate and public projects.
As a full-service firm, The Nomad Concept is present on projects from start to finish, ensuring that the ideas conceived in the beginning with clients are manifest at the end. .

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