Tension roofs or canopies are those in which every part of the structure is loaded only in tension,
with no requirement to resist compression or bending forces.
Classes of Tensile Structures:
1. Membranes – The structural membrane acts also as the weather shield Common materials for doubly curved fabric structures are PVDF-coated and PVC-coated polyester.
These are woven materials with different strengths in different directions. The warp fibres (those fibres which are originally straight—equivalent to the starting fibres on a loom) can
carry greater load than the weft or fill fibres, which are woven between the warp fibres.
2. Cable Nets – A separate grid of structural cables supports a non–structural weather shield.
3. Pneumatics –The tension force is created by an interior positive pressure and the membrane acts as the weather shield.
Shapes of Tensile Structures:
1. Anticlastic Surfaces – The centres of curvature of the membrane are on opposite sides of the membrane e.g. hyperbolic paraboloid, torus.
2. Synclastic Surfaces – The centres of curvature of the membrane are on the same side of the membrane. Ex: Sphere or balloon.
3. A Simple Paraboloid – Defined by a Minimum of Four Points w/ at Least One out of Plane.
1. Anticlastic Surfaces – The surface is stabilised because the tension forces from prestressing or load are in equilibrium.
2. Synclastic Surfaces – The surface is stabilised because the tension forces from internal pressure are in equilibrium with the tension forces in the membrane.
By definition, the membrane can only resist forces in tension, so forces perpendicular to the membrane cause large deformations and large membrane forces.
- Tensile roofs are susceptible to vibration.
- Reverse curvature and/ or prestressing can bring these forces to zero.
1. Uncoated fabrics
- Simple woven fibres
- Canvas, nylon
2. Coated fabrics
- High strength woven fibres with coatings to prevent UV– & environmental degradation and improve weather resistance.
- PVC–coated polyester: relatively short service life
- PVDF–coated: high strength, long service life
Rope Edge – A cable running in a sleeve and connected to a rigid structure at both ends
Hard Edge – A continuous connection to a rigid structure through clamping plates with or without pre-stressing devices.
- Masts capable of resisting compression and buckling forces.
- The magnitude of forces at the point support requires distribution via rings or umbrellas.
Line Shaped Supports:
- Arches – internal or External
- Valley and Ridge Cables