An Introduction to Tensile Fabric Membrane Structures

More and more projects like the Millennium Dome in London and the Denver International Airport are using tensile fabric structures. Tensile fabric structures are becoming more and more popular, as new structural design methods are becoming more prevalent and as their advantages are being realized. So... what makes tensile fabric structures so popular? Just what are tensile fabric structures?

Tensile fabric structures only carry tension and are commonly used as roofs or canopies for military hangers, athletic facilities, gaming and entertainment structures, etc. While tensile structures can include cable structures, we will only focus on membrane structures in this post.

Millennium Dome in London Source: http://www.skibbereeneagle.ie/

What are Tensile Fabric Structures?

Tensile fabric structures are thin shelled structures made out of polyester or woven fabric, protected with coatings or laminations. Membrane joints are usually glued or welded together, while clamp plates, bale rings, or membrane plates are used to connect the membrane to its supporting structure. Different arrangement and direction of fabric warps produce different strength values.

The three most commonly used membrane materials are PVC, PTFE, and ETFE.

1. PVC coated polyester fabric has low cost and life expectancy, so it is more for temporary structures.

2. PTFE coated fibreglass has high life expectancy and provides incombustible effect, so it is more for long lasting structures. Although PTFE is expensive, it provides good resistance to UV radiation, wear, weather conditions, and chemical attacks and can have long spans. PTFE does not creep over time.

3. ETFE have high tear strength, so they creep when loaded and yield when elongated.

Source: http://www.architen.com/

Some advantages of tensile fabric structures include

● Spans with large distances (from 3m to 200m) so less structural steel are needed for roofs

● High strength to weight ratio

● Can produce many shapes, such as arches, saddles, masts, or a combination of shapes

● Fast installation

● Not a lot of construction debris produced after demolition

● Seismic resistance due to its inherent light-weight property

● Free flowing designs for aesthetic look

● Provides space so only few support columns are needed

● Light-weight and flexible structure

● Provide translucency and great light transmission causing reduction in energy costs

● Fire resistance

● Proven durability against stain, UV radiation, corrosion, and weather conditions

Source: http://www.fabritecstructures.com/

The Science behind Tensile Fabric Structure

Pretensioning

When the membrane is unloaded, membrane stresses are in equilibrium. When loaded, the membrane stresses are not at equilibrium, therefore the membrane deforms until the stresses are in equilibrium. Therefore, tensile fabric structures need pretensioning to decrease the deflection of the loaded membrane and to increase stiffness of the membrane under imposed loads. Pretensioning also ensures the structure is always in tension. The typical prestress load is around 150-350 kg/meter.

Load Cases and Load Paths

● Wind is the critical load case for tensile fabric structures. A good membrane must have good tensile strength and inherent curvature to resist wind loads.

● Self-weight of membrane should be negligible due to its lightness.

● Seismic loads should also be negligible due to the membrane’s lightness.

● Rain ponding loads should be avoided in the design, since this can lead to deformation failure.

● Tensile fabric membranes should not be designed with concentrated loads. The material of the membrane cannot support high forces perpendicular to membrane. Tensile fabric membranes should only be designed with axial forces in mind.

Gravity loads transfer from the tensile membrane structure to the compression elements, which then transfer the loads to the foundation.

Compression elements resist and transfer lateral loads to the foundation.