TENSILE STRUCTURES

Avjeet Gur Prashad Harpreet Mehar Nikhil Shantanu

•The term tensile structures describes the category of buildings in which the load bearing capacity is achieved through tension stress in the majority of the components, such as cables, technical fabrics or foils. •It can also be defined as a structure where the exterior shell is a fabric material spread over a framework. The fabric is maintained in tension in all directions to provide stability.•The only exception is represented by rigid boundaries and structural members which are generally subjected to compression and bending. •Tension structures are commonly subdivided in boundary tensioned membranes, pneumatic structures and pre-stressed cable nets and beams (Lewis 2003).

Definition

DENVER INTERNATIONAL AIRPORTCHICAGO’S NAVY PIER

• FABRIC STRUCTURES CANNOT TAKE HEAVY WEATHER CONDITIONS IS AN INCORRECT STATEMENT.

• FABRIC IS ELASTIC AND STRETCHES. Fabric has a strong tensile strength and will creep (stretch very slightly) only a few percent over 20 years of use.

Common Misconceptions

•Tensile fabric structures have been with us since the Mongolians swept down from the plains, with their yurts as their transportable overnight accommodation.

•And there’s evidence that the Romans even covered the Colosseum with massive canopies, hoisted by an intricate system of pulleys, to protect the audience from the elements.

•But they really came into their own in the last half of the 20th century, when designs like this by Germany’s Frei Otto.

History

Frei Otto Olympic Stadium Colosseum roof

Why tensile are the shape they are?•Large flat pieces of fabric are very poor at resisting loads.

•Imagine four of you each pulling on the strings laced through a tennis ball. Fig 1. A fifth person pushing down on the ball can deflect it easily.

•Imagine a flappy marquee roof. Try lifting two opposite strings and lowering the other two. Fig 2. The ball is now locked in space. Apply this principle to fabric and you have created ‘anticlastic’ double curvature

Fig. 1 Fig. 2

• Saddle roof

• Mast supported

• Arch supported

•Combinations

Types of Fabric Structures

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Four or more point system when the fabric is stretched between a set of alternating high and low points

Saddle Roof

The roof plan, taken directly from the structural engineering working drawings, illustrates the roof configuration and its components.

The saddle-shaped roof of the stage cover nestles under the auditorium roof of the project. The leaning A-frames and the stay cables which hold them back are clearly visible, along with the radial cables which shape the tent units. The corner tripods, each consisting of a vertical mast and two sloping cables, are connected to concrete anchors rising from the water.

Section through the project showing the stage roof  tucked under the auditorium roof. 

•Tent-like in appearance, mast supported structures typically have one or sometimes several peaks that are supported by either interior or perimeter masts.

•The fabric is attached to the interior mast by special connections, usually a bale ring or cable loop. •Mast-supported structures can also be supported by adjacent buildings. The peaks of a mast supported structure are determined by the design and how the fabric is attached.

•Openings are typically ovoid or elliptical. The fabric that extends from the top of the opening is seamed and can necessitate patterning.

•Mast supported systems are suitable for long span roofs.

Mast Supported

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•Curved compression members are used as the main supporting elements and cross arches are used for lateral stability.

•In a plane arch, large differences between the thrust lines and the main geometry will produce large bending moments that in turn produce large changes in shape and high stresses in the arch chord section. One method to significantly reduce these effects is to tie or restrain points along the arch chord to reduce the initial large deformations of the chord

•The buckling length of the arch chord can also be reduced by discretely or continuously supporting the chord with tension elements or systems comprised of cables or membranes.

Arch Supported Roof

Typical arch shapes defined by physical and ergonomic constraints.

CombinationsCombination of several support types.

ComponentsConnection to concrete foundation pillar

•BASE PLATE

•MEMBRANESForms the enclosure of the structure. Connections can be glued or heat welded

Provide a link between the membrane and structural elements..• Bale rings are used at the top of conical

shapes. • Membrane plates accept centenary cables

and pin connection hardware.

•BALE RING/ MEMBRANE PLATE

• PVC coated polyester (polyvinylchloride)

• Silicon coated glass

• Teflon coated glass P.T.F.E (polytetrafluroethylene)

•TYPES OF FABRIC MEMBRANEPVC : Less expensive 15 to 20 year life span Easy to erectSILICON GLASS: Higher tensile strength

Brittle, subject to damage from flexing 30+ year life span

TEFLON GLASS: Similar to silicon glass, less brittle.

Specialized HardwareTripod head with centenary cables

Centenary cables at a side connection

Extruded section with membrane plate and centenary cablesTensioner

CABLE CLAMPS

Edge c able with c lamps. Used mainly for PTFE-coated fiber glass fabric , but also for PV C-coated poly ester fabric when edge spansare longer than 20 m.

Bale rings are a go o d way to c o ntro l stresses in fabric ro o f at high o r lo w po ints. Used at high po ints they must be c o v ered tomake the struc ture watertight. If used at lo w po ints, they c an b e used to gather rainwater and sno w fo r redistrib utio n o n site.

Channel (with grommets) and lacing. Used with PV C-coated polyester fabric where the edge has grommets spaced at frequent intervals.

Rope is laced thro ugh the grommets and to a tie rod within the channel.

Water dreainage via Membrane plates

Solar Behaviour

•Longer life cycles of materials.•Materials can be re-used in form.

•Most materials are completely recyclable.•Less impact on site.•Less construction debris after demolition.

• Unique designs

• Lightweight and flexible

• Environmentally sensitive

• High strength weight ratio

Advantages Disadvantages• Little to no rigidity

• Loss of tension is dangerous for stability

• Thermal values limit use

Tensile Structures In India