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Tensile Structures

Tanvi Choudhari Vrunda Pachchigar Namrata Vyas Digisha Sinvhal Devanshi Mehta

Tensile structures are characterized by the prevalence of tension force in their structural systems and by limitation of compression forces to a few support members Thus these lightweight structures do not require the considerable amount of construction material to absorb the buckling and bending moments in compression members.

Tension

Compression

8. 9.

categories of tensile structures are mast and cable supported membranes pneumatically inflated membrane.

1.mast and cable supported membranessimple saddle membrane with linear perimeter supports. Ridge type membrane with linear internal and perimeter support. Arch type membrane with linear internal support. High point type membrane with multiple internal support.

Types of structure with significant tension membersLinear structures Suspension bridges Cable-stayed beams or trusses Cable trusses Straight tensioned cables Three-dimensional structures Tensegrity structures Pre stressed membranes Pneumatically stressed membranes Cable and membrane structures

There are many different doubly-curved forms, many of which have special mathematical properties. The most basic doubly curved form is the saddle shape, which can be a hyperbolic paraboloid

Tensioned fabric structures True tensile fabric structures are those in which every part of the fabric is in tension. The fundamental rule for stability is that a tensioned fabric structure must curve equally in opposite directions, this gives the canopy stability. This is known as an anticlastic form and mathematically as a hyperbolic paraboloid. We put the fabric of a tensile structure under tension. We do not stretch the fabric into position. It is cut and bonded together to make its final shape

Pre-tension is the most efficient way of resisting live loads snow, wind etc. Design Fabrication Erection. Design factors Location (Wind and snow loads; Foundations Drainage

Fabrics :1.PVC (polyvinyl chloride) coated polyester polyester is the least expensive, design life of 15 to 20 years due to ultra violet attack 2. Silicon coated giass Silicon glass has higher tensile strength than polyester, but being glass it is brittle, subject to damage from repeated flexing. Not subject to ultra violet attack, 30+ year design life. 3. Teflon coated glass PVC coated Silicon and Teflon are almost completely chemically inert, resistant to moisture and micro-organisms and have self cleaning properties.

Internal fabrics :All types of fabric can be used if suitably fire retarded. The most commonly used is PVC coated glass cloth due to its easy maintenance and very good fire resistance.

Form finding :The final shape, or form, of a fabric structure depends upon shape, or pattern, of the fabric The geometry of the supporting structure (such as masts, cables, ring beams etc) the pretension applied to the fabric or its supporting structure

Advantages : Unique building medium. Lightweight and flexible, fabric interacts with and expresses natural forces. Tensile fabric structures are an environmentally sensitive medium. Tension is the most efficient way of using any material, it utilises the material at maximum efficiency rather than just the material at the extremes of the cross sectional form, as in bending and compression loads. Fabric structures have higher strength/weight ratio than concrete or steel. Most fabrics can be recycled. A fabric structure can be designed for almost any condition, heavier fabrics and more 3 dimensional forms will cope with extreme wind and snow loads.

Disadvantages : Fabric structures being mainly fabric and cables have little or no rigidity and therefore must rely on their form and internal pre-stress to perform the this function. As a rule of thumb spans greater than 15 metres should be avoided however, much greater spans can be achieved by reinforcing the fabric with webbing or cables. Loss of tension is dangerous for the stability of the structure and if not regularly maintained will lead to failure of the structure.

Cables : Cables can be of mild steel, high strength steel , stainless steel or polyester or aramid fibres. Structural cables are made of a series of small strands twisted or bound together to form a much larger cable

CABLE-NETSCable net structures are for covering large unsupported spans with considerable ease. The constructional elements are steel pylons, steel cable networks, steel or wooden grids, and roof coverings of acrylic glass or translucent, plasticreinforced sheeting. Cables are fastened into the edges of the steel network, and are laid over pin-jointed and usually obliquely positioned steel supports, and then anchored. Basic structure of the cable-net roof

OLYMPIC ROOF, MunichConstruction materials used: Masts cable net membrane panels covered area : steel : steel : acrylic : 74 000 m2

Construction

The cable net as built, the nets are formed of crossed pairs of strands spaced 750 millimeters in both directions. This spacing remains constant regardless of net shape, all changes of plane in the double-curved surfaces being accommodated by changes in the strand intersection angles . Intersections joints were formed by an automatic process, aluminum clamps with central holes being pressed on to all strands at exactly 750-millimetre centers under a defined level of pre-stress. The two sets of strands could thus be formed into a 750 x 750-millimetre mesh with no need for measurement, simply by connecting the aluminium clamps

Main and edge cables

The connections used one bolt per joint, resulting in a freely rotatable node that allowed the mesh to adjust to any angle of intersection. With regard to cable specification, a balance had to be struck between the need for cable flexibility (which favours a strand spun from many thin wires) and durability (which favours one spun from fewer thick wires). The decision was to form the net from strands spun 19 heavily galvanised 2,3- and 3,3-millimetre steel wires, with a lay length of 10 x the lay diameter . The main cables, composed of five strands formed from between 37 and 109 wires each, had to be held at high tension to control deformaton of the roof under snow and wind loads. Permissible load was 11,5 mN (1150tonf); where forces exceed this figure several ropes were coupled rather than increasing cable size. The edge cables vary in specification, a typical example being a locked-surface wire rope of 81 millimetres diameter. With a safety factor of 2 the permissible load is 3mN (300tonf) and again several The distance, parallel to the axis of the cable, in which a strand makes one complete turn about that axis is known as the lay length or pitch length

Erection on site: The cable nets were completely assembled on the ground, then lifted to their final positions.

Foundations and mastsTension foundations were needed to anchor the main cables down to earth. Upward pulls of up to 50mN (in the case of the big edge cable of the stadium) are exerted on such foundations, and three foundation types were used inclined slot foundations, working rather like tent pegs gravity anchor foundations, deriving their anchoring effects from self-weight plus the weight of the soil surcharge earth anchor foundations were needed to support the masts. To accommodate some movement these footings consist of rubber bearing pads on concrete bases. Temporary steel balls were provided under the rubber pads to allow rotation during assembly . Masts are cylindrical welded steel tubes up to 80 metres long and with a 50mN (5000 tonf) load capacity.

Roof coveringThe transparent roof covering was formed of 2.9 x 2.9-metre acrylic panels of 4 millimeter thickness, laid on the cable net and bolted to the intersection nodes. As the angles of intersection in the cable net change up to 6 degrees under load and temperature change, the rigid acrylic panels had to be flexibly connected to the net. This was done by supporting the panels on neoprene pesetals , allowing them to 'float', and sealing the joints between panels with a continuous neoprene profile clamped to the panel edges. The strip had to be thin and wide enough to absorb movements by wrinkling - unfortunately an inelegant detail.

Detail of how the acrylic plates are connected with each other . They are all framed in a steel square section and then connected with each other using bolt connection . Also the each of the acrylic plate rest on the net structure which is also made up of steel cables passing horizontally as well as laterally. None of the joint is continues with each other In order to gain more stable form .

passage on the top from where the people can pass through.

steel mast supporting the structure radially from the one of the end point of the sag . the part where the membrane is made to rise with the help of the mast there forms a slope and that slope is provided with a path .

CABLE-NET: ICE SKATING RINK (OLYMPIC PARK MUNICH) - 1983

MAST AND CABLE SUPPORTED MEMBRANES Arch type membrane with linear Archexternal support.

To enable the open ice-surface in the Olympic Park to be used all round the year, independently of the weather, a light roofing, naturally without supports, was required a steel-trussed arch of three chords. With a span of 100m and a height of roughly 19m at its apex, the arch is capable of transmitting any thrusts to two large concrete abutments. Two sets of cables hang in opposing curves from the arch, stabilizing it by their anchorage and forming a net. These symmetrical nets of cable have a grid of 75 x 75 cm and support a wooden lattice, upon which is attached a translucent plastic sheeting. At the roof's edges the cable nets are bordered by