Tensile Structures

Tanvi ChoudhariVrunda PachchigarNamrata VyasDigisha SinvhalDevanshi Mehta

Tension

Compression

• 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.

• categories of tensile structures are 1. mast and cable supported membranes2. pneumatically inflated membrane.

•simple 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.

1.mast and cable supported membranes

Types of structure with significant tension members

Linear structuresSuspension bridges Cable-stayed beams or trusses Cable trusses Straight tensioned cablesThree-dimensional structuresTensegrity structuresPre stressed membranes Pneumatically stressed membranesCable 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.

DesignFabricationErection.Design factorsLocation (Wind and snow loads;FoundationsDrainage

Fabrics :1.PVC (polyvinyl chloride) coated polyester

polyester is the least expensive, design life of 15 to 20 years due to ultra violet attack2. Silicon coated giass

Silicon glass has higher tensile strength than polyester, but being glass it is brittle, sub ject 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 ex tremes 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 fail ure 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 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, plastic-reinforced 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.

CABLE-NETS

Basic structure of the cable-net roof

OLYMPIC ROOF, MunichConstruction materials used: Masts : steelcable net : steelmembrane : acrylic panels covered area : 74 000 m2

ConstructionThe 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

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 ropes are coupled where forces exceed this figure.

Main and edge cables

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.

Tension 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 pegsgravity anchor foundations, deriving their anchoring effects from self-weight plus the weight of the soil surchargeearth 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.

Foundations and masts

The 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.

Roof covering

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 .

•Arch type membrane with linear Arch-external support.

MAST AND CABLE SUPPORTED MEMBRANES

CABLE-NET: ICE SKATING RINK (OLYMPIC PARK

MUNICH) - 1983

•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 garland-shaped cables which pass over adjustable angled supports of steel being anchored fast.

LAYOUT PLAN•The construction and form of the “hanging from the arch”•correspond to that of the roof edge.•a series of elliptically strung openings below the latticed arch . These are filled by "glass eyes" equipped with ventilators.

•The continuous "facades" between 3 to 5 m tall between the edge of the roof and the ground in the region of the angled supports incline from the eaves to the interior at an angle corresponding to that of these supports. •the first ever in itself , horizontally barred glass "façade" which is able to participate in the formal changes allowed by the anchoring cables.

ELEVATIONS

STRUCTURAL DETAILS

CABLE JOINERY DETAILS

• Vital Statistics:Location: Kobe and Awaji-shima, Japan

• Completion Date: 1998• Cost: $4.3 billion• Length: 12,828 feet(3910 m)• Type: Suspension• Purpose: Roadway• Materials: Steel• Longest Single Span: 6,527

feet• Clearance below :65.72

meters• Structural Type:

Suspension bridge gravity-anchored, deck truss

AKASHI KAIKYO BRIDGE, JAPAN

• the longest spanning suspension bridge in the world.

• The Akashi Kaikyo Bridge isn't just long -- it's also extremely tall. Its two towers, at 928 feet(283 m), soar higher than any other bridge towers in the world.

• Design had to take into account that the area had suffered several earthquakes, (measured 6 on the Richter scale).

• the complicated topography in the strait required anchoring one of the towers of the bridge's central span on a steep slope, while the other foundation was relatively flat.

The enormous wind forces, were especially relevant, as they are for all suspension bridges, because of their flexibility, in fact, the abundance of typhoons clearly made in-depth climatological studies necessary.

• Hurricanes, tsunamis, and earthquakes rattle and thrash the island almost annually.

• To examine these factors and investigate potential design criteria applicable to an area with such extreme conditions, an observation tower was built in 1964 near the future site of the bridge.

283m

SOLUTION• The structure of the bridge

strengthened with a truss, or complex network of triangular braces, beneath the roadway.

• The open network of triangles makes the bridge very rigid, but it also allows the wind to blow right through the structure.

• 20 tuned mass dampers (TMDs) placed in each tower. The TMDs swing in the opposite direction of the wind sway. So when the wind blows the bridge in one direction, the TMDs sway in the opposite direction, effectively "balancing" the bridge and canceling out the sway.

• the Akashi Kaikyo can handle 180-mile-per-hour winds, and it can withstand an earthquake with a magnitude of up to 8.5 on the Richter scale

Each section has a triangulated form. This means that weight is kept to a minimum and yet each section has maximum strength

• Here's how this bridge stacks up against some of the longest-spanning bridges in the world. (total length, in feet)

• Akashi Kaikyo Bridge 12,828' (3910 m)

•The bridge was designed with a two-hinged stiffening girder system, allowing the structure to withstand winds• The bridge also contains pendulums that are designed to operate at the resonance frequency of the bridge to dampen the forces.• The steel cables have 3,00,000 km of wire.•each cable is 112cm in diameter and contains 36,830 strands of wire. , a new low-alloy steel strengthened with silicon was developed; its tensile strength (resistance against pulling forces) is 12% greater than any previous steel wire formulation. On some suspension bridges, the steel wires forming the cables have been galvanized (coated with zinc).

Erection of Suspension Bridges

STAGES IN CONSTRUCTING THE AKASHI-KAIKYOSUSPENSION BRIDGE

• The two towers stand on two large circular foundations. The moulds for the two foundations were built in dry dock weighing 15 000 tonnes and 60 metres height.

• In March 1989 the moulds of foundations of the towers being towed out to their positions in the sea by numerous tugs. When in position the moulds were flooded with 250 million litres of water, taking eight hours to complete. By the time the moulds were full, they were resting on the sea bed.

• Each of the two foundations were filled with 265 000 cubic metres of concrete. However, ordinary concrete does not mix with water and so the Japanese had to develop special concrete which was capable of mixing with sea water. 

FOUNDATION

 

   In 1989 work on the two towers began.

• Each is nearly as high as the Eiffel Tower and is designed to have a 200year lifespan.

• The towers are 283 metres in height and if the foundations are included, this adds a further 60 metres.

• Each tower is made up of 90 sections and they were built with absolute precision as the design allowed only a 25mm offset at the top. In order to achieve this level of accuracy each of the blocks were ‘surface ground’ to a precise finish. 700 000 bolts were used to fix each of the towers together.

• Each tower is designed to flex / move in storm force conditions. They and even have a special mechanism that counteracts and dampens movement

PYLONS /TOWERS

•The pylon composed of columns and footing.• Double sets of 8 columns of a 4m diameter each are aligned in a rectangle with two 7m diameter columns at each center, and the footing caps atop the sets of columns. •These columns and footing are RCC structure. •These columns were composed of RC piers and steel-pipes, which were not strength members, but they were designed as RC piers protection members from outer damage. The steel pipe was regarded as strength member in the seismic analysis.

structure of the multi-column foundation.

FOOTING

• When the towers were completed a temporary cable stretched between both and a wire mesh gangway built so that workers could start construction of the main cables. Workers and machinery pulled the main cables from one tower to the other.

• Once the main cables and the vertical cables were in position the deck / roadway was fixed hanging below them,

• 290 sections make up the entire bridge.

• cranes in operation and the deck as it was fixed in position, section by section.

CABLES,(MAIN & TEMPORARY)

ARIAL VIEW OF AKASHI KAIKYO

VIEW OF AKASHI KAIKYO FROM BELOW THE DECK BRIDGE

DECK (ROAD)VIEW OF THE BRIDGE

The London Eye• Designed by: architects David Marks,

Julia Barfield, Malcolm Cook, Mark Sparrowhawk, Steven Chilton and Nic Bailey

• Height 135 metres (443 ft) ,biggest Ferris wheel in Europe

• most popular paid tourist attraction in the United Kingdom, visited by over 3 million people a year

• Described by its operators as "the world's tallest cantilevered observation wheel" (because the entire structure is supported by an A-frame on one side only). allowing the wheel to hang over the River Thames.

LOCATION:

London Eye is located at the western end of Jubilee Gardens, on the South Bank of the River Thames in London, United Kingdom, between Westminster Bridge and Hungerford Bridge.

• The rim of the Eye is supported by tie rods and resembles a huge spoked bicycle wheel.

The London Eye is an excellent example of a frame structure. Its steel design forms an "A" shape, with two large tapered legs at the base -- 20 meters apart and each over 58 meters in length. The legs lean toward the river at a 65-degree angle.

Cable backstays keep the frame from tilting into the river -- they're anchored to the top of the frame and then buried in a concrete foundation 33 meters deep.

The spindle itself is supported by the frame on one side only (cantilevered), and the frame holds the wheel over the river.

HOW LONDON EYE WORKS

Capsules • The London eye has 32 capsules.

Capsules have 360 degree views, a heating and cooling system and bench seating. It rotates at 26 cm (10 in) per second (about 0.9 km/h (0.5mph) so that one revolution takes about 30 minutes.

• Each capsule 24 people. • Instead of being suspended under the

wheel, the capsules turn within circular mounting rings fixed to the outside of the main rim.

• The result is a stunning 360 degree panoramic view from the top of the wheel.

COMPONENTS OF LONDON EYE IN DETAIL

2.Cables • six backstay cables holding the wheel in place

The wheel part of the London Eye resembles a bicycle wheel -- with a spindle and hub connected to the rim by 64 cables, or spokes.

16 additional rotation cables are attached to the hub at an

opposing angle holding the rim tight to the central spindle. to ensure there's no lag between the turning of the rim and the

turning of the hub.

6 BACKSTAY CABLES

3.Foundation • The main foundation for the London Eye is situated underneath the A-frame legs • it required 2,200 tonnes of concrete and 44 concrete piles - each of which is 33 metres

deep• The second foundation, the tension foundation holding the backstay cables behind

the wheel, used 1,200 tonnes of concrete.

4. Spindle • At the centre of the London Eye is the vast hub and spindle. • The main elements were manufactured cast in steel. • The spindle itself was too large to cast as a single piece so instead was produced

in eight smaller sections. • Two further castings, in the form of great rings form the main structural element

of the hub. • The hub has a rolled steel tube forming the spacer that holds them apart. • All the casting was carried out by Skoda Steel.  

•The wheel was constructed in sections which were floated up the Thames on barges and assembled lying flat on piled platforms in the river which made construction faster, easier and safer than if it had been built vertically.

CONSTRUCTION OF LONDON EYE

•Once the wheel was complete it was raised into an upright position by a strand jack system( hydraulic lifts and cables), being lifted at 2 degrees an hour until it reached 65 degrees.

• It was left in that position for a week while engineers prepared for the second phase of the lift. The total weight of steel in the Eye is 1,700 tons. Once it was in final position, the 32 capsules were attached to the rim, which took eight days.

During its construction, the London Eye underwent extensive safety monitoring, testing and evaluation.

The London Eye rotates around the hub like a MOTORIZED bicycle wheel. Hydraulic motors, driven by electric pumps, provide energy to turn the wheel. The

drive systems are located in two towers, one at each end of the wheel's boarding platform. How the wheel turns: Standard truck tires along the rim of the wheel act as friction

rollers. Hydraulic motors turn the tires, and the rotation of the tires turns the wheel. A computer controls the hydraulic motor speed for each tire.

• The project was truly European with major components coming from six countries: the steel was supplied from the UK and fabricated in The Netherlands by the Dutch company Hollandia, the cables came from Italy, the bearings came from Germany, the spindle and hub were cast in the Czech Republic, the capsules were made by Poma in France (and the glass for these came from Italy), and the electrical components from the UK.

The London Eye can withstand winds of a 50-year storm, the worst storm anticipated to occur once in a period of 50 years, and if it's ever struck by lighting, the strike would be conducted to the ground with no harm to passengers

MECHANISM OF THE LONDON EYE