Tensile Final

Tensile Structures

Tanvi Choudhari Vrunda Pachchigar Namrata Vyas Digisha Sinvhal Devanshi Mehta

•

•

• 8. 9.

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 mast and cable supported membranes pneumatically inflated membrane.

Tension

Compression

1.mast and cable supported membranes •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.

Types of structure with significant tension members Linear 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-NETS •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, 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, Munich Construction materials used: Masts cable net membrane panels covered area

Construction

: steel : steel : acrylic : 74 000 m2

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

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 .

Main and edge cables

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 masts 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 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 covering 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.

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

•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.

LAYOUT PLAN

•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

•

AKASHI KAIKYO BRIDGE, Vital Statistics: JAPAN 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 • the longest spanning truss 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 indepth 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,

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 itThis Each section has a triangulated form. can withstand an earthquake means that weight is kept to a minimum yet each section withand a magnitude ofhas upmaximum to 8.5 strength

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). • Here's how this bridge stacks up against some of the longestspanning bridges in the world. (total length, in feet) • Akashi Kaikyo Bridge

STAGES IN CONSTRUCTING THE AKASHI-KAIKYO SUSPENSION BRIDGE

Erection of Suspension Bridges

FOUNDATION •

•

•

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

 

PYLONS /TOWERS

   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

• Double sets of 8 columns of a 4m diameter each are aligned in a FOOTING 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 structure of the multi-column regarded as strength member in foundation. the seismic analysis.

• When the towers were completedCABLES,(MAIN & TEMPORARY) 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.

DECK (ROAD)VIEW OF THE BRIDG VIEW OF AKASHI KAIKYO FROM BELOW HE DECK BRIDGE

ARIAL VIEW OF AKASHI KAIKYO

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.

HOW LONDON EYE WORKS •

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.

COMPONENTS OF LONDON EYE IN DETAIL 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.

1.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

1.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.

1.

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.

• • •  

CONSTRUCTION OF LONDON EYE

•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

•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.

MECHANISM OF THE LONDON EYE 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