Long Span Structures - Vector Active, Form Active & Pneumatic Structures.-1

Long Span Structures Ar. Preeti Pawar - Kale

Vector Active Structures • There is a structure consisting of linear members joined together with hinge joints in which members develop either axial compression or tension but no bending , this type of structures are known as Vector active structures. • Vector Active System : 1. Plane Trusses -Flat trusses, Curved Trusses, Open Web Joist, 2. Space Trusses.

W – Load R – Reaction

Characteristics of Vector Active Structures: • Members of vector active structure develop only axial compression or tension and resist the same full cross – section comes into play, hence they need very small cross –section as compared to bulk active structures. Thereby self weights of these structures is very less and ideally suitable for long span structures. • Loads are applied at joints (nodes) only because of thin cross- section, members are not suitable for bending stress. • Being strong both in compression and tension , timber and steel are suitable materials for vector active system.

Plane Trusses • A plane trusses is used as one way spanning element and requires less volume of material as compared to space truss. • It can only resist forces lying in the same plane as itself. • The basic repetitive module is a simple triangle. • The joints are in form of pin connection. • Material – steel • It does not have lateral stability, it gets lateral stability from purlins.

Flat Trusess • All members and nodes lies within a 2D plane consist of straight members connected at joints. No member is continuous through a joint. • Where forces tend to pull the member apart, its in tension and when it compresses it is in compression. • Application – in building, to support roofs and floors, to span large distances and carry relatively light loads. In road and rail bridges, for short and intermediate spans and in footbridges.

Curved Trusses • Curved Trusses are trusses which are used to give a particular form to the building. • The material mostly used is steel for such construction. • Form is not organic but is in section.

Open Web Joist • A parallel chord truss supported at ends can also be looked upon as a perforated I beam, whose top chord goes under compression and the bottom chord is under tension when the truss is loaded. • Used in roofs.

Space Frame • Space frames are useful when used to form two way system. • These are also efficient when trusses are used in free standing way without beam like transmission towers etc. • It can resist force in any plane. • The basic repetitive module is tetrahedron. • All joints are in the form of pinned connections. • This truss extents into a third dimension and covers an area.

Space Trusses or Space Frames • Space Frame is a truss – like , lightweight rigid structure constructed from interlocking struts in a geometric pattern. • Space frames usually utilize a multi directional span, and are often used to accomplish long spans with few supports. • Strength is from triangular frame. • Bending moments are transmitted as a tension and compression loads along the length of each strut.

Form Active Structures • Structures which support external forces and self weight on account of particular form given to their material are known as Form Active Structures or Single Stress Condition Structures and are classified in two types: Cable Structure (Tensile Structure) Arches ( Compression Structures) Tent Structure Pneumatic Structures.

Cable Structures (Tensile Structures) • Cable structures are long span structures, subjected to tension and use suspension cables for supporting the loads. • Cable structures are highly efficient because of their large strength to weight ratio. • Cable structure includes: 1. Cable suspension bridges 2. Cable stayed roofs over large areas 3. Bicycle roof

Cable Stayed Roof • Inverted Barrel Roof with Vertical Columns: 1. It is the simplest roof and consist of series of cables suspended from end columns. 2. Cables are connected by straight beams there by creating inverted barrel surface on which roofing material is supported. 3. To avoid fluttering of roof under the action of wind the roofing material must be heavy. 4. Columns resist the entire horizontal thrust of cable by acting like a cantilever beam projecting out of the ground. 5. Significant bending is induced in column and to absorb the same, column cross section at bottom should be fairly large. 6. Foundation of column should also be designed to resist the over turning moment.

Cable Stayed Roof • Inverted Barrel Roof with Vertical Columns and Guy Cable 1. In this case cables instead of being suspended from top of columns passes over a pulley fixed to the top of column and are anchored to ground. 2. The cables between the column and ground anchor is known as Guy Cable. 3. Anchorage provided for Guy Cable is designed to resist the horizontal force in cable. 4. Angle of cable on either side of column is kept equal to avoid any bending moment in column and column is subjected to direct compression force only.

Cable Stayed Roof • Inverted Barrel Roof with inclined Columns and Guy Masts 1. This is further improvement on the above discussed systems. In this case part of the horizontal thrust is absorbed by inclined columns also known as Guy Masts and the balance horizontal thrust is resisted by the anchorage of Guy cable.

Cable Stayed Roof

• Double Concave Pre Tensioned Cable Roof 1. A double cable structure typically consist of two coupled pretension cables and related struts or ties that function together in carrying n external force. 2. Use of double cable system prevents roof flutter due to wind effect. 3. Pretension in upper cable is slightly different than lower cable, therefore each member has a different natural frequency of vibration.

Cable Stayed Roof • Suspended RCC Roof 1. This roof consist of prefabricated RCC beams and slabs, which are constructed on ground and the lifted and attached suspended from the vertical suspenders attached to main cable. 2. RCC Columns (also known as towers) are inclined outward (Just liked guyed masts) to absorb part of horizontal thrust of main cable. 3. Towers are also supported in their length by short RCC struts. Columns and struts are cast monolithic and have combined foundation. 4. Walls & pillars are designed to withstand horizontal wind load.

Cable Stayed Roof • Suspended Cantilever Roof : • In this case also the principal of suspended RCC roof is used. • This types of roof is highly suitable for air plane hanger which require one entirely openable wall to allow the entry and exit of airplanes as shown in fig.

• Bicycle Wheel Roof: 1. By creating a self balancing system composed of members in direct stress, an efficient structure for gravity loads is created to cover large circular areas. 2. In this case number of radial cables having sag of 1:8 to 1:10 3. Radical cables connect a lower circular tension ring of steel to an outer compression ring of concrete, which is supported on vertical supports. 4. Roof consists of prefabricated trapezoidal slab, which are attached to radial cables by hooked end of their reinforcement.

Cable Stayed Roof

Cable Stayed Roof • Bicycle Wheel Roof: 5. Cables are over stressed by placing of sand bags or blast bags on the slabs, thereby producing extra tension in cable, after this the radial and circular joints between these slabs are filled with cement mortar, on setting of cement mortar the entire roof becomes monolithic concrete dish. 6. On removal of extra load placed on slab the cables tend to shorten but are prevented from doing the same by monolithic RCC roof in which their bars are embedded there by prestressing the cables. 7. The drainage of this type of roof is obtained by pumping rain water to drain pipes located on outer rim of roof.

Arches • Arches are designed to support vertical loads primarily by axial compression. They use their curvilinear form to transform the vertical forces of a supported load into inclined components and transmit them to abutments on either side of the archway.

Arch Roofs • Vault and Groin Vault: • A vault is an application of the arch extended horizontally in two dimensions, the groin vault is the intersection of two vaults.

Arch Roofs • Barrel Roof: • A series of parallel arches connected by transverse roof elements (beams/ Slabs) constitutes a barrel roof. • At the ends the arches are supported on buttress or a continuous fixed RCC beam.

Arch Roofs • Diagonal Arch Roof: • To cover a rectangular area arches are placed along the diagonals of the area with roof material supported on these arches.

Arch Roofs • Radial Arch Roof: • Circular areas with curved boundaries may be provided with radial arches. • Arches may be by peripheral tension ring.

Arch Roofs • Lamella Roof: • This roof is suitable for rectangular areas, in this case a series of parallel arches skewed to the sides of rectangle are intersected by another set of skewed series of parallel arches.

Pneumatic structures • Pneumatic structures are membrane structures, which acquire stabilization through internal pressurization. • This is achieved by varying the internal and external pressure inside an enclosed volume. • Membrane is a thin sheet of material which is so thin that it can not develop any bending stresses but can only develop in plane tension like a piece of cloth or a sheet or paper.

Types of Pneumatic Structures • Air Supported Structures • Air Inflated Structures • Hybrid Structures

Air Supported Structures • An air supported structure consist of a single membrane enclosing the functional space. • Internal air pressure is slightly higher than atmospheric pressure to maintain tension in membrane, small pressure difference between inside and out side cause no discomfort to the people working in these structures.

Air Supported Structures • Air supported structure must be equipped with edge anchoring system that ties membrane firmly to ground to prevent any leakage that is to have an air tight seal. • The foundation should be designed to resist both vertical and horizontal forces to ensure its anchorage to ground. • For large span structures base containment ring is used to resist these forces, for low profile structure the horizontal forces are directed inward hence base containment ring will be in compression.

Air Supported Structures • Air supported pneumatic structure is a low energy structure internal pressure and stress in membrane is low, so incase of puncture the tearing of membrane does not propagate rapidly, rather the effect is localized.

Air Inflated Structures • Air Inflated structure is supported by pressurized air contained within building elements. • The internal volume of building remains at atmospheric pressure. • Structural forms of this structures form stabilized wholly or mainly by pressure differences of gases, liquid etc materials in bulk.

Air Inflated Structures • Inflated rib structure is made of a series of inflated tubes usually in the form of an arch and Dual wall Structure consist of parallel membrane system . • Air Inflated Structures is high energy system when punctured seems literally explode, crack caused by puncture propagates very rapidly. • Use of Materials – Fiberglass , Polyester, PVC & Teflon.

Hybrid Pneumatic Structures • Combine both structural qualities, like spanning potential of air supported structures and dual wall system for insulation and safety against collapse from air – inflated structure.

Advantages of Pneumatic Structures: • Elegant and dramatic design. • Most efficient structural form available in term of weight and material. • Combines tensile strength of materials with the structural efficiency of the shell form. • Able to provide free span coverage for a large area. • Supported by air, so requires no columns or beams. • Great roof heights can be achieved. • Portable structures, can be created / dismantled easily. • Lower cost of construction and supporting material.

Disadvantage of Pneumatic Structures: • Requires uninterrupted supply of air. • People , equipments are required to enter and leave the building through air locks only. • Life of available skin member is shorter then that of conventional building material. • Limitations in terms of shapes of structures. • Conventional shapes generally obtained by revolution about one axis. • Unintentional deflection due to accumulation of snow.