Write short notes on Form active systems.
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Answer: 1 THEORY OF STRUCTURES FORM ACTIVE STRUCTURES SUBMITTED TO: ER. BEENA VASHISHT SUBMITTED BY : ANMOL DEEP KAUR 2015ARA007 GUNJAN SHARMA 2015ARA011 NAVNEET KAUR 2015ARA023 SEHAJDEEP KAUR 2015ARA036 ZARNAIN 2015ARA040
2. SINGLE STOREY LONG SPAN STRUCTURE Classification of structural forms: • Form active systems • Vector active systems • Section active systems • Surface active systems FORM ACTIVE STRUCTURAL SYSTEMS . . . are systems of flexible, non-rigid matter, in which the redirection of forces is effected by particular form design and characteristic form stabilization Example of structures: 1. Arch structures 2. Tent structures 3. Pneumatic structures 4. Cable structures 5.Shelled structures 2
3. Parallel cable Radial cable Biaxial cable Illustrated examples of parallel cable structures 3
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5. Examples of cable structures formed by arch Examples of tent structures 5
6. Examples of pnuematic structures 6
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8. ARCH AS FORM ACTIVE STRUCTURE SYSTEM An arch is a curved structure that spans an elevated space and may or may not support the weight above it. Terminology: • KEYSTONE: the wedge shaped, often embellished voussoir at the crown of an arch. • VOUSSOIR: any of the wedge shaped units in masonry arch or vault, having side cuts converging at one of the arch centers. • SPRINGER: the first voussoir resting on the impost of an arch. • EXTRADOS: the exterior curve, surface or boundary of the visible face of an arch. • INTRADOS: the inner curve or surface of an arch forming the concave underside. • SPRING: the point at which an arch, vault or dome rises from its support. • RISE: the height of an arch from the springing line to the highest point of the intrados. • LINE OF THRUST: the set of resultants of thrust and weight each part of an arch imposes on the next lower one. For bending to be eliminated throughout an arch, the line of thrust must coincide with the arch axis. • ARCH AXIS: the median line of an arched structure. • THRUST: the outward force or pressure exerted by one part of a structure against another. • DRIFT: the thrust of an arched structure on its abutment proportional to the total load and span and inversely proportional to the rise. 8
9. TYPES OF ARCHES: Arches have many forms, but all fall into three basic categories: • circular • pointed • Parabolic Arches with a circular form were commonly employed by the builders of ancient, heavy masonry arches. Ancient Roman builders relied heavily on the rounded arch to span large, open areas. Several rounded arches placed in-line, end-to-end, form an arcade, such as the ”Roman aqueduct.” Pointed arches were most often used by builders of Gothic-style architecture. The advantage to using a pointed arch, rather than a circular one, is that the arch action produces less thrust at the base. This innovation allowed for taller and more closely spaced openings, typical of “Gothic architecture”. The parabolic arch employs the principle that when weight is uniformly applied to an arch, the internal compression resulting from that weight will follow a parabolic profile. Of all arch types, the parabolic arch produces the most thrust at the base, but can span the largest areas. It is commonly used in bridge design, where long spans are needed. LOAD MECHANISM: An arch is a pure compression form. It can span a large area by resolving forces into compressive stresses and, in turn eliminating tensile stresses. This is sometimes referred to as ARCH ACTION. As the forces in the arch are carried to the ground, the arch will push outward at the base, called THRUST. As the rise, or height of the arch decreases, the outward thrust increases. In order to maintain arch action and prevent the arch from collapsing, the thrust needs to be restrained, either with internal ties or external bracing, such as abutments. Instead of pushing straight down, load is carried outward along the curve of the arch to the supports at each ends. Abutments carry load and safely transfer it to the ground without spreading it. Since an action has a reaction, ground is squeezed and pushes back on the abutments. Ground pushes back on the abutment creates a resistance which is passed from unit to unit, until it is eventually pushing on the keystone which is supporting the load. 9
s not suitable to resist it.