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Composite Construction

Introduction

  • Composite construction refers to two load-carrying structural members that are integrally connected and deflect as a single unit.
  • Composite construction provides a method of using two materials together so as to utilize each material to its best advantage. Some of the construction problems associated with the normal steel-concrete composite beams are examined, including the placing sequence of deck concrete and the monitoring of deflections to insure proper slab thickness. New approaches to steel-concrete composite construction include a beam and column system and a box girder utilizing the void space as an air conditioning duct. Other types of composite construction are mentioned, including the timber-concrete slab bridge, a timber-steel composite truss utilizing high-strength bridge strand as the lower chord, a wood-steel open web joist which provides naiable chords, and light gage metal decking with a concrete slab.
  • Construction, as defined herein, is the use of a cast-in-place concrete slab placed upon and interconnected to a prefabricated beam, so that the combined beam and slab will act together as a unit. The prefabricated beam may be  a rolled or built-up steel shape, a precast reinforced concrete beam, a prestressed concrete beam, a timber beam, or even light-gauge steel decking. The interconnection to obtain the single unit action is by combinations of mechanical shear connectors, friction, and shear keys.
  • The most important and most frequently encountered combination of construction materials is that of steel and concrete, with applications in multi-storey commercial buildings and factories, as well as in bridges.
  • This has become a standard type of construction in high rise buildings selected by many architects, engineers and developers.

The Principle of Composite Action

  • The principle of composite action underpins the use of composite materials in construction. It relates to the interaction of two or more separate elements acting together and contributing together rather than separately. By physically connecting them, the strength of the beams and the resistance to bending, shear and torsion are significantly increased.

Why composite construction is good??

  • The reason why composite is often so good can be expressed in one simple way-concrete is good in compression and steel is good in tension. By joining the two materials together structurally these strengths can be exploited to result in a highly efficient and lightweight design.
  • The reduced self weight of composite elements has a knock-on effect by reducing the forces in those elements also offer benefits in terms of speed of construction. The floor depth reductions that can be achieved using composite construction can also provide significant benefits in terms of the costs of services and the building envelope.

Benefits of Composite Construction

  • The benefits of composite construction include speed of construction, performance and value. Steel framing for a structure can be erected quickly and the pre-fabricated steel floor decks can be put in place immediately. When cured, the concrete provides additional stiffness to the structure.
  • Additionally, the concrete encasement protects the steel from buckling, corrosion and fire. Service integration within the channels on the composite decks is another advantage to composite construction.
  • Building quality standards can be adhered to easily by the use of pre-fabricated decks. Excessive deflections can be controlled by cambering the beams or by shoring the metal decks to limit deflection when concrete is poured.

Failure of Composite Material

  • Failure under longitudinal compressive loading
  • Failure under longitudinal tensile loading
  • Failure under transverse compressive loading
  • Failure under transverse tensile loading

Advantages of Composite Construction

  • The concrete acts together with the steel to create a stiffer, lighter, less expensive structure.
  • Speed and simplicity of construction- faster to erect, nearly 25% faster then traditional construction.
  • Lighter construction than a traditional concrete building.
  • Less material handling at site.
  • Has better ductility and hence superior lateral load behavior; better earthquake resister.
  • Ability to cover large column free area in buildings and longer span for bridges/flyovers.

Disadvantages of Composite Construction

  • Provide misleading messages about quality if poorly constructed or misinterpreted.
  • Lead to simplistic policy conclusions.
  • Can be misused, if the construction process is not transparent and lacks sound statistical or conceptual principles.
  • Selection of metrics and weights can be challenged by other stakeholders.

Applications

  • In automobile industries.
  • Marine applications like shafts, hulls, spars.
  • Aeronautical application like components of rockets, aircrafts.
  • Communications antennae, electronic circuit boards.
  • Safety equipment like ballistic protection and air bags of cars.

Conclusion

  • The reduction in the dead weight of the steel framed structures is 32% with respect to R.C.C. frame structure and composite framed structure is 30% with respect to R.C.C. framed structure.
  • Axial forces in column have been reduced by average 46% in steel structure and reduced by average 7% composite framed structure as compared to R.C.C. framed structure.
  • Steel and composite structure gives more ductility to the structure as compared to the R.C.C. which is best suited under the effect of lateral forces.
  • Total saving in the composite option as compared to the R.C.C. results in 10% so as with steel it will be 6-7%.

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