concrete

Basic Structural Elements

An RC structure consists of different structural elements. It may also contain non-structural elements such as partitions and false ceilings. The function of any structure is to resist the applied loads effectively and to transmit the resulting forces to the supporting ground without differential settlement. At the same time the structure should also satisfy the serviceability requirements, be durable and should not pose problems of maintenance.

Footings:

Footings distribute the load they receive from columns or walls to the soil underneath in such a way that settlement, particularly uneven or relative settlement of the structure is limited and failure of the underlying soil is avoided. Hence, the size of footings is so chosen that the pressure under them is less than the allowable bearing pressure of the soil.

Among several types of RC footings in common use are the wall, isolated, spread, combined, raft foundations, pile, and pile cap types.

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Fig 1 – Types of footings.

Columns:

Columns are vertical structural elements that transfer the load from the beams to the foundations. When they carry only axial load they are are called axially loaded columns. However, in actual practice, there are no perfect axially loaded columns. Due to the eccentricity of loads, imperfections in their construction, and so forth, there may be secondary moments in the columns. There may be bending moments due to rigid frame actions or lateral loads. Such columns with large bending moments are called beam columns.

Reinforced concrete columns with rectangular or square shapes are often used because of the simplicity of constructing the formwork. However, when they are used in open spaces circular shapes are attractive. Square and Circular shapes are also preferable in high earthquake zones. L, Plus, T shaped columns are also used on rare occasions.

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Fig 2 – Different shapes of Columns.

Beams:

 Beam is a structural element that is primarily subjected to bending. Beam support the slabs and transfer the loads to the columns. Secondary beams may transfer the load to main beams, which in turn transfer the load to columns. RC beams are normally cast monolithically with slabs. As a result, the two parts act together to resist the loads. Hence, though beams are normally rectangular, some extra slab width at the top called flange, is assumed to act together in the design. The resulting beams are called L beams or T beams.

The most efficient cross section for a simply supported beam is in I or H section beam. I section and box beams are normally adopted in bridges.

Normally RC beams are designed for bending moments and shear forces. Longitudinal reinforcements are provided to resist the tension produced by the bending moments and stirrups are provided to resist the shear forces.

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Fig 3 – Different types of beams

Walls:

Walls are vertical elements and are of RC construction. Walls may be different types such as load-bearing walls, shear walls, retaining walls and partition walls. When they support gravity loads in buildings, they are called load-bearing walls, and when they resist lateral loads due to wind or earthquake, they are called Shear walls.

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Fig 4 – Shear wall

When walls are provided as non-structural dividing elements, their thickness is decided based on sound insulation and fire resistance requirements, and only nominal reinforcements are provided.

Walls which are used to retain earth is called Retaining Walls, these are vertical walls and are situated where there is abrupt changes in ground levels.The design of wall requires knowledge of later loads  exerted by  the soil.

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Fig 5 – Retaining Wall

Trusses:

A truss is essentially a triangulated system of (usually) straight interconnected structural elements.The principal force in each element in a truss is axial tension or compression.Trusses are used in a broad range of buildings, mainly where there is a requirement for very long spans, such as in airport terminals, aircraft hangers, sports stadia roofs, auditoriums and other leisure buildings.Trusses are also used to carry heavy loads and are sometimes used as transfer structures.

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Fig 6 – Typical Truss.

Slabs:

Buildings and bridges require a floor slab to provide protection for occupants and for the vehicles to pass safely through. Concrete is the ideal material of choice for the slab because its mass and stiffness can be used to reduce deflections and vibrations of the floor system and to provide the required fire protection. Slabs can be simply supported, continuous or cantilevered. Slabs are supported on beams which are in turn supported on columns.

Slabs are classified as one way, two way, flat plates, flat slabs, waffle slabs and ribbed slabs.

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Fig 7 – Two way slab with beams.

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