Composite Construction of Buildings
by Chandresh Choudhary
Introduction – Steel and concrete
- These two materials complete one another:
- Concrete is efficient in compression and steel in tension
- Concrete encasement restrain steel against buckling
- Concrete provides Protection against corrosion and fire
- Steel bring ductility into the structure
Composite Construction of Buildings refers to any members composed of more than one material. The parts of these composite members are rigidly connected such that no relative movement can occur. The main composite elements in buildings are 1.Steel Concrete Composite Beam 2.Composite Slab 3.Composite Column
Steel Concrete Composite Beam
Composite beams are normally hot rolled or fabricated steel sections that act compositely with the slab. The composite interaction is achieved by the attachment of shear connectors to the top flange of the beam. These connectors generally take the form of headed studs. The composite action increases the load carrying capacity and stiffness of the beam by factors of up to 2 and 3.5 respectively. It is normally designed to be unpropped during construction, and must be sized to support the self-weight of the slab, and other construction loads, in their non-composite state. Size of the steel section is governed by serviceability considerations because composite beams tend to be used for long span applications. Check that beam deflections during construction will not lead to significant additional concrete loads (due to ponding) that have not been allowed for in the design.The bending resistance of the section is normally evaluated using ‘plastic’ principles The plastic moment resistance is calculated using idealized rectangular stress Blocks. It is assumed that stresses of fyd and 0.85 fcd can be achieved in the steel and concrete respectively.
Composite beams are generally shallower (for any given span and loading) than non-composite beams, and they are used commonly in long span applications.
Shear Connectors These connectors are designed to
- Transmit longitudinal shear along the interface
- prevent separation of steel beam and concrete slab at the interface
- most common type of shear connector used in composite beams for buildings is a 19 mm diameter by either 100 mm or 125 mm long welded stud.
- efficiency of the shear connectors is reduced when the decking is orientated with the ribs transverse to the beam
- The property of shear connector most relevant to design is the relation-ship between the shear force transmitted, P, and the slip at the interface, s This load-slip curve should ideally be found from tests on composite beams.