Composite Beam- Supported Or Unsupported

This Short Article Covers “What Is Composite Beam” Under Supported Formwork Or Unsupported Formwork.

What Is Composite Beam

  • Floor construction in buildings and bridges often consists of a reinforced concrete slab supported on steel beams.
  • Earlier it was assumed that beams act independently on the floor slab, because the natural bond can not depended upon to develop the shear VQ/I on interface between slab and beam.
  • If the beam completely encased in concrete or a mechanical bond established by means of shear connectors, the two will act as a unit.
Composite Beam

Supported Or Unsupported Construction

  • During construction, steel beams placed on the supports with cranes. The concrete-deck formwork is then constructed on top of these beams and the concrete deck is poured.
  • During the deck placement, the steel beams may or may not have beam shoring along their length (supported or unsupported).
  • If the beams shored (supported) until the concrete of the deck cures, the resulting composite beam will be effective for the entire dead load of both the beam and slab, as well as live loads.
  • If the beams unshored (unsupported) during construction, then the steel beam by itself must support its own dead load, and the composite beam section will only be effective for the dead load of the deck and live loads.
  • Normally, the cost of shoring is not practical when compared with the small increase in material costs required for unsupported construction. Unless the method of construction definitely known, assume that unsupported construction methods were used.
Equivalent Flange Width
  • A composite floor assumed to act as a series of T beams.
  • The beams are analyzes by transforming the effective x-sectional area of concrete slab into an equivalent area of steel by the use of modular ratio.
  • n= Es/Ec where Ec=57,000/sq. root f’c
  • n= 500 /sq. root f’c

For Interior Girder Effective Flange Width lesser of,

Be<= L/4 , Or Be<= Bo

For Exterior Girder Effective Flange Width lesser of,

Be<= L/8 + (Distance From Beam Center To Edge Of Slab)

Be<= 1/2 Bo + (Distance From Beam Center To Edge Of Slab)

The effective flange width of slab to be smaller of

  • One-forth of beam span
  • Center to center distance of girders
  • Twelve times the thickness of slab

For girders having a flange on one side only

  • One-twelve of span
  • One-half the distance center to center of next girder
  • Six times thickness of slab

The effective flange width of slab on each side of beam center line must not exceed

  • One-eighth of beam span
  • One- half the Center to center distance of beams
  • For edge beams the distance to the edge of slab

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