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Lecture 12: Composite Beams . By: Prof Dr. Akhtar Naeem Khan chairciv@nwfpuet.edu.pk. Composite Beam. Floor construction in buildings and bridges often consists of a reinforced concrete slab supported on steel beams. Composite Beam.
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Lecture 12: Composite Beams By: Prof Dr. Akhtar Naeem Khan chairciv@nwfpuet.edu.pk
Composite Beam • Floor construction in buildings and bridges often consists of a reinforced concrete slab supported on steel beams
Composite Beam • Earlier it was assumed that beams act independently of the floor slab, because the natural bond cannot be depended upon to develop the shear VQ/I on interface between slab and beam. • If the beam is completely encased in concrete or a mechanical bond established by means of shear connectors, the two will act as a unit.
Composite Beam Supported and Unsupported Construction • During construction, steel beams are 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).
Composite Beam Supported and Unsupported Construction • If the beams are 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 are 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.
Composite Beam Supported and Unsupported Construction • 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 is definitely known, assume that unsupported construction methods were used.
Composite Beam Equivalent Flange Width • A composite floor is 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/Ecwhere Ec=57,000/f’c • n= 500 /f’c
Composite Beam Effective Flange Width: AASHTO
Composite Beam Effective Flange Width: AASHTO For Interior Girder Effective Flange Width lesser of: For Exterior Girder Effective Flange Width lesser of:
Composite Beam Effective Flange Width: AASHTO 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
Composite Beam Effective Flange Width: AISC/ASD & AISC/LRFD 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
Design of Composite Beam AISC allows two methods of design Method 1 • The beam may be sized assuming the steel sections to carry all loads applied prior to hardening of concrete and composite section to carry all dead and live loads acting after the concrete has hardened. If shoring is used, all loads are assumed to be resisted by composite section. Method 2 • The steel section alone may be proportioned to resist the positive moment due to all loads. If this method is used, shoring is not required.
Design of Composite Beam ASIC/ASD • Allowable stress 0.66Fy ………method 1 0.76Fy ………method 2 AISC/LRFD • b = 0.9 for method 1 and method 2 • Dead-load & Live- load factors are used
Design of Shear Connectors • The horizontal shear at the slab beam junction must be resisted by using shear studs to ensure composite action • Bond between concrete slab and steel beam can not be relied upon • Therefore mechanical shear connectors are required at the slab/ beam interface
Design of Shear Connectors Shear Strength of a single shear connector is given by: Qn = Nominal Strength of one stud Asc = Sectional area of stud in sq. in. fc’ = Compressive strength of concrete Ec = Modulus of elasticity of concrete Number of connectors required is given by: Which are to be provided between pt of Max moment to zero moment on each side of Mmax
Design of Composite Beams Deflection Considerations • If the dead loads are taken up by the steel beam, then the Dead Load Deflection is: • The Live load deflections are assumed to be resisted by composite action and moment of inertia of composite section is used for the purpose:
Design of Composite Beam ASIC/ASD • Estimate As based on ultimate moment with FOS=2.2 • Allowable stress in steel section 0.66Fy. • Allowable stress in concrete 0.45fc’.
Design of Composite Beam ASIC/ASD • Section properties for in unshored construction are computed by elastic theory. • Bending stress in beam is the sum of (1)Dead-load moment (that the steel beam alone resists) (2)Live-load moment (that the composite beam resists). • Allowable stress is 0.9Fy
Design of Composite Beam ASIC/LRFD • A good estimate of required As of steel section based on ultimate moment is given by
Design of Composite Beam ASIC/LRFD Positive moment • For plastic stress distribution on composite section hc/tw 640/Fy …………………..b=0.85 • For hc/tw > 640/Fy…………………..b=0.90 Negative moment • Mn is based on steel section alone …….b=0.90 • For plastic stress distribution…………….b=0.85
Design of Composite Beam ASIC/LRFD Steel section for unshored construction must be designed to support all loads applied before the concrete attains 75% of specified fc’
