N.W.F.P. University of Engineering and
Technology Peshawar
Lecture 12: Composite Beams
By: Prof Dr. Akhtar Naeem Khan
chairciv@nwfpuet.edu.pk
1
Composite Beam
 Floor construction in buildings and bridges
often consists of a reinforced concrete slab
supported on steel beams
CE-409: Lecture 12
Prof. Dr Akhtar Naeem Khan
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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.
CE-409: Lecture 12
Prof. Dr Akhtar Naeem Khan
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Composite Beam
Supported and Unsupported Construction
 During construction, steel beams are placed
on the supports with cranes. The concretedeck 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).
CE-409: Lecture 12
Prof. Dr Akhtar Naeem Khan
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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.
CE-409: Lecture 12
Prof. Dr Akhtar Naeem Khan
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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.
CE-409: Lecture 12
Prof. Dr Akhtar Naeem Khan
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Analysis and Design
methods of Composite
beams
CE-409: Lecture 12
Prof. Dr Akhtar Naeem Khan
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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/Ec where Ec=57,000/f’c
 n= 500 /f’c
CE-409: Lecture 12
Prof. Dr Akhtar Naeem Khan
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Composite Beam
Effective Flange Width: AASHTO
CE-409: Lecture 12
Prof. Dr Akhtar Naeem Khan
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Composite Beam
Effective Flange Width: AASHTO
For Interior Girder Effective Flange Width lesser of:
L
Be 
4
, or
Be  Bo
For Exterior Girder Effective Flange Width lesser of:
L
Be   (Distance from Beam Center to Edge of Slab)
8
1
Be  Bo  (Distance from Beam Center to Edge of Slab)
2
CE-409: Lecture 12
Prof. Dr Akhtar Naeem Khan
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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
CE-409: Lecture 12
Prof. Dr Akhtar Naeem Khan
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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
CE-409: Lecture 12
Prof. Dr Akhtar Naeem Khan
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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.
CE-409: Lecture 12
Prof. Dr Akhtar Naeem Khan
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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
CE-409: Lecture 12
Prof. Dr Akhtar Naeem Khan
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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
CE-409: Lecture 12
Prof. Dr Akhtar Naeem Khan
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Design of Shear Connectors
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Prof. Dr Akhtar Naeem Khan
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Design of Shear Connectors
Shear Strength of a single shear connector is given by:
Qn  0.5 Asc
fc' Ec  Asc Fub
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
Vnh
N
Qn
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
CE-409: Lecture 12
Prof. Dr Akhtar Naeem Khan
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Design of Composite Beams
CE-409: Lecture 12
Prof. Dr Akhtar Naeem Khan
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Design of Composite Beams
Deflection Considerations
 If the dead loads are taken up by the steel beam,
then the Dead Load Deflection is:
 DL

5 w L4

384 Es 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:
 LL
CE-409: Lecture 12
5 wL L4

384 Es Icomp
Prof. Dr Akhtar Naeem Khan
 Total
1

L
350
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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’.
CE-409: Lecture 12
Prof. Dr Akhtar Naeem Khan
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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
CE-409: Lecture 12
Prof. Dr Akhtar Naeem Khan
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Design of Composite Beam
ASIC/LRFD
 A good estimate of required As of steel
section based on ultimate moment is
given by
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Prof. Dr Akhtar Naeem Khan
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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
CE-409: Lecture 12
Prof. Dr Akhtar Naeem Khan
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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’
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Thanks
Design Example 1 ASD
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Design Example 1 ASD
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Design Example 1 ASD
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Design Example 1 ASD
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Design Example 1 ASD
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Design Example 1 ASD
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Design Example 1 ASD
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Design Example 1 ASD
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Design Example 1 ASD
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Design Example 1 LRFD
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Prof. Dr Akhtar Naeem Khan
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Design Example 1 LRFD
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Design Example 1 LRFD
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Design Example 1 LRFD
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Design Example 1 LRFD
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Prof. Dr Akhtar Naeem Khan
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Design Example 1 LRFD
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Thanks
CE-409: Lecture 12
Prof. Dr Akhtar Naeem Khan
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