OUTLINE: Introduction Shoring Process Effective Beam Flange Width Shear Transfer Strength Of Steel Anchors Partially Composite Beams Moment Capacity Of Composite Sections Deflection Design Of Composite Sections

1. COMPOSITE CONSTRUCTION:
COMPOSITE BEAM
1
Presented By
Eng. ZEINAB AWADA
Email
Zeinabawada198@gmail.com
MASTER OF CIVIL ENGINEERING -LEBANON
ADVANCED STRUCTURAL SYSTEMS COURSE
Sat, Oct 16-2021

2. OUTLINE
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❑ Introduction
❑ Shoring Process
❑ Effective Beam Flange Width
❑ Shear Transfer
❑ Strength Of Steel Anchors
❑ Partially Composite Beams
❑ Moment Capacity Of Composite Sections
❑ Deflection
❑ Design Of Composite Sections

3. ❖ What is a composite section !
1. INTRODUCTION
• A composite structure is made up of materials with different
strength acting together.
• If a concrete slab is supported by steel beams & If there is no
provision for shear transfer between the slab and the beam
→ Non-composite section
• When no bond exists between both, the load carried by the slab is
small and may be neglected.
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4. ❖ Advantages of composite construction
• The floor serves as a large cover plate for the upper flange of the steel
beam → increasing the beam’s strength.
• Longer spans for same sections.
• Composite sections have greater stiffness than non-composite sections.
• These sections have smaller deflections.
• Ability of composite structure to take overload is decidedly greater than for
non-composite structure.
• Increasing the load-carrying capacity of an existing floor system can be
handled by welding cover plates onto the bottom flanges of the beam.
• Possibility of having smaller overall floor depths – this is important for tall
buildings
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1. INTRODUCTION

5. ❖ Some types of composite constructions
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A. Composite building floors:
i. Floors encased in concrete: very rare due to expense.
ii. Non-encased with shear connectors: mostly used nowadays.
iii. Formed steel deck: used for almost all composite building floors
1. INTRODUCTION

6. 6
1. INTRODUCTION
❖ Composite vs non composite beam !
• For non composite beam:
Slip between two materials occurs due
to horizontal shear
• For composite beam:
No slip between 2 materials because
the shear studs resists the horizontal
shear

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❖ Elastic stresses in composite Beams !
1. INTRODUCTION
• Available strength of composite beams is usually based on conditions at
failure.
• In some cases the available strength is based on limit state at first yield.
• Flexural and shearing stresses in beams of homogenous materials can be
computed from the formulas:
• But a composite section is not homogeneous, so these formulas are not
valid.
• To be able to use them, an artifice known as transformed section is
employed to convert the concrete into amount of steel that has the same
effect as the concrete.
• This procedure requires the strain in the fictitious steel to be same as those
in concrete it replaces.
• Ec = modulus of elasticity of concrete
• n= Es/Ec = modular ratio

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❖ Elastic stresses in composite Beams !
1. INTRODUCTION
• “n in2” of concrete are required to resist the same force
as “ 1 in2” of steel.
• To determine this area of steel that will resist the same
force as the concrete, divide the concrete area by n,
replace Ac by Ac/n, the result is the transformed area.
• In the Determination of the effective flange width “b”,
first we should transform the concrete area Ac, the
most convenient way to do this is to divide the width
“be” by n and leave the thickness unchanged.
• To compute stresses in the homogenous steel section,
we calculate the neutral axis of this shape and compute
then the corresponding moment of inertia.
• M = applied bending moment
• Itr = moment of inertia about neutral axis
• Yt = distance from the neutral axis to the top of steel
• yb = distance from the neutral axis to the bottom of steel
• Ybar = distance from neutral axis to the top of concrete
• The bending stresses at the top of steel:
• The bending stresses at the bottom of steel:
• The stress in the concrete :

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❖ Flexural strength !
1. INTRODUCTION
• In most cases the nominal flexural strength will be reached when the
entire steel section yields and the concrete crushes in compression
(plastic stress distribution).
• For shapes with compact webs that is
The nominal strength Mn is obtained from the plastic stress distribution
• For shapes with that is
The elastic stress distribution corresponding to first yield of the steel
• For LRFD . Φb = 0,9 & For ASD Ω = 1,67
• When a composite beam has reached the plastic limit state, the stresses
will be distributed in one of the three ways shown to the right.

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2. SHORING PROCESS
Case 1
no shoring is used
steel beams must support all the loads as well
as their own weights.
Case 2
shoring is used
shoring supports the weight of wet concrete and
other construction loads
• Beams must be designed with enough
strength and stiffness to support wet
concrete and construction loads.
• Most specifications say that after the
concrete has gained 75% of its 28-day
strength → the section has become
composite
• Shoring does not support the weight of the
steel beams.
• When shoring is removed (after 75%) → weight
of the slab is transferred to the composite
section and not just to the steel beams.
• will be possible to use lighter and thus cheaper
steel beams.

11. • Camber is a curvature built into a member or structure so
that when it is loaded, it deflects to a desired shape.
• Camber is usually designed to compensate for deflections
caused by pre-composite dead loads.
• Cambering can be designed to compensate for either:
i. Certain percentage of dead load deflection.
ii. Full dead load deflection.
iii. Full dead load deflection as well as a % of live load deflection.
• Deflections of un-shored floors due to the wet concrete
sometimes can be quite large. → use cambering.
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2. SHORING PROCESS
❖ What is cambering of beams !

12. Case 1
Beams are closely spaced
Case 2
Large spacing between beams
bending stresses in the slab will be fairly uniformly
distributed across the compression zone
bending stresses will vary quite a bit nonlinearly
across the flange.
• The further a particular part of the slab or flange is away from the steel
beam → the smaller will be its bending stress.
• Specifications attempt to handle this problem by replacing the actual
slab with a narrower or effective slab that has a constant stress.
• This equivalent slab is considered to support the same total
compression as is supported by the actual slab.
• The portion of the slab or flange that can be considered to participate in
the composite beam action is controlled by the specifications. 12
3. EFFECTIVE FLANGE WIDTH

13. 1. AISC specifications I3.1a
The effective width of the concrete slab on each side of the beam center
line shall not exceed the least of the values to follow.
• The distance from the beam center line to the edge of the slab.
• 1/8 of the span of the beam measured C-C of supports for both simple and continuous spans.
• 1/2 of the distance from the beam center line to the center line of the adjacent beam (b/2).
❖ Determination of effective flange width
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3. EFFECTIVE FLANGE WIDTH
2. AASHTO
The Maximum total flange width could be distinguished as follows:
• May not exceed:(1/4 of the beam span, 12x the least thickness of the slab, or the distance C-C of the beams).
• If the slab exists only on one side of the beam, hence Its effective width may not exceed 1/12 of the beam span, 6x
the slab thickness, or 1/2 of the distance from the center line of the beam to the center line of the adjacent beam.
• The portion of the slab or flange that can be considered to participate in the
composite beam action is known as effective flange width

14. ❖ How is the longitudinal shear transferred?
Case 1
Beams are encased
Case 2
Beams are not encased
It can be transferred between the
two by bond and shear (and
possibly some type of shear
reinforcing), if needed.
Mechanical connectors transfer
the loads.
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4. SHEAR TRANSFER
Case 3
Case of Bridges
steel anchors are designed to
resist all of the shear between
bridge slabs and beams

15. ❖ Types of steel Anchors
• Various types of steel anchors have been tried, including
spiral bars, channels, zees, angles, and studs.
• Economic considerations have usually led to the use of round
studs welded to the top flanges of the beams.
• These studs are available in diameters from ½ to 1in and in
lengths from 2 to 8in, but AISC specifications (I8.2) states that
their length may not be less than 4stud diameters.
• This specification also permits the use of hot-rolled steel
channels, but not spiral connectors.
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4. SHEAR TRANSFER

16. • Shop installation initially is more economical
• Anchors may easily be damaged during transportation and setting of the beam.
• Anchors serve as a hindrance to the workers walking along the top
flanges during the early phases of construction.
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4. SHEAR TRANSFER
❖ Shop vs field installation of steel anchors
• If the plastic neutral axis (PNA) falls in the slab
→ maximum horizontal shear (or horizontal force on the plane between
concrete and steel) is said to be AsFy
• If the PNA is in the steel section → maximum horizontal shear
is considered to be equal to 0,85 f ’cAc.
❖ Maximum horizontal shear

17. AISC (I3.2d) states that:
For composite action, total horizontal shear between the points of maximum
negative moment and zero moment is to be taken as the least of the following,
where ƩQn is the total nominal shear strength of the steel anchors:
• For concrete crushing: 𝑉 ′ = 0.85𝑓′𝐴c
• For tensile yielding of the steel section: 𝑉 ′ = 𝐹𝑦𝐴𝑠
For hybrid beams: yield force must be calculated separately for each of the
components of cross section.
• For strength of steel anchors: 𝑉 ′ = Ʃ 𝑄n
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4. SHEAR TRANSFER
❖ Shear to be taken by the anchors

18. Manual calculation:
• Nominal shear strength in kips of one stud steel anchors embedded in
a solid concrete slab is to be determined by this eq.
Normal shear strength:
• Fu : specified minimum tensile strength of the steel stud in ksi (MPa).
• Ec : modulus of elasticity of concrete in ksi(MPa).
• w: unit weight of concrete in lb/ft3.
• Asa: cross-sectional area of the anchors in (in2).
• f ’c : specified compressive stress of concrete in ksi.
• Rg : coefficient to account for group effect.
• Rp : position effect factor for shear studs.
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5. STRENGTH OF STEEL ANCHORS
❖ Steel headed stud anchors ! According to AISC specification I8.2a

19. Tabular Values
Values of Qn are listed in Table 3-21 in AISC manual.
These values are given for different stud diameters,
for 3 and 4 ksi normal and lightweight concrete
weighing 110 lb/ft3, and for composite sections with
or without steel decking.
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❖ Steel headed stud anchors !
5. STRENGTH OF STEEL ANCHORS

20. ❖ What is partially composite section !
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6. PARTIALLY COMPOSITE BEAMS
• The partially composite section, is one that does not have a
sufficient number of anchors to develop the full flexural strength
of the composite beam and to completely prevent slip between
the concrete and steel.
• for all partial composite beams, the PNA is located within the
steel member. Since there is not enough shear studs to transfer
the total compressive force into the slab, there must be some
steel in compression and therefore making the PNA within the
steel.
• Whenever a fully composite beam has excess capacity, the design
can be fine-tuned by eliminating some of the studs, thereby
creating a partially composite beam

21. 8.MOMENT CAPACITY OF COMPOSITE SECTIONS
• The plastic neutral axis may fall in the slab or in the flange
of the steel section or in its web.
• h : the distance between the web toes of the fillet: h=d-2k
• tw : web thickness.
• Fyf : yield stress of the beam flange.
• E: modulus of elasticity of steel = 29000 ksi (200 GPa).
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❖ The nominal flexural strength of a composite beam !
• The positive nominal flexural strength, Mn of a
composite section is to be determined, assuming a
plastic stress (Mn=Mp) distribution if:

22. ❖ Neutral axis in concrete slab
• If a is equal to or less than the slab thickness → PNA will fall in the slab
• The concrete slab compression stresses vary somewhat from the PNA out to the top of the slab.
• For convenience in calculations, they are assumed to be uniform, with a value of 0,85 f ’c over an
area of depth a and width be (effective flange width).
• The value of a can be determined from the
following expression, where the total tension in the
steel section is set equal to the total compression in
the slab:
T = C →
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8.MOMENT CAPACITY OF COMPOSITE SECTIONS
plastic moment capacity

23. • If a is calculated and is greater than the slab thickness t
→ PNA will fall down in the steel section.
• If this happens → it will be necessary to find out whether
PNA is in the flange or below the flange.
• Suppose, we assume that it’s at the base of the flange.
• We can calculate the total compressive force C above the
PNA:
• Total tensile force can be calculated as follows:
Af : area of the flange → 𝐴𝑓 = 𝑡𝑓.𝑏𝑓 23
8.MOMENT CAPACITY OF COMPOSITE SECTIONS
❖ Neutral axis in top flange of steel beam

24. ❖ Neutral axis in top flange of steel beam
• If C > T → PNA will be in the flange.
• If C < T → PNA is below the flange.
• Assuming that we find the PNA is in the flange, we can determine its location, letting 𝑦 be the
distance to the PNA measured from the top of the top flange, by equating C and T
• Y can be calculated as follows:
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8.MOMENT CAPACITY OF COMPOSITE SECTIONS
• Then the nominal or plastic moment capacity of the section:

25. • If we find that a is larger than the slab thickness, and if we then assume
that the PNA is located at the bottom of the steel flange and we calculate
C and T and find T is larger than C → PNA will fall in the web.
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8.MOMENT CAPACITY OF COMPOSITE SECTIONS
❖ Neutral axis in web of steel section
• Then the nominal or plastic moment capacity of the section:
• Y can be calculated as follows:

26. ➢ For common cases of simple-span beams and I-shaped
members and channels, the maximum vertical deflection Δ:
❖ Deflection !
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• M=maximum service load moment, kip-ft
• L=span length, ft
• Ix= moment of inertia, in4
• C1=loading constant which includes the
numerical constants appropriate for the
given loading pattern, E (29000 ksi)
9.DEFLECTIONS
Case 1:Unshored composite construction:
• Final deflections will equal the initial deflections caused by wet
concrete calculated with moments of inertia of the steel
beams, plus the deflections due to the loads applied after
concrete hardens, calculated with moments of inertia of the
composite sections.
Case 2: Shored composite construction:
• All deflections will be calculated with moments of inertia of
the composite section.

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• When W section is insufficient to support the loads anticipated for certain span,
several possible alternatives may be taken, Perhaps the most economical solution:
1. the use of one of a higher strength steel W section.
• If this is not feasible, we may make use of one of the following:
1. two or more regular W sections side-by-side (an expensive solution)
2. a cover-plated beam
3. a built-up girder
4. a steel truss.
10.DESIGN OF COMPOSITE SECTIONS
❖ Cover plates Why !

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• An expression for the required area of one flange cover plate can be
developed as follows
• The total Z of the built-up section must at least equal the Z required.
• It will be furnished by the W shape and the cover plate as follows:
10.DESIGN OF COMPOSITE SECTIONS
❖ Cover plates Area!

29. ❖ Lower bound moment of inertia !
Manual
• The lower bound moment of inertia is computed as follows:
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10.DESIGN OF COMPOSITE SECTIONS
Tabular
• a table of lower bound moment of inertia values
is presented to the right.

30. THANK YOU
30
THANK YOU
Hope you enjoyed my presentation !
Contact me for any help
zeinabawada198@gmail.com

31. ✓ Structural steel design,5th edition, jack C.Mc Cromac,S tephan F. Csernak
✓ American institute of steel construction, Steel construction manual, 14th edition
✓ Chapter 9
❖ References
31

32. 32
• Note, however, these numerous advantages for composite beams is only true when the concrete is subject to
compressive forces. Concrete is a brittle material whose tensile strength is quite variable and fractures (or
cracks) with little or no warning. As a result, concrete is assumed to have no tensile strength and is ignored
when it is in tension.
• On the other hand, concrete is very good in compression. This means that composite action is only of benefit
in POSITIVE moment regions where concrete is on the compression side of the beam.
• In negative moment regions, where the concrete is on the tension side of beam, the concrete adds nothing
to the strength or stiffness of the steel beam.
• Since composite action is of most benefit to simply supported, single span beams where all the moment on
the section is positive.
• For continuous beams, the maximum moments tend to be negative and located over supports. Beams sized
for these negative moments will tend to more than adequate to handle the positive moments so composite
action is not required.
NOTES

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• The cover plate must extend, at a minimum, over the
distance where the moment demand exceeds the
moment capacity provided by base section without the
cover plates.
• In the example shown the moment capacity of the base
section is sufficient near the ends of the member where
the moment is low but needs to be enhanced where
moment demand exceeds the capacity of the base
section. The intersection of the moment demand and
moment capacity curves can generally be determine
mathematically since it is possible to write equations for
both curves
NOTES

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• By statics, V' equals Cc and/or Ts. When the plastic neutral axis is in the steel beam, Cc is at its
maximum value (Cc = .85f'cAc) since Ac is at its maximum value. When the plastic neutral axis is
in the slab, Ts is at its maximum value (Ts = AgFy) since the entire section is in tension. Since
Cc always equals Ts when the plastic neutral axis is in the slab and is at its maximum when the
plastic neutral axis is located in the beam, the maximum value of V' will be the lesser of the
maximum values of Cc or Ts. The smaller value controls.
• For a section to be "fully composite" the shear connectors must provide strength that equals or
exceeds the maximum V' resulting from concrete crushing (Ccmax) or tensile yielding (Tsmax) as
discussed above. If the shear strength provided by the shear connectors (V' = sum of the
strength of the shear connectors located between the location of maximum and zero moments)
is less than what can be developed by concrete crushing or tensile yielding then the section is
said to be "partially composite" and it's strength must be determined by the limit imposed by
the shear connector strength.
NOTES