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Composite Floor Systems – Background, Design
Approaches, Case Studies
This Presentation is split into three
parts.
(1) The Background which explains
the reasoning for composite floor
systems and the different types of
composite floor systems
(2) The Design which will focus on
the design of composite steel floor
deck and the composite beam with
deck because those are commonly
used types of composite floor
systems.
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(3) The Case Studies which show
how the composite beam with deck
design is modified to accommodate
an innovative composite beam
called the TEC beam.
Bernard Lai and Felix Lam
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Background – Composite Floor Systems
In the construction industry,
composite floor systems typically try
to make use of the qualities of two
materials: concrete and steel.
Concrete is strong in compression
Steel is strong in Tension
By isolating the compression and
tension zones, the efficiency of each
material can be maximized.
Some common composite floor
systems include the Composite
Steel Floor Deck, Composite Beam
with Deck, Composite Open Web
Steel Joists/Trusses, and the StubGirder Floor System.
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Background – Composite Floor Systems
Composite Steel Deck
This is a composite steel deck which
is an efficient one-way slab system.
The loads are transferred along the
flutes. This type of composite
system usually acts as the floor for
steel frame buildings.
Composite Beam with Deck
In a composite beam, the steel
shape, usually W-shapes, is used to
resist tension and shear while the
concrete slab is used to resist
compression.
The advantages of composite beam
construction include savings in steel,
overall reduction of depth, increased
floor stiffness and increased load
capacity.
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Background – Composite Floor Systems
Composite Open Web Steel Joists/Trusses
Composite Open Web Steel
Joists/Trusses is another alternative
for composite flooring systems. It’s
advantage is that there are openings
for mechanical ducting while offering
substantial strength and stiffness.
Stub-Girder Floor System
The Stub-Girder Floor System is an
assembly with a W-shape steel
bottom flange and a concrete deckslab top flange. There are short
intermittent connections of W-shape
steel connected to the top and
bottom cords for shear transfer. This
system allows for easily adjustable
beam depths for mechanical ducting.
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Design – Composite Steel Floor Deck and
Composite Beam with Deck
The design portion of this
presentation will be divided into two
sections.
(1) The design of composite steel
floor deck via deck charts.
(2) The design of composite beams
with deck via the equivalent block
method.
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Design - Composite Steel Floor Deck 1
Steel floor deck is used with concrete topping
The steel deck acts as formwork for the wet
concrete and as positive reinforcement for the
composite slab.
Design and Construction of Composite Floor
Systems by Chien and Ritchie outlines the
following procedure for designing composite floor
deck:

Above: Composite floor deck
before concrete placement.
The rebar is in place and
chaired. Note the indentations
in the deck to make it act in a
composite manner.
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a. “Check deflection under fresh concrete including
ponding (accumulation of concrete due to deck
deflection). A deflection of L/180 or 20mm is a
normal limitation.
b. Check effects due to construction load during slab
pouring.
c. Check effects due to concentrated construction
load
d. Check shear –bond capacity of composite section.
e. Check maximum concrete compressive stress in
composite section.
f. Check maximum steel deck tensile stress in
composite section.
g. Check live load deflection of composite section.”
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Design - Composite Steel Floor Deck 2
Fortunately, when you purchase floor
decks, the manufacturers provide deck
charts which simplify the calculations.
The determination of the deck
capacity from these charts requires
the following information:
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a) The thickness of the slab which
includes the topping and the deck
itself. (A composite slab of thickness
141mm has 76mm deck with a
minimum 65mm topping)
b) The deck gage is given at the
extreme left – the usual choice being
0.91mm deck.
c) The span of the deck - determined
from the layout of beams
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Design - Composite Steel Floor Deck 3
Extra Notes and
Considerations
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If there is no capacity value provided
for a span, the deck needs to be
shored for concrete placement and it is
usually not economical to shore metal
deck for concrete placement. The
contractor usually expects that your
deck design does not require shoring.
The 1-Span, 2-Span and 3-Span refers
to the system when the deck is acting
as a form for wet concrete and
construction live load. The deck acting
alone as a form (non-composite) is
better able to carry loads if it is in a
three–span.
The load capacity of the composite
deck section is not dependant on the
number of spans (after concrete is
hard) – the composite deck is assumed
to be a series of simple span concrete
beams with the deck acting as the
reinforcing steel.
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The different stress regimes in the deck is
shown below:
The composite deck system can carry
much more load than the deck alone. The
full deck section is in tension, and the
leaver arm between the centroid of
compression and tension forces is larger
in the composite system.
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Design - Composite Steel Floor Deck 4
Example of Composite Steel Floor
Deck Design
Verify that VicWest HB308 (75mm deck)
with .91mm thickness and ZF75 Galvanizing
and 65mm cover slab can be used for 3.2m
spans. The deck is designed for a live load
of 4.8 kPa and a partition / mechanical load
of 1.0 kPa. The deck will be placed in a
three spans continuous configuration.
Solution: The first and easiest solution is to
examine the VicWest tables and examine
the load capacity listed.
75mm Deck + 65 Topping = Overall
thickness of 140mm
Span / Depth = 3200 / 140mm = 22.9 (OK
<32)
Determine the specified load:
Live Load
4.8 kPa
Partition / mechanical Load 1.0 kPa
Total Specified Load
5.8 kPa
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Note that the self-weight of the slab
and the deck is NOT included in this
calculation as the charts have
subtracted this weight out when
computing capacity.
Using the VicWest HB308 tables with
ZF75 Galvanizing, with 141mm slab
thickness and 0.91mm deck 3200mm
span 2 or 3 spans continuous get
maximum specified load as 8.8 kPa so
OK.
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Design - Composite Beam with Deck 1
Consists of a concrete slab or
composite deck resting on steel
girders
Calculates not only the decking, but
the girder support as well
Need to check 5 important points:
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Effective width and depth of concrete
Strength of shear connectors if used
Horizontal and vertical shear
capacity of the system
Percent shear transfer provided
Lateral buckling of girder
4 different cases to design for
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Design - Composite Beam with Deck 2
Case 1 – Full shear connection and
plastic neutral axis in slab:
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Qr > ΦAsFy and ΦAsFy < α1Φcb1tcf’c
where;
 Qr is the sum of all factored resistances
of all shear connectors between points of
maximum and minimum moment
 b1 and tc defined in Clause 17.2 and 17.4
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Depth of compression block “a” can be
found by equating Cr’ and Tr
Mrc = Tre’ = ΦAsFye’ where
 e’ = distance between compression block
resultant Cr’ and tension block resultant
Tr
 e’ = d/2 + to + a/2
 to = overall depth of deck+slab
α1 = 0.85-0.0015f’c
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Design - Composite Beam with Deck 3
Case 2 – Full shear connection and
plastic neutral axis in steel web (Cr >
ΦsbtFy) :
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Qr > α1Φcb1tcf’c and α1Φcb1tcf’c <
ΦAsFy
Still have Cr’ which is compressive force
in the concrete, but now have to take
into account compressive force in steel
Cr
Cr’ = α1Φcb1tcf’c
Cr = (ΦAsFy – Cr’)/2
Moment equilibrium:
Mr = Cre + Cr’e’
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Design - Composite Beam with Deck 4
Case 3 – Partial shear connection:
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Qr < α1Φcb1tcf’c and ΦAsFy
Plastic NA always in steel
a = Qr/(α1Φcf’cb1)
Cr = (ΦAsFy – Qr)/2
e and e’ calculated the same as case 2
except with tc replaced with “a”
Mr = Cre + Qre’
What if NA is in steel flange? Simple.
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Redesign beam to avoid this
Trust us calculating e is not pretty
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Design - Composite Beam with Deck 5
How to calculate Qr?
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% shear connection defined as
minimum of Qr / α1Φcb1tcf’c or Qr /
ΦAsFy
Qr is the sum of all factored
resistance of shear connectors
Usually provided by shear studs
 qrs = 0.5ΦscAsc(f’cEc)0.5 < ΦscAscFu
 Only valid for solid slabs
 Fu = 450 MPa in 10th Edition of code
Must check horizontal and vertical shear
capacity of deck
 Horizontal shear defined as:
Vh = ΦAsFy
Vh = α1Φcb1tcf’c
Vh = Qr
For cases 1, 2 and 3 respectively
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For ribbed slabs, easier to just look
at tables provided on page 5-18
 Need to be concerned about width to
height ratios of deck flutes and pull
out area of the studs
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Vertical shear resistance defined by
 Vr = ΦAwFs
Fs defined in clause 13.4.1.1
Equations to calculate qrr found in
clause 17.7.2.2
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Design - Composite Beam with Deck 6
Simplest way is to use lookup tables
in handbook
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Usually design load is known or
given -> Find maximum design
moment
Factored moment resistance given
as a function of % shear connection
and effective slab width b1
For intermediate values, interpolate
Check if beam meets
geometric/layout constraints
Rinse and repeat!
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Case Studies – TEC Beam 1
Composite beams with deck is one of the
most widely used floor system in steel
structures because they require less labor,
no formwork, less quality inspection, and
less construction time. On the contrary,
there are also some disadvantages to the
conventional composite beam because it
requires tremendous depth (vertical shear
studs, fireproofing for exposed steel, and
inefficient use of the top flange). Structures
which use concrete – flat slabs, compared
with composite beams with deck, have the
advantages of decreased storey heights and
less fire-proofing requirements. The concept
of the TEC beam is to merge the qualities of
both precast systems and steel systems by
creating a new type of composite beam.
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In the TEC beam, the top flange is cutoff
because it is inefficient in moment
resistance and it will allow for a shorter
overall depth. Instead of vertical studs
which are attached to the top flange, the
TEC beam has horizontal studs attached to
the web so that there is a further reduction
in overall depth. There is precast concrete
placed between the bottom flange and the
slab which act as fireproofing and to
support the deck. The precast concrete is
integrated to the cast-in-place concrete slab
via minimum spacing stirrups. In fact, the
precast concrete can prevent lateral
buckling of the web because it provides
lateral support.
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Case Studies – TEC Beam 2
Ju and Kim summarize the
advantages of the TEC beam as
follows: “(i) construction cost lower
than that of the reinforced concrete
or steel frame structures; (ii) less
construction time than that of
reinforced concrete structures; (iii)
higher construction quality control
and construction management tha
that of reinforced concrete
structures; (iv) flexible planning; and
(v) lower storey height due to
reduced beam depth.”
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References
Chien, E. Y. L., and Ritchie, J. K., “Design and construction of composite floor
systems”, Canadian Institute of Steel Construction, 1984.
Metten, Andrew. Design of Steel Decks for Vertical Loads UBC Civl 432 –
Advanced Structural Steel Design. 2010.
Handbook of Steel Construction,10th Edition. Ontario: Canadian
Institute of Steel Construction, 2010.
Ju, Young K., and Kim, Sang-Dae, “Structural behavior of alternative
low floor height system using structural “tee,” half precast concrete, and
horizontal stud.”, NRC Canada, 2005.
Kim, Sang-Dae , Ju, Young K., and Jung, Kwang-Ryang, “An
Experimental Investigation on the Structural Behavior of TEC Beam
System”, Steel Structures, 2001.
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