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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 ... – PowerPoint PPT presentation

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Title: 1 of 18


1
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.
  • (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.

2
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 Stub-Girder Floor System.

3
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.
  • 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.

Composite Beam with Deck
4
Background Composite Floor Systems
Composite Open Web Steel Joists/Trusses
  • Composite Open Web Steel Joists/Trusses is
    another alternative for composite flooring
    systems. Its advantage is that there are
    openings for mechanical ducting while offering
    substantial strength and stiffness.
  • The Stub-Girder Floor System is an assembly with
    a W-shape steel bottom flange and a concrete
    deck-slab 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.

Stub-Girder Floor System
5
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.

6
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
  • 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.

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.
7
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
  • 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

8
Design - Composite Steel Floor Deck 3
  • Extra Notes and Considerations
  • 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 threespan.
  • 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.
  • 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.

9
Design - Composite Steel Floor Deck 4
  • 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.
  • 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 lt32)
  • Determine the specified load
  • Live Load 4.8 kPa
  • Partition / mechanical Load 1.0 kPa
  • Total Specified Load 5.8 kPa

10
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
  • 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

11
Design - Composite Beam with Deck 2
  • Case 1 Full shear connection and plastic
    neutral axis in slab
  • Qr gt FAsFy and FAsFy lt a1Fcb1tcfc 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
  • Depth of compression block a can be found by
    equating Cr and Tr
  • Mrc Tre FAsFye where
  • e distance between compression block resultant
    Cr and tension block resultant Tr
  • e d/2 to a/2
  • to overall depth of deckslab

a1 0.85-0.0015fc
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12
Design - Composite Beam with Deck 3
  • Case 2 Full shear connection and plastic
    neutral axis in steel web (Cr gt FsbtFy)
  • Qr gt a1Fcb1tcfc and a1Fcb1tcfc lt FAsFy
  • Still have Cr which is compressive force in the
    concrete, but now have to take into account
    compressive force in steel Cr
  • Cr a1Fcb1tcfc
  • Cr (FAsFy Cr)/2
  • Moment equilibrium
  • Mr Cre Cre

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13
Design - Composite Beam with Deck 4
  • Case 3 Partial shear connection
  • Qr lt a1Fcb1tcfc and FAsFy
  • Plastic NA always in steel
  • a Qr/(a1Fcfcb1)
  • Cr (FAsFy 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.
  • Redesign beam to avoid this
  • Trust us calculating e is not pretty

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14
Design - Composite Beam with Deck 5
  • How to calculate Qr?
  • shear connection defined as minimum of Qr /
    a1Fcb1tcfc or Qr / FAsFy
  • Qr is the sum of all factored resistance of shear
    connectors
  • Usually provided by shear studs
  • qrs 0.5FscAsc(fcEc)0.5 lt FscAscFu
  • Only valid for solid slabs
  • Fu 450 MPa in 10th Edition of code
  • 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
  • Equations to calculate qrr found in clause
    17.7.2.2
  • Must check horizontal and vertical shear capacity
    of deck
  • Horizontal shear defined as
  • Vh FAsFy
  • Vh a1Fcb1tcfc
  • Vh Qr
  • For cases 1, 2 and 3 respectively
  • Vertical shear resistance defined by
  • Vr FAwFs
  • Fs defined in clause 13.4.1.1

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15
Design - Composite Beam with Deck 6
  • Simplest way is to use lookup tables in handbook
  • Usually design load is known or given -gt 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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16
Case Studies TEC Beam 1
  • 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. 
  • 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.

17
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.

18
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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