CFS-CID MD White Paper

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Openings in composite floor decks • 06/2019
Structural considerations for openings in composite oor decks
b) design of the composite slab for superimposed loads after
concrete hardened.
Note that the design of the steel deck to accommodate as a
form is usually more critical than the design of the composite
floor for superimposed loads since the steel deck profile and
thickness shall be chosen such that the un-shored span of the
steel deck can support the construction loads.
2.1.1 Design of the steel deck as a form for
construction loads:
As a formwork during concreting, the steel deck should be
designed to resist the anticipated construction loads applied
to the steel deck. A design meeting the minimum design
loads specified in the Standard for Composite Steel Floor
Deck-Slabs, ANSI/SDI C-2011 includes evaluation of two
separate load combinations: the dead weight of concrete and
20 psf uniform construction live load; and the dead weight of
concrete and 150 lbs concentrate load per foot width of deck.
The design of deck as a form is a straight forward engineering
exercise. Since the deck is no more than a cold–formed steel
beam spanning between the support framing, the provision of
the North American Specification for the Design of Cold-
Formed Steel Structural members, AISI S100, is typically
used to determine the strength of the deck. The bending
moment, web shear, and reactions are determined using
engineering mechanics for slender beams. The maximum
moment, web shear, and reactions are then checked against the
strength of the deck to determine the appropriate deck profile,
thickness, and allowable un-shored span for a project. The
reactions of the steel deck at supports must also be checked
to ensure that the webs of the steel deck do not buckle. The
decking is normally intended to be used without temporary
propping since propping reduces the speed of construction and
therefore affects the construction sequence and economy. The
maximum un-shored span of the steel deck is usually provided
by the deck manufacturer for each deck type. Therefore, deck
thickness and profile is typically selected such that shoring is
not required.
Figure 5 — Construction of composite floor deck
The designer also needs to check the deflection of the deck
at the construction stage to limit excess deflections which can
lead to ‘ponding’ of the concrete, which can cause unintended
dead loads on the structure. Deflection limits for the decking are
given in SDI C-2011. The calculated deflection of the deck as
a form is based on the weight of the wet concrete and the self-
weight of the steel deck uniformly loaded on all spans and is
limited to the lesser of 1/180 of the clear span or ¾”.
2.1.2 Design of the composite deck for
superimposed loads:
After the concrete has cured, the decking acts ‘compositely’
with the concrete to resist the loads on the floor (please see
Figure 3). Composite action is obtained by shear bond and
mechanical interlock between the concrete and the decking.
The design of composite steel deck-slab system reflects the
engineering concepts used to design reinforced concrete
beams. The concrete acts as the compression material, and the
steel deck bonded to the bottom of the concrete acts as the
tension rebar.
The bending capacity of the composite steel deck shall be
sufficient to resist out of plane gravity loads on the deck, which
are typically superimposed dead and live loads in addition to
the concrete and deck self-weight. There are various methods
described in ANSI/SDI C-2 011 to determine the strength of
the composite deck-slab such as Prequalified section method,
Shear bond method, and Full scale performance testing. Also,
while generally not the controlling design factor, deflection of
the deck under superimposed loads shall be checked according
to the building codes, which typically limit deflection to L/360
during service live.
Figure 6 — Out of plane gravity loads on the deck
In addition to providing resistance against out of plane vertical
loads, the composite decking is used as a horizontal shear
diaphragm to stabilize the building and transferring in-plane
shear loads (such as wind and seismic forces) to the buildings
main frame (walls and columns). For this purpose, composite
deck shear diaphragm is modeled as a horizontal beam with
interconnected floor deck units that act as the beam web.
Intermediate joists or beams function as web stiffeners and