Composite Slab
Design Checks &
Loads
• Serviceability limit state (SLS): maximum deflection, maximum steel
stress, maximum concrete stress – adding up of values at construction
& composite stages, load = Gk+Qk (new value: composite cross-section
second moment of area Ic)
• Ultimate limit state (ULS)
- Construction stage: concrete has not gained strength & stiffness, so
design for steel section only. Load = 1.35Gk+1.5Qk, Gk=self weight only
with no allowance for permanent fitting, Qk (construction load) =
1.5kN/m2
- Composite stage: composite action, load=1.35Gk+1.5Qk (new value:
composite cross-section plastic bending moment capacity)
Plastic bending moment resistance –
complete shear interaction
• The concrete slab between
the steel beams is
continuous in two
directions. Under load, it is
under bending with
longitudinal stresses. The
stress distribution is not
uniform, being higher near
the steel beam and lower
further away. An effective
width is used to represent
the concrete slab
contribution. Within this
effective width, the
concrete is assumed to be
under uniform stress.
Effective width of concrete slab
• The properties of a composite cross-section are not sensitive
to the exact value of effective width.
• A value of beam span/8 is assumed on each side of the
beam where concrete is present.
• The maximum effective width value is half of the slab
spacing between two steel beams.
• If the decking spans parallel to the steel beam, the
longitudinal local bending stress in the slab between the
supporting steel beams is in the same direction as the
longitudinal bending stress from composite action.
Therefore, the slab contribution is reduced. The maximum
effective width is 0.8*half slab spacing between two steel
beams.
Plastic analysis of composite section – Case 1
• The maximum tensile resistance of the steel section is Ts=
• The maximum compressive resistance of the concrete slab is Cc=
0.45=0.6/1.5, where 1.5 is material safety factor, concrete bending stress~0.6 cube
strength
• If Rs<Rc, plastic neutral axis is in concrete slab. Depth of concrete in compression:
• Plastic BM resistance:
Case 2: PNA in steel flange (Rs > Rc  Rw)
Case 3: PNA in steel web (Rc < Rw)
Example