D1.C.9 Composite Beam Design per the AISC LRFD 3rd edition code
The design of composite beams per the 3rd edition of the American LRFD code has been implemented. The salient points of this feature are as follows:
Nomenclature of composite beams
Parameter Name  Default Value  Description 

RBH  0.0 in.  Rib height for steel form deck. 
EFFW  Value used in analysis  Effective width of the slab. 
FPC  Value used in analysis  Ultimate compressive strength of the concrete slab. 
Theoretical Basis

Find the maximum compressive force carried by concrete as:
0.85 f_{c}⋅b⋅t

Find the maximum tensile force carried by the steel beam as:
A_{s} . f_{y}
Tensile strength of concrete is ignored.
 If step 1 produces a higher value than step 2, plastic neutral axis (PNA) is in the slab. Else, it is in the steel beam.
Location of the Plastic Neutral Axis (PNA) defines the moment capacity:

Case 1: PNA in the slab
Find the depth of the PNA below the top of the slab as:
0.85f_{c}⋅b⋅a = A_{s}⋅f_{y}
Rearranging terms:
a = A_{s}⋅f_{y}/(0.85f_{c}⋅b)
Plastic neutral axis in the concrete slab
Lever arm
e = d/2 + h_{r} + t  a/2
Moment Capacity
φ_{b}(A_{s}⋅f_{y})e

Case 2: PNA in Steel Beam
Define:
 C_{s} = Compressive force in slab = 0.85f_{c}⋅b⋅t
 C_{b} = Compressive force in steel beam
 T_{b} = Tensile force in steel beam
C_{s} + C_{b} = T_{b}
Since the magnitude of C_{b} + magnitude of T_{b} = A_{s}⋅f_{y}
Substituting for Tb as (A_{s}⋅f_{y}  C_{b}) gives:
C_{s}+ C_{b} = A_{s}⋅f_{y}  C_{b}
Rearranging terms:
C_{b}= (A_{s}⋅f_{y}  C_{s})/2
Determine whether the PNA is within the top flang of the steel beam or inside the web:
 C_{f} = Maximum compressive force carried by the flange = A_{f}⋅f_{y}
Where:
 A_{f} = Area of the flange
If C_{f} ≥ C_{b}, the PNA lies within the flange (Case 2A)
If C_{f} < C_{b}, the PNA lies within the web (Case 2B)

Case 2A: PNA in Flange of Steel Beam
Calculate:
y = C_{f}/(b_{f}⋅f_{y})
Where:
 b_{f} = width of the flange
The point of action of the tensile force is the centroid of the steel are below the PNA. After find that point, e_{1} and e_{2} can be calculated.
Plastic neutral axis falls within the top flange
Moment Capacity
φ_{b}(C_{f}⋅e_{1} + C_{s}⋅e_{2})

Case 2B: PNA in Web of Steel Beam
Plastic neutral axis falls within the web
C_{w} = Compressive force in the web = C_{b}  C_{f}
g = C_{w}/(t_{w}⋅f_{y})
Where:
 t_{w} = thickness of the web
The point of action of the tensile force is the centroid of the steel area below the PNA. After finding that point, e_{1}, e_{2}, and e_{3} can be calculated.
Moment Capacity
φ_{b}(C_{s}⋅e_{2} + C_{f}⋅e_{1} + C_{w}⋅e_{3})
Utilization Ratio = Applied Moment / Moment Capacity
Notes

Rib Height is the distance from top of flange of steel beam to lower surface of concrete.

If the slab is flush on top of the steel beam, set the Rib Height to zero.
Steel deck form ribs

For moments which cause tension in the slab (called positive moments in STAAD convention), design of the beam is presently not carried out.

Shear connectors are presently not designed.

Member selection is presently not carried out.

In order to design a member as a composite beam, the member property specification during the analysis phase of the data must contain the CM attribute. See TR.20.1 Assigning Properties from Steel Tables for details.