Name:  Composite section 
Description:  Several approaches to the modeling of composite sections. 
Program: 
SAP2000

Version:  12.0.0 
Model ID:  na 
This article is for SAP2000, but the same concepts apply also to CSiBridge.
On this page:
SAP2000 provides various ways to model the composite behavior of a beamslab assembly. Attached are four SAP2000 models which demonstrate various approaches. The geometric and material properties of the model used for comparison are listed as follows:
 Slab width = 2.0m
 Slab thickness = 0.2m
 Total girder height = 1.2m
 Top and bottom flange width = 1.0m
 Top and bottom flange thickness = 0.1m
 Web thickness = 0.1m
 Applied load at midspan = 100kN
 Modulus of elasticity, E = 33000000 kN/m^{2}
 Span length L = 20m
 Boundary condition: fixed at both ends
Midspan deflections are calculated as follows:
Naked girder, Δ = 0.0018m
Composite girder, Δ = 0.00083m
Model overview
Figure 1  Model overview
Approaches to modeling composite behavior
Figure 2  Modeling composite behavior
Fixedbeam model
The eight approaches to modeling composite behavior, described above, are applied using eight different beam models which are fixed at either end. Results are summarized as follows:
Beam Designation  Behavior  Midspan Deflection  Comments  

Theoretical Beam  composite  0.8181  Theoretical deflection is based on the PL^3/192EI formulation. Please note that SAP2000 calculations produce slightly greater values because shear deformation is considered in deflection.  
Beam 1  nocomposite  1.7938  The deckslab center line coincides with the section neutral axis. Therefore, the deckslab contribution to section flexural stiffness will be negligible. Further, because there is no composite action, midspan deflection should be close to that of a naked girder.  
Beam 2  composite  0.8313  In this model, slab shell objects are drawn at the girder center of gravity (COG), and then offset vertically, above the girder, to model composite action. The shells are offset such that the slab soffit is located above the girder top flange.  
Beam 3  composite  0.8313  In this model, the girder and the slab are drawn at their respective centerlines. The corresponding girder and slab joints are then connected through body constraints.  
Beam 4  composite  0.8313  In this model, composite action is modeled using frame insertion points.  
Beam 5  noncompostite  1.7938  Equal constraints are used to model noncomposite behavior.  
Beam 6  noncomposite  1.7938  Links are used to model noncomposite behavior.  
Beam 7  partially composite  1.0302  Links are used to model partially composite behavior.  
Beam 8  composite  0.8313  Links are used to model composite behavior. 
As implied by the list above, the composite action of a beamslab assembly may be modeled using either area offsets, body constraints, frame insertion points, or links.
Simply supported beam model
The eight approaches to compositebehavior modeling, described above, are applied using eight different simply supported beam models. Results are summarized as follows:
Beam Designation  Behavior  Midspan Deflection  Comments  

Theoretical Beam  composite  3.2725  Theoretical deflection is based on the PL^3/48EI formulation. Please note that SAP2000 calculations produce slightly greater values because shear deformation is considered in deflection.  
Beam 1  nocomposite  7.1752  The deckslab center line coincides with the section neutral axis. Therefore, the deckslab contribution to section flexural stiffness will be negligible. Further, because there is no composite action, midspan deflection should be close to that of a naked girder.  
Beam 2  composite  3.2624  In this model, slab shell objects are drawn at the girder center of gravity (COG), and then offset vertically, above the girder, to model composite action. The shells are offset such that the slab soffit is located above the girder top flange.  
Beam 3  composite  3.2624  In this model, the girder and the slab are drawn at their respective centerlines. The corresponding girder and slab joints are then connected through body constraints.  
Beam 4  composite  3.2624  In this model, composite action is modeled using frame insertion points.  
Beam 5  noncompostite  7.1752  Equal constraints are used to model noncomposite behavior.  
Beam 6  noncomposite  7.1752  Links are used to model noncomposite behavior.  
Beam 7  partially composite  3.5036  Links are used to model partially composite behavior.  
Beam 8  composite  3.2624  Links are used to model composite behavior. 
Attachments
 Modeling composite behavior in SAP2000.zip (zipped SDB file), which contains:
 SAP2000 V12.0.0 file which demonstrates the modeling of composite behavior for eight fixed beams.
 SAP2000 V12.0.0 file which demonstrates the modeling of composite behavior for eight simply supported beams.
 Sketches which illustrate the modeling of composite, noncomposite, and partially composite behavior.