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There are various ways of modeling composite behavior of beam-and-slab assembly in SAP2000. For your reference and study, we have attached four SAP2000 models that show different ways for modeling composite action for girder-and-slab assembly. The model description used for the comparison is as follows:

The Slab width = 2 m
Slab thickness = .2 m
Girder total height = 1.2 m
Top and bottom flanges width = 1 m
Top and bottom flange thickness = .1 m
Web thickness = .1 m

The applied load at mid span is 100 kN
Modules of elasticity E= 33000000 kN/m2
Span length = 20 m, fixed at both ends

The deflection at mid-span is calculated for the naked girder and composite girder as 0.0018m and 0.00083m respectively.

Model Overview

Model1.SDB

In this model, the center line of deck slab coincides with the neutral axis of the section. Hence, the contribution of deck slab to the flexural stiffness of the section will be negligible and the deflection at mid-span for this model should be close to the deflection for naked girder (i.e. no composite action). In this case, the deflection at midspan is 0.00179m.

Model2

In this model, the shell elements representing the slab are drawn at the CG of the girder and then offset above the girder to model the composite action. The shells are offset such that the soffit of the slab is at the top of the girder top flange. The deflection at midspan in this case is 0.0008313m.

Model3

In this model the girder and slab are drawn at their respective centerlines and then the corresponding joints of the girder and the slab are connected together through body constraints. This is another way to model the composite action. The deflection at midspan, in this case is also 0.0008313 as expected.

Model4

In this model the concept of frame insertion points is used to model the composite action. The deflection at midspan, in this case is 0.0008313 which is same as for Model2 and Model3.

In view of the foregoing, you can use either area offsets or body constraints or frame insertion points to model composite action of the beam-and-slab assembly.

Attachments

Modeling composite behavior in SAP2000.zip - revised models for fixed beam and simply supported beam with additional modeling approaches using springs and detailed sketches illustrating modeling composite, noncomposite and partially composite behavior in SAP2000 (Zipped archive with two SDB model files and a PDF file)
Composite section models V12.0.2.zip - Models 1 to 4 SAP2000 V12.0.0 files (Zipped SDB files)

Composite section