Composite Construction (Steel)

Steel girders with in-situ concrete
Steel girders with in-situ concrete

Composite steel structures, for example, steel girders with in-situ concrete, are normally modeled through the representation of beam and area elements with different properties. Together with the construction stages the resulting creep redistributions can also be easily calculated. As concerns the interpretation and further processing of the results, there is the problem that reactions are determined separately for the beam and area elements.
With the so-called design objects the reactions of any elements at defined sections can be sum up to 'gross section internal forces'. These are required, for example, for the section-related checks.

The described procedure is illustrated in the following simple example:

Twin-span Composite Girder

For the illustrated twin-span girder the stress and internal forces are to be ascertained from the load as well as the redistribution from creep and shrinkage.
This example is taken from the article Verbundkonstruktionen – Bemessung auf der Grundlage des Eurocode 4, Teil 1 (Roik, K., Bergmann, R., Haensel, J., Hanswille, G., Betonkalender 1999, Teil II, Ernst & Sohn, Berlin) (Composite Constructions | Design Based on the Eurocode 4, Part 1 (Roik, K., Bergmann, R., Haensel, J., Hanswille, G., Betonkalender 1999, Part II, Ernst & Sohn, Berlin)) and should enable the comparison with other solutions.

System and load
System and load
Composite section of the design object
Composite section of the design object

A possible structure model is shown in the next figure. The steel girder is represented by a rolled girder HEA 400 and the concrete section by an eccentrically linked polygon.

Model of the load-bearing system
Model of the load-bearing system

This system delivers internal forces and stresses for the rolled section and the concrete section.

Stress curve in the concrete section
Stress curve in the concrete section
Stress curve in the rolled section
Stress curve in the rolled section

To determine the total internal forces from the concrete and rolled sections, a so-called design object is defined. With its help the stresses of all the elements can be integrated into internal forces and applied.

Integrated internal forces on the design object:

Design object with sections
Design object with sections
My as a result of the line load
My as a result of the line load
Qz as a result of the line load
Qz as a result of the line load

The redistributions from the creep and shrinkage of the concrete represent a statically indeterminated share of internal forces in the total section:

My as a result of C&S
My as a result of C&S

Single design for the composite section at midspan:

Listing from single design
Listing from single design