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Composite Construction
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.
This is
where the so-called design objects come into play, as they can be
used to sum up the reactions of any elements at defined sections
into '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.
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| E-modulus concrete |
33,500 |
MN/mē |
| E-modulus steel |
210,000 |
MN/mē |
| Creep coefficient phi |
2.5 |
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| Material shrinkage eps. |
-60 * 10-5 |
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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.
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 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:
My 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
Single design for the composite section at midspan
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