Composite Construction

Composite structures, for example, precast 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 integrate 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 small example:

Prestressed, Precast Girders with In-situ Concrete

The following figure shows the section of a simple one-span bridge, built with prefabricated parts and a subsequently applied slab of in-situ concrete.

A possible structure model is shown in the next figure. The prefabricated parts are modeled using beam elements, and the roadway slab using eccentrically coupled prismatic shell structure elements.



This system delivers the internal forces in the prefabricated parts and the in-situ slab.

In the first construction stage only the prestressed, precast girders exist (beam elements). These carry their dead load and the fresh concrete from the in-situ concrete slab. The illustrated internal forces exist after 50 days of creep and shrinkage.

Bending moment My	Normal force Nx

In the second construction stage the in-situ concrete slab is activated. At first it is free of stress. Only through the redistribution from the creep and shrinkage do reactions arise, as indicated below:

Normal forces in the slab after creep and shrinkage

In this way any creep stage can be investigated for the different components.

Further loads affect the entire system and result in reactions in the precast girder and the slab.

The internal forces of the 'gross section' made up of the precast girder and the slab is required to perform the checks on the composite girder. Toward this end a design object is defined. With its help, the stresses of the girder and slab are integrated into internal forces, which are then combined according to standards to make the checks possible.

The 'gross section internal forces' curve is applied to the design object:

Bending moment My	Normal force Nx
Lateral force Qz