In solid construction calculation models are used that are based on a combination of beam and area elements or, less commonly, solid elements. These models make possible, for instance, the representation of construction stages and deliver a detailed picture of the stress acting on the structure. They cannot, however, be used directly for reinforced concrete or prestressed concrete checks. The reason for this is that the checks are usually section oriented. For instance, for the checks at the ultimate limit state, several elements must be merged into one object with respect to geometry and stresses. This is done using the so-called "design objects".
A design object is a linear element with a start and end point. The connection between start and end point defines the element axis and at the same time the local x axis. The local z axis typically corresponds to the global z axis, although it can be defined to have an angle of tilt. Sections can be defined at any point along the element axis. The sections are perpendicular to the axis. At the point of each section the FE program integrates the stresses and internal forces of the elements that lie within the enclosing rectangle of the section. The coordinate system of the design object determines the orientation of the internal forces.
The illustration shows the section of a simple two-span bridge, built with prefabricated parts a and subsequently applied in-situ concrete slab.
A possible structure model is shown in the next figure. The prefabricated parts are modeled using beam elements and the roadway slabs using eccentrically coupled prismatic shell structure elements.
This system delivers the internal forces in the prefabricated parts and in-situ concrete slab. The internal forces for the design of the central composite girder are to be determined as the next step. For this purpose a design object is defined.
The following illustrations show several internal forces in the area of the central girder.
The input function is started from the FEM menu (’Design objects’). After the start and end points have been defined, the following dialog field appears for entering the additional object properties. The angle of tilt refers to an additional rotation of the object along its longitudinal axis compared with the standard orientation. When in standard orientation, the local y axis runs parallel to the global XY plane.
To edit a design object, select it and open the following dialog using the shortcut menu.
The location of the sections is determined by the insert point, described by xi (0 to 1), y, z as well as an optional subsequent rotation of the object axis.
A graphical function for manipulating the location of the section is available using "FEM/Design Objects/Move Section".
Integration & Checks
During the integration of the internal forces or stresses at the location of the section, the following elements are considered:
- Beams whose axis pierce through the enclosing rectangle of the section.
- Area elements whose lines of intersection with the section plane lie within the enclosing rectangle. Thereby also the parts of cut elements are taken into account.
- Solid elements whose planes of intersection with the section plane lie within the enclosing rectangle. Thereby also subareas are taken into account.
After the integration of the internal forces, the combinations and all reinforced concrete and prestressed concrete checks can be carried out.
During the calculation of the statically determined and undetermined part of prestressing the tendon groups, who have an angle smaller than 45° with the design object axis at the intersection point of the enclosing rectangle, are taken into account.
For a composite section that consists of several smaller sections with different materials, including, for instance, construction steel, it is possible to determine the internal forces, perform combinations and create concrete designs in ULS.