|
Design Objects
Purpose
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".
Definition
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.
Design object
|
Enclosing rectangle
|
Example
The following 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.
Nx in the prismatic shell structures above the central girder [kN/m]
My in the beam elements of the central girder [kNm]
My in the design objects [kNm]
Input
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.
Editing
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
During the integration of the internal forces or stresses at the location of the section, the following elements are considered:
- Beams whose axis lies within the rectangle that encloses the section.
- Area elements whose intersection with the section plane lies completely within the rectangle that encloses the section.
- Solid elements whose plane of intersection with the section plane lies completely within the rectangle that encloses the section.
Handling of the area and solid elements during integration. The elements Fn and Vn are not considered.
State of Development
Currently the determination of the internal forces, the combinations and all reinforced concrete and prestressed concrete checks can be carried out for design objects.
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 single designs.
|
