3.2.4: Prescribed displacements

Supports as described in section 3.1.16 are a special case of displacement boundary conditions: They set the corresponding degree of freedom of a node to zero. The more general “Prescribed Displacement”-component lets you preset arbitrary displacements at nodes. Fig. 3.2.4.1 shows a beam with prescribed, clockwise rotations at both end-points.

The term “displacement” as used throughout this manual includes translations and rotations.

The “PreDisp”-component resembles the “Support”-component to a large degree: Nodes where displacement conditions apply can be selected via node-index or nodal coordinates. The “Plane”-plug can be used to define an arbitrarily oriented coordinate system for the application of support conditions. In order to find out the index of a specific node enable the “node tag”-checkbox in the “ModelView”-component.

Input-plug “LCase” lets you set the index of the load-case in which displacements shall have a specified value. The default value is “−1” which means that the displacement condition is in place for all load-cases. It is not possible to have displacement boundary conditions active in one load-case and completely disabled in others: For load-cases not mentioned in “LCase” the “PreDisp”-component will act like a simple support with fixed degrees of freedom equal to zero.

Fig. 3.2.4.1: Deflection of a beam under predefined displacements at its end-supports

The “Trans”- and “Rot”-input-plugs expect vectors. They define nodal translations and rotations either in global coordinates or in the coordinate system defined by the plane fed into the “Plane”-input plug. Translations are to be given in meter (or feet), rotations in degree. The X-component of the rotation vector describes a rotation about the coordinate systems X-axis. A positive value means that the node rotates counter-clockwise if the X-axis points towards you. Analog definitions apply to rotations about the Y- and Z-axis. Karamba3D is partly based on the assumption of small deflections. Thus be aware that large prescribed displacements and rotations give rise to incorrect results in case of geometric linear calculations. For approximating effects due to large displacements see e.g. section 3.5.4.

Displacements can only be prescribed if the corresponding displacement degree of freedom is removed from the structural system. This means you have to activate the corresponding button in the Conditions-section of the “PreDisp”-component. The first three buttons stand for translations the last three for rotations. In addition to the radio bottons the prescribed degrees of freedom can also be specified parametrically using the input-plug "Dofs". It expects a list of integer values where the numbers '0' to '5' stand for Tx to Rx. Right-click on the component and select 'Expand ValueLists' to get a ValueList-component as shown in fig. 3.2.4.1.

Only those components of the “Trans”- and “Rot”-vectors take effect which correspond to activated supports.