Explanation of Support Nonlinearities on Example 1.1 | Translation

Technical Article

In practice, an engineer often faces the task of representing the support conditions as close to reality as possible in order to be able to analyse deformations and internal forces of the structure subjected to their influence and to enable as much cost‑efficient construction as possible. RFEM and RSTAB provide numerous options for defining nonlinear nodal supports. The first part of my article describes the options for creating a nonlinear free support and provides a simple example. For better understanding, the result is always compared to a linearly defined support.

General

Each nodal support has its own local axis system. The axes are designated as X', Y' and Z'. By default, this support axis system is based on the global axis system of the RFEM or RSTAB file. However, it is also possible to define a custom axis system or a rotation. In the example provided here, the support axis systems are shown for all nodal supports.

The options of the individual nonlinearities are shown for the displacement in X'. For the other two support axis directions, similar definitions apply.

Note: The nonlinearity always refers to the acting support force.

Failure if Negative PX'

Figure 01 - Failure if Negative PX'

If the acting support force is directed towards the axis X', the support fails in the direction X' and all the other support conditions act linearly.

Failure if Positive PX'

Figure 02 - Failure if Positive PX'

If the support force is positive and thus aligned with the support axis X', the support fails in the direction X' and all the other support conditions act linearly.

Failure All if Negative PX'

Figure 03 - Failure All if Negative PX'

If the acting support force is directed towards the axis X', the support fails completely. In this case, no forces or moments are absorbed by this support.

Failure All if Positive PX'

Figure 04 - Failure All if Positive PX'

If the support force is positive and thus aligned with the support axis X', the support fails completely. In this case, no forces or moments are absorbed by this support.

Partial Activity: Slippage

Figure 05 - Partial Activity: Slippage

The partial activity is defined in an additional menu. Here, it is possible to independently define the support for the positive zone (positive support force P‑X' and positive displacement uX') as well as the negative zone (negative support force P‑X' and negative displacement uX'). The settings are then displayed graphically in a diagram.

If the support has been defined as ‘fixed’ in X', the support node with the defined ‘complete’ and ‘slippage’ support activity is deformed until the defined slippage occurs. Then, the acting support force will be transferred completely. If a spring has been defined, it is effective after reaching the defined slippage.

Partial Activity: Yielding and Slippage

Figure 06 - Partial Activity: Yielding and Slippage

By selecting this option, you can define a limit support force and a value for the slippage. Again, this can be done independently for the positive and the negative zone. If a greater deformation than the defined slippage is reached, the support can only transfer the defined limit support force. If the acting support force exceeds the limit support force, the deformation increases further without increasing the support force.

Partial Activity: Spring and Slippage

Figure 07 - Partial Activity: Spring and Slippage

If a spring constant has been defined for the support, an additional option ‘Fixed from support displacement u+’ is available for ‘Partial activity’. As already described, it is possible to define a limit value for the slippage. Furthermore, the activity of the defined spring is now limited by the limiting value ‘displacement’. A linear translational spring operates between the limiting value ‘Slippage’ and the limiting value ‘Displacement’. If the deformation becomes greater than the limiting value ‘Displacement’, the support force is absorbed completely without a further increase of the deformation. As in the case of all other options, this can also be defined separately for the positive and the negative zone.

Partial Activity: Tearing from Support Force

Figure 08 - Partial Activity: Tearing from Support Force

If a spring constant has been defined for the support, it is also possible to specify ‘Tearing from support force P+’ for the support. This option can again be combined with slippage. The support force increases accordingly with regard to the spring constant until the defined limit support force is reached. If the limit support force is exceeded, the support fails suddenly for this direction.

My next article will describe the other options that have not been explained here.

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