What Member Fixities mean and how they are identified in Structural 3D

Member End Fixities control how members are connected to each other or to the end nodes and include conditions such as a fixed, appuntato, or spring connection.

This Member setting controls how the end of the member connects to its end node, and therefore how members connect to each other. A fixity code is a 6 character code specifying the 6 degrees of freedom for translation and rotation.

Member-end fixity codes accept ‘F’ (Fisso) and ‘R’ (Rilasciato) valori. This code refers to the member end’s connection to its end node for each of the 6 degrees of freedom in the following order:

• Local x-axis Translation
• Local y-axis Translation
• Local z-axis Translation
• Local x-axis Rotation
• Local y-axis Rotation
• Local z-axis Rotation

NOTA: Member-end fixities are with respect to the member’s local axis system rather than the global axis system.

Per esempio, a fully fixed connection between two members (vale a dire. any forces or moments are transferred from one member to the next) will be denoted as ‘FFFFFF’ whereas a hinge connection, which cannot transfer moments in the y or z direction, would be coded as ‘FFFFRR’. The ‘Frame’ and ‘Truss’ buttons are preset end fixity conditions commonly used in the analysis:

Telaio members have a fully fixed, rigid connection specified by a fixity code of ‘FFFFFF’.

travatura members have a hinged connection denoted by a code of ‘FFFFRR’.

To specify custom values, ensure that the costume button is selected. This enables the end fixity fields to be edited.

How to tell Member End Fixity while looking in the model space

When modeling with members, it is convenient to know the members’ end fixity without having to click on each member individually. Per impostazione predefinita, when members are modeled they have fully fixed, o Telaio, end fixities (FFFFFF). The member looks like a single line between nodes:

When any of the degrees of freedom are released, the member will have a small dot near the end of the member (as shown below circled in red). Per esempio, for members with pinned, o travatura, end fixities (FFFRRR), the member would look like this:

Further Explanation

For more information on member fixities, SkyCiv has a technical article (including a video) nel nostro blog, CREDIAMO IN UN SOFTWARE MIGLIORE how to model member fixities:

Esempio: How to model pin connections or hinges

In questo esempio, creeremo una semplice connessione pin, come una cerniera. Inizia creando 3 nodi e unendoli con 2 membri. In questo esempio node, 2 sarà il cardine.

Seleziona membro 1 per richiamare il menu per le sue impostazioni. Clicca il “costume” option to allow you to edit the fixity values. We want the hinge to be at node 2, which for member 1 è il “nodo B” end of the member. This means we need to edit the “Nodo B Fixity”. Since we want the hinge to be able to rotate about its local z-axis, we set the z rotational fixity (the 6th character code) from F to R.

Seleziona membro 2 to bring up its menu. Clicca il “costume” pulsante. Note that node 2 è il “nodo A” end of this member. This means we need to change the “Nodo A Fixity”. Come prima, cambia il suo valore di fissità rotazionale locale-z da F a R.

Il cardine che ruota attorno al proprio asse z locale è ora completo. Notare che un punto appare alla fine del membro che ha un rilascio di rotazione.

Maggiori informazioni

Per maggiori informazioni, SkyCiv has a further explanation on how to model a hinge, utilizzando il software di analisi strutturale.

video: Spiegazione dei gradi di libertà e dei codici di fissità