1. General
By default, every beam, column, plate, wall or support you model in FEM-Design is connected to every element it touches. Trusses are an exception, because they are only connected by their end points and cannot be connected through any other point on the truss member. The default behavior also means that elements that do not touch each other are not connected.
We can change the default behavior and disconnect objects from each other even if they touch or we can connect elements to each other even if they are far apart.
2. Disconnecting bar elements (beams and columns)
As mentioned before, bar elements (here beams and columns) are connected by default by end points or any other point on them. In figure 1 is an example of the default behavior.
Figure 1. Default behavior of bars connected together.
It can be seen from the displacement result that all bars move together even though the load is acting only on bar B.2. Bars B.2 and B.3 are connected by end point both to each other and also to bar B.1, while bar B.1 is connected to bar B.2 and B.3 by an arbitrary point on that bar. We have three options for bar elements to disconnect them from something.
3. First option for disconnecting bar elements: End point releases
If the bar is connected to something else by end point(s), like bars B.2 and B.3 in figure 1, then the easiest way to disconnect it is to use the properties dialogue window and set the releases (figure 2).
Figure 2. Bar B.2 end points can be released from environment using the properties window.
In this dialogue you can set the release to any of the degrees of freedom, or to disconnect the bar totally, you can set all releases to 0.
Idea. You do not have to disconnect both ends, otherwise the bar will be disconnected from the support as well and will "fall down". To set only one end point release uncheck the box "The same at both ends" and click on the button "Start" or "End" to define release at the appropriate end point.
Idea. To see which is the start or end point of the bar, just turn on the local coordinate system for bars. It can be done in the Settings menu, Display, Bar and then tick the option "Display local system"Local coordinate system will show which is the start and end point. For example, here in bar B.2 the start is on the left:
Idea. This little icon shows which end of the bar has some/any/all releases:
Setting all releases of bar B.2 "End" point to 0 will disconnect that bar from both others. This also means that the force is no longer being transferred to other objects and they will not deform. See figure 3 below:
Figure 3. Displacement result when bar B.2 has been disconnected from all others.
Here in this example bar B.1 and B.3 are still connected together, but B.1 has been disconnected from both. For bar B.3, we can use the exact same procedure to disconnect it from B.1 and B.2.
Bar B.1 cannot be released from bars B.2 and B.3 in the same way. The reason is, that its end point is not the "connecting" point. Its end point hangs free in the air and is not connected to anything. If we use the release on bar B.1 end point, then nothing will change in the results. To release bar B.1 from the others, we must use option 2.
Setting the releases for any bar element in this way will disconnect it from all elements!
4. Second option for disconnecting bar elements: Point connections
If the bar element is connected to other elements by something else than its end point, for example by an arbitrary point on that bar (like B.1 in figure 1), then it can be disconnected using the Point connection tool.
To disconnect bar B.1 from B.2 and B.3 but at the same time keep B.2 and B.3 connected to each other, we must first remove the releases on bar B.2 (because we set it as released in the previous example. Now we will set it as "rigid". This will bring B.2 back to the default behavior) and then we can add a "release" to bar B.1 with the Point connection tool.
To use the tool, we must set the default values as with any other tool (figure 4).
Figure 4. Point connection tool and its default settings.
We can set the "Springs" values to 0 for all the degrees of freedom manually, or by pressing the "Free" button in the bottom.
Now we must apply the connection to an actual element. We must do it in three steps:
a) Firstly, select the element that we want to connect (disconnect in our example). We select bar B.1.
b) Secondly, define Master point. That is the point on the object that we want to connect/disconnect. In our example it will be a point on bar B.1. We must select the point where bars B.2 and B.3 are connected to B.1.
c) Finally, define Other point. That is a point on the other object(s) that we want to connect to or disconnect from. In our example, it will be the end point (meeting point) of bars B.2 and B.3. So, we click on the exact same position as in step "b".
Now that the connection with "free" springs is defined, we can see for example from displacement result, that the bar B.1 is disconnected from the common point of bars B.2 and B.3 (figure 5).
Figure 5. Bar B.1 is disconnected from B.2 and B.3.
We can see from figure 5 that while bar B.1 is disconnected from the other bars, bars B.2 and B.3 are still connected to each other and work like they should.
5. Third option for disconnecting bar elements from shells: Line connections
Similar operation is possible for bars that are connected to shell elements (plates and walls). For example, in figure 6 there is a bar and a shell with their default behavior. By default, the bar will be connected to the shell with every point they have in common (all finite element nodes along bar).
Figure 6. Default behavior of bar and shell connected together.
To disconnect bar B.4 from plate P.1 we must use the Line connection tool. To use the tool, we must set the default values as with any other tool (figure 7).
Figure 7. Line connection tool and its default settings.
We can set the "Springs" values to 0 for line connection the same way as we did for point connection.
Now we must apply the connection to an actual element. We do it in similar way to the point connection. We must do it in three steps:
a) Firstly, select the element that we want to connect (disconnect in our example). We select bar B.4.
b) Secondly, define the Master line. That is a line on the object that we want to connect/disconnect. In our example it will be a line on bar B.4. We start the line from end of the bar B.4 and end the line at the last common point of bar B.4 and shell P.1 (see figure 8).
c) Finally, define the Slave line. That is a line on the other object(s) that we want to connect to or disconnect from. In our example, it will be a line on shell P.1. So, we click on the exact same position as in step "b".
Figure 8. Start and end point of the line connection for disconnecting the bar.
Now that the connection with "free" springs is defined we can see for example from displacement result, that the bar B.4 is disconnected from the shell P.1 (figure 9).
Figure 9. Bar B.4 is disconnected from shell P.1
Idea. You do not have to disconnect the whole bar from the shell, but instead the line connection can be smaller to disconnect only part of the bar from shell like this:
6. Disconnecting shell elements (plates and walls)
As it is with bars, shell elements (here plates and walls) are also connected by default by end points or any other point on them. In figure 10 is an example of the default behavior.
Figure 10. Default behavior of shells connected together.
It can be seen from the displacement result that all shells move and bend together even though the load is acting only on some of them. We have two options to disconnect a shell from something else.
7. First option for shell elements: Edge connections
If the shell is connected to something else by edge(s), like the plates and walls in figure 10, then the easiest way to disconnect it is to set up an edge connection to that shell and define springs stiffness in it. It must be done separately for plates and walls - there is no option to set it for both at the same time (no one tool is available).
Edge connections for plates are made with Plate tool and connections for walls are made with Wall tool. Here we make an example how to define an edge connection to the wall W.1 (figure 11).
Figure 11. Setting up an edge connection to a wall.
To set up the connection we must follow these steps:
a) Select Wall tool then select the Edge connection properties tool from Plane wall toolbar.
b) Select wall that you want to define the edge connection to. I select wall W.1.
c) Select an edge on the wall you just selected. I select the top edge of the wall W.1
d) Now we can set the "Springs" values to 0 for all the degrees of freedom manually, or by pressing the "Free" button in the bottom.
Idea. There is a visual indicator on the edge now showing that there is some sort of release or modified stiffness in any/all directions. It can even be a 1kN/m/m difference and it is not considered "rigid" anymore and thus the edge indicator will show on the wall.
Idea. The edge connection has its own local axis along which the "Springs" are defined.
It can be seen by turning on the setting in the Settings menu, Display, Connection, diaphragm and then tick the option "Display local system"
Now that the connection with "free" springs is defined we can see for example from displacement result, that the wall W.1 is disconnected from the plate P.2 (figure 12).
Figure 12. Wall W.1 is disconnected from plate P.2.
8. Second option for shell elements: Line connections
Like it is with the bar and shell example above (chapter 5), we can use the line connections between shell objects as well. To do that we follow exactly the same procedure as we did in the third option for bars (chapter 5).
In this example, we will disconnect plate P.2 from wall W.3. We use the Line connection tool. To use the tool, we must set the default values as with any other tool (figure 7).
We can set the "Springs" values to 0 for line connection the same way as we did for bar/shell connection above.
Now we must apply the connection to an actual element. We do it exactly the same way as above. We must do it in three steps:
a) Firstly, select the element that we want to connect (disconnect in our example). We select plate P.2.
b) Secondly, define the Master line. That is a line on the object that we want to connect/disconnect. In our example it will be an edge line on the plate P.2. We start the line from one end of the connection between plate P.2 and wall W.3 and end the line in the other end of that connection (see figure 13).
c) Finally, define the Slave line. That is a line on the other object(s) that we want to connect to or disconnect from. In our example, it will be a line on wall W.3. So, we click on the exact same position as in step "b".
Figure 13. Start and end point of the line connection for disconnecting the plate P.2
Now that the connection with "free" springs is defined we can see for example from displacement result, that the plate P.2 is disconnected from the wall W.3 (figure 14).
Figure 14. Plate P.2 is disconnected from wall W.3 using Line connection tool
9. Connecting elements together (bars and shells)
As mentioned before, structural elements (beams, columns, plates and walls) can be connected to each other even if they are far apart.
Here is an example with bars. By default, they are of course disconnected from each other like in figure 15.
Figure 15. Default behavior of bars that are not connected together.
We can use the Point connection or Line connection tool to connect them. To define the connection element itself, look at the description above about disconnecting the bars from each other and disconnecting the bars from shells. The process for defining the connection is the same as in these examples above, but of course the "Springs" must be set to rigid or some other stiffness instead of 0.
In this example, I set connection between bar B.5 and B.6 with line connection and between B.5 and B.7 with point connection.
Figure 16. Connections between bars
From figure 16, it is clearly visible, that adding either line or point connections between bar elements will force them to work together as if they had physical connection between them.
Here is an example of a plate that we want to connect to beams. By default, they are of course disconnected like in figure 17.
Figure 17. Default behavior of bars and shells that are apart from each other.
I will add a point connection between bar B.9 and the plate P.3 and a line connection between bar B.8 and the plate. So, we end up with something like this in figure 18.
Figure 18. Bars and shell connected using point and line connections.
From figure 18 it is again clearly visible that adding connections between elements made the structural elements working together. Here the force from bar B.9 is transferred through the shell P.3 to bar B.8.
We can do similar operations on shells as well. Here in figure 19 is an example of four shells (two plates and two walls) connected together using Point connection and Line connection. Point and line connections work exactly the same way as they did for bars and bar/shell examples above.
Idea. You might be tempted to use Surface connection tool for connection between walls W.4 and W.5, but this is not possible. Surface connection is only made for soil connections.
So, connection between walls W.4 and W.5 is made using line connections on all four sides of wall W.5.
Figure 19. Connections between shell elements
In figure 20 we can see how the connections affect the shells and transfer the loads from one element to the others.
Figure 20. Deformed shape of the structure.
10.Red dot and red line in connection elements
In the examples above there is a red dot visible for point connections and red line for line connections. In FEM-Design this is called interface point or interface line. This is the place that has the "release" or the stiffness we set in the "springs".
To make this clearer let us look at the example in figure 20 above. The point connections there have a black dot (the Master point), a grey dot (the "other" point of that connection), dashed line (the line showing which elements are connected) and the red dot (interface position). We can move the interface point (or interface line in line connections) by setting a ratio in the connection's settings. In figure 21 is an example of point connection's settings for the interface.
Figure 21. Interface position of point connection.
This ratio (proportion) is a value between 0 and 1 (including 0 and 1). Value 0 will put the interface point to the same position as the Master point (black dot in figure 20) and value 1 will put it to the "other" point (grey point in figure 20). Value 0.5 will set it exactly in the middle of the connection.
The Master point and "other" point are always rigid (their stiffness cannot be set) unless the interface point happens to be right on their position (value 0 or value 1). Also, in the first examples about disconnecting bars, we set the Master and "other" points to be exactly in the same physical location, which also means that the interface point value did not matter at all and the interface point was also in the exact same location (all three points in one location). Therefore, the connection was not rigid (although Master and "other" points themselves are rigid), but because we had the interface point there as well, we could set it as "free" instead.
Also, it is good to note that if we look results for connections (for example connection forces) we will see the result in the interface point like in figure 22.
Figure 22. Connection force result is in the interface point.
To learn more about the interface point/line and connections, please follow the two links to manual
Link 1: about point connections.
Link 2: about line connections.