One of our clients asked the following question:
Could you explain to me how I can put a restraint on 30 nodes moving altogether?
Let’s say I have 5x6 pillar axes and I would like to enter the condition that they all move with the same head path as if they were joined by an infinitely stiff slab.
Occasionally we have received some models from clients in which they tried to simulate the membrane effect of a stiff slab between pillars using infinitely stiff cross bracing struts made by simple elements with simple cross section but which stiffness was modified, multiplying it by 1E5, for example.
This models tend to end up having an unnecessary convoluted and packed look. Maybe the displacements and forces on the elements of the building are similar to the ones obtained with stiff joints between nodes (the ones we will explain in this article) but without a doubt it is an unsophisticated way of solving the problem, which leads to heavy and inconvenient models.
Using STATIK it is possible to make every type of connection between nodes, for example:
- Generic connections between nodes (the displacement in Z of a node is equal to the displacement in X of another one plus 4.25 times the displacement in Z of a third one).
- Make certain parameters equal, like the displacement is X and Z.
- Make certain parameters equal, in the case of double nodes (at strained joints)
- Stiff joints, very useful to model membranes in buildings.
As shown in this image from the manual, the membrane effect is normally represented by a master node and a series of slave nodes with the same DX, DY and RZ as the master.
To successfully run this operation, a series of precautions must be taken:
- A master node cannot be a slave node
- The parameters of the slave node cannot make it a support, in other words, we cannot use supports and make them move because of a connection, since that would not be possible.
- The connections or restraints of some nodes that are going to be configured must be defined as so, that is, they cannot be nodes generated by the program, like the joints between members (in case there are a lot of them, we will teach you a trick to turn these nodes created by the program into nodes entered by the user).
- Beware of the name of the nodes, especially of the master, since all of them will refer to it in the definition of the connections.
Thus, in a simple case like an initial client, we will be able to create a node in each pillar head in the process of creating the model or creating one and duplicating it as many times as necessary:
Then, we will select the 29 nodes (all except the master) and we will define the following stiff connection:
The expected result should be this movement against a horizontal load:
Instead of the result that we would obtain if there was no membrane connection between nodes:
In the case of a complex building, like the one we showed with the cross bracing struts, it is incredibly useful to obtain the position of all the automatically generated nodes of the model to exactly enter some nodes there but defined by us and so we would only have to generate one membrane per storey like we have done with the previous structure.
For that purpose, we visualize all the nodes generated by the structure (possible after checking the structure): they will show up as green points when enabling the ‘intermediate node’ layer.
We select all the points using Ctrl+A, then we refine the selection so that we only keep the intermediate ones (right click → modify selection) and then we copy them using Ctrl+C.
After that, we go to the node creation tool and when it asks for the position we press Ctrl+V.
That would mean we have created 300 nodes at the same time and we now only have to create one membrane per storey! Easy, right? And with the help of the clipping boz, even more!
The structures generated like this are obviously cleaner:
And they behave as we want them to: