Connection problems top the list at IES technical support for errors in customer models. There are four primary causes for connection problem:
- Mistaking external supports for moment connections in a frame.
- Misunderstanding the nature of the connection between a plate element and a beam element in the same plane.
- Unconnected members that appear to be connected.
- Members along the boundary of a plate mesh that are only connected to the plates at their end-points.
We will look at each of these errors and how to solve them.
- PROBLEM: All Nodes Fixed
- PROBLEM: Members Not Connected
- PROBLEM: Members Not Connected to Plate Mesh
PROBLEM: All Nodes Fixed
The first problem consists of misunderstanding how beam elements connect in a frame model. When you first create a model in VisualAnalysis, you are asked to specify whether the model is a truss or a frame. In truss models all joints are pure, frictionless hinges. In a frame model all joints, by default, are rigid moment-type connections. Some customers have missed this distinction and try to create moment connections in a frame by using external supports. Typically every node in the structure is marked as "fixed". The confusion probably is due to terminology changes from one software program to the next, or perhaps this is a first-time use of finite element software.
There are two ways to identify this situation in a model. First, when you try to apply a nodal load to the model, you will get an error "Nodal loads may not be applied in the direction of a nodal support." Another symptom of this error is that all of the results (nodal displacements) are zero.
The "all nodes fixed" problem is easily solved by removing all of the external supports on nodes with the exception of nodes that truly have external support. In a typical building frame this is just the nodes at the base of the structure where the frame sits on a foundation or where the foundation rests on the ground.
PROBLEM: Members Not Connected
Another common problem is to create a model that looks like it has connected members, but the members do not in fact share any nodes. This is something that VisualAnalysis will normally warn against as you draw or copy members.
You will often get an error when you try to analyze an unconnected model: "Members exist in your model which are not connected. Would you like them deleted?" If you say yes, you will find out immediately which members are missing. (Just make sure your model is saved before you say yes, so that you don't lose a lot of work. You will probably get a message after they are removed that your structure is no longer stable!
The solution is in understanding that the finite element formulation requires members to connect at nodal points. So if a member crosses another, it must be split and both members joined at common nodes.
Note: You can later "recombine" the columns as combined members for simpler reporting!
PROBLEM: Members Elements Not Connected to Plate Mesh
The final example of connection problems again involves both plate and member elements. When creating a floor slab it is almost always incorrect to use one single plate element. You should use a mesh of plate elements. Even if you have a mesh it is very tempting to keep the model simple and to create single beam elements that span between columns. The problem is that without the intermediate connections between the beam element and the mesh, you will not get consistent deflections or any load transfer between the two element types, except at the end points.
When you inspect results you will find that beam elements and plate elements deflect differently. If you have loaded the plates they will deflect but the beam will not. If you have loaded the beams, then the reverse will be true.
The solution in this situation is to split the member elements along the plate mesh boundary to get common nodes connecting the two types of elements. When you create a plate mesh by splitting a single plate element, VisualAnalysis will automatically do this for you. Note: You may use Combined Members to see a chain of members as a single member, while preserving the correct connectivity.
Connection errors are surprisingly easy to create, yet easily solved. With some basic knowledge of finite element behavior and implementation, connection problems can be avoided altogether.
For more information about FEA modeling issues and errors, IES recommends the following textbook:
Finite Element Modeling for Stress Analysis
By Robert D. Cook
1995, John Wiley & Sons