Element Definition Solutions
After you have built dozens or even hundreds of models, it is easy to get
careless in your modeling. The class of problems we discuss in this section
represent simple oversights or mistakes. They are subtle problems that can cause
some strange analysis results. While they are easy to solve you may not know
where to start looking. This section will provide some practical ways to quickly
eliminate trouble spots in your models.
- PROBLEM: Members are Oriented Incorrectly
- PROBLEM: Members have the Wrong Material
- PROBLEM: Elements Have Zero Area
PROBLEM: Members are Oriented Incorrectly
Member orientation refers to the directions of the strong and weak axes. There are four common reasons that members are created with an incorrect orientation. All four are easily solved once you understand how members are oriented in VisualAnalysis and how to check their orientations.
1. Columns are rotated 90 degrees
2. Model is imported from a DXF file (Z is vertical)
3. Model is created in VA with vertical Z axis
4. Roof purlin is not aligned with the roof slope
If you have columns rotated the wrong direction you may get large story drifts. If you have beams rotated the wrong direction, you will probably see large bending stresses or deflections.
Many of these types of problems do not have obvious symptoms. You will need to watch out for these conditions without any warnings or errors from the software.
The solutions to member orientation problems are an understanding of default orientation for members in VisualAnalysis, and knowing how to check the orientations. The strong axis of a member is bending about local z.
How to Rotate Members: Use the beta angle in a space frame to rotate the member local coordinate system (and the cross section). To edit the rotation angle use the Modify tab of Project Manager. Note: Members cannot be rotated in Plane Frames or in truss structures.
How to Check Member Orientation: One of the quickest ways to check member orientations is simply look at the member in a Picture View. Using the zoom and rotate commands you can quickly scan your structure and make sure the members are all correct. You can also display the beta angle as a number on each member in a Model View. Another way to check in a space frame is to turn on the Local Coordinate filter item. This will display the local coordinate system of each member graphically in a Model View. If the view or local coordinate system is skewed it may be difficult to see the exact orientation.
PROBLEM: Members have the Wrong Material
VisualAnalysis is a fairly smart program. If you choose a W12x14 from the AISC shape database, VisualAnalysis will automatically recognize that this member is steel and will set the material accordingly. (At least most of the time!) However, it is entirely possible to create a concrete W12x14, or a steel 2x4.
If you do not catch this problem before analysis, you may get some rather strange deflections. Otherwise the problem can go unnoticed. The software may protest by ignoring this member during unity checks.
One quick way to check for this type of problem is to look at your model and ensure members of the same materials are the same colors. Members and plates are colored according to their material and you will quickly spot the "bad" element. Here is an example:
The Find Tool window will also show members or plates in a list and you can sort them based on material type.
PROBLEM: Elements Have Zero Area
It is possible to create a four-node plate element that used the same node for two corners. When you create a three-node plate element you define node 1 the same as node 4 and this is OK. But if any other two nodes (1,2), (2,3), or (3,4) are identical, this is an error condition and will result in a plate element with a zero or negative area.
VisualAnalysis will optionally perform an element check before the analysis and will stop with an error indicating a plate element has a zero area or length.
The simple solution is to redefine the plate element's nodal connections. You can do this by editing the element. This is the best solution if you have loads defined on the element. Although it may be easier to delete and redraw the element, than to figure out the names of the nodes it should be connected to, doing so will delete the loads on the plate.