March 9, 2011

How Do I Investigate My RISA-3D Model Using the Deflected Shape?

When your RISA-3D model is not behaving as you anticipated, one of the best tools you have is viewing the deflected shape. Displaying the deflection graphically will help you visualize how the model is behaving and will often times bring to light modeling errors.

Turn on the deflected shape by clicking on the Result View Settings button on the Home tab.

Or by clicking on the Deflection button found on the View Toolbar.

After the deflected shape is enabled, it is possible to increase the magnification factor of the deflection so that individual locations can be evaluated in more detail.

The deflected shapes pictured below illustrate two simple braced frames. These two images are a perfect example of how you can get seemingly good results including reactions, code checks and even Joint Deflections for both frames.

However, the right frame above shows that the beam is not connected to the column or brace. You need to display the deflected shape in order to catch this modeling error.

Tags: RISA-3D, RISA-2D, New, Instability

August 9, 2012 | RISA-3D, Walls, New

How to Model a Column within a Wall in RISA-3D

While RISA-3D does not currently have pilaster design, there are still plenty of situations where you would want to have a column embedded within a wall. The idea behind this is that the column handles the gravity loads for any beams which frame into it, while the wall handles lateral loads. No load sharing is intended between them, although they do occupy the same space. The problem with simply modeling a column within a wall is that the column is (by default) a Physical Member and it therefore is fully merged with the wall along its height. In the example below, a beam frames into two wide flange columns which are embedded within masonry walls. The green arrows in the image below represent the reactions at the base of the wall. As you can see, of the 20k of applied load, only about half of that load is carried through the columns. The other half has bled out into the walls, distributing by arching action into a distributed load at the base of the wall. This is not a realistic distribution unless the column has shear studs embedded into the masonry. You can get around this by changing the columns…

June 13, 2011 | RISA-3D, New, Instability

How Do I Avoid Instability Warnings in a RISA-3D Model?

Have you ever received an instability warning when running a three dimensional RISA-3D model? This is because RISA-3D cannot build the stiffness matrix with the configuration you have modeled. In some cases, your model is truly unstable and in others it’s a matter of correctly modeling your structure for a 3D finite element analysis program. Below are a couple of tips to avoid instabilities in your RISA-3D models. Every joint in your model should have at least one fixed Member End Release framing into it. A good example of this is a truss. For more information on modeling boundary conditions for a 2D truss within RISA-3D see the Support article listed below. Link: Tips & Tricks: Truss Boundary Conditions When only one connecting member end releases is modeled as fixed (all others are modeled as pinned), no moment can be transferred at this joint. However, it allows the program to assign rotational stiffness to this joint which decreases your chances of getting an instability warning. Ensure that members framing into Boundary Conditions also follow Rule 1. This means that a column with a pinned Boundary Condition should have a fixed end release. Again, this ensures the program can assign rotational…

April 18, 2011 | RISA-3D, Loads, New

Using Sum of Reactions in RISA-3D

RISA-3D will now check your model for errors by summing the reactions in your model and comparing them to the applied loads. This occurs for the global X, Y, and Z directions. If RISA identifies that the reactions do not equal the applied loads then the software will show a warning message to the user. There are three main reasons why this might happen: You have used a Fixed call out in the Boundary Conditions spreadsheet: There are instabilities in your model, causing locked joints: These locked joints may have an actual reaction coming out, but because the program was forced to lock it there is no calculation of this reaction. Fix the instabilities and the message will likely go away. Areas of high rigidity simulating boundary conditions (sometimes referred to as ghost reactions): If you have an area of your model that has a concentrated amount of very stiff elements and/or single elements that have very high stiffness, then these areas may approach the stiffness of a boundary condition and erroneously take load out of your model. Common reasons why this might happen: Setting the E and I values for your Material or Section Set overly high. A rule…