September 11, 2014
How Do I Create Offset Connections in RISA-3D?
By default RISA-3D draws all members as line elements located at the centroid of the cross-section. However, connections between members are not always aligned with a member’s centroid. There may be horizontal or vertical offsets in the connection. To account for these you can add rigid links to create this eccentricity. More information about rigid links can be found in the following article:
Before proceeding with these offsets, ask yourself:
- Do I need to account for these offsets in my structural model?
- Will these offsets significantly affect the analysis?
In many circumstances the answer to both of these questions is no. In such cases, it is not worth the extra modeling to obtain the minimal change in the analysis results. However, if the eccentricity causes a significant effect to the analysis then you may want to account for the eccentricities. This decision is up to your engineering judgment.
For example, below a beam is offset from a column and is supported on a haunch off of the column flange. The approach is simple:
- Rather than drawing the members connected to one another, draw them in space as they actually are. If there is an offset then these members will not actually connect to one another. Here is what the two members would look like:
- As you can see above, there is no connectivity between the members. Thus, there will be no load transfer and likely an unstable model. To address this, add a horizontal rigid link member between the column and the beam:
- Now, a load that is applied to the beam will transfer into the column at this eccentricity. This will produce an axial force as well as a moment into the column.