Can you help me understand how the Load Group should be used in the Staad Physical Modeller?
When you create a Load Group in the Physical Modeler, it is imported into the Analytical Modeler as a Reference Load. If you use Load Case option in the Physical Modeler, it is added as a Primary Load case into the Analytical Modeler. More details can be found in the Physical Modeler Help section Applying Loads > Working with Load Groups.
I'd like to open this question back up. I'm trying to gain an understanding of when to use load groups and when to use load cases + combinations and using "options - REPEAT Primary Load Cases". I'm going to talk through a few situations and apricate any feedback.
Let's assume that I'm going to want to run a 2nd order analysis, so I want the combinations to be repeating. I have dead, live, snow, wind, and EQ.
Add all loads as separate load cases - dead, live, snow, wind in 4 separate directions and EQ in 4 separate directions. I can then use the Automatic Load combinations to create load combinations. All these will be sent to the analytical modeler as Repeat Load cases. The analytical modeler will produce results for all my load cases and the load combinations Even though I typically only want the design to be for the combinations.
I assign my dead, live, snow, wind and EQ in 4 directions to load groups (instead of load cases). These are now load definitions. I cannot use load combinations in this case because load combos only use load cases. Instead, I would have to create a load case and apply factors to each load group. I don't think I would need to use Options -> REPEAT Primary Load Cases because all loads would be applied for each load case. Seems that one benefit to this work flow would be that you would remove all the individual load cases from your results, so it might speed up computation time. However, you would still need to create a bunch of custom load combinations. This doesn't seem worth the tradeoff. Is there another reason to use load groups? Also I'm not sure what you would set the Load Case category to. Maybe "None"?
Is there an example you could provide me where you would use some load groups in combination with load cases and load combinations?
One example that comes to mind would be the case of skipped live loads or live loads on cantilever spans. Maybe you want to create different conditions where live load is on the entire span vs only the cantilever span. Is this the intent of load groups or is there another way to accomplish this?
Upon some research, some reasons for Load Groups
This video by BentleyStructural talks about load groups at the 13:50 (LINK). It only explains there use for defining Mass which is then used by other load definitions.
Also Reference Load cases are discussed here LINK. In this article the primary reason for reference loads (or load groups) is to make the program analyze the combination directly without analyzing the individual component cases. This is what I discuss in workflow 2. However, in using Options -> REPEAT primary Load Cases, it doesn't seem like you would need Load Groups for this purpose, unless the individual load case was causing an error with the analysis.
Could use a load group for defining "Heavy" and "Light" dead load cases. For example - green roof loads or mechanical Equpment. You want the load to be applied in combination with live, snow, etc. However, you want to capture both a "light" case and "heavy" case. Therefore, you would define a "heavy" load case, then factor up your Load Group which has the dead loads for the green roof or mechanical equipment.
I noticed that there is "include load groups" option in the Automatic load combinations dialogue box. Would this allow you to have a work flow where you don't use "options REPEAT load cases". Instead, you would use load groups and load combinations? My first thought is, not really, because the load combination is not going to come in as a repeated load.
Hoping to "bump" this question. Again, just looking for a good example of when to use Load Group vs When to use Repeat Load Cases. If I assume that I'm always going to want to capture the non-linear behavior that is possible with Repeat Load Cases then would I only be using "Load Group" for mass (typically)?
Load Groups in the Physical Modeler are the equivalent to the Reference Loads in the Analytical Modeler. The main difference between Reference Loads and simple Load Case is that Reference Loads are not analyzed by the program. Some more details on this can be found here: communities.bentley.com/.../reference-load-case
When creating Load Combinations in the Physical Modeler, you can select an option "Include Load Groups" which allows to use Load Groups (or Reference Loads) as well as Load Cases in the Load Combinations. As you noted, if you will be performing a non-linear analysis, it is necessary to set the option of Combination Load Cases to "REPEAT Primary Load Cases" in the File -> Options -> Analysis model. This will combine the Load Groups or Load Cases as Repeat Loads rather than usual Load Combinations.
So for complex large models with non-linear analysis using the Load Groups will save the analysis time. If the model is simple and takes several seconds to analyze it, then using Load Case will be fine too.
You may check this Wiki as well for more details on Reference Loads: Reference Loads An Introduction - RAM | STAAD | ADINA Wiki - RAM | STAAD | ADINA - Bentley Communities