RAM Elements Load Combos [FAQ]


 Product(s):RAM Elements; RAM Connection
 Version(s):All
 Environment: N/A
 Area: Analysis; Design; Modeling

What is the load combination "Type" for?

In RAM Elements (and Ram Connection), every load combination is given a type that determines when the load combination is utilized. The figure below illustrates how several different types of combos could be used in a single file.

 

When I analyze the model all the combinations are grayed out. Why?

On the Analysis tab of the Process – Analyze model dialog box, the user indicates if second order analysis will be considered. This is required when using tension-only members or compression-only springs, but may be turned on for any model.

Whenever a second order analysis is performed, the program individually analyzes each selected load case and combination that is checked on the Condition tab.

If second order analysis is NOT performed, then the program only analyzes the selected load cases. The results for all combinations are generated using the principal of superposition, so the user can no longer uncheck individual combos.

Furthermore, if the user opts not to analyze certain load cases, any combinations including those load cases will not have any results available.

When I try to optimize including deflections, I cannot select any load combos. Why?

The program uses "Service" or "Service-Steel" type load combinations for checking beam deflections. If a model has only "Design" or "Design-Steel" combinations and no service combinations, this feature will not be available. Add at least one service combo (e.g. 1.0 DL + 1.0 LL) to access this feature.

When I try to enter loads on members or nodes I can't because the spreadsheet is grayed out. Why?

In the lower right hand corner of the program window is the Current Load Condition selection. If the selection is a load case, then you can enter loads for that load case on members, nodes, shells etc. If that selection is a load combination then you can view, but not edit or add to the existing loads.

Change the condition to a valid load case in order to add loads. 

Load cases that are associated with dynamic response spectrum analysis should not have any static loads at all. 

Can I copy and paste load case or load combinations from one file to another?

Yes, you can literally copy the data (ctrl+C) from one file to paste (ctrl+V) into another. You can also copy the data and paste into Excel for future use in another file. Note, the list box data like "Type" will be converted into integers during this process.

You can also save your load conditions from a complete model and then recover those load conditions in a new file. Those options, as well as the option for generating load combinations, are all under the Home Ribbon Menu - Load Conditions group.

When I use Skip Loading in the Continuous Beam module, why does the program combine all the load patterns into one (and over design the beam)?

In the continuous beam module, if you have skip loading defined with any percentage it's important to generate combos using the Continuous Beam Combo Generator designed for skip loading (.cbg files). If you use the general RAM Elements Combo Generators (.rag files) you get something like this:
 
1  DL
2  DL+LL+SK1+SK2+SK3+SK4
 
i.e. with all the live load cases (LL,SK1, SK2…) together. The result is a combination which effectively doubles the live load.
 
It should be more like this:
 
1  DL
2  DL+SK1
3  DL+SK2
4  DL+SK3
 
i.e. with each skip load case used independently, which is what you get using the .cbg combo generators.

Do I need to include load combinations with negative factors on lateral loads?

Default program generated load combinations only include positive factors on lateral wind or seismic loads. If the model is subject to equal magnitude, opposite direction lateral loads, then additional combinations should be manually added using negative load factors on those load cases. Or a custom Load Combination generator (.rag file) can be created to generate the additional load combinations.

If the opposite direction lateral loads should be applied to different members, as is often the case with wind loads which vary from the windward to the leeward surface, or models with tension-only members, then an additional negative direction load case should be added instead.

This is contrary to the behavior in RAM Structural System - Ram Frame, where rigid diaphragms and linear-elastic braces are the norm and hence load combinations with negative terms are the default.

Can I customize the Load Combination Generation template files?

Yes, in the Load Combinations Generation Dialog, at the lower left is a folder selection which defaults to C:\Program Files\Bentley\Engineering\RAM Elements\ComboGenerators\. in the main module the program will list each file with a .rag extension for selection. These *.rag files are plain text files with simple code to express how load combinations should be generated using constants, Boolean operators (AND, OR), and load types.

For full details on the .rag file format refer to the program help, by pressing F1 while this dialog is open.

See Also

For information on load combinations for Connection Design see Ram Connection Capabilities and Modeling

[[Ram Elements - Tension Only Members]]