Single span members can be made inactive for analysis and design while still being kept in the model for load distribution and for determination of effective lengths.
This feature is particularly useful for industrial structures, as frequently not all the secondary beams are included in the analysis and design model, but they are still required to distribute load in the structure and act as restraints to supporting members.
An inactive member is totally ignored in the solver model as is any load carried by it. Therefore, to allow the load to be distributed it is instead decomposed to nodal forces in a pre-analysis decomposition stage - this happens for all applied loads on inactive members on the basis of the members being simply supported.
Which members can be made inactive?
Only single span beams, braces, analysis elements, purlins and rails have the potential to be inactive. They each have an Active setting in the properties which defaults to 'on' but can be unchecked.
- they are multi-span, or,
- they support another member (active or inactive)
To make a member inactive
By selecting the Show/Alter State Active attribute in a Review View, inactive members are color coded allowing the active/inactive setting to be toggled graphically.
Alternatively, you can change the setting manually in the Properties window as follows:
- Select the member in a 2D or 3D view.
- In the Properties window deselect Active.
Inactive member load decomposition
The first stage of load decomposition treats active and inactive members in exactly the same way.
- End reactions are calculated for the inactive members.
- These are then applied as nodal forces to the supporting members.
After decomposition has taken place analysis can proceed with the inactive members removed from the solver model.
In the second stage the end reactions of the inactive beam have been applied as nodal forces to the supporting edge beams in the first bay causing moments to be generated in them, (without this second stage the two supporting beams would have otherwise been completely unloaded.)
No moments are generated in the inactive beam itself because after the decomposition has taken place it does not exist in the solver model.
Typical usage cases for inactive members
There are potentially many uses for inactive members, the following being some examples.
Inactive members can be used in conjunction with ancillaries in order to apply loads to the structure while not themselves participating in the analysis.
Purlins and Rails
In large portal sheds, purlins and rails may be included in the model to act as a means of load distribution; the engineer would not want to see the forces in them when reviewing forces in the main structural members, or consider the small axial loads that develop in purlins and rails when gravity loadcases are analysed.
By setting the purlins and rails to be inactive these goals are achieved.
Design can be speeded up by enabling group design and limiting the number of active members in a group.
- You are responsible for choosing critical beam(s)
- Primary beams cannot be set to inactive