'Linked' Microstran Model

You should probably attach the model in question, or at least some images of the problem using the advanced reply editor. The best way to diagnose it in my opinion is to see if the two variations of the model lead to different member forces or not. If so, then the links are changing the analytical behavior by connecting members differently. The check input command might help too. If the forces are the same, but the designs are different, then I have to assume it's due to a change in the default unbraced length.

 

I hope this is attached properly.

I do believe they are changing the default unbraced length. But how can we later add a twist restraint? it seems like we can add a lateral restraint, which doesn't change the default unbraced length (I assumed it would have).

I also add that the bending moment is the same for both situations indicating that it is changing the default braced length. But why does it do this, and when is it appropriate to link or not link the members?

Thank you :)

Generally speaking, you would initialized members as "linked" if collectively they form a design member, if you initialized members as "unlinked", then each analytical member in Microstran would be one design member.

Use the Initialize Design Members > Not Linked command if you want each selected analysis member to be a design member. Twist restraint is then assumed at each end of each analysis member. If this twist restraint does not exist unlinked initialization is not valid and you may obtain unconservative results (For example, truss with unsupported chord.)

In Microstran, every design member that is not a cantilever has twist restraint at each end. Interior nodes that do not have twist restraint make it necessary to link analysis members together forming a design member. On initialization, no twist restraint is assumed at intermediate nodes in linked members.
You can add restraints in the Steel Restraint Dialogue.

The restraints implicit with Linked vs Unlinked becomes apparent when you view the restraint data. To illustrate this, you can view the restraints prior to design by going to View-->Virtual Reality, try this for both Linked scenario as well as Unlinked scenario.

You want to ensure that the restraints presented in these views, are indeed the intended physical restraints.

For example, if you have a linked design column that spans many levels, then Virtual Reality view will show that it is completely unrestrained all along its length. But each level will provide restraint to the column, thus dramatically increase its design capacity. Design Restraints such as Column Buckling Restraint and Flange Restraint can be added by going to the Restraint Dialogue which can be accessed by going through the dialogues via Design-->Design Data-->Input/Edit.
For further reference, Chapter 16 of The Microstran Manual also goes through this process in detail.