In RAM Structural System we make the distinction between gravity-only members and lateral members (lateral members are really lateral and gravity members) so that the size of the finite element matrix used in Ram Frame is compact, excluding all the gravity-only members which don't offer any significant stiffness. STAAD.pro does not have the same division of the structure into gravity and lateral, but it does include a command called inactive member that can be applied to the gravity only members for the lateral load cases to yield a similar result. Here's a saved search you can use to find out more.
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