-ve

I also need to define a Gap NLlink to model the gap between a shelf and an adjacent wall. How do you determine the stiffness of the gap?

It depends on the connection details between the shelF and the wall. Without knowing the details, the Gap cannot be defined.

Gap is a compression only spring, and SAP2000 provides an option to allow you to control when it come into functin by defining a real seperation (gap) between the Gap NLink element and the supported Joint. The defined stiffness only come into effect when the deformation exceed the defined seperation.

Can someone please explain why SAP2000 need to activate Modal analysis (Ritz method) such that nonlinear Gap elements are effective.

Please help!

In another software SpaceGass, compression-only members uses an iterative analysis method, SPACE GASS puts it into a slightly different category to the other non-linear effects and makes it available in either a linear or a non-linear static analysis.

During the iterations SPACE GASS disables compression-only members which have either end in tension.

It seems this is totally different from SAP2000 which use Modal analysis.

Which method (iterative or Modal) is better?

jacky, you only need Ritz vectors if you're doing a time history or other dynamic analysis. SAP does not require you to run a modal analysis unless you're doing dynamic analysis. And with version 9, SAP started offering direct integration time history which does not require modal.