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Dear stko team
Is there any material in stko that can simulate nonlinear spring?（Using zerolength element）
Its spring stiffness can not be a constant, but a customized curve

There are many. Try the Pinching4

Dear stko team
i have two qusetions
（1）I saw physical properties from Pysimple and Tzsimple in stko.
If I build a two-dimensional model and want to use them to simulate the zerolength connection between the liquefied soil layer (quadup element 3dofs) and the underground structure (electrical beam column element 2dof)
Can I only use the zerolengthcontactASDimplex element? Is there any other zerolength element that can be used?
（2）If the underground structure is not a pile foundation, but a tunnel or subway station, can Pysimple1 and Tzsimple1 materials be used to simulate the contact between soil and structure?

（1）I saw physical properties from Pysimple and Tzsimple in stko.
If I build a two-dimensional model and want to use them to simulate the zerolength connection between the liquefied soil layer (quadup element 3dofs) and the underground structure (electrical beam column element 2dof)
Can I only use the zerolengthcontactASDimplex element? Is there any other zerolength element that can be used?

The zeroLengthASDimplex is for contact. If you want to use those 2 uniaxial material, use them with a standard zeroLength element.

（2）If the underground structure is not a pile foundation, but a tunnel or subway station, can Pysimple1 and Tzsimple1 materials be used to simulate the contact between soil and structure?

Yes, with a standard zeroLength element as exaplained above

ummm..actully what I want to ask is whether the zeroLengthASDimplex element is the only zeroLength element used to connect 2dof and 3dof nodes

Oh yes, it's the only element that supports different DOFs at the 2 end-nodes.
With other elements, you need to duplicate nodes, and use the EqualDOF command to connect corresponding DOFs