Modeling a negative stiffness isolator in ETABS involves the following steps:

1. Create a new ETABS model: Open ETABS and create a new model. Set up the units and coordinate system based on your project requirements.

2. Draw the floor plan: Draw the floor plan of your structure using the ETABS drawing tools.

3. Define materials and sections: Define the materials and sections of the structure, including the negative stiffness isolator.

4. Assign loads: Assign loads to the structure, including gravity and lateral loads.

5. Define properties of the negative stiffness isolator: In the property dialog box, define the properties of the negative stiffness isolator, including its stiffness, damping, and effective mass.

6. Specify constraint conditions: Specify the constraint conditions to model the negative stiffness isolator.

7. Assign support conditions: Assign support conditions to the structure, taking into account the negative stiffness isolator.

8. Perform the analysis: Analyze the model to determine the structural response, including the displacement and stresses of the negative stiffness isolator.

9. Review the results: Review the analysis results to ensure that they are consistent with the expected behavior of the negative stiffness isolator.

10. Make adjustments if needed: If necessary, make adjustments to the properties or the modeling approach to achieve more accurate analysis results.

By following these steps, you can effectively model a negative stiffness isolator in ETABS and analyze its impacts on the structural performance of the building.

Regarding the second part of your question:

ETABS provides an option to incorporate nonlinear material behavior in the model, including the behavior of shape memory alloys (SMAs). As SMAs have unique characteristics of super elasticity and shape memory effect, using them as a material for base isolators in ETABS can provide excellent performance in mitigating the effects of seismic loads.

To use SMAs as the material for base isolators in ETABS, you need to create a new material property in the Material Property Editor. In the Type tab, select 'Nonlinear Time History (Fiber)' or 'Nonlinear Static (Fiber)' as the material type, depending on the analysis you are performing. Then, go to the Property tab and select 'Shape Memory Alloy' from the drop-down list of materials.

In the SMAs' properties, you need to specify constants such as austenite start temperature, austenite finish temperature, martensite start temperature, and martensite finish temperature, etc. Once you have defined the SMA material properties, you can use it in the base isolator definition.

Next, you will define the geometry, including the mechanical properties, of the base isolation system. You can assign the base isolator material property that you created earlier to the isolator element.

After defining the base isolation system geometry and attaching it to the structure, you are ready to perform the structural analysis.

Note that to use SMAs as the material for base isolators in ETABS, you need to have a thorough understanding of the behavior of these materials. Additionally, you may need to use specialized software to model the behavior of these materials accurately.

You can use