Each material has unique damping properties. If you want to analyze a structure composed of both steel and concrete you will have two different RS data with different damping properties for each material.
For example the first floor of a building is made of concrete and the floors above are made of steel. You can set different damping values for each material, but the RS curve to be used would only have a single damping value. To solve this you can define two different RS data pairs with unique damping properties for each material and assign them to their respective elements for analysis.
This video will walk you through the procedure to define multiple RS data sets with different damping properties and instruct the program to analyze a structure with multiple damping properties.
