How to evaluate spring stiffness of exisiting steel structure?

[SilverBeam75]

My project involves to build a new steel structure for an elevator which will be attached to the existing structure. Each node which will be attached I would like to model it as a spring. But how do you evaluate the stiffness of an existing structure ? If I design my structure without any attachment, it will sway a lot because its very narrow and high. Any suggestions? (see image)

Thank you,