Why is the stiffness prioperties of Lead Rubber Bearing different under the headings  'Linear Properties' and 'Non- Linear Properties'?

Question:

In the attached screenshot of Lead Rubber Bearing Properties.  , in the Linear Properties, the stiffness is defined as 336.1 tonf/m, then in the Nonlinear properties the stiffness is defined as 1099 tonf/m. How come the stiffness differs in both cases? as it changes from 336.1 to 1099? shouldn't the stiffness be the same?? Explain this in detail.

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Answer:

The linear effective stiffness of LRB is used to compute the fundamental natural period of a base isolated bridge. It is determined as below. Therefore, the linear effective stiffness can be different from the initial stiffness of nonlinear properties.



You can refer to the articles below for more information.
http://www.iitk.ac.in/nicee/wcee/article/13_3332.pdf
http://www.iitk.ac.in/nicee/wcee/article/WCEE2012_5756.pdf