Regarding pile stub creation in SACS

Hi friends,

I would like to find natural frequency for a tower along with the linearised pile. (by creating a pile stub)

While creating pile stub, I would like to know why stiffness matrix is created only for lateral loads at pile head joint. In my case, while calculating natural frequency I don't have any lateral deflection/force at pile head joint. In this case, people in the forum kindly guide me how to proceed further. 

Also while assuming pile stub behavior, kindly explain on how this rigid link and elastic behavior is assumed.Thank you in advance.