thank you for reply, 

from your answer what i understand that i have to copy the face of solid body which is in contact with fin then use share topology, i did in this way but next thing is i need to give thickness in mechanical and created the bonded contact between the solid and base surface it does work but result was totally different than what i was getting with bonded cotact bewteen shell edge and solid surface , so i decided to use this option only.

Next thing i observed that when i using share topology option form workbench then the share topology option which is in analysis is not available, Why is this so? also if is use the share topology available in Analysis option then it will create one group of geometry in Mechanical geometry tree which is good otherwise every other is any where in geometry. I meant to so geometry is very well sorted out like in spaceclaim but workbecnh all parts arranged in random way in geometry tree.

As now i need to use only bonded contact for all of the situation this and other like where surface of shell bodies is in contact with solid surface so can you please tell me can i use project option to create the surface are which is exactly in contact so that i can give bonded conatct only in that part of area? 


While working on this types of problems i came across one interesting thing is that if shell surface is like C section as shown in below fig the the share topology option is working , i have attached the result here see there is no zero frequency.



Note: Please excuse me for these many questions