Does Fastener Member Stiffness Need to be Multiplied by 2 in Calculations?

[djdoug99]

I am going through Shigley's Mechanical Design book and have a quick question on fasteners. Sometimes when calculating the spring rate for a member, you have to multiply the rate by 2, why is this? For instance in this problem
http://imgur.com/2ug9AaR

When the spring rate is calculated for both the cylinder head and cylinder, both spring rates are multiplied by 2 and then added together to get the total Km. (This is not a homework problem)

Thanks.

 

[pbuk]

I'm not familiar with the precise context, but could it be because the objects can expand/contract radially in two directions (whereas if fixed at one end could only expand/contract axially in one)?