Can you focus on the feeling of your body on the chair for reaching the jhanas? I feel like focusing on the feeling of my feet on the ground is easier than focusing on the sensations of the breath. Is this a viable object for access concentration/jhana?
Jhana is not "access" concentration. Jhana is called "attainment" concentration.

Focusing on the feeling of feet on the ground may result in access concentration (because the mind may be relatively open, pliant &amp; non-attached when focusing on the feeling of the feet) however: 


The focus should not be overly forced or rigid because, for access concentration to occur, the mind must have the pliancy to fall into the body. 
When access concentration occurs, the mind will automatically experience the breathing.


Methods such as feeling the feet on the ground; listening to silence via the ears; or sitting with a very quiet non-doing non-ambitious mind are often more effective in manifesting access concentration (than clumsy direct attempts at watching breathing). Because the Buddha actually taught his Path is the abandonment of craving, often practise is counter-intuitive.