Design of structure

Ok, start the design. No need to inform us, you can design any structures that you want.

I would suggest hiring the services of a structural engineer to help you out. A specific design such as this can not be accomplished over an internet forum, especially with the lack of information given, for example:

• How tall of a structure is required?
  
• What is the structure going to be mounted to? Concrete foundation? Wooden roof?
• What are the size/dimension limitations to install this structure?
• What does this loading arm do? Does it lift 2000 lbs? Does it reach out 10'?
• Where are you? What codes are to be followed?
  
• ..................
• ...................
• ....................
  

Thinking along the lines of a jib crane assembly here:

Your solution may be as simple as this example, depending on your exact loadings (moments created) and requirements:

Take a ten foot length of off-the-shelf 12" XXS or SCH. 160 (perhaps even lighter such as XS or SCH. 80) pipe, weld it vertical to a baseplate (say 3/4" thick), attach with a few gussets, anchor the baseplate into concrete with a few anchor bolts, mount your "loading arm assemby", and call it day