Is this type of wing design structurally sound?
The simple answer is yes. There are many examples of working tube and fabric aircraft, both ultralights and larger designs, that make use of wing designs similar to your example.

The more complex answer is that it depends on a lot of things:


Did the designer anticipate flight loads correctly?
Did the designer model flight loads correctly when validating the wing design?
Do the materials used to construct the wing have physical properties close enough to the model used during design?
Was the aircraft built correctly according to the design?
Was the aircraft damaged such that the structure was compromised?


I don't want to discourage you from designing and building your own aircraft--one of the most exciting things about ultralights is how accessible they are to people with little or no formal training. Before you build and fly though, you should either learn how to determine the safety of a wing design using analytical methods, or stick to copying proven designs exactly.