Cutting door or window openings into the sides of any Quonset hut severely weakens the structure. This weakness must be compensated for, or you risk total structural failure and collapse of the hut. When you order a brand new Quonset hut building and include side openings in your order, the factory will account for the openings in their engineering, and the structure should be sound if erected carefully following their instructions.
To add side openings to an existing building, the safest way to do it is to contact the factory that originally engineered the structure, and ask them. They should be able to review the records and structural calculations for your building, and in many cases the will probably be able to sell you a window kit to match your building's arch profiles that will safely accommodate the structural requirements. They might need you to replace the arches on either side of the new window with heavier gauge arches to help compensate for the weakness induced by the window hole. In our Headquarters Hut at the Quompound, the arches on either side of the windows are heavier gauge than the rest of the building, to help compensate for this weakness. This is pretty common with side windows from what I understand.
The thing you have to realize about the structure of a Quonset hut, which isn't immediately obvious, is that in addition to the gravitational force of the building pushing straight down, the arched shape also induces an outward thrust at its base. The slab foundation that is most commonly used with a Quonset hut is actually under tension, serving to tie the outer foundation lines and preventing the bottom of the arch from springing outward. Imagining the base of the arch wanting to spring outward, now imagine cutting away two or three of the arches to make a window opening. You have now created a section of the Quonset shell where, at the top of the window, there is nothing to resist the outward push naturally exerted by the arch shape.
I have seen, with my own eyes, a Quonset hut that collapsed because of an improperly engineered side window. Thankfully no one was hurt; it was not a pretty sight. Cutting holes in a Quonset hut without involving the original factory that manufactured it is not a risk that I would advise anyone to take. The collapsed building was not one of my own projects or clients; I was simply allowed to view it before it was demolished and replaced.
So again, side windows can be added to existing Quonset huts, but I would never attempt it without the full cooperation of the original factory engineers.