There are major flaws in your structural system. There is no load path for transferring lateral loads from top to foundation. They way you have modeled it, you are relying on moment connections, which, I think is not practical, and also not applicable for open web joists.
Why have you selected open web joist for 15 ft span. You can use hot rolled section
You have not modelled bracing required to prevent local buckling of compression flanges of your flexural members. You can ensure buckling restraint by running beams in longer directions of the shed as well; Span between the beams will depend upon required unbraced length. You will also need to put cross bracing in the plane of roof deck, as well in the plane of column grid-lines (in both direction) , at-least in one bay, to improve global stability of the system.
You will need to check, among others, if the model is using correct values of unbraced length, moment gradient factor (Cb) and effective length factor. If you want to use open-web joist, then you need to ensure that Etabs knows that it is built up section.
I suggest you to select compact section for you flexural members, and ensure that slenderness ratio of elements of column-section are low enough to ensure inelastic behavior.
You should be using 2D model.