Given your elevation, wind and snow load, I would look at steel trusses. My best advice is to go with a reputable builder with a good reputation, references, etc. They most likely will not be the cheapest bid, but will build to code and not take shortcuts. My trusses changed 3x before construction as the permitting dept. kept upping the snow load and wind shear. Fine with me! Really didn’t want the roof to fall on me. If you need ground excavating work done, it helps to have someone the builder has worked with before so they can coordinate.
Mine is 20x40. Works just fine. You can fit slanted kickwalls inside a 70’ x 136’ building and have a true 20m x 40m riding area. Plenty of room for a few jumps (make sure your trusses have enough head clearance).
Worry about your footing later. Construction will require ALOT of clean up before you can ride, so hold off on footing. Think about a building pad. If you need to raise the area the arena will be built on, you can have that first layer or two be part of your arena base. We started with some extra dirt, excavated from another place on the property to level the area, then 4-6" (can’t remember) of 2 1/2" minus rock, then built the arena. By the time the arena was built, that first layer was very compact. After clean up, including magnets to find nails, we built up the rest of the base. Then the footing.
Congrats on your arena!
