ShopShoe's suggestion is not just good, but essential if you want to be able to adjust this, or to be able to move it without a forklift. Beyond that, I am not in a position to offer advice on materials. I am not an engineer, and am not qualified to judge the strength of various materials and construction techniques - and even if I were, I'd need a lot more information, e.g., what is the maximum unsupported span, what is the maximum weight and how will it be distributed, etc.
As a home hobbyist, making my own shelves, I can over-engineer a shelf for the expected load, WITH triangular bracing, of course. (The experience that ShopShoe shares could easily have been fatal for one or more persons!!) But honestly, I don't know for sure my home-built shelves are truly sufficient, and I am careful how much I stress them. And if worst comes to worst and they fail, the only one that will be hurt, or suffer property damage, is me. I'm not willing to take that risk for someone else.