I have been thinking for years about building an outside stair / loft ladder to climb from a platform to another one, one stair higher (3 meters). However, I never studied material strength, and I don't know anything about how to calculate my needs. Especially since talking with a friend, he mentioned that aluminium becomes brittle in cold weather and he has witnessed multiple accidents with ladders breaking in cold weather.
Here is the kind of material I wanted to buy
And here is the strength data