A steel door built in the early '80s has held up fine structurally in most cases, but the bottom panel is the one that takes the abuse - road salt kicked up off a plow blade, a bike pedal clipping the edge, a car bumper backing in too far. Ontario sees the same cold-severe winter exposure as the rest of the area, and salt spray plus freeze-thaw cycling is what eats through the galvanized coating on a bottom panel faster than anywhere else on the door. Once the coating goes, rust works from the inside of the steel skin outward, and by the time you see bubbling paint, the panel's usually thinner than it looks. Upper panels rarely see this - they fail from impact, not corrosion, so a cracked top section is almost always a ladder, a stored item falling, or a spring that let go and dropped the door hard.