'Extreme' turbulence is a derived vertical gust velocity of >3,000 fpm (50 fps) that causes a vertical G-load of >3. The limit turbulence intensity for a modern day airliner is 90 fps. (there is likely a 50% safety factor, so the plane is capable of withstanding more).
Lateral turbulence is the type that puts most stress on things like engine pylons and the vertical tail, rather than the wings. An accident involving lateral turbulence is BOAC Flight 911 and the B-52 crashes (Savage Mountain and Elephant Mountain).