Can a stair be designed only to go up but not down?

This depends on how theoretical or abstract you want to get.

You can have a mechanical system that moves upward faster than you can get down (think of a fast escalator), or some kind of impediment making travel in one direction difficult to the point of near impossible (think gates or barrier you must move past).

You can design a stair that most people would prefer to only travel in a single direction (people will go up steep stairs more readily than go down out of fear of falling).

The most direct version of such a thing would be found in platform style video games. Only a few steps would be available to walk upon at any given moment. As you proceed along the intended direction steps would be removed from the anti-direction and added to the positive direction. As such you would have a stair traveling in only one direction.

Of course, such a thing would be difficult to build, expensive and have limited value to users. Unless you like video game scenarios.80 viewsView Upvoters