I seam to remember seeing a split tread ladder-way on one of the museum submarines I toured I think it may have even been semi spiral. And on a couple off shore rigs some smart tailed engineer saw fit to put some in the most inopportune places the only easy way to descend them was to hook your boots on the railing and slide down at the risk of being written up of course. Then there was that architect friend of mine who had me build him a spiral stair without a center pole splitting the treads to access his bunker style basement situated 30 feet under his house to have an escape way in case his elevator failed