With the principal structure of columns, beams, slabs, and load bearing walls you can form an elementary structural unit capable of enclosing space for in-habitation. But with this comes the different kind of spans. Those two supports are the horizontal and the vertical span which are both supported by different structural elements. Horizontal may be crossed by concrete slabs or by a layered joist supporting planks. The vertical span may be provided by load-bearing walls or columns and beams. With these spans come the one-way system and the two-way system. Each of these systems have their moments where they’re most effective.