HORIZONTAL STRUCTURES

Horizontal systems come in two types: one way and two way. Two way systems are only  efficient for spaces with about equal span in both directions; as described below.  The  diagrams here show one way systems at left and two way systems at right

1  Plywood deck on wood joists
2  Concrete slab on metal deck and steel joists
3  One way concrete slab
4  One way beams
5  One way rib slab
6  Two way concrete plate
7  Two way concrete slab on drop panels
8  Two way concrete slab on edge beams
9  Two way beams  
10  Two way waffle slab
11 Deflection ∆ for span length L1
12 Deflection ∆=16 due to double span L2 = 2 L1

Note: Deflection increases with the fourth power of span.  Hence for double span deflection  increase 16-fold.. Therefore two way systems over rectangular plan are ineffective  because elements that span the short way control deflection and consequently have to  resist most load and elements that span the long way are very ineffective.