Design Overview – How It Works

In Positive Bending


Standard Reinforced Concrete Deck


In a standard reinforced concrete deck, in positive bending, the concrete at the bottom of the deck is considered 'cracked' and provides no practical benefit. Thus, the effective depth and (stiffness) of the slab is reduced, and the entire bridge – superstructure and substructure – has to carry the dead load of this 'cracked' concrete.

Exodermic® Deck


In an Exodermic® deck in positive bending, essentially all of the concrete is in compression and contributes fully to the section. The main bearing bars of the grid handle the tensile forces at the bottom of deck. Because the materials (steel and concrete) in an Exodermic® deck are used more efficiently than in a reinforced concrete slab, an Exodermic® design can be substantially lighter without sacrificing stiffness or strength.

The neutral axis is located near the welds and punch-outs of the grid. This keeps the live load stress range low at these locations, generally eliminating fatigue as a limiting factor in design (though it is always checked).

In Negative Bending


Standard Reinforced Concrete Deck


In negative bending, a standard reinforced concrete deck handles tensile forces with the top rebar; concrete handles the compressive force at the bottom of the deck.

Exodermic® Design


Similarly, in an Exodermic® design, the rebar in the top portion of the deck handles the tensile forces, while the compressive force is borne by the grid main bearing bars and the full depth concrete placed over all stringers and floorbeams. Rebar can be selected to provide significant negative moment capacity for longer continuous spans and sizable overhangs.

Again, the welds and punch-outs in the grid are located close to the neutral axis, so fatigue, though checked, rarely limits design.