Filled and Partially Filled Metal Grid Deck
A filled metal grid deck is a deck in which the entire depth of the grid system is completely filled with concrete, as shown in Figure 11. Filled grid systems were first introduced in the 1930’s as a method to increase speed of construction on large scale bridge projects. Filled grid decks can utilize precast or cast-in-place methods for the reinforced concrete. Mild reinforcement is required to satisfy concrete serviceability and durability requirements, and is typically located perpendicular to the main supporting bars. Thin gage sheet metal rests between the bottom flanges to support the full depth concrete.
A partially filled metal grid deck is a deck in which only a portion of the depth of the grid system is filled with concrete, as shown in Figure 12. Partially filled grid systems were first introduced in the 1950’s as a method to further reduce weight by eliminating concrete from the bottom of the deck which is in tension in simple span applications. Partially filled grid decks can utilize precast or cast-in-place methods for the reinforced concrete. Mild reinforcement may be used to satisfy concrete serviceability and durability requirements, and is typically located parallel to and in between the main supporting bars. A rib, located near the mid depth of the primary I-beam in the deck system is used to support thin gage sheet metal. The thin gage sheet metal supports the wet concrete, creating a partially filled system.
Per Article 9.8.2.3.1 of the AASHTO LRFD (7th Edition, 2014), a 1.75 inch thick structural overfill should be provided when possible on filled and partially filled metal grid decks, although a 2 inch overfill is more common. Overfill is taken as the concrete thickness above the metal portion of the grid deck. The concrete overfill provides protection for the steel portion of the grid deck from chloride ion attack. The steel grid deck should also be galvanized or painted to further protect the deck system. The overfill and galvanizing (or painting) limits the corrosion in the grid deck steel. Heavy corrosion of the grid deck can lead to a phenomenon often called “grid growth,” which impacts the performance of the grid deck.
Filled and partially filled grid decks can be attached to the supporting components (girders, stringers, cross beams, etc.) by welding, or more commonly by shear stud connectors, to transfer the shear between the deck and the supporting component. If shear studs are employed on the supporting components, a haunch should be provided so that the shear studs are completely encased in concrete. The shear studs should extend into the grid deck similar to a conventional reinforced concrete deck slab, but typically do not need to extend into the overfill.