Open Metal Grid Deck
An open grid deck consists of the primary and secondary steel members only, as the system does not include any reinforced concrete. Figure 10 shows a section of an open steel grid deck. In this figure, the primary supporting bars are I-shaped, while the secondary members are simple rectangular bars that are perpendicular but interlock with the primary members. Additional simple rectangular members interlock with the secondary members, and run parallel to the primary members. These members help to provide the ride surface of the open metal grid deck. With open grid decks, serrations are typically provided on the top of the grid members to enhance vehicle traction.
Per Article 9.8.2.2 of the AASHTO LRFD (7th Edition, 2014), open metal grid decks must be connected to the supporting components (girders, stringers, cross beams, etc.) by welds or mechanically fasteners at each primary member of the grid deck system. Using shear studs and a narrow, full depth concrete pour over the supports is another method of connecting open grids, if the minimal additional weight is permitted. Furthermore, welding within the open grid decks should typically be considered as a Category E detail.
An advantage of the open steel grid deck is that it is lightweight compared to conventional reinforced concrete deck slabs or filled metal grid decks. However, it also has several disadvantages including a perceived unpleasant ride quality, additional noise, possible safety issues when wet, and debris, road salts, and water passing through the deck and onto the features below the bridge. Also, open metal grid decks are prone to fatigue issues, both internally and in their connections to girders or stringers. Open grid decks are rarely specified on new construction projects, and are typically only used for in-kind replacements. The use of open metal grid decks is typically limited to situations in which weight savings is the main driver, so as to increase the live load carrying capacity of the structure, or where these systems were installed many years ago. Older truss bridges and movable bridges are examples of where one may see these systems employed.