When contractors plan bridge deck forms, they must carefully weigh the advantages and disadvantages of the available forming systems and materials. Many bridge span superstructures incorporate concrete or steel girders on piers. These girders sustain the deck's formwork. Supporting members of the formwork are usually attached in one of two ways: They either hang from the beam's upper flange or they sit on the beam's lower flange. Of course, these two general methods represent interior formwork construction in its broadest sense; contractors also incorporate a wide range of formwork detail, depending on the circumstances of each project. The common features of bridge overhangs - guardrails and sidewalks - require their own type of formwork, but they are also supported from the flanged concrete or steel girders. Sidewalks, for example, are usually cast at the same time as the deck; however, compared to interior formwork, their formwork requires more hardware, such as brackets, to support the system. Manufacturers of bridge deck forms often include such accessories in their systems.