Where possible, bridges should be made continuous. Continuous spans are less prone to catastrophic collapse from loss of substructure support due to stream erosion, earthquake, or vehicle or vessel collision. Bridges with multiple simple spans must have two lines of bearings and an expansion joint at each intermediate support. Two lines of bearings, each having the required capacity for the end of a simple span, will almost certainly be more expensive than the single line of bearings required for continuous spans. Expansion joints are expensive and in most geographic locations should be sealed against storm drainage and intrusion of debris, which further increases their cost. (Even the manufacturers of sealed expansion joints agree that the best joint is no joint.) Aesthetically, continuous bridges are generally superior, especially if constant depth is maintained, and do not require the cosmetic plates or other devices that have sometimes been used to conceal the gaps between simple spans.
Continuous bridges are generally more economical than simple-span bridges because of the reduction of mid-span moments. Most bridges can be designed continuous for live load, and some bridges may be designed continuous for dead load as well. In the case of precast prestressed-concrete bridges, it is generally more convenient and economical to place the deck slab concrete while the beams are supported on their bearings, without temporary intermediate shoring, so that the beams are not continuous until the deck slab has acquired its strength and top longitudinal reinforcing bars are present in the composite section over the piers to resist negative moment. Therefore, these bridges are designed continuous for live load and for superimposed dead loads (loads above the deck slab) only. Note that this type of construction unavoidably requires two lines of bearings at intermediate supports because practical prestressed-concrete design and construction require that the spans be simple initially. This is called “made-continuous” construction.
There are situations where simple spans are preferable. Examples include situations where adjacent spans are unavoidably different in length and depth, or where adjacent spans have widely different geometrics with beam layouts that do not lend themselves to continuity, such as varying beam spacing or splayed framing. Simple spans may also be preferable where the bridge is part of a facility, such as an interchange, where stage construction will require future removal or addition of one or more spans. Simple spans are also desirable where differential substructure settlement is anticipated.
