All new signs mounted on bridge structures shall be positioned such that the bottom of the sign or lighting bracket does not extend below the bottom of the bridge as shown in figure 10.1.2-1. The position of the sign does not need to allow for the future placement of lights below the sign. If lights are to be added in the future they will be mounted above the sign. To ensure that the bottom of sign or lighting bracket is above the bottom of the bridge, the designer should maintain at least a nominal 2 inch dimension between the bottom of sign or lighting and the bottom of bridge.
Sign brackets shall be designed to account for the weight of added lights, and for the wind affects on the lights to ensure bracket adequacy if lighting was attached in the future.
The AASHTO Standard Specifications for Structural Supports for Highway Signs, Luminaires and Traffic Signals specifies that the bottom of the sign or lighting brackets be 1.0 ft above the bottom of the bridge structure to insure that high vehicles will strike the stronger overpass structures first. There is no guarantee that the 1.0 ft dimension insures the overpass will be struck first. With sign sizes becoming larger, there are structural impacts when trying to place the signs higher on the bridge. Thus, reducing the 1.0 ft dimension to a minimal value is desirable.
cc: Mohammad Sheikhizadeh, Bridge Construction - 47354