Bridges and other related major structures play an important role in defining the manner in which a highway affects the aesthetic, scenic, historic, and cultural resources of the corridor within which it is located. Indeed, some of the distinguishing features of a number of major cities are their bridges. When one thinks of San Francisco, one of the first images that come to mind is the Golden Gate Bridge.
General Guidelines for the Geometrics of Bridge Design
The geometric criteria in the AASHTO Green Book for new or replacement bridges deal primarily with the width of the bridge deck and its relationship to approach roads. Early design coordination is important when establishing the width of a new or replacement bridge and in determining its horizontal and vertical alignment. Road engineers, architects, and landscape architects, as well as members of the community, can provide input to help the bridge designer determine the appropriate geometric dimensions and overall appearance of the bridge. The AASHTO Green Book presents a range of options for traveled-way widths for bridges with a span of less than 30 m, depending on functional classification and average daily traffic.
On urban collectors and arterials, the AASHTO Green Book recommends that the minimum clear width for new bridges be the same as the curb-to-curb width of the approach street.
In addition to determining the width of the travelway, a bridge designer must consider the need for pedestrian and non-vehicular traffic over the bridge and the most appropriate method for accommodating it. This could include a wide shoulder, a raised sidewalk, or both. If sidewalks are on the approach road, continuity of the sidewalk over the bridge is important.
For existing bridges that do not meet the criteria for travelway width, the AASHTO Green Book recognizes that those that tolerably meet the criteria may be retained. It identifies some of the factors in considering the retention of existing bridges, including “the aesthetic value and the historical significance attached to famous structures, covered bridges, and stone arches.” Because of this, AASHTO has criteria for minimum roadway widths and minimum structural capacities for bridges that are to remain in place. It is important to consider this option for each aesthetically and historically significant bridge on a case-by-case basis, before deciding to demolish and replace it.
Source: Flexibility in Highway Design pp. 101-103
Published: 1997
