Bridge: Horizontal and Vertical Alignment

Horizontal and Vertical Alignments

Primary Guidance When possible, avoid locating bridges on curved horizontal alignments, since curved bridges are more expensive than straight bridges . Cross slopes should be constant across a bridge. Bridges should not be located in a sump (low point) in the roadway profile. For additional information, refer to Horizontal Alignments and Vertical Alignments in the Roadway section.

Discussion Curved Horizontal Alignments If possible, avoid locating bridges on curved horizontal alignments, since curved structures are more expensive to fabricate and are more difficult to construct than straight structures. If a bridge falls within a curved roadway alignment, see if a tangent (straight) section can be introduced into the curve which encompasses the limits of the bridge. If the radius of the curve is large enough, consider constructing the bridge with straight girders and deck, with variable width shoulders. To determine if this is feasible, determine the ordinate between the inside curve gutter line and the chord created by connecting the location of the gutter line at the center line of bearing at each abutment. If this value is less than 1’-0”, then the structure should be laid out with straight girders and deck. Cross Slopes The preferred minimum cross slope on a bridge deck is 2%, which will facilitate drainage from the travel lanes and helps to avoid an icy bridge deck. Constant cross slopes are preferred across the entire bridge length to reduce complex deck construction operations. It is possible to accommodate transitional cross slopes in a bridge deck, however the transition from normal crown (2% in each direction from the crown) to plane incline requires a longitudinal joint at the crown/pivot point and complicated camber in the girders to account for the unusual deflections that will occur. It is also possible to have transitioning superelevation across a bridge, which would occur if it was in a reverse curve. This should be avoided when possible since it will result in a flat area on the deck when the superelevation is 0%, which does not facilitate the drainage of water from the travel lanes and can lead to safety and maintenance issues. If it is not possible to adjust the alignment so the bridge is not within the area of transitioning superelevation, scuppers should be placed on either side the flat area to capture drainage from the bridge deck before it reaches this point. Sumps in Roadway Profile Bridges should not be located in a sump (low point) in the roadway profile. Sumps on bridge decks will result in an area where water will collect and can lead to safety and maintenance issues. Even when the sump is moved off the bridge, the roadway elevations should be checked to ensure the bridge deck is not essentially flat as you approach the sump. If it is not possible to avoid a sump on the bridge, scuppers should be placed at the sump and on either side of the sump to ensure sufficient drainage.

See Also

• Roadways: Vertical Alignment