Bridge Width. Roadway width on bridges is the inside measurement to the bottom of the sidewalk curb or the bottom of the safety barrier. For bridges on roads where sidewalks are not provided, the bridge width is made equal to the approach roadway width including shoulders, so that the bottom of the barrier curb or the near face of the railing is aligned with the face of the barrier rail at the outside edge of the shoulder.
In the past, policy did not always permit full shoulders to be accommodated on bridges. Often the roadway was made narrower, particularly on longer bridges. This was done strictly to reduce bridge cost. From the traffic operations standpoint, however, it was an unwise practice. Disabled vehicles could not find refuge on the shoulder, and a full shoulder was not available for temporary maintenance of traffic during road rehabilitation or repaving. It is now recognized that a bridge is an integral part of a highway system when it comes to roadway width. The FHWA requires a minimum shoulder width of 8 ft (2.4 m) on each side of the roadway on federally funded projects.
Bridge Horizontal Clearance. For bridges over streams, the location of substructure units, and therefore the length of spans, is controlled by hydraulic requirements and by navigation clearance requirements established by agencies such as the U. S. Coast Guard and the U. S. Army Corps of Engineers. For bridges over navigable waters, the bridge designer should also consider the possibility of collisions from vessels. Refer to AASHTO’s Guide Specifications and Commentary for Vessel Collision Design of Highway Bridges.
For crossings of highways, the bridge columns or pier walls should clear the traveled way, shoulders, ditches where required, barrier rail, and any additional width required to provide a safe clear zone from edge of pavement. A minimum clearance of 30 ft (9.1 m) from edge of pavement is required except where this clearance is impractical, in which case the pier or wall may be placed closer to the edge of pavement, with barrier rail 2′-0" (610 mm) minimum from edge of shoulder, and pier or wall 2′-0" (610 mm) minimum from face of barrier rail. The barrier rail offset from face of pier or wall will be further controlled by the dynamic deflection of the particular system used. (See Chap. 6 for additional information.)
For crossings over railroads, the horizontal clearance requirements are usually set by the railroad company or by the state public utilities commission. In addition to clearance for safe operation of trains, including allowance for accidentally overhanging cargo, railroad companies are cognizant of the importance of trackside drainage and require that drainage ditches be accommodated where present. In addition, a maintenance roadway for off-track equipment is often required. A horizontal clearance of 25 ft (7.6 m) from the centerline of the track is desirable and will obviate the need for pier crash walls.
If a pier adjacent to a railroad track is located closer than what is considered to be an adequate distance to prevent derailed cars from striking the pier (generally 25 ft (7.6 mm) from centerline of railroad track), the pier is required to be of heavy construction, or a substantial crash wall is required to be constructed to protect the pier and prevent catastrophic collapse of the bridge. This wall should be aligned with the pier. For additional details, refer to the American Railway Engineering and Maintenance-of-Way Association (AREMA) Manual for Railroad Engineering.
AASHTO LRFD Specifications require abutments and piers located closer than 30 ft (9.1 m) to the edge of the roadway or closer than 50 ft (15 m) to the centerline of a railway track to be designed for a vehicular collision load defined in the specifications, unless protected by an embankment or a structurally independent, crashworthy, ground — mounted barrier.
Bridge Vertical Clearance. Generally, a clearance of 16 ft (4.9 m) plus an allowance for resurfacing should be provided over major state, U. S., and interstate highways, over the entire width of roadway. Over less important highways, a clearance of 14 ft (4.3 m) should be provided. These are AASHTO requirements. Published state standards, if different from AASHTO, should be followed.
The above vertical clearances apply to vehicular bridges. Because pedestrian bridges are narrower and lighter in weight, and therefore more vulnerable to major damage or collapse in the event of collision from overheight vehicles passing under the bridge, states are beginning to require an additional clearance of 1 ft (300 mm) for pedestrian bridges. This additional clearance is also recommended for overhead sign structures.
Vertical clearance requirements over railroads, like horizontal clearances, are set by the railroad company or state public utilities commission. A minimum clearance of 23 ft (7.0 m) above high rail is common for new bridges over nonelectrified racks. If the tracks are electrified, an additional 1-ft (300-mm) minimum clearance is required for catenary wires. Widened or rehabilitated bridges will generally be allowed to maintain the existing clearance, but no less.
Vertical clearance requirements over navigable waterways are subject to bridge permits by U. S. Coast Guard (USCG). USCG has established guide clearances for particular waterways. They are not regulatory in nature and greater or lesser clearances meeting reasonable needs of navigation for a particular location may be required or approved by USCG.
