Bridge: Length/Span Configuration
Bridge lengths shall be sufficient to span over the roadway or facility being crossed. Longer bridges with short abutments are preferred since they create an open structure while providing added safety in the ability to see beyond the bridge. Although this results in a bridge that is longer than it would need to be with full height abutments, the tall abutments and foundations costs generally offset the longer superstructure costs. Tall abutments, especially those located adjacent to a paved roadway, can also be visual obstructions and require additional impact damage protection. Shorter abutments make inspections easier, and the additional length provided under the bridge could be used to accommodate future widening, if needed.
All bridges are rated for functional criteria to determine if a bridge is classified as Functionally Obsolete. This includes horizontal under clearance, which evaluates clearance from the bridge substructure unit to the through roadway. The required minimum horizontal under clearance is based on the roadway type. A rating value from 0 to 9 is assigned with any rating below a 4, resulting in the bridge being classified as Functionally Obsolete. In order to avoid jeopardizing federal funding, ensure the minimum tolerable limit is met for new or replacement bridges. At a minimum, establish a bridge length that will not result in the bridge being classified as Functionally Obsolete.
Roadway Configuration Under a Bridge
The lane, shoulder, and sidewalk widths under the bridge should be consistent with the lane, shoulder, and sidewalk widths on the roadway beyond the bridge. The exception is when determining shoulder widths under a bridge. In these cases, the starting point should be the minimum horizontal under clearance to a substructure unit or concrete barrier. In general, shoulders under bridges should match the roadway width at the bridge if this is wider than the NBI requirements. (Add clearance to railroad tracks)
The bridge span configuration should be evaluated to provide the most cost effective structure. For long bridges, options should be investigated with fewer piers, which would have lower substructure costs and higher superstructure costs, and compared to options with more piers, which would have higher substructure costs and lower superstructure costs, in order to find the most cost effective span configuration. For bridges crossing roadways, shoulder piers should be avoided since they create an obstruction and may prohibit future roadway widening. For bridges crossing waterways, piers should be kept out of the waterway if possible since they require dewatering and cofferdams for construction, are susceptible to scour, and can cause debris jams restricting flow potentially resulting in flooding. Removing debris is difficult for maintenance forces, as access to these areas is limited and often requires a permit. Keeping piers out of waterways is also a desirable practice from an environmental permitting standpoint. Refer to Bridge: Stream Crossings for more information on Piers in waterways.
Design for Future Considerations
Prior to and throughout design, contact the appropriate MDOT regional planning office to identify any proposed future capacity improvements for the roadway or other facility the bridge is spanning that would result in the need for a longer bridge. The time frame for the proposed improvement should be considered. If the roadway or facility widening is likely in the next 20 years, then accommodating the additional width with a longer bridge may be warranted depending on the project budget. In general, if the time frame is uncertain then the scope of the project may be limited to address only the current needs and requirements or consideration should be given to a structure design that will enable the bridge to be lengthened in the future.
Capacity Improvement Project
Major capital improvement projects often include replacing and lengthening existing bridges. When a roadway widening, project impacts an existing overpass bridge, the initial thought may be to replace the bridge to span whatever roadway typical section is proposed, which may result in unnecessary cost. Depending on the existing bridge superelevation or profile, widening the roadway under it may reduce the vertical underclearance, making a full bridge replacement necessary. However, if this is not an issue and the bridge is in good condition and not expected to need major rehabilitation in the next 20 years, keeping the bridge may be an option. If the answer is yes to any of the following questions, then eliminating the bridge replacement should be strongly considered:
Grading Under the Bridge
Often proposed roadway typical sections include safety grading and drainage ditches, which can significantly increase bridge lengths. For bridges spanning over roadways, coordinate with the roadway designer to see if the safety grading and drainage ditches under the bridge can be eliminated and replaced with a concrete barrier at the shoulder edge that retains the fill slope. This will shorten the bridge length while still providing an open bridge. Similarly, when there are sidewalks under a bridge, consider placing a concrete barrier at the sidewalk edge with fill behind it, to help minimize the bridge length.
Environmental entities often request that bridges be made longer to avoid impacts to wetlands, forests, floodplains, etc. Before agreeing to this, the benefits of avoiding the environmental features should be compared to the additional construction and maintenance costs of a longer bridge. For example, simply lengthening a bridge may not save the wetland if the wetland will be in the shadow of the bridge, causing all the vegetation to die. It may be more economical to build a shorter bridge and provide mitigation for the impacted wetland at another location, especially if the area of the wetland is relatively small. Another frequent request from environmental entities is to span the entire floodplain, resulting in bridges much longer that required from a structure hydraulics standpoint. Every effort should be made to keep the bridge only as long as it needs to be to meet the design requirements, while taking into account stream geomorphology, recreational use, and wildlife requirements.