In accordance with National Bridge Inventory (NBI) requirements, all bridges are rated for functional criteria to determine if a bridge is classified as Functionally Obsolete; this includes deck geometry which evaluates a bridge’s clear roadway width (curb to curb). The required minimum bridge width is based on traffic volume, number of lanes, and the type of roadway. A rating value from 0 to 9 is assigned with any rating below a 4 resulting in the bridge being classified as Functionally Obsolete. This minimum width is described in Recording and Coding Guide for the Structure Inventory and Appraisal (SIA) of the Nation’s Bridges as the “the minimum tolerable limit”. For new or replacement bridges, Federal funding may be jeopardized by not meeting the functional criteria. At a minimum, establish a bridge width that will not result in the bridge being classified as functional obsolete.
Lane and Shoulder Widths
The AASHTO Policy on Geometric Design of Highways and Streets provides guidance roadway for widths based on the functional classification of the roadway. The recommended roadway widths often exceed the criteria established in the NBI. While most of the Interstate system and roadways constructed over the last 50 years were designed with shoulders meeting AASHTO criteria, a large portion of the system constructed prior does not. There are many roadways and bridges that only provide 24 feet of clear roadway where AASHTO criteria would call for 6-10 foot shoulders. Reconstructing miles of these roadways to meet these standards is not feasible given the financial, environmental and right of way constraints. Therefore, when establishing bridge width, the starting point should be to first meet the minimum NBI requirements for deck geometry. Second, the lane widths on the bridge should match the approach roadway lane widths. In general, shoulder widths on bridges should match the approach roadway width at the bridge if this is wider than the NBI requirements. If the existing approach roadway narrows for a short distance (<200 feet) to tie in at the bridge, then the bridge typical section should be widened to match the predominant approach roadway section beyond the short narrowed section right at the bridge. For longer bridges (length greater than 200 feet), consider carrying a reduced shoulder width across the structure, as allowed by NBI. Any reduction in shoulder width should take into consideration the functional classification of the roadway, traffic volumes including percentage of heavy trucks, accident data, sight distance for curved bridges, and the need to accommodate bicycles.
Provide a minimum 3 ft. wide shoulder to accommodate bicyclists even in locations where there are less than 3 ft. wide shoulders on the approach roadway. Providing a three foot shoulder on the bridge gives a bicyclist a refuge or buffer area on the bridge between the traffic and curb or barrier.
Where sidewalks extend along the approaches to the bridge, provide sidewalks on the bridge. Sidewalks should connect to existing facilities that have logical termini. The minimum sidewalk width shall meet ADA requirements.
Design for Future Considerations
Prior to starting design, contact the appropriate state or local planning office or agency to identify any proposed future capacity improvements that would result in the need for a wider bridge. The time frame for the proposed improvement should be considered. If the widening is likely in the next 20 years, with planning activities and/or preliminary engineering underway, then accommodating the additional bridge width may be warranted depending on the project budget. At minimum, the bridge should be designed in such way as accommodate the future widening (i.e. look at beam/girder spacing and pier configuration for the ultimate bridge). If the time frame is uncertain and no planning or preliminary engineering has been completed to establish the future needs, then the scope of the project should be limited to address only the current needs and requirements.
Capacity Improvement Project – Bridge Widening
Major capital improvement roadway projects often include widening existing bridges. When a roadway widening project impacts an existing bridge, the initial thought may be to widen the existing bridge to match whatever roadway typical section is proposed, this may result in unnecessary cost. Depending on the proposed super elevation, widening an existing bridge may reduce the clearance over the roadway, waterway, or railroad the bridge crosses potentially turning a bridge widening into a bridge replacement significantly increasing the cost of the project. If the answer is yes to any of the following questions, then eliminating the bridge widening should be strongly considered.
Is the bridge being widened just to provide shoulders?
Can the wider approach shoulders be tapered in to match?
Can the travel lanes be narrowed across the bridge?
System Preservation Project – Bridge Replacement, Bridge Deck Replacement, Bridge Superstructure Replacement
Bridge system preservation projects are focused on addressing bridges that are rated structurally deficient or are close to becoming structurally deficient. A bridge is classified as structurally deficient if one of its major rating elements (deck, superstructure, substructure, culvert), which are rated on a scale of 0-9, is rated a 4 or less. The goal of these projects is to address the deficient element(s) and the scope of the project should be established with this goal in mind. There are situations were minor widening of the bridge as part of the deck or superstructure replacement can address a sub-standard shoulder or sidewalk. These are instances where the bridge can be widened one to two feet by just extending the deck overhangs without widening the bridge substructure and adding additional beams/girders. The cost of this minor widening is minimal and may address a functionally obsolete element, meet ADA requirements, or improve bicycle compatibility.
Widening to Minimize Construction Staging
Often, bridge projects involving multi-lane facilities along high volume routes that require the work to be completed in multiple stages of construction. This involves narrowing lanes and shifting traffic to one portion of the bridge while another portion of the bridge is replaced or reconstructed. Minimizing the number of stages of construction reduces cost and the overall duration of construction. In these situations, it may be advantageous to build a wider bridge during the first stage of construction so that more lanes of traffic can be shifted onto this new portion in the subsequent stage of construction. The cost of the wider bridge is offset by eliminating a stage of construction, increasing worker safety by eliminating having the work zone between split traffic, reducing the overall duration of the project six to nine months per stage of construction, and in many cases the additional bridge width will accommodate future widening of the roadway.