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2006 EDITION Chapter 10 Bridges 10.1 Introduction The primary function of bridges is to carry pedestrians, bicycles, and/or vehicles over various types of transportation facilities or natural features. Bridges come in a wide variety of configurations and structure types. This chapter provides a brief introduction into the topic of bridge planning and design including discussion of applications of bridges, contextual influences on bridge design, preliminary design guidelines, major bridge eleme
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  2006 EDITION Chapter 10 Bridges 10.1   Introduction The primary function of bridges is to carry pedestrians, bicycles, and/orvehicles over various types of transportation facilities or natural features.Bridges come in a wide variety of configurations and structure types. Thischapter provides a brief introduction into the topic of bridge planning anddesign including discussion of applications of bridges, contextualinfluences on bridge design, preliminary design guidelines, major bridgeelements, and the inventory and management of bridges.For detailed information on bridge design, please refer to the MassHighway Bridge Manual  which, along with Chapter 2 of this HighwayDesign Manual, also provides information on process and roles andresponsibilities. 10.2   Applications of Bridges The various bridge applications are described in the following sections. 10.2.1   Pedestrian and Bicycle Facility Bridges Bridges can be constructed to carry pedestrian and bicycle traffic over anobstacle, usually vehicular traffic, a railroad, or a watercourse. Given thatextra travel distance is more acceptable for vehicular travel than forpedestrians and cyclists, it may be appropriate to include a separate bicycleand pedestrian crossing at locations where the existence of schools,churches, parks and open spaces, and other land uses generate largevolumes of pedestrians or cyclists. Other factors affecting the decision toprovide a pedestrian or bicycle bridge include:    A large number of children crossing;    Unacceptable traffic conflicts due to roadway width, high trafficspeeds, and high traffic volumes; and    Cost. January 2006 Bridges 10-1  2006 EDITION Pedestrian routes on bridges have the same requirements foraccessibility for people with disabilities as other sidewalks. Slopes canfollow the roadway alignment but cross-slopes cannot exceed 2percent in the built condition (1.5 percent in design). Curb cut rampsare required at intersections. Where a bridge is not along the roadwayright-of-way, its slope cannot exceed 8.33 percent in the builtcondition.Preliminary design features such as vertical and horizontal clearanceand cross-section should be considered. Pedestrian passages under aroadway are discouraged unless the highway lanes are on a fill sectionof 15 feet or more. This type of structure presents problems fordrainage and lighting and creates a condition where policing is difficult. 10.2.2   Highway Grade Separations A grade separation where a roadway passes over an intersectingroadway or railroad is called an overpass or over-crossing. A gradeseparation where a roadway passes under an intersecting roadway orrailroad is called an underpass or under-crossing. These definitionsapply for the roadway in question and are the opposite for the crossingroadway.Most grade separations and interchanges are located on freeways ormajor arterials. These structures allow the highway to safelyaccommodate high volumes of traffic through intersections. Somecontrolling factors in the planning of a highway grade separationinclude highway geometry and the available right-of-way. Highwaygrade separations are often combined with ramp systems to forminterchanges described in Chapter 7.To present consistent visual cues to drivers, grade separationstructures should conform to the highway alignment and cross section,and also provide the required vertical clearance. Its profile must belimited to grades that allow sufficient stopping sight distance. Thetransition from roadway to grade separation should be designed suchthat the driver 's behavior is not altered. Other considerations thatshould be integrated into the design of highway structures include thefollowing:    Generous lateral clearances to structural elements and otherfeatures; 10-2 Bridges January 2006      2006 EDITION    Structural elements of the bridge should be shielded from potentialimpact by errant vehicles either through grading of roadwaymedians and off-shoulder slopes, or through the provision of barrier systems and impact attenuators as described in Chapter 5;    The design should support projections of activity levels by all users,consistent with state, regional and local plans and policies (seeChapter 3);    Aesthetically pleasing highway architecture should be considered;    Underpass structures should be as open as practical to allow lightpenetration, air circulation, and maximum visibility. In the case of long underpasses or tunnels, consideration must be given to theinclusion of rescue assistance areas, fire suppression, andventilation;    Bridges over navigable rivers should be as open as possible torecreational (canoe/kayak) passage.    All structures should be designed with shoulders, curbing, lightingand other highway elements that exist on the approaching roadwayor that may be expected to be provided on the approachingroadway with future improvements. Bridges should also includesidewalks and bicycle accommodation even where such facilitiesare not provided on the approaching roadway. These facilities areneeded to provide pedestrian and bicycle access across barriers,such as water, railroads and highways and to assure facilitiesconsistent with potential future roadway and sidewalkimprovements on the approaching roadway.With respect to highway operations, there is no minimum spacing orlimit to the number of grade-separated cross streets, however,considerable savings can be achieved by terminating some of the lessimportant cross streets or combining nearby streets into singlecrossings.Engineering studies should be conducted to determine the effects of termination and the mitigation required to maintain continuity, safetyand access requirements for area roads. In some instances, frontageroads may be installed along the mainline to provide connectivity forusers from terminated street to through cross streets. Factors thatmay affect the number and spacing of cross streets include: January 2006 Bridges 10-3  2006 EDITION    Network connectivity and development characteristics;    Activity levels (pedestrian, bicycle and traffic volumes);    Location of schools, recreational areas, hospitals and other publicfacilities; and    Emergency service   routes.The availability of adequate right of way may limit the possiblestructure types. Moreover, the construction process can also beadversely affected by the lack of right of way and can require stagedconstruction. Additionally, considerations such as the bridge spanlength, soil characteristics, and skew may also affect the structure'sdesign.In some instances, particularly in developed areas, the grades of localroads cannot be changed due to the surrounding context. In thesesituations, it may be necessary to depress or raise the entire throughroadway into a boat section, tunnel or viaduct. These facilities aremuch more expensive to construct than simple overpasses andunderpasses and may require drainage pump stations, control of groundwater, underpinning of nearby structures, special lighting, andvideo systems for security monitoring. 10.2.3   Railroad Grade Separations Structures that carry a roadway over railroad traffic are referred to as railroad overpasses . Conversely, railroad underpasses arestructures that pass roadways under railroads. Some considerationswhen planning a railroad overpass or underpass include the selectionof the structure type, the horizontal and vertical clearance to thecenterline of the track, the available right-of-way, drainage, trainmovements, and required coordination with the railroad company.The selection of the type of structure, either overpass or underpass,usually depends on the existing topographical conditions. Railroadunderpasses often present drainage problems, sometimes requiring theuse of pump stations which can be costly and require ongoingmaintenance.Proper clearances are an important consideration in the early planningphase. In order to determine vertical clearance, it is important todetermine the top of high rail elevation for approximately 500 ft. in 10-4 Bridges January 2006  
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