Bridges carrying or anticipated to carry rail-transit vehicles become more commonplace in crowded metropolitan areas as an alternative to relieve traffic congestion and provide additional transportation choices. In many cases, there are bridges designed to carry the rail-transit vehicles only, but there are instances where rail transit is designed to occupy a dedicated lane or perhaps mix with regular highway traffic. While there is a wide spectrum of railway systems, light rail transit is distinct from heavy rail (i.e., metro rail systems, commuter rail, and railroad freight systems). Heavy rail entirely operates in exclusive rights-of-way while light rail transit can operate in shared rights-of-way (i.e., mixed with traffic). The American Public Transportation Association (APTA) defines light rail transit as, "an electric railway system characterized by its ability to operate single or multiple car consists (trains) along exclusive rights-of-way at ground level, on aerial structures, in subways or in streets, able to board and discharge passengers at station platforms or at street, track, or car-floor level and normally powered by overhead electrical wires."
For bridges designed for light rail transit systems, the responsible agency often requires that such bridges be designed to satisfy owner-specific and local design codes, various AASHTO bridge specifications, and the Manual for Railway Engineering (MRE) by the American Railway Engineering and Maintenance of Way Association (AREMA). However, neither AASHTO bridge specifications nor the AREMA manual specifies the light rail transit loads. In addition, design of bridges for light rail transit systems involves a number of additional conditions that affect bridge design and may not be familiar to highway bridge engineers such as the details of the trackwork design and the interaction between the rails and structure. State DOTs have an immediate need for commonly accepted design procedures for bridges carrying light rail transit systems.
The objective of this research is to develop proposed AASHTO LRFD Bridge Design Specifications for bridges carrying only light rail transit vehicles and bridges carrying both light rail transit vehicles and regular highway traffic. As a minimum, the specifications shall specify transit load characteristics (e.g., loads and forces, load distribution, load frequency, dynamic allowance, and dimensional requirements), load factors and combinations, analysis requirements, and detailing requirements. Design examples should be developed to illustrate the proposed specifications.
Accomplishment of the project objective will require at least the following tasks.
Task descriptions are intended to provide a framework for conducting the research. The NCHRP is seeking the insights of proposers on how best to achieve the research objective. Proposers are expected to describe research plans that can realistically be accomplished within the constraints of available funds and contract time. Proposals must present the proposers' current thinking in sufficient detail to demonstrate their understanding of the issues and the soundness of their approach to meeting the research objective.
(1). Conduct a critical review of relevant specifications, technical literature, different light rail equipment, and owner and industry experiences. The review should cover research findings from both domestic and foreign sources. (2). Propose a methodology based on the concept of LRFD to specify the light rail transit load characteristics considering track configuration requirements, rail-structure interaction forces, and the expected interaction between transit and highway traffic (e.g., live load models and multiple presence factors). (3). Propose a detailed outline with annotated description for the developed specifications.
(4). Prepare Interim Report No. 1 that documents Tasks 1 through 3 and provides an updated work plan for the remainder of the project no later than 4 months after contract award. The updated plan must describe the process and rationale for the work proposed for Phases II through IV
PHASE II—Methodology Development and Calibration
(5). Execute the approved work plan to develop the proposed methodology. Limited calibration based on current design procedures is recommended. However statistical calibration is not required.(6). Prepare Interim Report No. 2 that documents Task 5 and provides an updated work plan for the remainder of the project no later than 15 months after Phase I approval. The updated plan must describe the process and rationale for the work proposed for Phases III and IV
PHASE III—Proposed Specifications Development and Design Examples
(7). Develop proposed AASHTO LRFD Design Specifications and commentary. (8). Develop design examples to illustrate the proposed specifications.(9). Prepare Interim Report No. 3 that documents Tasks 7 and 8 no later than 6 months after Phase II approval.
PHASE IV—Final Products
(10). Update proposed development to the AASHTO LRFD Bridge Specifications after consideration of review comments and prepare ballot items for AASHTO Highway Subcommittee on Bridges and Structures consideration. (11). Prepare a final report that documents the entire research effort.
STATUS: the final report will be published as NCHRP Report in July 2017