The NCHRP Report 851 provides proposed AASHTO LRFD Bridge Design Specifications for bridges carrying light rail transit loading, including those subjected to both light rail and highway traffic loadings. The proposed specifications and design examples are based on comprehensive response monitoring of five bridges carrying light rail transit vehicles in Denver, Colorado, and analytical programs for investigating the behavior of light rail bridges, live loads and associated forces, rail–train–structure interaction, and load factor calibration to implement the Load and Resistance Factor Design (LRFD) method. A standard live load model was developed to generate uniform design outcomes, regardless of transit agency. Load factors are calibrated, particularly for the strength and fatigue limit states of light rail bridges. The material in this report will be of immediate interest to bridge engineers.

Bridges carrying or anticipated to carry rail–transit vehicles have become more common-place in crowded metropolitan areas as an alternative to relieve traffic congestion and provide additional transportation choices. In many cases, bridges are 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 Rail-way Engineering and Maintenance of Way Association (AREMA). However, neither AASHTO bridge specifications nor the AREMA manual specifies the light rail transit loads. In addition, designing 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.

Research was performed under NCHRP Project 12-92 by University of Colorado Denver 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. The proposed specifications 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.

The 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 will be published by AASHTO if the AASHTO Subcommittee on Bridges and Structures approves them. The Design Examples are available on the TRB project website.