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Suggested Citation:"Chapter 1 - Introduction." National Academies of Sciences, Engineering, and Medicine. 2017. Proposed AASHTO LRFD Bridge Design Specifications for Light Rail Transit Loads. Washington, DC: The National Academies Press. doi: 10.17226/24840.
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Suggested Citation:"Chapter 1 - Introduction." National Academies of Sciences, Engineering, and Medicine. 2017. Proposed AASHTO LRFD Bridge Design Specifications for Light Rail Transit Loads. Washington, DC: The National Academies Press. doi: 10.17226/24840.
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Suggested Citation:"Chapter 1 - Introduction." National Academies of Sciences, Engineering, and Medicine. 2017. Proposed AASHTO LRFD Bridge Design Specifications for Light Rail Transit Loads. Washington, DC: The National Academies Press. doi: 10.17226/24840.
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Suggested Citation:"Chapter 1 - Introduction." National Academies of Sciences, Engineering, and Medicine. 2017. Proposed AASHTO LRFD Bridge Design Specifications for Light Rail Transit Loads. Washington, DC: The National Academies Press. doi: 10.17226/24840.
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2C h a p t e r 1 1.1 Background Light rail systems are an important transportation mode in many urban centers. According to the American Public Transportation Association (Parsons Brinckerhoff et al. 2012), light rail transit is defined as An electric railway system characterized by its ability to operate single or multiple car consists 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. Because of prompt operation and reasonable construction costs, light rail transit is particularly recommended for large- and medium-size cities (Black 1993). Electrically propelled rail vehicles are environmentally friendly (no emission), reliable, and affordable (Fatemi et al. 1996; Kim et al. 2009). Various light rail trains operate in the United States and Canada, as shown in Figure 1.1 and Table 1.1. The design of bridges subjected to light rail transit has frequently been conducted using the AASHTO Specifications. TCRP Report 57 (2000) mentions that old transit systems, such as Chicago and New York, were designed using the AREMA manual, whereas new systems, such as Atlanta and Baltimore, were designed pursuant to the AASHTO Specifications. Some state agen- cies permit the ASD method for light rail structures, such as the one in Washington (STL 2012). In 2007, the FHWA mandated that light rail bridges be designed as per the AASHTO LRFD BDS (Parsons Brinckerhoff et al. 2012). TCRP Report 155 states that there is no design code for light rail transit in the nation (Parsons Brinckerhoff et al. 2012). It also discusses that the AASHTO Specifications and the AREMA manual cannot fully address the need for designing light rail bridges. The reason is that most light rail trains are heavier than HL93/HS20 (AASHTO), but lighter than Cooper E80 (AREMA), as shown in Figure 1.2. Several transit agencies have developed their own design specifications for bridges carrying light rail transit loading. Depending upon geometric and structural requirements (e.g., span length and service envi- ronments), typical superstructure for light rail transit comprises prestressed concrete girders, steel plate or box girders, and cast-in-place concrete multicell boxes. Loads and forces to be considered in light rail structures include dead load, live load (train) with DLA, longitudinal and centrifugal forces, derailment, wind, earthquake, and so forth. Some loads are fundamen- tally identical to those of highway bridges (e.g., earth pressure, buoyancy, shrinkage, creep, and temperature); however, live load and train-structure-interaction forces need additional con- sideration. Due to the absence of standard specifications for light rail transit loading, design of bridge structures is site-specific and largely dependent upon the criteria of individual transit agencies. Several state departments of transportation (DOTs) have material requirements for light rail construction, which are basically the same as those of highway construction (AECOM Introduction

Introduction 3 Figure 1.1. Light rail trains operated in Denver, CO. City/System One-way line (mile) Cars per mile Weekday rides (×103) Service productivity Passengers per mile Passengers per car Baltimore/Central Corridor 30.0 1.8 27.2 938 513 Boston/Green Line & Mattapan 25.6 7.8 233.3 10,188 1,051 Buffalo/Metro Rail 6.4 4.2 24.5 3,828 907 Charlotte 9.6 1.7 14.6 1,521 730 Cleveland/Blue & Green 15.3 3.1 15.1 981 315 Dallas/DART LRT 85.0 1.9 83.4 1,078 878 Denver, RTD LRT 47.0 3.6 66.8 1,925 1,363 Houston/MTA 12.8 2.4 36.1 4,813 2,006 Jersey City/NJ Transit 21.3 3.4 41.9 1,958 574 Los Angeles/Blue, Green, & Gold 61.7 2.2 154.5 2,201 1,277 Minneapolis, Metro Transit 12.3 2.3 30.3 2,463 1,263 New Orleans/Streetcars 22.3 7.6 19.7 1,387 298 Philadelphia/City & Suburban 47.6 3.7 110.1 2,591 692 Phoenix 20.0 2.5 41.3 2,107 826 Pittsburgh/South Hills 26.2 3.5 24.2 968 292 Portland/MAX 52.4 2.4 114.5 2,189 1,090 Portland/Streetcar 7.2 2.5 12.0 2,857 1,714 Sacramento/RT LRT 38.6 2.0 45.6 1,178 600 St. Louis/MetroLink 46.0 1.6 52.3 1,149 630 Salt Lake City/UTA LRT 44.8 2.1 59.1 1,655 1,478 San Diego/Trolley 53.5 3.0 103.4 1,970 772 San Francisco/Muni 34.6 4.4 162.4 5,290 928 San Jose/VTA LRT 42.2 2.4 32.9 780 329 Seattle/Tacoma 17.3 2.1 27.8 1,782 428 Calgary/C-Train 35.0 5.4 263.9 9,527 1,692 Edmonton/LRT 13.1 4.6 93.3 7,122 1,261 Toronto/Streetcars 51.0 5.1 299.8 6,131 1,209 Total 878.8 89.3 2190 80,577 25,116 Table 1.1. Database of light rail transit in North America based on various sources. 2008). Although transit structural design is often carried out per the AASHTO Specifications or the AREMA manual, their applicability to light rail structures is limited. For example, the train load and geometric configurations specified in the AREMA manual do not represent light rail transit (Parsons Brinckerhoff 2000), and the dynamic characteristics of trains in the AREMA doc- ument (e.g., impact) are not relevant to light rail transit (Harrington and Dunn 1981; Niemietz and Neimeyer 1992).

4 proposed aaShtO LrFD Bridge Design Specifications for Light rail transit Loads Most critical load conditions are determined when designing bridge structures that carry light rail transit, including dead loads, train loads, DLA, engaged forces, and their combinations. Dead loads typically encompass the self-weight of the structure, trackwork, utilities, and other fixtures. These loads are readily obtainable for design. By contrast, live loads and their effects require extra attention owing to a dearth of standard light rail train models and related forces. If a light rail transit system is constructed in a seismic area, earthquake loads must be consid- ered. Some design guidelines state two-level requirements: a maximum design earthquake load (MDE) and an operating design earthquake load (ODE). The design concept of these two cat- egories is that MDE provides sufficient safety against structural collapse, whereas ODE addresses seismic issues without disrupting service (STL 2012). The seismic effects specified in AASHTO LRFD BDS (AASHTO 2016) offer valuable information on structures in a seismic region (i.e., hazard characterization and design forces). As such, depending upon technical subjects, the AASHTO document may still be usable for light rail bridges. The function of an existing bridge may change from highway to light rail or vice versa. Struc- tural strengthening may be necessary to accommodate the increased live load, including support- ing girders and connection details (Parsons Brinckerhoff et al. 2012). A unified consideration on these distinct load configurations is, therefore, helpful when designing bridges in urban areas. The City of Edmonton specifies that bridges for light rail transit must also be designed for high- way vehicle loads (COE 2011). Challenges in such stringent and practical requirements involve design criteria for light rail transit that are not fully developed, and existing design manuals sometimes conflict each other (AECOM 2008). Rigorous effort is imperative to develop standard specifications for practicing engineers interested in the design of light rail structures. 1.2 Research Objectives The research aims to develop design specifications for bridge superstructures carrying light rail trains, and those carrying both light rail train and highway traffic loadings. The specifications stipu- late technical contents that need to be considered in design of light rail bridges, including a standard light rail train load and associated force characteristics. The specific objectives of the research are: • To characterize the load effect of light rail transit on the behavior of bridge superstructures, such as standard train loading, DLA, live load distribution, multiple presence, and load factors; • To examine the interaction between light rail loadings and supporting structures, in conjunc- tion with various aspects to be considered in design (e.g., longitudinal and centrifugal forces, user comfort criteria, and rail–structure interaction); and E80 railroad HS20 highwayLight rail vehicle Figure 1.2. Comparison of vehicle load (reproduced based on Harrington and Dunn 1981).

Introduction 5 • To propose a unified design approach for light rail transit and highway traffic loadings, and corresponding design articles and commentaries, including design examples for practitioners. 1.3 Research Approach A comprehensive study was conducted to accomplish the aforementioned research objectives, including: • State of the art review: Published research papers and design guidelines were collected and reviewed to understand the current state of light rail bridge design and to identify technical gaps to be filled by additional research. • Response monitoring of constructed bridges: Five light rail bridges in Denver, CO, were instru- mented and their responses monitored. These bridges include various superstructure types and track configurations. Statistical properties were characterized, which was necessary for reliabil- ity analysis. The in situ data were employed to calibrate finite element models. • Finite element modeling: To predict the behavior of bridges subjected to light rail train and highway vehicle loadings, implicit and explicit finite element models were developed. Global (bridge superstructure) and local (wheel-rail) investigations were conducted. • Development of a standard live load model for light rail transit: A live load model is devel- oped using probability theory in tandem with the finite element and field monitoring data. The model was assessed with 33 light rail trains operated in the United States and four in Canada. • Characterization of live load effects: A variety of live load effects were characterized, such as live load distribution, DLA, and multiple presence factors. Existing design provisions were comparatively evaluated. User comfort criteria were taken into consideration based on the frequency and deflection of light rail bridges. • Rail–structure interaction: Forces associated with the interaction between the rails and struc- ture were studied, such as centrifugal and longitudinal forces, thermal effects, and rail break. • A unified approach for bridges carrying light rail and highway traffic loads: A statistical inves- tigation (analysis of variance: ANOVA) was conducted to establish a unified design approach for bridges carrying light rail and highway traffic loads. • Load factor calibration: Load factors were calibrated for strength and fatigue limit states. A safety index of b = 3.5 was employed in compliance with AASHTO LRFD BDS. 1.4 Organization of the Report This report consists of five chapters. Chapter 1, Introduction, discusses the background of research, objectives, and approaches. Chapter 2, State of the Art Review, elaborates on exist- ing knowledge about how rail bridges relate to conventional trains, high-speed trains, and light rail trains, with an emphasis on tracks and decks, the use of mixed load configurations, in situ response of bridges, live load and associated effects, train-structure interaction, load and resistance factors, and summary and challenges. Chapter 3, Research Program, encompasses technical subjects on the response monitoring of constructed light rail bridges, finite element modeling, development of a standard live load model for light rail transit, characterization of live load effects, a unified design approach for light rail and highway traffic loadings, and load factor calibration. Chapter 4, LRFD Guide Specifications for Bridges Carrying Light Rail Transit Loads, summarizes design articles developed based on the present research and existing guide- lines. Chapter 5, Summary and Conclusions, provides a grand summary of the research program with important findings.

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TRB's National Cooperative Highway Research Program (NCHRP) Research Report 851: Proposed AASHTO LRFD Bridge Design Specifications for Light Rail Transit Loads provides proposed specifications for bridges carrying light rail transit loading, including those subjected to both light rail and highway traffic loading. 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 generates uniform design outcomes for any transit agency. Load factors are calibrated, particularly for the strength and fatigue limit states of light rail bridges.

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