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Proposed AASHTO LRFD Bridge Design Specifications for Light Rail Transit Loads (2017)

Chapter: Chapter 5 - Summary and Conclusions

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Page 193
Suggested Citation:"Chapter 5 - Summary and Conclusions." 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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Page 193
Page 194
Suggested Citation:"Chapter 5 - Summary and Conclusions." 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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Page 194

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193 C h a p t e r 5 This report has discussed technical details and requirements concerning bridges subjected to light rail transit loading and to both light rail and highway traffic loadings. A comprehensive lit- erature review was conducted to understand the state of the art of rail bridges at large (published literature on light rail bridges was limited relative to that of bridges carrying conventional heavy- haul and high-speed trains). All technical findings were integrated to develop the LRFD Guide Specifications for Bridges Carrying Light Rail Transit Loads. The following are specific summaries and findings: State of the Art Review • In design of light rail bridges, the AW4 category was used to include the weight of trains, fully seated passengers, and standing passengers. Typical deck types for these bridges were embedded, ballasted, and direct fixation. Although light rail bridges were exclusively loaded with light rail trains, there were practical needs to consider light rail and highway traffic loadings. • The majority of in situ monitoring data was concerned with bridges subjected to heavy-haul and high-speed trains, including acceleration, displacement, strain, and live load measure- ment. Scarce information existed on the behavior of bridges under light rail loading. The test data were complemented by analytical modeling to better elucidate the response of constructed rail bridges. • Design and practice relied upon the live load models of individual transit agencies, and the standard live loads of AASHTO LRFD BDS and the AREMA manual. The responses of exist- ing bridges were, therefore, agency specific. A standard live load model should be developed to accomplish uniform design outcomes, which can facilitate the design of light rail bridges and can better manage the performance of constructed bridges from nation-level perspectives. • Various components of train-structure interaction were examined in conjunction with rail–structure interaction. DLA or impact was of interest to address the amplified load effect induced by the movement of trains. Existing light rail design guidelines showed a DLA range between 10% and 40%, or suggested to follow the provisions of other specifications such as AASHTO LRFD BDS or the AREMA manual. Centrifugal and longitudinal forces were inves- tigated experimentally and numerically; however, there was no information dedicated to light rail bridges. The effect of rail break caused by thermal loading at geometric discontinuities along a rail was evaluated. No consensus was made between transit agencies on these train- rail–structure interaction subjects. • Although the LRFD philosophy was adopted by several light rail agencies, the factors were arbi- trarily taken or modified from those of AASHTO LRFD BDS. Some agencies employed the ASD method in tandem with the AASHTO Standard Specifications. Refined calibration was required to propose load factors (LRFD) dedicated to light rail bridges, because the level of uncertainty associated with light rail transit loading is different from that with highway traffic loading. Summary and Conclusions

194 proposed aaShtO LrFD Bridge Design Specifications for Light rail transit Loads Research Program • Five constructed light rail bridges in Denver, CO, were instrumented to monitor the behavior with an emphasis on in situ train loadings, girder strains, displacements, dynamic character- istics such as frequencies, and live load distributions. The occurrence of multiple presence of light rail trains was also examined. Statistical properties were acquired. • Finite element models, validated with the site data, were developed to predict the behavior of bridges subjected to light rail train loadings and to combined light rail and highway traf- fic loadings. Implicit and explicit modeling approaches were employed, depending upon the nature of technical subjects. • In accordance with benchmark light rail bridges consisting of five superstructure types (i.e., steel plate girder, steel box girder, prestressed concrete I girder, prestressed concrete box girder, and reinforced concrete girder bridges), parametric investigations were conducted. • A standard live load model (LRT-16) was developed based on a probability-based load inference technique using 48,256 loading cases. The load model was further assessed with 660 load cases resulting from 33 light rail trains, covering a span length of up to 300 ft. • Deflection and user comfort criteria were examined to propose design recommendations for service. The existing deflection control criteria were substantially conservative (e.g., L/800 in AASHTO LRFD BDS, where L is the span length of the bridge). Passenger and pedestrian comfort was taken into consideration. • Live LDF were calibrated to overcome the limitations of existing design approaches. A similar approach was taken to develop skew correction factors. The format of these proposed equa- tions was in conformance with that of AASHTO LRFD BDS. • A DLA of 30% was probabilistically determined based on 2,960 finite element models, including a safety margin of 5% to address potential uncertainties on site. The proposed value was evalu- ated against design provisions and explicit finite element results. Unlike the varying MPFs in AASHTO LRFD BDS, a single value of 1.0 was recommended for light rail bridges. • Design equations for centrifugal and longitudinal forces were derived with the aid of finite ele- ment modeling and analytical approaches. Thermal forces alongside rail break were of interest in studying the rail–structure interaction. • Statistical analysis was carried out to establish a unified design approach for bridges carrying light rail and highway traffic loadings at a significance level of 0.05 (95% confidence level). • Load factors were calibrated for light rail loadings, particularly for strength and fatigue limit states. Because the characteristics of light rail trains are different from those of highway vehicles, the proposed factors were not the same as those listed in AASHTO LRFD BDS.

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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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