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191 C h a p t e r 4 All technical findings from this research are integrated to develop guide specifications for bridges carrying light rail loading and light rail and highway traffic loadings. A holistic summary of the specifications is briefed in this section, while detailed contents are available in a separately published AASHTO document. 4.1 General The scope of the guide specifications is introduced along with notations and definitions. It is noted that the specifications are a supplement to AASHTO LRFD BDS providing comprehensive technical information necessary for bridge design. 4.2 Design Philosophy In compliance with AASHTO LRFD BDS, the design methodology of the specifications is based on LRFD. Various limit states are defined for service, strength, extreme events, and fatigue. Load factors and their combinations for bridge design are listed, depending upon loading con- figurations. To address the comfort of users, design criteria are specified from pedestrian and passenger standpoints. 4.3 Loads A variety of loads and associated forces to be considered in design of light rail bridges are discussed, including permanent loads and live loads. A standard live load model is presented to facilitate design processes, leading to uniform design outcomes. The DLA of light rail trains is stipulated, in conjunction with additional considerations on derailment events. Provisions on centrifugal force and braking force are followed. Several design articles are taken from AASHTO LRFD BDS, because these are both applicable to light rail and highway bridges (e.g., wind load on structures, earthquake effects, and thermal loading). This section concludes with articles related to railâstructure interaction and rail break. 4.4 Structural Analysis Analysis methods are concisely provided, followed by material behavior, modeling geometry, and boundary conditions. The contents of the approximate method of analysis encompass bridge decks, live load distribution for moment and shear, skew correction, and curved bridges. LRFD Guide Specifications for Bridges Carrying Light Rail Transit Loads
192 proposed aaShtO LrFD Bridge Design Specifications for Light rail transit Loads General discussions on the refined method of analysis, dynamic analysis, and analysis by physical models are given in accordance of AASHTO LRFD BDS. 4.5 Design Examples Six design examples are illustrated to help design professionals understand the use of the specification articles. Specifically, the following examples are worked: (1) simple span com- posite steel plate girderâStrength I moment, (2) continuous span composite steel plate girderâStrength I moment, (3) simple span composite precast prestressed girderâservice stress checks and Strength I moment, (4) simple span composite steel plate girderâStrength I moment subjected to both light rail and highway traffic loadings, (5) simple span composite steel plate girderâother considerations, and (6) simple span composite steel plate girderâ interior deck design