AASHTO LRFD aims for reliability-based design, balancing safety and cost-effectiveness in bridge structures. It provides important design criteria to engineers by evaluating both Ultimate Limit States (ULS) and Service Limit States (SLS). ULS ensures structural safety, while SLS focuses on performance under service conditions, such as deformation, cracking, and vibration.
AASHTO LRFD allows for the application of load and resistance factors to evaluate various load combinations. For instance, during the design of a PSC box girder, both dead and live loads are combined to meet safety and durability requirements. Designers must make careful selections of design parameters and conduct thorough load assessments.
Two common bridge types that employ AASHTO LRFD are PSC girders and steel composite girders. In PSC girder design, prestressed concrete is primarily used to achieve stiffness in long-span structures. On the other hand, steel composite girders maintain structural stability and achieve cost efficiency by combining steel and concrete.
PSC Box Girder Design: PSC box girder design focuses on crack control and long-term durability through prestressing. AASHTO LRFD helps ensure structural safety by thoroughly evaluating ULS and SLS. PSC girders are generally analyzed with respect to torsional stiffness, axial stress, and load combinations due to dead and live loads.
Steel Composite Girder Design: For steel composite girders, AASHTO LRFD defines design parameters considering the composite behavior of steel and concrete. The designer needs to understand local buckling in steel and stress distribution due to composite action. Evaluating loads and abnormal load combinations during construction is crucial to ensuring reliability. Construction-stage load evaluation is an essential process for improving the reliability of steel composite girder designs.
Bridge load rating is a key step in assessing the condition of a bridge during its service life. AASHTO LRFR provides a systematic approach for load rating both PSC and steel composite bridges. The primary focus during load rating is to analyze the differences between design load combinations and the actual load conditions encountered in service.
PSC Bridge Load Rating: The load rating of PSC bridges involves evaluating prestress losses and crack resistance to verify whether the design’s intended safety and durability are maintained during operation.
Steel Composite Bridge Load Rating: For steel composite bridges, AASHTO LRFR requires examining local buckling and fatigue. Ensuring long-term safety involves fatigue damage assessment, which is essential in maintaining the bridge. Additionally, any load changes after construction or those occurring during maintenance should be considered.
This design guide aims to explain the design algorithms applied to finite element analysis and design systems. The guide provides users with enough information to understand the scope, limitations, and equations used in the AASHTO LRFD design of 2020, while serving as a reference to the relevant provisions of the design standard.
Table of Contents of the Guide:
Chapter 1: Provides details on the design parameters used for PSC girder design based on AASHTO LRFD, ULS/SLS evaluations, and detailed descriptions of design outputs.
Chapter 2: Covers the design parameters used for steel composite girder design according to AASHTO LRFD, ULS/SLS evaluations, and detailed descriptions of design outputs.
Chapter 3: Provides the design parameters used for load rating of PSC bridges following AASHTO LRFR, ULS/SLS evaluations, and detailed descriptions of rating outputs.
Chapter 4: Discusses the design parameters used for load rating of steel composite bridges based on AASHTO LRFR, ULS/SLS evaluations, and detailed descriptions of rating outputs.