Load and Resistance Factor Design (LRFD) For Highway Bridge Superstructures - Reference Manual

Grubb, Michael A.; Wilson, Kenneth E.; White, Christopher D.; Nickas, William N. · 2015 · ROSA P / National Highway Institute (U.S.)

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Summary

This document is a comprehensive reference manual titled "Load and Resistance Factor Design (LRFD) for Highway Bridge Superstructures," published by the Federal Highway Administration (FHWA) in 2015. It serves as a technical guide for bridge engineers implementing the AASHTO LRFD Bridge Design Specifications, Seventh Edition (2014), including interim revisions through 2015. The manual addresses the need for standardized, probability-based design methodologies to ensure uniform safety levels across diverse bridge structures, replacing older, less precise methods like Allowable Stress Design (ASD) and Load Factor Design (LFD). The manual is structured into eight chapters that systematically cover the theory, methodology, and application of LRFD for both steel and concrete superstructures. Chapter 1 introduces the LRFD design philosophy, explaining its basis in reliability theory and risk assessment, and details the primary limit states: service, fatigue and fracture, strength, and extreme event. Chapter 2 outlines general design objectives, including safety, serviceability, constructibility, economy, aesthetics, and security, while also introducing Accelerated Bridge Construction techniques. Chapter 3 provides detailed criteria for permanent, construction, and live loads, including vehicular, pedestrian, wind, seismic, and blast loads, along with the specific load factors and combinations required for various limit states. Chapter 4 covers structural analysis methods, ranging from approximate techniques to refined analysis, addressing dead and live load distribution, influence lines, and effective width calculations. Chapters 5 and 6 focus on the specific design of concrete and steel girder superstructures, respectively. Chapter 5 details materials, preliminary design decisions, flexural design of prestressed I-girders, shear and torsion design, prestressing losses, and reinforcement detailing for various concrete bridge types, including box girders and spliced precast girders. Chapter 6 covers steel girder design, including material properties, composite and noncomposite sections, and verifications for constructibility, service, fatigue, flexure, and shear. It also addresses specific details such as shear connectors, bracing, connections, and splices. Chapter 7 examines deck systems, covering traditional and empirical design methods for concrete slabs, metal grid decks, orthotropic steel decks, and other alternative materials like FRP and wood. Chapter 8 concludes with the design of bearings and joints, detailing requirements for elastomeric and pot bearings, as well as joint selection and installation. The significance of this manual lies in its role as a definitive resource for ensuring the structural integrity and safety of highway bridges. By providing detailed design examples, commentary, and a glossary of terms, it aids engineers in correctly applying complex LRFD specifications. The manual supports the transition to more reliable, probability-based design practices that account for variability in both loads and material resistance, thereby promoting consistent safety margins and efficient construction practices across the nation’s transportation infrastructure.

Key finding

The document is a technical reference manual providing theory and application guidelines for LRFD bridge design rather than a research study reporting empirical findings.

Methodology

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• road geometry