Review of Load Rating Highway Bridges: In Accordance with Load and Resistance Factor Rating Method by Lubin Gao

There are few books available to bridge engineers dedicated to the subject of bridge load rating. Load Rating Highway Bridges is a book that helps fill this void by compiling into one book all of the technical aspects an engineer must consider when load rating a bridge. The book is divided into three sections. The first two sections cover rating vehicles, resistances, structural reliability, load rating methodologies, and structural analysis. The third section provides illustrative load rating examples of the most commonly found bridge types in the United States.

The first chapter provides an overview of the National Bridge Inspection Standards (NBIS) and National Bridge Inventory (NBI) data. A historical perspective showing the percentage of structurally deficient bridges over the past 100 years is presented, the various load rating methods are explained, and a historical review of the AASHTO Manual for Bridge Evaluation (MBE) is provided.

Chapter 2 demonstrates how structural reliability theory relates to the AASHTO load and resistance factor design (LRFD) specifications and to load and resistance factor rating (LRFR) calibration. The parameters used in the calibration—redundancy, deterioration, loads, load effects, and resistances—are well explained. Procedures for calculating the reliability index and computing load factors based on a target probability are also demonstrated. Several illustrative examples show how to calculate the reliability index (margin of safety) using normal and log-normal distributions and a Monte Carlo simulation. Chapter 2 is a valuable resource for developing an understanding of how reliability is the basis for LRFD/LRFR methodology.

Chapter 3 explains allowable stress rating (ASR), load factor rating (LFR), and load and resistance factor rating (LRFR) methodologies. The LRFR design, legal, and permit rating procedures are explained in detail. All of the parameters required for a LRFR load rating are addressed, including dead and live loads, load factors, load effects, structural analysis, resistances, dynamic load allowance, structural resistances, condition factors, material properties, and limit states. ASR and LFR methodologies are only briefly explained with some illustrative examples, as the objective of the book is LRFR methodology.

The chapter also compares ASR, LFR, and LRFR ratings, highlighting the variability in the reliability index inherent with ASR and LFR methodologies. Of interest are several illustrative examples demonstrating how structural deterioration and redundancy impact the reliability of a structure and how this issue is addressed in the MBE through the condition and redundancy factors.

Chapter 3 also includes a good discussion on state-specific legal loads and provides several examples. The advantages and disadvantages of using state-specific legal loads to load post bridges are presented, along with a brief discussion on how to use site-specific traffic data to determine the load effects caused by site-specific legal loads. The load effects of specialized hauling vehicles (SHVs) and how they compare to LFR/ASR HS-20 loading are discussed. Bridge load posting and closure are also explained, with examples of why safe load posting is nonlinear in relation to the rating factor.

The structural analysis of bridges is covered in Chapter 4, which introduces the reader to the three common methods of structural analysis: empirical, simplified, and refined. The chapter begins with a description of the building block of structural analysis, section property calculation. Illustrative examples highlight the most common beam types: steel, pre-stressed I-beam, and concrete box girders.

The chapter also covers the structural analysis of several different bridge types, ranging from analysis of a simple span concrete slab to an empirical analysis of a metal plate box culvert (a structure type not often covered in textbooks). The chapter also provides a good example of analyzing a concrete box culvert using the moment distribution method. The theory behind line girder analysis, one of the most common methods used in bridge load rating, is thoroughly explained, including how to model the material and section properties, restraints, and application of the LRFD live load distribution factors.

The chapter also covers the fundamentals of several more advanced analysis methods: grillage, two- and three-dimensional frame finite element, general finite element, and nonlinear. The chapter introduces the reader to the theory behind these analysis methods and is a good reference for further investigation by the reader.

Chapters 5 through 9 demonstrate how to rate the most common bridges types utilized throughout the United States. These include steel, reinforced and precast prestressed concrete, culvert, and timber structures. For each structural type, failure modes, limit states, resistance factors, and flexural and shear strength resistances are explained. Each chapter contains several illustrative examples that take the reader through the design load rating, legal load rating, and the load posting process. A couple of examples not usually found in textbooks include the rating of a steel box beam superstructure and a steel plate box culvert. The variety of examples allows the reader to refer to the bridge types of particular interest.

In summary, the book is an excellent load rating reference guide that provides readers with a good understanding of how LRFR methodology utilizes statistical reliability. Some of the more expansive subjects, such as statistics and finite element analysis, will require further research by the reader, aided by this excellent reference guide.