Abstract
The intent of Bridges: Their Engineering and Planning is to understand bridge construction through the lens of multiple disciplines and fields, such as engineering, financial analysis, transportation planning, environmental studies, and public policy, in only 180 pages. In their preface, the authors outline two goals for their book: (1) to inspire students to take up the study of bridge engineering and infrastructure planning, and (2) to inform citizens and public officials about the complexity of bridge construction and planning. The book is not a guidebook for bridge engineers nor is it a groundbreaking contribution to the literature of infrastructure planning. It is a book for the general audience. Accordingly, the authors faced two big challenges. First, they had to persuade the readers with “the eye accustomed to the little (shiny) screen and uneducated in the meanings and value of the built environment” (157) to appreciate bridges almost as much as a specialist and convince them that bridges are more than structures that “seem to just sit there.” It is not an easy task to keep a specialist reader, let alone a non-specialist, excited enough to read an entire book about any type of infrastructure. Second, the authors aimed to create an unorthodox but valuable link between engineering and planning perspectives on this subject. Therefore, they covered a wide range of topics from basic engineering principles, types of bridges, cost–benefit analysis to transportation modeling, and the meaning of a sustainable bridge, all the while keeping the structure, cohesion, fluidity, and clarity of their book. Overall, the authors did an admirable job overcoming both of these challenges.
The book is organized into eleven chapters, but might best be understood as two distinct parts: bridge engineering and bridge planning. The authors begin their book with some facts about the number of bridges and bridge construction in the United States. Of the more than six hundred thousand bridges in the United States, about 13 percent are medium-span (100 to 329 feet) and one-fifth of 1 percent are long-span (330 feet or longer). The peak of bridge building occurred in the 1960s and 1970s, and subsequently many of these longer and more expensive bridges are becoming deficient or obsolescent. Although the United States has succeeded in reducing the percentage of structurally deficient bridges in recent decades, the authors warn us that a bridge infrastructure crisis is looming because of our aging bridges.
The next four chapters explain the basics of bridge engineering (Understanding Stresses and Strains, Bridge Types and Sites, Making Strong Bridges: Dealing with Uncertainty, Resisting Extreme Events). First, the authors briefly define the kinds of stress produced by different forces: compressive, tensile, shear, basic bending, basic torsion, and combined forces. The engineer’s task is to ensure that each structural member withstand these forces that the loads (the structure itself, the deck and pavement, automobile traffic, intermittent heavy truck traffic, wind, earthquake, etc.) will exert on it. Then, the authors describe different types of bridges: culver and slab, girder, arch, truss, suspension, and cable-stayed. A basic criterion by which to select a bridge type is the length to be spanned. The authors briefly discuss other main criteria used for selecting a bridge type—community preference, the clearance needed for tall ships or trucks, geological conditions, traffic projections, and environmental concerns, such as floods, preservation of natural shoreline, and protection of endangered species, among others.
In deciding the kind of bridge that fulfills a community’s needs at the lowest cost, uncertainties must be taken into consideration. Engineers deal with uncertainty by adjusting load and resistance with uncertainty factors. They rely on the Load and Resistance Factor Design (LRFD) method to figure out whether a proposed bridge is meeting safety standards. Safety is a relative term, since engineers are unable to predict, quantify, and take into account the various uncertainties, such as natural disasters. In practice, the value judgment of engineers sets the level of failure that is acceptable. The authors present some statistics about common causes of failure of bridges, such as flooding, the most common reason in the United States, followed by collision, overload, design, and construction problems. The authors recommend equal bridge-failure probabilities across hazard types, to better adapt bridge costs to safety needs.
Bridge planning is discussed in the next four chapters (Is It Worth It? Costs, Benefits, and Tough Decisions, Traffic Across the Bridge, the Bridge in the Environment, Delivering the Bridge). In chapter 7, the authors introduce cost–benefit analysis to decide whether the bridge is worth building. The fundamental ideas of cost–benefit analysis are straightforward, yet the problem is the underlying assumptions and data that may be wrong. Through an example of a hypothetical cost–benefit analysis, the authors explain the concepts of internal and external costs and benefits, the discount rate and present value, and the issue of addressing concerns about neighborhood, traffic volume, economic development, environment, and intangible effects, such as lasting legacy and visible identity in the decision-making process.
Chapter 8 is on transportation planning. It uses an extended example to analyze whether traffic pressures justify a new bridge or not. The authors explain the four-step process of modeling trip patterns: trip generation, trip distribution, modal split, and trip assignment. They clarify how the modeler can provide forecasts of the effects of a new bridge on traffic. The authors also highlight the concept of induced demand as a caution for decision makers. This phenomenon happens when additional lanes or a new bridge induces additional driving by potential motorists, who previously avoided car travel because of slow traffic.
Chapter 9 is about the meaning of a sustainable bridge. The discussion begins with introducing the environmental impact statement (EIS), a series of studies and public reviews to assess the environmental impact of bridge projects. EIS is not just examining the environmental impact of a bridge, but also studying the project’s fit with state and federal laws and assessing the social and economic impacts of the project. At the end of this chapter, the authors define the concept of embodied energy to elaborate the idea of a sustainable bridge. Embodied energy is the total nonrenewable energy that was consumed for the material, the construction process, the maintenance, the eventual demolition or disassembly, and the disposal of that waste. Clearly, a bridge that fulfills the needs with less embodied energy is more sustainable than its alternatives.
Chapter 10 is about the construction process from proposal to completed construction. According to Lee and Sternberg, any major bridge development has six stages: (1) initiation, (2) scoping (one to two years), (3) preliminary design and environmental review (two to ten years), (4) detailed design and agreements (two to three years), (5) bidding and contracting (six months), and (6) construction (two to five years). The project delivery can take between 7.5 and 20.5 years in total, a long time. At the end, the authors encourage policy makers, planners, and engineers to learn how to deliver projects more cost-effectively and faster to meet the current infrastructure needs of the United States.
The strength of this book is that although the concepts are diverse, surprisingly, the reader does not struggle to make sense of the collection. The book has a clear structure, and illustrations and tables have been used wisely to clarify the ideas of the text. Overall, it can serve as a useful resource for students, citizens, and public officials to learn more about bridge engineering, planning, as well as making infrastructure decisions in general. In this reviewer’s opinion, there are two areas where the book could be improved. First, the authors raised some debatable and stimulating questions, such as how to consider the intangible impacts of bridges in the cost–benefit analysis or how to make the delivery process faster and more effective, but did not provide answers to these questions. Second, and more significantly, an urban design point of view is missing in this collection. There are some references to bridge design (in an architectural sense), pedestrian bridges, bridges as public space, and bridges as place makers scattered throughout; however, these topics are of such importance, especially for citizens and public officials, that they deserve their own chapters. Obviously, this reviewer has his own bias, but a future edition may include chapters written in collaboration with an urban designer. One reason for the absence of this topic may be that there are not as many good examples of pedestrian bridges, which are also public destinations in the United States (unlike in Europe and Asia). The lack of an elegant pedestrian bridge that acts as a public space, public destination, and contributes to a public identity does not necessarily mean that our communities cannot start thinking about having one, especially when one of the goals of our current planning practice is to decrease vehicle dependency and promote walkability.