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Today’s highway bridge is an offspring of the rapid development of the modern transportation network. In the United States, this development took the form of what is known as the U.S. Interstate system, a highway system composed of over 46,500 miles (74,800 km) of roadway. The history of the Interstate system is germane to our discussion of bridge design because its development parallels the growth of bridge engineering in the second half of the twentieth century. The evolution from the design of new structures in almost assembly-line like fashion, to the detailed design of a bridge rehabilitation, did not occur overnight. Indeed, the creation of modern standards and specifications in place today, central to the design sections of this book, are an out growth of the efforts of an entire generation of civil engineers who grew up professionally during the formative years of what was, and still is, the largest public works project in the U.S. history.

The Interstate system was funded as a response to the growing U.S.economy after World War II. Although the plan to build some major form of highway system that would link the major U.S. metropolitan areas existed before World War II, the impetus for the plan did not gain strength until 1956.One of the principal impacts of the Interstate system on highway bridges is in its servicing of long-distance trucking. It is this function that would serve as one of the overriding design constraints in all highway bridge structures. At a variety of levels, the construction of the Interstate highway system affected the way we build bridges today. Whether it is the width of the structure set to allow multilane travel over a bridge or its clearance, defined to accommodate the passage of large military vehicles under it, the Interstate was the primary influence on the functionality of the modern highway bridge.

Before the Interstate took hold, most small bridge structures were designed to handle low-level vehicular traffic. The advent of the Interstate greatly impacted the need for structures to carry heavier and heavier loads. It was also the construction of the Interstate on a national level that led to the adoption of uniform design standards across the states, bringing about the many advantages of standardization enumerated in our discussion of the slab-on-stringer structure. In short, the development of the Interstate system has had the following effects on highway bridges :

• Through federal funding, the Interstate system financed the construction of a large number of the highway bridges in use today.
  

• The Interstate system spurred the research and development of highway bridge design and construction which has led directly to many of today’s common design and construction practices.
  

• Because of the national concept of the Interstate system, a refined and common design standard was developed. The detailed design standard, which was once a reality for a few major states like New York, California, and Ohio, now became accessible throughout the United States (and even to other countries throughout the world) that could not afford to finance the high level of research and effort required to produce such specifications.

All of these factors have coalesced to form the science of bridge engineering as we know it today and make it unique from any other type of structural design. If we make an analogy to building design, it can be recognized that the design of highway bridge structures could never be facilitated in the same fashion as one would engineer a building. Although buildings and their associated sites incorporate many, if not all, of the same concepts and design principles as bridges, they are often unique with specific solutions designed on a site-by-site basis with code and specifications varying dramatically from municipality to municipality. Imagine a highway network populated with bridges in this same fashion with its thousands upon thousands of structures. The Interstate system, as a result of its magnitude, forced the issue,if you will, by making the various state and local agencies adopt a uniform approach to the engineering of highway bridges. While some may argue that this has depleted bridge design of its flare and creativity, the reality is that construction of such a large number of structures in so short a time frame could never have been undertaken any other way.

Many engineers, both new and experienced, view the heyday of the Interstate in the late 1950’s and 1960’s as the golden age for civil engineering.It is difficult, in today’s litigious environment, for civil engineers to fully appreciate the velocity with which Interstate development took place and,with this growth, the number of bridges constructed in so few years. In truth,many of today’s rigorous rules were born out of the problems associated with moving so quickly in the early days. The alacrity with which new standards were created, as engineers began to more fully understand the impact which the new level of traffic would have on their designs, was so great that plans would literally have to be changed from the time design was completed to the time the project was let for construction. For the bridge designer, it is almost amusing to note that it only took 10 or 11 plan sheets to build a new structure in the 1950’s and today it takes almost 40 just to repair it! The early years of Interstate development, however, also represented a time when there was considerable public approval for the building of roads and bridges,which facilitated the speed with which projects came off the boards and into the field. The environmental movement in the mid 1960’s followed by public apathy (if not downright disapproval) toward new highway projects effectively ended this heyday and ushered in the era of maintaining what our predecessors have built.

Yet make no doubt about it, rehabilitation is neither trivial nor mundane.An engineer at the Kansas Department of Transportation was quoted as saying about rehabilitation work that “It’s a lot more fun to build something new than it is to try to refurbish something that is already there.” Where there is certainly nothing exciting about performing surface concrete patching on deteriorated pier columns, the scope and magnitude of a rehabilitation design can often exceed that of the design of a new structure. Bill Cosby once noted in a monologue that having only one child does not constitute being a parent because “too many things are left out”. In a similar fashion, we can say the same thing about the modern bridge designer because, like having only one child to blame when a vase is broken, not having to worry about staged construction, maintaining traffic through a project site, dealing with lead paint and other hazardous materials put in the initial design, etc., just leaves too many things out of the experience of being involved in bridge design today.

In reality, today’s bridge designers should be envious of their predecessors for the relative ease with which they could throw a bridge up. Modern design requires a higher level of details than the engineers of the 1950’s and 1960’swould have ever imagined. Many states require that details, like the design of temporary earth support systems and reinforcing steel bar schedules, which were once left to the contractor at the site, be rigorously detailed by the design engineer. Indeed, the number of managers and agency personnel involved in any one bridge design project today most likely exceeds the number of people it took to make high-level decisions on where to locate whole sections of the Interstate.

As with the Germans, who owe a great deal to the building of the Autobahn, we owe much in the United States to the development of the Interstate system, for it was major public works projects like these that created a workable and efficient method of erecting bridges in a manner that was both consistent and manageable. It is also important to always keep in mind that the highway bridge and its many components is but itself one piece in an entire transportation network. The importance of the role the bridge plays in this network was never so vividly demonstrated than when the bridge carrying the New York State Thruway over the Schoharie Creek collapsed in the rushing waters of a near record flood. The earth beneath the piers washed away(scoured), the footings became undermined, careening out of place, and, in an instant, this tragic failure effectively cut the state’s transportation trunk in half and in the process cost 10 people their lives.