Bridging the Gap

Introduction

Have you ever placed a board or log over a stream so you can get across without getting wet? Have you ever noticed when traveling in a vehicle how many bridges you cross to get to where you are going? Have you noticed the different types of bridges? What do they look like? Bridges are part of our everyday lives; they usually go unnoticed and are taken for granted. Most bridges do not receive any kind of recognition. Often, they are only thought of as being part of the roadway, when in fact they contribute heavily to commerce, urban development and social life. Bridges also impact cultural and environmental development. Some bridges even serve as national landmarks, symbols of their respective cities. Just as the Eiffel Tower in Paris, draws worldwide interest and testifies to engineering excellence, many bridges leave a similar impression and maintain a magnificent presence. For example, the Golden Gate Bridge in San Francisco, the Tower Bridge in London (see Figure 2), or the Brooklyn Bridge in New York City. These bridges, when built, caught the attention of the entire nation and the world. Not only do they make possible transport between places for commercial and social purposes, but they bring people together, serving as significant attractions, generating tourism and pride in one's city. Such bridges become symbols of their cities and are certainly unique.

However, when you think about it, all bridges are unique. All bridges pass over a different roadway, river, bay, valley, canyon or railroad tracks. All of these obstacles are different and therefore the bridges that cross them must be designed for those particular obstacles. In addition, a variety of geologic and environmental factors must be considered during the design process. This includes anchoring the foundation, minimizing weathering from local climate conditions and minimizing environmental impacts during and after construction, which includes sediment and airborne particle control.

It is also important to consider why bridges have been constructed or used in the past. Some of the earliest bridges were no more than fallen (or cut) trees or logs. They were positioned to cross over small rivers and ravines, so people could access other communities or resources for commerce, trade or social visits. We call these natural bridges. While natural bridges are inexpensive and effective for human travel, they are almost impossible for moving heavy loads.

Later bridges were made of rope with wooden boards. These bridges made it possible for more people to cross, and transport materials, carts and wagons. However, heavy loads were restricted from these bridges due to the relative weakness of early rope. How would wind or rain affect these rope bridges? High winds might cause the bridge to sway and heavy rains could wash out the soil supporting the bridge, or even the bridge itself. Stone arch bridges, primarily built during the Roman Empire, are some of the strongest and most durable bridges. Even today, many are still standing and being used in many parts of Europe, Asia and the Middle East.

In addition, other alternative bridge designs have been constructed, such as wire bridges for crossing ravines, canyons and gorges (see Figure 3). In Nepal for example, some communities constructed suspended wire bridges that span canyons and other extreme land features. In some cases, these bridges resemble gondolas, and in other cases they are simple zip lines used for passenger and material transport. Today, the use of steel, concrete and cables lends greater material strength and enables the construction of even larger bridges capable of supporting many automobiles, passengers as well as the weight of the bridge itself. Currently the state of Colorado has more than 8,000 bridges and the U.S. has more than 500,000 bridges that are more than 100 feet (30 m) long! Civil and structural engineers are responsible for the design of bridges. They work in teams to determine the bridge type, design and materials; analyze site conditions, geologic and environmental factors; and establish detailed design plans and budget/funding schedules

Indeed, bridges are more than materials, designs and budgeting. Engineers must not forget the customers — the people and the community that use the bridge. Who might benefit from using a bridge and why? (Possible answers: Themselves, their families, businesses — for commerce, social gatherings, visiting family and friends, transportation to other areas, travel, etc.) Some people want a bridge for the access it provides to resources, for expansion, for trade or industry, for the purpose of being connected to another community, city or region

What would it be like if we didn't have any bridges? Would we be able to get our favorite video games or shop at our favorite stores or visit our friends? Do you think we could effectively function as a society without bridges? People living during the 1850s were faced with this same situation when heading west to explore the U.S. frontier. They walked and traveled with covered wagons pulled by horses or oxen. Often they would either have to cross streams or rivers or travel many kilometers out of the way to find an adequate place to cross. If a river was particularly high or flooding, they were forced to wait days or weeks. For them, crossing rivers without bridges was dangerous and often disastrous. Countless examples throughout history are evidence that bridges serve as links for survival, an essential connection for the prosperity and well being of a city or community.

Another example includes the Roman Empire, which relied heavily on the construction of roadways and bridges to expand the limits of its domain and, in one well documented case, the removal of one such bridge spared the lives of many by eliminating access by their warring neighbors, the Etruscan's.