Bridge Design and Barodynamics

Summary

Barodynamics is the study of the mechanics of heavy structures that may collapse under their own weight. In bridge building, barodynamics is the science of the support and mechanics of the methods and types of materials used in bridge design to ensure the stability of the structure. Concepts to consider in avoiding the collapse of a bridge are the materials available for use, what type of terrain will hold the bridge, the obstacle to be crossed (such as river or chasm), how long the bridge needs to be to cross the obstacle, what types of natural obstacles or disasters are likely to occur in the area (high winds, earthquakes), the purpose of the bridge (foot traffic, cars, railway), and what type of vehicles will need to cross the bridge.

Definition and Basic Principles

Barodynamics is a key component of any bridge design. Bridges are made of heavy materials, and many concepts, such as tension and compression of building materials, and other factors, such as wind shear, water pressure, and torsion come into play in bridge building.

Bridge designers and constructors must keep in mind the efficiency (the least amount of material for the highest-level performance) and economy (lowest possible costs while still retaining efficiency) of bridge building. In addition, some aesthetic principles must be followed. Public outcry can occur when a bridge is thought to be “ugly.” Conversely, a beautiful bridge can become a landmark symbol for an area, such as the Golden Gate Bridge has become for San Francisco.

The four main construction materials for bridges are wood, stone, concrete (including prestressed concrete), and iron (from which steel is made). Wood is nearly always available and inexpensive but is comparatively weak in compression and tension. Stone, another often available material, is strong in compression but weak in tension. Concrete, or “artificial stone,” is, like stone, strong in compression and weak in tension.

The first type of iron used in bridges, cast iron, is strong in compression but weak in tension. The use of wrought iron helped in bridge building, as it is strong in compression but still has tensile strength. Steel, a further refinement of iron, is superior in compression and tensile strength, making it a preferred material for bridge building. Reinforced concrete or prestressed concrete (types of concrete with steel bars running through concrete beams) are also popular bridge-building materials because of their strength and lighter-weight design.

Background and History

From the beginning of their existence, humans have constructed bridges to cross obstacles such as rivers and chasms using the materials at hand, such as trees, stones, or vines. In China during the third century BCE, the emperor of the Qin Dynasty built canals to transport goods, but when these canals interfered with existing roads, his engineers built bridges of stone or wood over these canals.

However, the history of barodynamics in bridge building truly began in Roman times, when Roman engineers perfected the keystone arch. In addition to the keystone and arch concepts, the Romans improved bridge-building materials such as cement and concrete and invented the cofferdam so that underwater pilings could be made for bridges. These engineers built a network of bridges throughout the Roman Empire to keep communication with and transportation to and from Rome intact. The Romans made bridges of stone because of its durability, and many of these bridges are still in use in modern times.

In the Middle Ages, bridges were an important part of travel and transportation of goods, and many bridges were constructed during this period to support heavy traffic. This was also the period when people began to live in houses built on bridges, in part because in walled cities, places to build homes were limited. Possibly the most famous inhabited bridge was London Bridge, the world's first stone bridge to be built over a tidal waterway where the water rose and fell considerably every twelve hours. However, in Paris in the sixteenth century, there were at least five inhabited bridges over the Seine to the Île de la Cité.

The first iron bridge was built in 1779. Using this material changed the entire bridge-building industry because of the size and strength of the structure that became possible. In the 1870s, a fall in the price of steel made bridges made of this material even more popular, and in 1884, Gustave Eiffel, of Eiffel Tower fame, designed a steel arch bridge that let wind pass through it, overcoming many of the structural problems with iron and steel that had previously existed. Iron and steel are still the most common materials to use in bridge building.

Suspension bridges began to be quite popular as they are the most inexpensive way to span a longer distance. In the early 1800s, American engineer John Augustus Roebling designed a new method of placing cables on suspension bridges. Famous examples of suspension bridges include the Golden Gate Bridge (completed in 1937) and Roebling's Brooklyn Bridge (completed in 1883).

Girder bridges were often built to carry trains in the early twentieth century. Though capable of carrying heavyweight railroad cars, this type of bridge is usually only built for short distances as is typical with beam-type bridges. In the 1950s, the box girder was designed, allowing air to pass through this type of bridge and making longer girder bridges possible.

How It Works

The engineering principles used to construct even a simple beam bridge are staggering. Supports must be engineered to correctly hold the entire structure's weight and any traffic that will cross the bridge. The bridge itself, or span, must be strong enough to bear the weight of traffic and stable enough to keep traffic safe. Spans must be kept as short as reasonably possible but sometimes must be built across long distances, for example, over deep water. Historically, bridges were primarily made of wood, stone, iron, and concrete. In the twenty-first century, most bridges are steel, prestressed concrete, and reinforced concrete.

Arch. The Roman arch concept uses the pressure of gravity on the material forming the arch to hold the bridge together with the outward thrust contained by buttresses. It carries loads by compressing and exerting pressure on the foundation, which must be prevented from settling and sliding. This concept allowed longer bridges than ever before. For example, a surviving bridge over the Tagus River in Spain has two central arches 110 feet wide and 210 feet above the water level. These arches are made of uncemented granite, and each keystone weighs eight tons. This type of bridge is constructed by building a huge timber structure to support the bridge during the building phase, then winching blocks into place with a pulley system. After the keystone to the arch is placed, the scaffolding is removed, leaving the bridge to stand alone.

Beam. This is the most common form of bridge and may be as simple as a log across a stream. This type of bridge carries a load by bending, which horizontally compresses the top of the beam and simultaneously causes horizontal tension on the bottom of the beam. A bridge with two or more beams joined over supports is called a continuous beam bridge, while a bridge made of beams that spans two supports is a supported bridge.

Truss. A truss bridge is popular because it requires a relatively small amount of construction material to carry a heavy load. It works like a beam bridge, carrying loads by bending and causing compression and tension in the vertical and diagonal supports.

Suspension. Suspension bridges are essentially steel-rope bridges. Thick steel cables are entwined like ropes into a larger and stronger steel cable or rope. These thick, strong cables then suspend the bridge itself between pylons that support the weight of the bridge. A suspension bridge can be thought of as an upside-down arch, as the curved cables use tension and compression to support the load.

Cantilever. Cantilevered means something that projects outward and is supported at only one end (similar to a diving board). This type of bridge is generally made with three spans, with the outside spans supported on the shore and the middle span supported by the outside spans. This is a type of beam bridge that uses tension in the lower spans and compression in the upper span to carry a load.

Pontoon. A pontoon bridge is built across the water with materials that float. Each pontoon, or floating object, can support a maximum load equal to the amount of water it displaces. If the load placed on one pontoon-supported section exceeds the water displaced, the pontoon will submerge and cause the entire bridge to sink.

Applications and Products

Bridges are continuously being built to cross physical obstacles, and as the nature of materials changes, the ability to cross even larger obstacles becomes a reality. Nature is the defining force on a bridge. Most bridges fail because of flooding or other natural disasters.

Improvements in building materials are ongoing. For example, the Jakway Park Bridge in Buchanan County, Iowa, was the first bridge in North America to be built with ultrahigh performance concrete (UHPC) with pi-girders. This moldable material combines high compressive strength and flexibility and offers various design possibilities. It is very durable and has low permeability. Self-healing concrete contains bacteria that produce limestone, which fills cracks as they develop. Superelastic reinforcement, or shape memory alloy, replaces steel in reinforced concrete because it will return to its original shape following high stress.

Bridge-building products can be developed to help the environment. For example, the rebuilt I-35W bridge in Minnesota uses concrete that is said to “eat smog.” The concrete contains photo-catalytic titanium dioxide, which accelerates the decomposition of organic material. Other materials like this may change the future of bridge building. Another type of concrete used in modern bridge designs is self-healing concrete. This material contains limestone-producing bacteria, which can help fill cracks.

Careers and Course Work

Those who engineer and design bridges may have backgrounds in various fields, including architecture and design. However, those involved in the barodynamic aspects of bridge building are engineers, usually civil engineers, materials, or mechanical engineers. Earning a degree in one of these fields is required to get the training needed in geology, math, and physics to learn about the physical limitations and considerations of bridge building. Many bridge engineers have advanced degrees in a specific related field. After earning a degree, a candidate for this type of job usually works for a few years as an assistant engineer in an apprenticeship, learning the specifics of bridge building, such as drafting, blueprint reading, surveying, and stabilization of materials. To become a professional engineer (PE), one must take a series of written exams to obtain a license.

Social Context and Future Prospects

Barodynamics is a rapidly changing field. As scientists develop new materials and existing materials change, the possibilities for future improvements increase. The development of future lightweight materials may change how bridges are engineered and designed to stabilize the structure and avoid collapse. As innovations in iron refinement altered the face of bridge building in the late 1800s, new materials may further refine and improve bridge building, bringing more efficient and economic bridges.

Materials engineers usually provide the technological innovation to create these new materials. They examine materials on the molecular level to understand, improve, and strengthen materials to create better building materials for structures like bridges. Possible future bridge-building materials include ceramics, polymers, and other composites. In the late 2010s, federal legislation was introduced to allow for more innovation in building materials and techniques geared toward economic longevity for public works and infrastructure projects such as bridges. Many engineers argued that composites, including reduced maintenance needs, were crucial to achieving these goals.

The potential of three-dimensional (3D) printing technology in constructing bridges is being explored. In 2021, four robots with welding torches created the first 3D-printed steel bridge in Amsterdam, the Netherlands. Biomaterials and nanomaterials are other rapidly growing materials affecting bridge aerodynamics. Engineers, designers, architects, and construction professionals create 3D models using a building information modeling (BIM) technology platform called MIDAS CIM (CIVIL Information Management System).

In March 2024, the Francis Scott Key Bridge in Baltimore, Maryland, collapsed after the Neopanamax container ship MV Dali struck it. The steel truss, arch-shaped bridge was the third longest in the world and crossed one of the most important shipping routes in the United States. Many bridges built across shipping routes in the twenty-first century have stout piers or underwater structures surrounding the bridge's structures to prevent vessels from colliding with them. However, the Francis Scott Key Bridge was built in 1977, when these structures were less common because shipping vessels were generally smaller than modern container ships. The National Transportation Safety Board (NTSB) investigated the incident, which the US Department of Transportation used to consider future bridge safety regulations.

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