How to Decide What Type of Bridge to Design
By Muna Mitchell, PE
Bridges are critical parts of our infrastructure. They help people move around safely and affect our perception of the built environment. Bridge design has important implications for safety and community. Initially, decisions around choosing what type of bridge to design lead to more questions.
What is a Bridge?
When people think of bridges, they might picture grand monumental structures like the Golden Gate Bridge in San Francisco or the Brooklyn Bridge across the East River. These structures are marvels of human ingenuity and iconic landmarks, but bridges are all around us. The official definition of a bridge comes from Title 23 of the Code of Federal Regulations: “a structure including supports erected over a depression or an obstruction and having a track or passageway for carrying moving loads, and having an opening measured along the center of the roadway of more than 20.” Essentially, a bridge is a connection between two locations that have an obstruction between them. What distinguishes them from other structures is moving loads, like traffic or pedestrians, and having a minimum opening.
Who Will Use the Bridge?
We consider what type of traffic will use a bridge in deciding what type of materials to use. Most vehicular bridges, by design, accommodate passenger vehicles and heavy trucks. Many are also designed with alternate forms of traffic to include paths for pedestrians and bicycles. Other bridges are limited to pedestrian and bicycle traffic, with a light truck for maintenance, while others are for trains or light rail. The weight of the traffic, the patterns it can form, and how it moves all affect the choice of bridge.
What is the Bridge Crossing?
Being well above obstructions like water, highways, and railways is safer for the traffic using the bridge. How far away the bridge should be from the obstruction depends on what it is. When it is water, we want to be above the expected flood level and wide enough to let the flood water pass through. For highways, we want to clear the tallest truck with room to spare and be wide enough that cars and trucks have plenty of room to pass underneath without getting too close to the bridge. Railroads have special requirements for height and width to make sure that the trains and all their cargo stay well away from any part of the bridge. How wide and how high the bridge needs to be can change the type of material used.
What is the Longest Span?
The widest opening of the bridge—known as the longest span—is dictated by the width of the obstacle and the clearances around it. Materials have limits on how far they can span. The iconic bridges mentioned earlier are suspension bridges, using a cable technology that allows the bridge to span long distances while still being well above the obstacle. Shorter spans can use simpler technology, like steel I-beams or pre-stressed concrete that comes in many shapes and sizes. In some cases, timber or masonry could be used to make an architectural statement.
Where is the Bridge?
Some materials are more readily available in some regions than in others, as is the labor needed to construct using different materials. These considerations can affect whether concrete or steel beams are used and if more sophisticated technologies like cable-supported systems are reasonable. Some materials require sophisticated equipment to install that might not be available locally or difficult to get close to the bridge site. Some regions require designs which consider special events like earthquakes that can affect the materials and geometry best suited to resist those events.
What is the Budget?
Every project has a budget, and the type of bridge has a large effect on how much the bridge will cost. The materials, labor, and equipment needed to fabricate and install the pieces of the bridge, and the details of how they all come together, are considered in the design.
What Materials Should Be Used?
The first bridges used local natural materials like stone and wood. Wood is still used in some pedestrian bridges. Modern highway bridges use combinations of steel and concrete based on span length, location, and budget, among other factors. Steel girders are often used for very long spans or curved spans. Some concrete options are also possible for very long spans using post-tensioning technology, for instance in segmental bridges. Record-setting span lengths require the use of cables and taller structures such as suspension and cable-stayed bridges.
All of the factors discussed are critical in determining which bridge solution best fits the needs of a project and which the most practical solution for crossing obstacles is.