When you picture a bridge, what does that structure look like? For many people, the first image that comes to mind is the Golden Gate Bridge in San Francisco or the famous Ikitsuki Bridge in Japan. Despite being built almost 60 years apart and with an ocean between them, these two bridges share one important thing in common: They both rely on trusses. The Golden Gate Bridge has a deck truss system, though it is primarily considered a suspension bridge.
History of Truss Bridges
Historically, bridges built before the 19th century were primarily made of stone. Despite being solidly constructed, stone bridges cannot be as long as most modern day bridges, and they do not resist compression or extreme tension as well as other materials like iron or even wood.
In the United States, the plentiful supply of wood made it a go-to building material. As the United States became more industrialized, it moved to using iron in the latter half of the 19th century. Wrought iron made way for steel toward the end of the 18th century. In some places in the United States, truss bridges were still primarily being built in the 1930s.
What Is a Truss Bridge?
By way of a somewhat redundant explanation, a truss bridge is a bridge that is mainly built using truss structures. A truss structure is comprised of components that form triangular units. This structure is:
- Able to transfer the load from the top of the triangle to the base
Thanks to this transfer of weight, the load is shared by a much wider area than it otherwise would be. Due to the effective use of materials, a truss bridge is easier to build than its stone predecessors and far cheaper in terms of materials and construction hours. Another important advantage to truss bridges is that they can be constructed to be much longer than bridges built before the Industrial Revolution. Why can a truss bridge be longer than a stone bridge? It all boils down to simple physics.
Lighter truss bridges can transfer a load over a wide area far more effectively than stone bridges can. This means that a truss bridge needs less support and can be suspended further and longer than any bridge that came before it.
What Is a Truss Beam?
A "truss" is a name given to a structure that is made from triangular units connected at joints that are called nodes. In any truss structure, those triangular units are to be slender and straight. You can categorize truss design into what is known as a planar frame and a space frame. Planar trusses have a two-dimensional design, while space-frame trusses are a three-dimensional form.
In modern bridges, a truss is manufactured from the groupings of triangles made of straight steel bars. These bars are connected with pin joints. In construction, pin joints are where the straight parts of a truss structure meet. By ensuring these elements are in the construction of a bridge, it allows only forces of compression, the act of pushing down, or tension, the act of stretching.
Longest Bridges in the World
The longest bridge in the world, the Danyang–Kunshan Grand Bridge in China, is actually a long viaduct supporting a network of different types of bridges, primarily truss style. Out of the other longest bridges in the world, variations of the truss style are the most common. These variations came to be for a multitude of reasons, including the load a bridge needs to carry. For example, an arch-style truss is a bridge that can be very strong and is primarily used for trains or heavily loaded bridges.
About Suspension Bridges
Another bridge type that can be long while still managing to carry the weight required of it is another variation of the truss, known as the suspension bridge. The suspension bridge is typically what comes to mind first when people think of bridges because many of the world's most famous landmark bridges are suspension bridges, such as the George Washington Bridge and the Golden Gate Bridge.
The beautiful form of suspension bridges makes them a popular choice for many cities. Suspension bridges are designed so that the deck (load-bearing part or road) can be hung below suspension cables that are on vertical suspenders. These decks usually rely on truss supports along the span. Less famously, there are simple suspension bridges that do not make use of vertical suspenders and are historically put to use in mountainous regions.
Suspension cables need to be anchored at each end of the bridge so that the load will be transferred into tension in the main cables. These cables are the ones that extend past the bridge and are anchored into the ground. The roadway itself will be supported on vertical suspender cables that give those world-famous bridges an iconic look. These vertical cables are called hangers.
Purpose of a Truss
When building a truss, forces of tension and compression work well due to the high ratio of strength to weight. What that means is that the truss bridge itself is far lighter and stronger than its stone counterpart. Thus, the truss allows a bridge to be built longer with fewer points on the ground.
When you consider what is the most famous bridge in the world, many truss bridges likely come to mind. The most famous of the truss bridges, such as the Golden Gate Bridge, were built inexpensively due to the effective utilization of materials possible with a truss deck. Because the truss is versatile, today there are many forms of truss bridges. For example, truss bridges can have a road on the top, middle or bottom of the design.
Beam Bridge Advantages and Disadvantages
Beam bridges are also known as girder bridges and are among the oldest type of bridge design. They are simply constructed with beams (load-bearing sections) that are supported by pillars and an abutment, a structure built to support the bridge's lateral load. The first beam bridges were made by prehistoric humans to imitate the bridges that can occur naturally, such as fallen trees between two hills.
To this day, the most common bridge design is the beam bridge. Their ease of construction makes them popular for short spans, such as over roadways. You can extend a beam bridge further than other types of bridges, provided you have solid piers supporting each section of the bridge. Long bridges may sag toward the middle, and the greater the load, the greater the tendency of a beam bridge to dip low in the middle.
However, beam bridges do have some disadvantages. Even for short spans, a beam bridge may be costly due to the price of the steel required to construct them. Concrete can be used, it isn't anywhere near as strong as steel and may not be able to withstand the tensile forces that the beams need to transfer. For this reason, steel mesh-reinforced concrete is typically used in construction.