7 Types of Bridges Every Engineer Should Know About
September 9, 2021
There are many different types of bridges, each with their own advantages and disadvantages.
If you are an engineering contractor or civil engineer intending to work on bridge projects, here is a refresher of the basic principles behind each type. A quick read-through may help to ensure you’re well informed when discussing bridge projects with your colleagues and customers.
In case you were wondering, there is no single answer to the question ‘What is the best type of bridge?’ This is because many factors need to be considered, such as location, the span required, weight and volume of traffic, resources, and the budget available.
Below we have listed the 7 main types of bridges.
7 Different Types of Bridges
- Arch Bridge
- Beam Bridge
- Cantilever Bridge
- Cable-Stayed Bridge
- Suspension Bridge
- Tied-Arch Bridge
- Truss Bridge
Arch bridges use one or more arches as the main structural component, with the arches positioned beneath the deck. This method dates back many thousands of years, with stone and brick being the most commonly used materials. However, in modern times you will see arch bridges constructed from concrete.
Whatever material is used the principle remains the same: An arch bridge uses compression – downward pressure from the deck travels laterally towards the keystone and to the supporting structures at each end of the bridge (the abutments).
Advantages of an Arch Bridge
- High levels of strength and resistance (many Roman bridges still exist).
- Adapts to local environmental conditions well.
- Greater span compared to beam bridges (though less span than cantilever and suspension types).
- Can be constructed from many different materials; stone, brick, concrete, iron, steel.
Disadvantages of an Arch Bridge
- Creating a long span length requires more arches.
- Time-consuming to construct and maintain.
- Requires strong side support to complete a successful span.
- Requires considerable expertise to build.
The beam bridge is the simplest type of bridge. In its most basic form, all that is needed is a crossbeam long enough to cover the span, and support from abutments under each end. To achieve a longer continuous span, (e.g., over 250 ft (80 m), piers need to be added to provide extra support. When doing this it means you create a series of bridges joined together.
A beam bridge may include several beams running in parallel to support the deck above. This is often referred to as a simply supported structure.
Advantages of a Beam Bridge
- Simplicity and quick construction.
- Inexpensive (if no piers needed).
- Modules can be prefabricated away from the bridge location.
- Versatile – can be used in many locations.
- Multiple types of material can be used.
Disadvantages of a Beam Bridge
- Limited span length between supporting structures.
- They can start to sag as they age (Weight limits are sometimes needed).
- Poor aesthetics.
- Costs are affected by fluctuations in steel prices.
- Maintenance and painting costs of steel bridges is expensive and time-consuming.
If you aren’t familiar with the cantilever principle, think of a diving board that is supported at just one end. A cantilever bridge is built using pillars securely anchored to the ground. The structure is then constructed outwards from each pillar with the horizontal beam often supported using diagonal bracing.
The famous picture above shows Forth Bridge engineers Sir John Fowler, Sir Benjamin Baker, and Kaichi Watanabe demonstrating the principle of the cantilever. In this example, Fowler and Baker are the cantilevers, and Watanabe represents the load on the suspended central span. The bricks on either end represent the anchor for the cantilever beams.
A cantilever bridge can be made using different materials, such as structural steel, or box girders of prestressed concrete for larger bridges that carry road or rail traffic.
Advantages of a Cantilever Bridge
- A good method for creating long spans.
- Support is required only on one side of each cantilever.
- Suitable for deep rocky valleys and flood-prone areas where supporting structures cannot be built.
- Little or no disruption to traffic underneath the bridge.
- On bridges with multiple spans, the cantilevers can be built simultaneously to reduce time.
Disadvantages of a Cantilever Bridge
- Complex to construct and maintain.
- Requires a heavy structure, so more material drives up the cost.
- Not suitable for excessive climate conditions or earthquake-prone areas.
- Experiences a high amount of tension during the construction (aka Negative moment).
- Stability relies on balancing compressive and tensile forces.
The cable-stayed bridge dates back to the 16th century and remains a popular design for spans greater than those of cantilever bridges – but shorter than the longest suspension bridges.
This design uses deck cables connected to one or more vertical columns, towers, or pylons which can be connected in either a fan or harp configuration. Although the deck relies on the cables for support, this method should not be confused with the suspension bridge that uses vertical cables between the deck and the main support cable.
The most common build materials used in cable-stayed bridges are steel or concrete pylons, post-tensioned concrete box girders, and steel rope.
Advantages of a Cable-Stayed Bridge
- Fast construction time compared to other bridge types.
- More rigid than suspension bridges.
- Can be aesthetically pleasing.
- Multiple design options: (e.g., side-spar, cantilever-spar, multiple-span, cradle-system designs).
Disadvantages of a Cable-Stayed Bridge
- Suitable for short to medium distances (Shorter span than suspension bridges).
- Difficult to access in some areas, which means higher-than-average costs of maintenance.
- Cables can be prone to corrosion and high levels of fatigue.
- Easier to vandalize (by cutting cables).
- Not suitable for excessive climate conditions or earthquake-prone areas.
The principle of a suspension bridge is not new – early designs simply used rope and wooden planks. Today, they are a popular design for long road bridges – thanks to the ability to create large spans across broad channels. Probably the most famous of all is the Golden Gate Bridge in San Francisco, US.
A suspension bridge uses vertical towers or pylons constructed from steel or reinforced concrete.
Attached Between the pylons are the main supporting cables. These cables are made from galvanized steel wire, and including their casing, can measure as much as 36 3/8 in (0.92 m) diameter.
Advantages of a Suspension Bridge
- Best bridge type for creating the longest spans with minimum piers.
- Regarded by many as aesthetically pleasing – creating a landmark for the community.
- Waterway can be left open while the bridge is under construction – (almost all of the work takes place on the top of the bridge).
- Flexibility – design allows for deck sections to be replaced.
- Can be built with a high deck, allowing plenty of clearance for passing ships.
Disadvantages of a Suspension Bridge
- Not as robust as some bridge types.
- Combination of vertical pressures and extreme side wind speeds can lead to a failure of the span.
- High winds can cause a suspension bridge to start vibrating.
- They can struggle to support focused heavy weights (e.g., trains).
- Failure of just one cable can be enough to cause the entire bridge to collapse.
- Access below the deck is difficult during construction and maintenance.
- Some suspension bridges require extensive foundation work if the ground is soft.
A tied-arch or bowstring bridge uses features seen in both a suspension bridge and an arched bridge. However, unlike a traditional arched bridge, the arch is positioned above the deck and uses vertical cables attached to support the deck.
This arch (or bow) uses the tension of its vertical cables, together with the compression of the arch, to support the load – keeping the bridge very stable.
Advantages of a Tied-Arch Bridge
- Very strong.
- Less force on abutments.
- Can be built off-site and transported into place.
Disadvantages of a Tied-Arch Bridge
- Regular maintenance is required to ensure stability of the hangers and arch.
- If one hanger breaks the entire structure can be adversely affected.
- Size of each span is limited as compared with suspension bridges
- More expensive to build as compared with other types of bridges the same length.
There are many different configurations of a truss bridge, but you will notice they all use triangular sections typically bound together by welded or riveted joints. The trusses are constructed vertically and horizontally which absorb tension and compression. The end result is a structure and decking area capable of withstanding relatively strong winds.
The truss design is reasonably inexpensive and has been around for a long time. In the early days during the 19th century, most were built of wood, before later shifting to iron and steel.
Advantages of a Truss Bridge
- The strongest type of bridge.
- Can be built off-site.
- Lighter than other bridge types.
- Withstands extreme weather conditions.
- Versatile – able to carry its roadway above (deck truss), along the middle (through truss), or on a lower truss sitting below the major structure.
Disadvantages of a Truss Bridge
- Truss bridges require perfect construction to work. They must distribute weight evenly.
- Older truss bridges designed for light traffic can have a lower weight tolerance.
- Requires a lot of space. Interconnecting triangular components need to be large to bear and distribute heavy loads.
- There are width requirements in relation to span length to achieve required strength
- Requires architectural and engineering specialists – less use of general laborers and steelworkers.
The descriptions covered here have hopefully provided an insight into the main types of bridges. There are, of course, many more possibilities that use a combination of each method, along with several types of moveable bridges, and floating pontoon bridges
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