Introduction to steel bridges
Steel bridge as a structure type is extremely popular all over the world, and it can have an almost limitless number of variations due to a large number of structural forms possible, as well as different bridge spans, different purposes for a bridge, and so on.
Any steel bridge automatically receives every advantage from a long list of benefits that steel as a material has – including construction speed, strength-to-weight ratio, ductility, mass production and prefabrication capabilities, and so on. These kinds of advantages are what made steel the best material for bridges (especially when it comes to larger bridge construction projects), as well as many other structure types.
Since steel as an alloy can have a multitude of different variations created by adding various elements to it, it is rather difficult to provide specific parameters, but we can safely say that regular, basic steel has about a hundred times more tensile strength than medium concrete, as well as over ten times more compressive strength. There is also the added factor of ductility – steel’s capability to deform before failing since steel as a whole would start to yield quite considerably when above a certain stress level threshold.
Numerous advantages of steel as a construction material for bridges
We have already mentioned some of the more notable advantages of steel as a whole, and how these advantages are beneficial for steel bridges specifically – such as steel’s general strength and durability, as well as its predictability and high level of quality due to its prefabrication/mass production capabilities. However, these are not the only advantages that steel has in this context:
- Steel production in general is a very environmentally-friendly process since structural steel is 100% recyclable and over 90% of all steel produced in the U.S. is already created using recycled materials in the first place. Steel scrap – the leftovers from the original prefabrication process – can also be used to produce more steel elements, and steel also has a rather low carbon footprint as a whole, minimizing the environmental impact of steel as a construction material.
- Not only steel is a prime material for larger bridges, but it is also an extremely useful material when a quick bridge replacement is needed for some sort of overpass or a small bridge – providing a cost-effective solution in regard to smaller bridges with basic bridge structures. Steel can also be used to fix existing short-span bridges in a small time frame, making it a modular construction process.
- Since all of the steel bridge components can be fabricated with high accuracy, steel has the biggest creative freedom in the field, allowing for steel structure variations that would be borderline impossible with other material types.
- Steel structures of all types can be reinforced and repaired with relative ease when needed, and this also applies to steel bridges – including both reinforcement for specific parts of a bridge as well as general regular maintenance to improve the bridge’s overall lifespan.
- The advantage of high strength is what makes steel bridges the best possible fit for both transit and railroad applications – steel bridges are great at supporting rail live loads, even though these tend to be several times more intensive than highway live loads.
- Steel’s many different components and bridge variations make it the only material type that is capable of creating some of the biggest bridges in the world, with crossings spanning over 500 feet, and with the help of elements such as trusses, plate girders, suspension bridges, and more.
- The combination of relatively light weight and high strength as a whole makes steel bridges perfect for regions with high seismic activity since lighter superstructures suffer reduced seismic forces as a whole.
- Steel bridges can not only be repaired but also modified at any point in the future, making them incredibly adaptable and exceptionally durable in terms of a general lifespan.
Different components and elements that can be a part of a steel bridge
Since there are so many variations of steel bridges and a countless number of circumstances every different bridge would have to be built in, it is only natural for steel bridges to consist of a multitude of different elements that are connected to each other – just like almost any other structure made of steel.
It is worth noting that some of these steel bridge elements and components are used in almost any bridge type, while others would be extremely case-specific and only usable in a short list of situations.
An integral part of basically any regular steel bridge, the foundation is pretty much every part of a bridge that is placed below the ground level. There can be multiple different foundation types depending on circumstances and bridge type – with pile foundation, well foundation, and open foundation being some of the biggest examples.
Some of the foundation types would have to have an additional element added to them called a cap (well and pile foundations are good examples of these cases). The main purpose of a cap is to serve as a connection between the bridge components that are placed above the ground level, transmitting weight loads from the highest elements of a steel bridge to the lowest – such as the aforementioned foundation.
A deck is one of the main parts of a bridge – its surface. A deck can not only be made using steel, but there can also be other materials in its place, such as wood, concrete, or open grating. A deck is where all of the specialized covering goes, be it asphalt concrete for cars/pedestrians, railroad bed for trains, and more.
A deck can act as one of the primary structural elements in some specific cases – mostly with cable-stayed and tied-arch bridges – supporting the entire bridge span by carrying compressive or tension forces. A concrete deck can also be supported by other elements instead – these supporting elements can be girders, I-beams, etc.
The biggest use case for beams is to act as supporting structures, but they can only be used for smaller bridges with relatively short bridge lengths since beams do not have built-in supports for themselves. The main deciding factor for any beam bridge is what is the distance between support elements since this distance has a direct correlation to how strong this bridge is going to be.
As such, some of the more common examples of beam bridges usually go for about 80 meters or less. However, this does not mean that it is impossible to build longer bridges with this structure – it only means that you would have to use multiple beam bridge segments that are connected to each other (the term “continuous span” is used for these kinds of bridges).
A girder is a variation of a regular steel beam that shares a lot of its characteristics – but there are also differences. For example, a girder can be made of both steel and concrete (and the variation of a girder that uses a steel beam enclosed into a concrete box is incredibly popular in rural areas that have a lot of corrosion-friendly environments). However, a lot of the similarities are still there, with the girder’s strength being mainly decided using parameters such as the girder’s height, material type, girder weight, girder shape, and so on.
A specific element that provides a resting surface between the bridge deck and that bridge’s piers is called a bearing. The main purpose of a bearing is to reduce the potential stress by allowing for controlled movement of the entire bridge’s structure. There can be many sources of stress that a bearing can help with, be it internal stress (fatigue, contraction, shrinkage, etc.) or external (seismic activity, ground settlement, thermal expansion, and so on).
A pier is an element that has only one purpose – to transfer either horizontal or vertical loads from the superstructure to the foundation and below. Piers are also used to accommodate for several other factors, be it water, wind, or vehicle collision. Piers can be made in several different shapes, ranging from relatively simple to rather complex and composite.
Substructure and superstructure
A substructure is a part of a steel bridge that consists of everything below the level of bearings, including bedblock, piers, footings, shafts, coping, and many other elements that are located in the lower part of a bridge.
Alternatively, a superstructure is the upper part of a bridge – every part of the structure that receives the direct live load of the bridge itself. It is the main supporting structure of the bridge as a whole – and it uses its own supporting elements to transfer the load to lower steel bridge elements.
It is a relatively simple combination of a hanger and two pins, and its main purpose is to be used in specific configurations of a bridge – mainly with a suspended span or articulated span. The placement of these elements is up for debate for each specific use case.
A welded connection is a rigid one-piece construction that uses two different parts fused in one joined part. Welded connections exist to connect elements or to make structural connections as a whole. There is also a difference between smaller temporary welds and proper welding processes since temporary welds (tack welds) only exist for a specific time frame and do not have the strength of a fully-welded connection.
As with the aforementioned beam bridge use case or with many other use cases, there might come a need to create multiple bridge spans that are connected to each other. These kinds of connections are called “splice connection”, and they can be performed both in the field during the construction process and in the fabrication shop beforehand. Splice connections can be created with welds, rivets, or bolts, with bolted splice connections being the more common ones.
In a traditional sense, an anchor is what holds a floating object of sorts in a single place, be it on the ground or connected to another physical object. In the context of steel bridges, an anchor is a series of cantilevers that are used to keep specific parts of the bridge in place.
A suspension cable is what connects an anchor to the bridge part, transmitting part of the overall load to tension via these cables. There can be multiple different types of suspension cables, such as main cables, vertical suspender cables, and so on. Suspension cables can also support either one or two bridge spans with the help of support pillars (for highway-related bridges or bridges that go over canyons).
Examples of steel bridges
Of course, this is quite a lot of information to take in at once, especially since the topic itself is not that easy to begin with. To make it easier to understand some of the more basic steel bridge components, we have collected a number of examples that showcase different bridge use cases and different steel structures that are used to create a steel bridge.
Here you can see a prime example of why steel bridges are so valued and popular all over the world – a structure like this would be a lot harder to recreate without all of the steel’s advantages as a material. You can see here that the bridge’s superstructure relies a lot on a multitude of suspension cables, and the substructure is full of welded connections, splice connections, bearings, and so on. We can’t also forget about a massive deck made specifically for automotive vehicles and their respective weight in mind.
Not all steel bridges have to be these complex structures with hundreds of meters in span, there are many use cases for smaller bridges, too – and this is one such example. Its superstructure is far less complex than the example above, with this one mainly relying on interconnected steel beams and other elements to create a rigid steel frame of sorts. The substructure mainly consists of a set of beams with no supporting structures in the middle of the bridge itself.
Additionally, not all steel bridges are built for automotive or pedestrian-related purposes, it is also fairly common to create bridges for the sole purpose of creating a pathway for trains over some sort of obstacle – in this example that obstacle is a body of water. This bridge uses a relatively common structure in general, with one large nuance – the entire deck of the bridge is replaced with a railroad bed, making this bridge suitable for trains only. In this image, we can also see a superstructure similar to the previous example – a steel frame that is used to distribute the transport’s load and transfer it to the lower parts of the bridge.
Steel bridges can be quite versatile when they need to be, offering passage to multiple types of people or transport at the same time. In this particular example, we can see that a regular automotive-related bridge with asphalt as its main material has also created a separate pathway for pedestrians to use – and this part’s desk is made of wood, not steel or concrete. This kind of versatility is great for bridges in all kinds of active and popular locations, combining multiple types of pathways in one location.
Our last example is another showcase of steel as a material for bridge building – a massive structure several hundred meters in a span that offers several lanes of vehicle crossing space along with a walkway for pedestrians. You can see that this structure relies a lot on bearings and suspension cables to distribute its load, and there are also anchors, piers, welded connections, and other steel bridge elements that help this bridge perform at its best at all times.
There are many reasons why steel bridges are so commonly used all over the planet, and the main reason for that is a large list of advantages that steel itself has as a material. Steel can be customized, it can withstand massive loads, it can be used to create extremely long bridges, the overall speed of construction is higher than the competition, and so on. In this article, we have managed to showcase steel’s advantages as a whole, list a lot of different steel bridge elements, and even showcase some examples of steel bridges with different designs and composition ideas.
- Introduction to steel bridges
- Numerous advantages of steel as a construction material for bridges
- Different components and elements that can be a part of a steel bridge
- Pile cap
- Substructure and superstructure
- Hanger/pin connection
- Welded connection
- Splice connection
- Suspension cable
- Examples of steel bridges