Since the earliest recorded history, modern man has sought to cross chasms.
In fact, there is an element of pride as well as function in separating moving features like water and traffic above and below one another. Engineers have long developed specialized methods for designing many types of bridges, and they still push out the envelopes for bridge lengths, spans, heights, and other characteristics.
We’ve had the pleasure of designing many bridges, and we love to talk about bridges.
So if you love bridges as much as we do, then let us describe the 6 different types of bridges:
- Girder (Beam)
- Truss
- Arch
- Cantilever
- Suspension
- Cable Stayed
Girder (Beam)
A traditional girder bridge transfers load from the road surface into horizontal girders and into vertical piers and abutments
Most highway and city bridges are built with girders (beams) spanning the opening and traffic crossing on a bridge deck.
The deck is a flat, horizontal element that transfers the traffic load into long, linear elements underneath called girders. The girders then transfer the load into vertical members called piers (abutments at the bridge ends) at each end which transfer it into the ground via piles or footings. The girders can be simple, with individual girders spanning from pier to pier, or continuous across many piers, but expansion and contraction due to temperature requires a joint about every 100 m.
They can be built any length, width, deck type, or material, from a small creek bridge to a large highway bridge. Technically, a log placed across a stream is a girder.
There is no real restriction on the number of girders, width, deck type, or railing type. All of the options can be built relatively easily with the cost being the only major differentiator.
However, the size of the girders is a major factor in the cost of the bridge. As you might imagine, the longer the span is, the larger the girders need to be, and hence the bigger the cost per square foot of bridge. Vehicular bridges require significantly smaller girders than pedestrian bridges.
Ideally the number of piers is kept to a minimum, however if the opening being spanned is too large for a single span bridge, piers can introduce a trade-off: The shorter spans allow smaller girders, thereby reducing cost, but the additional piers add a corresponding cost.
Truss
Trusses can be very aesthetic as well as efficient in distributing load to the primary members.
They work by distributing the traffic load from the bridge deck to the side elements. The members immediately underneath the traffic serve only to transfer this load to the edges. Then, as the bridge flexes, the top members of the truss are in compression and the bottom members of the truss are in tension. Generally, the top and bottom members of the truss carry the majority of the load, and the interior cross-bracing members require significantly less load carrying capacity. They serve mostly to hold the truss together.
In that sense, the truss works like a very high (deep) beam. The most efficient beam size is one that is very narrow and tall, but this is not always practical when the beam is underneath the traffic. Thus, a truss allows this efficient section to be moved to the edges where it can maximize its efficiency.
Trusses are generally made out of steel. Although wooden trusses can be built, the relative weakness of the connections would limit the bridge spans. Composite materials like fibreglass can be used as well, but they have generally been limited to pedestrian bridges. The maximum length and load bearing capacity is entirely dictated by the material.
Trusses can almost always be built with a camber (vertical curve) to allow for more water flow or clearance underneath the bridge.
Ideally, trusses are fabricated in the plant and delivered to the site as one piece. However, in some cases the bridge is too long or in the backcountry and the truss must be built in pieces and assembled on site. Steel structures have two options for making the connections:
- Welding
- Bolting
Arch
Arch bridges can be very aesthetic. They consist of corrugated steel or precast concrete plates on cast-in-place foundations, and they require good quality backfill around the structure. They can have an aesthetic retaining wall on the outside face, rock riprap, or just grass.
Arches can make excellent pedestrian structures but they are not always an option. Depending on the geometry of the site, an arch might require importing alot of gravel backfill or it might be a very long, dark structure for pedestrians.
Manufacturers of arches will generally include the design of the structure in their prices. However, they will not do any design outside of the “backfill envelope,” site plans, or the like. If you need anything more than a structural design of the structure and related backfill, their insurance requirements will force them to decline and you will need to hire an engineer.
Cantilever
The Quebec Bridge supports a middle span from two “cantilevers.”
A cantilever is an engineering artifact in which a structural element is supported on one end but not the other, such as a diving board. Likewise, a cantilever bridge is one in which the bridge piers support the bridge span.
Cantilever bridges are usually large trusses because the cantilever requires strong support to project out across the river or road. Often there is a middle span supported by the cantilevers, thereby requiring an even stronger cantilever.
Cantilever bridges are rare, but they are called “long span bridges” by engineers because they can span significantly longer distances than girder or simple truss bridges.
Cable Stayed
The Bob Kerrey pedestrian bridge in Omaha, Nebraska is the longest pedestrian bridge in the world at 914 m. It has two rigid tower piers from which the bridge hangs.
This type of bridge utilizes a tower from which the bridge surface “hangs.” Cable stayed bridges use a similar system to suspension bridges however it is more basic – the bridge is directly connected to a rigid tower via a line of cables.
Cable stayed bridges are long span bridges and can be built in spans up to about 500 m. In fact, the world’s longest pedestrian bridge, the Bob Kerrey pedestrian bridge in Omaha, Nebraska, as a cable stayed bridge.
Cable stayed bridges are very attractive and users tend to love the aesthetics of the structure.
Suspension
In a suspension bridge, the deck hangs from vertical cables, which in turn hang from the rigid tower supports.
Finally, the longest spans in the world have been suspension bridges in which the bridge hangs from large cables that are draped between rigid piers. The Golden Gate Bridge is the most famous suspension bridge.
Suspension bridges have the longest span possibilities of any bridge, hence the longest bridge span in the world is the Akashi-Kaikyo suspension bridge in Kobe, Japan, at 1,991 m.
Suspension bridges are generally considered very aesthetic, although their design and shape are derived from function rather than form.
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