As we go, we’ll identify different types of bridges and their unique characteristics.
Tarr Steps, Somerset, England (Clapper Bridge)
Tarr Steps is a clapper bridge that crosses the River Barle in Somerset, England. Clapper bridges can be found in many parts of the world. They are among the earliest known bridges.
Experts believe they were first developed in prehistoric times, though most surviving examples date from the medieval period. The earliest record of Tarr Steps bridge dates from the 16th century. At 55 metres, it is the longest clapper bridge in Britain.
The middle part of the bridge was washed out by floods twice in recent times, first in the early 1940s and then again in the early 1950s. Both times, the bridge was reconstructed using the original stones.
Slabs and Piers
Tarr Steps bridge is made from stones without mortar to hold them together. Pillars, or piers, sit on the riverbed about 2 metres apart. They hold flat slabs of stone—the clappers—that span the water. There are 17 spans altogether.
Iron Bridge, Telford, England (Arch Bridge)
Completed in 1779, this single-arch bridge spanning the Severn River was the first bridge in the world to be made of iron. In the late 1700s, the Severn Gorge was known for the high-quality iron produced there.
The Severn River was one of the busiest rivers in Europe, and it needed a bridge. A single-arched bridge that wouldn’t get in the way of river traffic, and one made of locally-produced iron, was a perfect solution.
The weight on top of an arch bridge is carried outward along the curve of the arch and transferred to the supports, or abutments, at either end. This prevents the bridge from collapsing at its centre or spreading out at its ends.
An advantage of single-arch bridges is that there are no support pillars rising from the river and getting in the way of boats. The main arch of the Iron Bridge has 5 ribs and spans 100 feet.
The Iron Bridge is an arch bridge, and, more specifically, it is an under-arch truss bridge. The arch structure is constructed with diamond and triangular shapes to form trusses. Among the geometrical shapes, these shapes are the strongest.
The parts of the Iron Bridge were cast by Abraham Darby III at his family’s ironworks close to where the bridge was erected. Darby’s grandfather had developed a way of smelting iron with coke instead of charcoal, which made the process cheaper.
Hengoed Viaduct, Hengoed, Wales (Arched Viaduct)
A viaduct is a bridgelike structure, often with semi-circular arches between towers. Viaducts carry roads or railways over water, valleys, or other roads. Often, viaducts cover long distances, and they can be stunningly tall.
Along with aqueducts built to carry water, viaducts were developed by the ancient Romans. The Hengoed Viaduct was built between 1853 and 1858 to carry a railway line. Today it is open to walkers and cyclists.
Height and Length
The Hengoed Viaduct is 36.5 metres tall at its highest point and about 260 metres long. It crosses the river Rhymney and two railway lines. It’s hard to tell from this angle, but the viaduct has a slight curve.
The first step in building the viaduct was to establish a stone quarry at nearby Maesycwmmer. Most of the viaduct, including the piers, spandrels and parapets, is stone. The soffits—the ceilings of the arches—are brick.
Arches and Piers
The viaduct’s pillars, or piers, are tapered—they are 10 feet wide at the bottom and 5 feet 6 inches wide where the arches begin. There are 16 arches, and each one covers a span of 40 feet.
Tay Rail Bridge, Dundee, Scotland (Truss Bridge)
Opened in 1887, Scotland’s Tay Rail Bridge connects Dundee on the northern bank of the Tay and the suburbs to the south of the river. A classic segmented truss bridge, it was constructed to replace an earlier bridge that was brought down by a storm in 1879.
The bridge has 85 spans and covers just under 4.5 kilometres, making it the longest rail bridge in the British Isles. It was refurbished and strengthened in a project that began in 2003.
The approach to the bridge on the southern end is a arched brick structure. In all, over 10 million bricks were used in the bridge.
Brick and concrete piers hold steel beams and wrought-iron trusses. Some materials from the original bridge were used in the construction of the new bridge.
Look closely at the trusses and you’ll see that the basic shape involved is the triangle. Triangles are rigid—you can’t wiggle a triangle. Weight placed on a triangle is distributed evenly through all 3 sides.
Forth Bridge, Fife, Scotland (Cantilever Bridge)
The Forth Rail Bridge crosses the Firth of Forth to connect North and South Queensbury in Fife, Scotland. Completed in 1890, it was the first steel bridge and one of the first cantilever bridges ever constructed.
In fact, this bridge is a hybrid of 3 common bridge types. Three cantilevered arms are connected by and support 2 simple beam bridges. All 5 elements employ trusses—they have rigid frames made of pieces joined to form triangular shapes.
Each cantilevered segment has 4 towers that extend vertically from a granite pier. Diagonal steel tubes project from the tops and bottoms of the towers. They support the deck with double rail tracks that runs horizontally through the centre.
The 3 massive cantilevered beams are connected by 2 simple beam bridges that look almost dainty in comparison. Their only support is the cantilevered beams, so they were designed to be as light as possible.
The bridge was made with 54,000 tons of steel and many thousands of tons of granite and other stone, concrete and cement. The steel components are held together by nearly 7 million rivets!
Beginning in 2002, the bridge was repainted. First the old paint was stripped off and any steel that needed repairs was attended to. Then 240,000 litres of new paint went on in 3 layers. The restoration took 10 years.
Tower Bridge, London, England (Bascule Bridge)
Tower Bridge is one of 33 bridges that cross the Thames River in London and one of the city’s outstanding landmarks. It was completed in 1894, toward the end of the Victorian era—a time when no construction challenge was too great for Britain’s architects and engineers.
The bridge’s combined suspension and bascule, or drawbridge, design allowed cargo ships to pass up the river to the busy wharves of the Pool of London.
Spans on either end of the bridge are supported by suspended cables. The massive cables consist of many strands of wire. This type of suspension bridge had been around for only about 30 years when Tower Bridge was built.
The twin towers are 61 metres tall. Their design was meant to compliment the nearby Tower of London. Materials used in their construction include Portland stone, Cornish granite, and roof tiles of Welsh slate.
The towers are connected above by 2 pedestrian walkways. The walkways were closed not long after the bridge was completed and reopened again in 1982. In 2014, part of the floor in each walkway was replaced with glass.
Between the towers, the two halves of the deck—the bascules—pivot up from their bases at the towers. Inside the towers, counterweights weighing over 400 tons each balance the bascules as they are raised and lowered.
Friarton Bridge, Perthshire, Scotland (Beam Bridge)
As bridges go, beam bridges are pretty simple: a series of vertical pillars, or piers, are connected on top by horizontal beams that carry the roadway. The Tarr Steps clapper bridge is a primitive form of beam bridge. But modern beam bridges can be vast and complex.
Think of the networks of raised motorways with their sweeping slip roads and cloverleaf interchanges that skirt our cities. Completed in 1978, the Friarton Bridge in Perth, Scotland, is a classic beam bridge.
The Friarton Bridge is a segmental beam bridge. It is made of repeated elements—vertical piers and horizontal beams—that connect to form the completed structure.
The Friarton Bridge is also a box girder bridge. The beams are massive steel girders in the shape of a hollow box. They sit on reinforced concrete piers and carry a lightweight reinforced concrete roadway.
None of the bridge’s piers are in the river—a single span crosses the river completely. This was an important aspect of the bridge’s design because the river is narrow, and it is busy with cargo vessels and dredgers.
Although it isn’t obvious in the panorama, the deck of the Friarton Bridge is actually two separate roadways with a gap in between. The roadways carry traffic in opposite directions for a total length of 831 metres.
Humber Bridge, Hull, England (Suspension Bridge)
With a centre span of 1,410 metres and a total length of 2,220 metres, the Humber Bridge near Kingston upon Hull in England is the longest single-span suspension bridge in the UK.
For many years, it was the longest in the world. Completed in 1981, it crosses the Humber Estuary to connect Yorkshire and North Lincolnshire. It is a classic suspension bridge with no frills. Two towers hold cables that, in turn, hold up the deck, or roadway.
The reinforced concrete towers are 155.5 metres high. Because of the curvature of the earth and because the towers are so far apart, they are slightly further apart at the top than at the bottom.
Each of the main cables is a bundle of 14,948 separate wires. The cables run over the towers and are attached to massive anchorages at either end. They transmit the force of their load to the anchorages.
The deck is about 30 metres over the water. It is made from steel and concrete and supports a dual carriageway with 2 lanes going in either direction. It also has footpaths/cycle tracks running its entire length on both sides.
Second Severn Crossing, England/Wales (Cable-stayed Bridge)
Opened in 1996, the Second Severn Crossing spans the Severn River to connect England and Wales. For most of its 5.128 kilometres, it’s a concrete and steel beam bridge.
But its 948-metre central span is a cable-stayed bridge that even has its own name—Shoots Bridge. In a cable-stayed bridge, each cable is attached at one end to a tower, or pylon, and at the other end to the deck. The towers bear the weight of the bridge.
The pylons are made from reinforced concrete. Their tops are 137 metres above water level and 100 metres above the deck. Each pylon has 2 hollow vertical shafts connected by crossbeams, one above and one below the deck.
A total of 240 cable stays connect the deck to the pylons. The main cables are connected by thinner vertical cables to keep them from moving too much. The bridge can carry a full load of traffic even if one of the cables is detached.