Footbridges

Introduction

The building of motorways, ring roads, and many other roads, large and small, has cut across existing rights of way, such as footpaths and farm roads. In earlier times, railways did the same thing. Communities and farms have been separated or cut in two. Many small bridges and subways were needed in order to overcome the problems. Railways can present less of a problem. Level crossings with automatic barriers can cope in the case of flat ground. They are the equivalent of traffic lights for roads, but the frequency of road traffic is such that pedestrians are not often allowed across major roads, and across motorways, never.

Footbridges are small, but important, being prominent in a town-scape. The appearance of footbridges, and indeed any other bridges, in a town, is a major concern of designers. People have to live with these structures, often seeing them every day. Conflicting demands may arise, especially in areas which are of architectural or scenic interest.

Footbridges, in fact, can be elegant or beautiful, and are built on a more human scale than large road and railway bridges. Railway footbridges tend to be somewhat utilitarian, dating as they generally do from earlier times. Apart from those in stations and in towns, they are generally not much seen, even by the passengers who go under them.

As you approach or enter a large town, you are likely to go over or under a railway track, a bypass, a ring-road, or a large road such as a motorway. If you go over a bridge, then unless the route below is in a cutting, your own route will have embankments and bridges, which exert a significant effect on the area. If you are going under the other route, the visual impact of your own entry is affected.

In many cases, large roads pass over smaller ones on beam bridges, which cut straight across the view, and if the major road has four or six lanes and a central reservation, the effect will be almost that of a short tunnel. Seldom do the designers make a slot in the central reservation, separating the bridge into two halves, and letting some light into the tunnel.

Here are some examples of bridges over roads into towns.

Luckily the impact of small footbridges is much weaker, but still important, especially as they are structures that people will walk over. They will spend a longer time near or on a bridge than a motorist will, and so they will be more aware of its appearance. Habitual use may of course render it almost invisible, but there may be some subliminal effect. Being generally invisible is almost a compliment to a structure if it means that nobody is complaining. If a designer's bridges were never noticed, it would surely mean that they were never ugly. And a footbridge is no more inferior to a giant suspension bridge than a short story is inherently inferior to a novel. In both any diminutive form the small scale can actually present the greater challenge. The resulting pleasure in contemplation can be greater because the whole thing can be taken in more easily.

Footbridges have taken almost every possible form, including arch bridges, beam bridges, cable-stayed bridges, cantilever bridges, portal frames, propped beams, suspension bridges, and trusses. If you read right through this page, you will notice that the numbers of bridges vary strongly, depending on the type. Why is this?

The designer has the advantage that loads will not be heavy, and he or she may be innovative in solving the problems. On the other hand, he or she will be working to a small budget.

There are two basic situations, road in a cutting, and road level with surroundings. The first is easier for two reasons; no piers are needed at the ends, and no ramps are needed. Ramps present a particular problem. If the ramps are too steep they will present difficulties for people who are carrying or pushing, and people who are infirm or disabled. If the ramps are made very gentle they will require more space, and lengthen the crossing. They may be smooth, or they may have shallow steps, not necessarily with horizontal treads. In any case, ramps are difficult to integrate into a design.

Ramps may be straight, cornered, zig-zag, or helical. If cornered or zig-zag they they will have two or more flights. On a railway station or a typical road, constraints of space may dictate straight ramps parallel with the route. With a little more lateral space, zig-zags or helicals may be possible. If the bridge joins parkland or other open spaces, the ramps may continue the line of the bridge.

Although footbridges are often intended to connect residential areas, their very existence draws attention to the division between them, emphasising the separation.

Some motorway footbridges have no ramps: they have steps on either side. Three-pinned arches can be used to advantage in such cases. A footbridge over the M5 near Patchway, Bristol, uses an arch with a curve that hints at the difficulty of obtaining the required clearance while getting the curve down at the sides in a reasonable distance and achieving an elegant shape. Such an arch must sustain bending moments, because it cannot be anything like a catenary, for the reasons given.

Once you depart from a mathematical shape such as a circle or an ellipse, you have the problem of creating a convincing shape. One solution is to compose the shape of arcs of circles with deliberately very different radii, as in the tudor arch. Using a single mathematical formula for the entire curve will almost always generate a smooth curve. What is difficult to bring off is the creation of a shape composed of several segments.

Here are links - Pic1 - Pic2 to an interesting footbridge, designed by David Price, AWOLPaintball, which meets all its design requirements:

To cross a nine-foot deep gully containing a stream,

To be constructed within the small available budget and with the available materials, mainly wood,

To be easily constructed by the three people available,

To be safe, reliable and long-lasting,

To look good.

The design has the advantage of being easily visible from a distance, so that if it is infrequently used and the path is overgrown, it can be easily found, making detours unnecessary. The A-frame makes it rigid, forming, with the ground, a triangle, and is in a sense the simplest possible truss. Never forget the ground as a structural element, as long as you know its properties. Another nice feature is that the deck is not split into three equal spans, but has a shorter one in the middle. Those who like looking for golden section might think that this is quite close to it. The portal provides a real sense of crossing from one place to another, which a flat structure would not have done.

Some basic data are -

Maximum depth and width of creek to date - 6 feet/1.83 m and 10 feet/3.05 m

Length at deck level - 22 feet/6.7 m

Approximate height of deck above water - 9 feet/2.74 m

Designs for Footbridges - Arches

Before the industrial revolution, masonry arches were common for small spans. They are labour intensive, and their role has largely been taken over by other designs. For the smallest streams, large corrugated pipes are often used. These are then covered with concrete or rubble, and the whole is encased in tidy facings.

But for wide road crossings, various arch forms are still popular. The three-pinned arch offers the possibility of transporting members no longer than about half the span, and assembling the bridge with minimal disruption, if the road already exists.

The designs of many pre-stressed and steel bridges appear to hover between arch and portal frame, because of the necessity of obtaining clearance without excessive length of span, and while keeping reasonably close to the funicular. This topic is discussed later in this page. See also Arches.

Let us not forget the small bridges. This one is purely ornamental, as you can walk across without using it. It is a place to stand and view the pool. The design of the railings makes a nice contrast with the shape of the bridge. Because the bridge is at the edge of the pool, it looks slightly different from the two sides, making for a slightly lop-sided appearance as seen from this side.

This bridge spans a stream where it opens out into an artificial lake in a park.

Here is a three arch bridge in brick. The invention of brick making was a huge step in technology. Cutting rocks to shape needs tools, time and a lot of energy. Cutting them small enough for easy handling needs more time and energy than leaving them in large blocks. Bricks can be moulded to any reasonable shape, and a variety of materials can be used to achieve different colours and textures.

Bourton-on-the-Water is a well-known place for tourists. The little river is crossed by several bridges.

Here is one of several fairly similar bridges in Birmingham. Are they arches, beams or trusses?

Next to Presernov Trg in Ljubljana, a masonry road bridge with two arches, built in 1842, crosses the Ljubljanica. On either side of this is a footbridge, added later. Each footbridge is double, having steps which give access to levels below the road, where you can find restaurants.

Click here for pictures of an interesting and attractive footbridge built in 2002.

This is the "mathematical bridge" or "Newton's bridge" over the river Cam in Cambridge. Both names are totally misleading.

Here is an interesting bridge in Edinburgh.

These pictures show a bridge that is identical to one shown earlier. The difference is that in February 2004, this one was hit by a crane being carried on a truck. The crane and the truck were damaged, and a notch was knocked out of the bridge. The authorities quickly erected a temporary support and closed the bridge while the damage was being investigated.

Designs for Footbridges - Beams

Books tell us that the earliest bridges were probably beams, possibly in the form of fallen trees. For small spans, from one metre to perhaps ten metres, beams are simple and cheap, as the large number of pictures suggests. There is little point in complication in a very small structure. The design of small bridges owes as much to the need for rigidity as to the need for strength. A tight-rope would do the job, but few people would use it.

Some of the smallest bridges are stone slabs, giving a span of up to a metre or so. Such bridges can be reassembled quickly if damaged by floods, though in fact they are usually across quiet streams.

Wood is also used extensively for bridges across streams, and can look well in many different surroundings. The parts for a wooden bridge can be made on-site, or made off-site and transported easily. This is important for bridges on farms; the parts often have to be taken along unmetalled tracks.

Wood has been used extensively in China and Japan, sometimes in curved bridges which look rather like arches, but with intermediate supports. These are actually beams or trestle bridges.

Reinforced concrete may be used across small streams in towns, though corrugated tubes, covered with rubble, with concrete or masonry facing for the bridge, are now common.

For footbridges across large roads and motorways, pre-stressed concrete beams are common. Some take the whole road in one span, giving the advantage that the beams can be moved into position with minimal disruption, and no construction in the central reservation. This is especially important for bridges built after the road has opened.

Beams with sloping and vertical struts merge into portal frames and arches: as in many other cases, categories can have ill-defined boundaries. For more information see Beams.

Bridges in towns often join buildings on opposite sides of a street. If the design is not done carefully, the abrupt appearance and disappearance of the bridge at the ends can be quite disconcerting. The first picture shows two examples in Cheltenham, and the second shows a bridge in Gloucester.

Click here for about sixty photographs of beam bridges carrying footpaths.

Examples of Beam Designs

Here is a very unusual footbridge. A concrete wall dams a very small stream in a shallow valley, forming a lake. On either side of the dam, the wall is higher, and a footpath is attached to the top, crossing the gap as a short beam bridge. Because the flow in the stream is never large, the gap above the dam need not have been very high, and the footpath could have rested on the wall throughout its length. The effect would have been much less attractive.

The attachment of the path to the wall could have been done in several ways. The two extremes are as follows. One way is to fix the path rigidly to the wall, cantilevering out on either face. The other is to rest the path on the wall with no rigid attachment, and to make the beam stiff enough to take torsional forces back to the abutments. The actual construction may well use both ideas. One picture shows that the wall is cracking quite badly.

This is a poor picture of a bridge. That is because it isn't a picture of a bridge: it's a picture of a piece of wood - the one that's a different colour from the rest. This bridge was built in 2003, on two steel I-beams after the previous wooden bridge had been deliberately damaged to the point where it was unusable. The pale piece of wood replaces the first damage the new bridge, done after less than a year of use. In time of war, bridges are often among the early targets. They are often among the late ones, too, when retreating troops destroy their own structures behind them. But this is damage for its own sake. Many objects in public places are now built in steel, or even stainless steel, to reduce the chance of damage.

This bridge is a typical very small beam footbridge over a stream, comprising two main beams on edge with slats on top. The left side guard rail is utilitarian. That on the right was probably identical, but was replaced by a fence because the land on that side is owned by a householder. This may not have been the case when the bridge was made. This does not rank high in the scale of technical importance, but it does illustrate the idea that from the highest to the lowest, things are subject to change.

Designs for Footbridges - Cable-stayed

For bridges across the larger rivers, as well as across major roads, the cable-stayed bridge, often of asymmetrical design, has become popular, at the expense of the suspension bridge. Using only one tower is cheaper than two, at the expense of a somewhat higher tower, and possibly slightly stronger cables.

Here is a cable-stayed footbridge over a canal in Birmingham. The city has done a tremendous job in regenerating the area.

This is a cable-stayed foot-bridge at Bridgenorth, seen from the east bank of the river Severn. There is a cable-stayed foot-bridge a little way downstream from the famous iron bridge at Coalbrookdale.

This is Bell's Bridge over the Clyde in Glasgow, near the exhibition centre and the Science Mall. It rests on old piers. Part of the bridge rotates about a vertical axis to open a waterway for boats and ships. A canopy is cantilevered from both sides of a central bar.

The Sabrina bridge is a beautiful and interesting little footbridge to the north of the railway bridge in Worcester, joining Le Vésinet Promenade to the west bank of the river. It is an asymmetrical cable-stayed bridge with one tower. An ingenious feature is the use of hinges where the cables join the deck. This allows the use of rigid trusses without the necessity for extremely precise setting of the cable lengths. With a through truss, inexact cable lengths would produce uneven tension in the cables, and unwanted bending stress in the deck. The web-page about indeterminacy discusses this topic in more detail.

A new bridge in Swindon, Wilts.

There is even a cable-stayed railway bridge across the M25 motorway. Cable Stayed Bridges