Footbridges and Accommodation Bridges
Part Two - Part One
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Cantilever construction is very popular for footbridges across wide roads and motorways. The individual parts may be shorter and lighter than for beams of the same span. The effects of subsidence are less damaging than with through beams, though provision for jacking is often incorporated. Cantilevers
Designs for footbridges - Portal Frames
A portal frame is perhaps intermediate between a supported beam and an arch. The presence of arch action depends on the presence of outward thrust, which in turn depends on the presence of inclined supports. Even then, arch action will change bending moment in the deck if it is flat.
Lucky is the town that is graced by a wide river (if it never floods) and a suspension bridge to cross it. This bridge between Chester and a suburb across the River Dee is asymmetrical. At the Chester end the deck slopes up from the bridge to the higher bank. Such bridges are often deeply trussed for their size, making them rigid, a desirable attribute that the "Millennium bridge" in London did not possess.
This bridge spans the Clyde in Glasgow.
These pictures have been very kindly donated by Michael Parry. They show the graceful Apley bridge over the river Severn between Ironbridge and Bridgenorth. Note the truss and towers which are typical of suspension footbridges. Such bridges are probably unnecessarily stiff from a technical point of view, but most pedestrians probably prefer things that way. London's Millennium bridge, a very flat suspension bridge, erred too far in the other direction.
The beautiful river Wye is graced by several suspension bridges. That furthest downstream is at Biblins. It possesses many of the attributes of large suspension bridges, except that the hangers are all the same length, so the deck dips with the cables. In these pictures, you can see the main cables, the saddles which carry them over the towers, and one of the attachments to the anchorages, which are heavy concrete blocks. One the features which is now seldom seen is the series of cables which pass outwards and downwards from the deck, to limit the amplitude of wind-induced oscillations. Such cables were sometimes employed during the 19th century. A notice reminds people that oscillation is bad for the bridge, though some engineers from eight to eighty years old might find it difficult to resist the temptation to see how easy it is to excite a small amount of sway. Here are some pictures of the bridge at Biblins.
A few miles upstream from Ross-on-Wye, Foy bridge, a small suspension span, crosses the river. Trees prevent a good photograph being obtained. The deck is stiffened by a truss. The cables are slightly kinked at the centre. The plaque reads "ROWELL & Co Ltd ENGINEERS WESTMINSTER".
The next bridge upstream on the Wye, at Sellack Boat, is also a suspension span.
The anchorages are unusual: the eyebolts that hold the main cables, also link to cables that are connected to the feet of the towers. Perhaps this defines the geometry of the whole bridge when the eyebolts are tensioned. Another unusual feature is the use of long bolted clamps, not only near the anchorages, but near the towers as well. Short cables hang on the towers; these are clamped to long sections that hold the deck and shorter sections that connect to the anchorages. The deck has no stiffening, and will oscillate gently in both torsion and translation, though with strong damping. The inscriptions on the towers read "LOUIS HARPER A.M.I.C.E. MAKER ABERDEEN".
In Hereford, across the beautiful river Wye, we come across that rare find, a pleasant suspension bridge in a position where it can be well seen and photographed. This example has tie-bars rather than cables. Note the unusual truss work on each side of the span. The trussed tower is typical of small suspension bridges.
Sarawak rain forest
These two pictures show a footbridge in the tree canopy in Sarawak, on the beautiful island of Borneo. The bridge was hung from a number of trees, and had little tendency to vertical oscillation. Laterally, however, it was a different story. It was difficult to avoid walking in step with the movements of the bridge induced by the previous person, producing the same phenomenon as in the London millennium bridge. One solution was to take longer and slower steps, and to put the pressure on the bridge gradually at each step. Keeping a greater distance from the previous person was also helpful. The deck was in part made from wooden spars, and in other from aluminium ladders with planks on top.
See also Suspension bridges.
It must be a footbridge:you can walk across it. There is a footpath along each side.
Trusses are quite common in motorway footbridges and railway footbridges. If built from closed sections like the one on the left, the appearance can be clean and elegant. The truss is useful when the bridge has to be enclosed against the weather. The next example uses this principle. See also Truss bridges.
This bridge connects Les Avanchets, a very large complex of apartment buildings in Geneva, with Balexert, a shopping centre. The cylindrical metal tubes produce a tidy appearance, but the curved transparent walkway does not fit neatly into the triangulated trusses.
This footbridge connects Podsmead with Tuffley, in Gloucester. Unusually, the ramps are in line with the main bridge, because there is open land on each side of the main road, so that houses are away from the noisy traffic. The bridge has been repainted in red.
Here is a bridge in north Oxford. The bridge and its ramps are quite neatly designed, but the undulating decoration seems unrelated to the bridge: it doesn't even have a wavelength that matches the truss panels. What does it add to the design?
This bridge is very similar to the previous one, but it is made of wood. It is in Slovenija, where wood is plentiful, and is therefore an economical choice. This type of truss design is common in traditional buildings in Slovenija.
This recently built bridge is made of steel, apart from the wooden deck slats. The use of steel in small bridges and other objects in public places may be a response to the frequency of deliberate damage in the UK. An unusual feature is the high guard rail, which seems out of proportion to the slender trusses below. Do you think the rail contributes to the stiffness of the bridge?
An interesting triangular truss bridge over a canal in Birmingham. The use of tubular members helps to achieve a tidy appearance, just like the use of square sections in the example at the top of this page.
More examples of truss bridges in Birmingham are shown below.
Here is a modern footbridge on a very old station which retains one of the original buildings. Note the design of the chimney. This is a rare example of an original station design of the Great Western Railway.
Plan Views of Designs for Steps and Ramps
Designing a flight of steps or a ramp is not a difficult task. What is difficult is to build the access into the space available, integrate it into the bridge design in a satisfactory manner, and make it reasonably easy to use by most people.
One solution is to make a sharp demarcation between bridge and ramp, as restricted space often dictates in any case.
Another solution, available where space is plentiful, is to make bridge and approaches in one straight line in the plan view. The example shown below connects Podsmead and Tuffley in Gloucester.
This picture illustrates the difficulty facing the designer. If he or she designs a simple ramp, it will be steep: if steps are provided, they break the rhythm of walking. This example has shallow steps, connected by ramps of reduced slope. Any bridge that starts at ground level presents a barrier to old people or those with difficulty in walking, and to people with push-chairs and shopping bags. And any shop-keeper knows the effect of distance on frequency of visits - probably a strong function like an exponential.
Here are some pictures showing the entire bridge, and the road that it crosses. Given that the road is a barrier to communication, we may as well consider the possibility of separating the communities further, by incorporating some open ground, as here, which has the merit of keeping the houses away from the noisy road.
North of Gloucester, on the northern bypass A40, a very long footbridge connects Longlevens with Innsworth. It is a continuous concrete beam on several piers, with a haunched main span, almost like a scaled-down road bridge. This is an elegant structure. Slightly non-vertical surfaces for both the beam and the deck reduce the potential monotony. Most of the structure, unfortunately, is hidden from the road by the trees which were planted to absorb the noise of the traffic, there being houses not far from the road on both sides. Before the trees grew up, the bridge was a splendid sight. It is probably one of the most elegant structures in and around Gloucester.
To see the bridge from a distance you have to go up a hill about two miles away. Then you find that some pylons are in the way. The size of the pylons relative to that of the bridge is made clear by the fact that one pylon is behind the bridge. The choice of a simple structure was possible because it crosses fields on both sides of the road, and the designer has made the most of the freedom. Truly an excellent design. In these pictures, the suburb of Longlevens is on the left, and the village of Innsworth is on the right. People on the bridge can be seen on some of the original slides.
Here is an aerial view of the same bridge from a great distance on the opposite side, showing the green belt between the communities and the road. This green area enabled designers to align the ramps and the bridge, while keeping a reasonable gradient.
In both of the last two examples, the bridges connect communities that are not quite adjacent, but in many other places, the length of the footbridge seems to emphasise the fact that new bypasses, ring roads and main highways connect some people at the cost of separating others. But without these roads, the streets many towns would be dominated by noisy and smelly lines of slowly moving vehicles.
Ramps and Steps
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