Manchester Bridges: 17. Three bridges in Marple

It's stretching it a bit to call these "Manchester" bridges, but it's as convenient a tag as any. These are essentially rural bridges, in and around the town of Marple, near Stockport (which is part of Greater Manchester).

All three bridges, and several others, can be readily reached on a morning or afternoon's walk, and one of the links below leads to suitable walking directions. All three are Grade II Listed Buildings, and there are plenty of other Listed bridges in the vicinity, including the Marple Aqueduct.

Marple's "Roman Bridge" is a pretty packhorse-style masonry arch spanning the River Goyt, near the so-called Roman Lakes. Neither the Bridge nor the Lakes date back to Roman times, it was merely a name attached to them when they became briefly popular with leisurely Victorians. The Roman Bridge itself is believed to date from the mid 18th-century, and was at one time called Windy Bottom Bridge

The engineering perspective is hardly relevant, but it's a segmental arch with a single arch ring, with the spandrels rising to a level a little below the arch crown. It currently has lightweight iron railings, although probably had masonry parapets at one stage. It spans 13.7m and is 1.75m wide.

It's a sweet little structure in a picturesque spot. As a bridge designer, it begs the question of how modern designers could emulate its charm. Simplicity isn't enough - the attraction of a bridge like this lies in its shape, its transparency, and its texture, not all of which are easy to achieve in modern materials.

Another attractive bridge in Marple is Sutton's Lane Bridge No. 1, better known as the Roving Bridge, although it is only one of several such bridges. It spans the Macclesfield Canal at its junction with the Peak Forest Canal - both canals have further roving bridges, including a brick example nearby on the latter.

It's engineer was William Crosley, and it was built around 1831. The arch is an attractive horseshoe shape.

The edges of the arch barrel are protected with metal bars at lower level, as can be seen on the photograph to the right. These are to prevent damage to the brickwork from ropes running between horses on the towpath and barges on the canal. As can be seen in the photograph (click any image for a full-size version), there has been sufficient contact over time that even the metal bar has become grooved through repeated rubbing.

The bridge gets its name as a "roving bridge" from the spiral layout of the approach ramp on one side. This allows a horse towing a barge on one side of the towpath to cross the canal and switch to the opposite towpath without ever having to unhitch the tow-rope. It makes for a very quaint, charming bridge.

Immediately north along the Peak Forest Canal, there is a flight of locks each with a small brick footbridge, ending at a span called Posset Bridge No. 18 (or perhaps Possett Bridge, depending on the source). This was designed under the auspices of Benjamin Outram and Thomas Brown, and carries a minor highway over the canal.

It was reportedly built on behalf of the local industrialist Samuel Oldknow, and named the Posset Bridge because he arranged for the workmen to be supplied ale posset from a nearby inn.

It has been widened on its north side with a metal beam footbridge, which makes it less than attractive viewed as a whole. However, on its west side, there is a short horse tunnel, in a narrow horseshoe or egg shape, used originally to allow horses to pass below the highway while boats negotiated the adjacent lock. As at the Roving Bridge, there are clear grooves on the edges of the tunnel cause by rubbing from tow-ropes. A smaller boatman's passage also exists, providing access from the lock gates down to the lower towpath.

Further information:

• Google maps: Roman Bridge / Roving Bridge / Posset Bridge
• Bing maps: Roman Bridge (hiding beneath the trees) / Roving Bridge / Posset Bridge
• Engineering Timelines: Posset Bridge / Posset Bridge Horse Tunnel
• CanalPlanAC Gazetteer: Roving Bridge / Posset Bridge
• British Listed Buildings: Roman Bridge / Roving Bridge / Posset Bridge
• Walk Around Marple - Roman Bridge and the Lakes
• Marple Locks Heritage Trail [PDF]
• Historic Roman Bridge photos from Virtual History Tour of Marple and District
• Guide to the Packhorse Bridges of England (Hinchliffe, 1994)

The Ha-Ha Bridge, Yorkshire Sculpture Park

This was one of the earliest bridges I featured on this blog, which I recently visited at its home in the Yorkshire Sculpture Park. Designed by the artist Brian Fell, it's nearly little more than a conceptual joke - the Ha-Ha embedded in the two sides of the bridge is simply the term for the ditch below.

I still like the way each "parapet" is the reverse of the other, stencil-style. It's clear that the arch does little to support the bridge - the deck is a steel plate, supported on either edge by the lower part of the fascia plate, which returns below the bridge to form a sort of channel section. Plate diaphragms are welded at regular intervals, stiffening the deck plate and stabilising the arches.

It may only be an artist's conceit, but it still has its charm.

Further information:

• Google maps / Bing maps (aerial image is very old)
• Brian Fell
• Yorkshire Sculpture Park

Mailbox Footbridge, Birmingham

Here are some photos taken on a visit to Birmingham, of a modern footbridge across a canal. It's a rather unattractive bridge, but there are several features worthy of comment.

I haven't been able to confirm the identity of the designer or the builder, although there is a very similar design by specialist footbridge manufacturer CTS Ltd not far away.

I don't know that there's a word for this type of bridge. Most truss bridges are "through" or "half-through" structures, where the bridge deck passes between the supporting truss girders. This maximises one of the advantages of the truss form, which is its ability to accommodate a very shallow depth of structure between the footway or roadway surface and the underside of the bridge. This is often required to maintain headroom below while minimising the length of approach ramps.

This bridge, which sits just to the east of the shopping, restaurant and office development known as The Mailbox, has a three-dimensional truss supporting the deck from below. It is triangular in cross-section, with the web members arranged in Warren truss form. The deck is also fully braced, ensuring that overall the truss has sufficient torsional stiffness. As the deck is supported from its two upper chords, it is also secure against overbalancing.

However, you can see on the photo on the left that the ends of the truss terminate in a very awkward way. It's unclear quite how the truss is supported, with the bearings hidden behind a covering plate. Also, because the bottom chord is a constant section and the truss a constant depth, the entire bridge looks visually "stiff", ending abruptly where you might expect the bottom chord to curve upwards.

I believe some of this is just poor design, but some of it is due to the difficult construction history - the bridge was built before the adjacent building was started. It's far from clear how well coordinated the different construction operations and designs will have been. Whatever, the reason, the ends of the bridge look very clumsy.

The bridge parapets are in a fairly standard contemporary style, with horizontal wire infill, although the boxes for the lighting units are perhaps overly intrusive.

The decking is a cheese-grater type material, visibly perforated from below but appearing very solid from above. It will be free-draining into the canal, and may therefore be partly responsible for the poor condition of some of the steelwork below.

Underneath, the structure looks particularly unattractive, with what I presume are lighting cables and power supply units dangling from the industrial walkway material and the patchy steel frame.

There are two or three other interesting footbridges over the canals in central Birmingham, which I hope to visit and then cover here when I get a chance.

Further information:

• Google maps / Bing maps
• Canalplan gazetteer
• Construction photograph

London Bridges: 10. Chelsea Bridge Wharf Link Bridge

Nestling just below the south side of Chelsea Bridge, there's a modern walkway which carries the Thames Path past the bridge and along this bank of the river (although not very far along - the Path is currently blocked in front of Battersea Power Station by construction works).

This structure was installed in 2004. It was fabricated by Littlehampton Welding, and erected by being floated along the river. The designer was Whitbybird, now part of Ramboll. The structure is about 4m wide, 65m long, and cost £650,000.

The walkway structure cantilevers out from the bank either side of the Chelsea Bridge's abutment, assisted by two extremely slender supports founded on the river bed. The structure is designed to resist uplift in flood conditions, and so that it will not collapse even if one of the two supports is destroyed by boat impact.

The walkway itself consists of a shallow steel box girder, sufficiently shallow that vibration was readily perceptible on the structure even with only one or two people using it when I visited it.

There are holes in the edge of the deck, which puzzled me when I visited the bridge, but which I gather allow concealed lighting behind to "punctuate" the line of the bridge at night.

The parapets use alternately slanting posts in a sort of "V" configuration, which I have seen on other Whitbybird / Ramboll designs, both built and planned. Presumably this adds sufficient longitudinal stiffness to justify using more slender posts than would be possible if they were vertical.

The parapets have stainless steel hand and bumper rails, and the main parapet infill comprises tensioned wires drawn through the parapet posts. There is a clear rust spot at every intersection, which may be attributable to poor fabrication (lack of paint penetration into the hole), design detailing (how could paint ever fill the hole?), or even to bimetallic corrosion (lack of an insert to prevent contact between dissimilar metals). Whichever is the cause, it will be awkward to rectify.

There's clearly no engineering reason why the walkway has to cantilever so far from the Chelsea Bridge's abutment - this seems to me a conscious design choice to facilitate the more attractive curved alignment and create a visual distance which respects the gap in age of the structures. It also minimises the sense of it being a "pedestrian subway" on a structure in close proximity to the dark underside of the bridge above, by establishing open space around the walkway rather than clinging to a wall.

This is no major footbridge, but a positive example of simple, well-organised design appropriate to its location and pleasingly lightweight in conception.

Further information:

• Google maps / Bing maps
• Ramboll
• Metalworks magazine, Corus (2004)

London Bridges: 9. Chelsea Bridge

Chelsea Bridge is the second in a trio of structures spanning the River Thames in London, all of which can be seen from each other.

Perhaps one of my more knowledgeable readers can tell me whether it is Britain's only self-anchored suspension bridge, or whether there are other examples? I'm not thinking of bridges like the Roxburgh Viaduct Footbridge, or the Royal Albert Bridge, although they are indeed self-anchored bridges with suspension systems, but structures which resemble the more conventional suspension bridge in form.

In a conventional suspension bridge, there are two (or rarely, more) towers, from which suspension cables are hung, with the bridge deck passing below and supported by (usually) vertical hanger cables. The main suspension cables are anchored into foundations which may either consists of ground anchorages, or massive blocks constructed with sufficient weight to restrain the pull of the cables. There are two significant advantages. The first is that the main structural elements are purely in tension, which allows a much lower weight of material than for elements subjected to compression and the attendant risk of buckling. The second is that the span can be constructed without the use of any temporary supports below the deck, minimising both cost and disruption to the obstacle spanned.

The self-anchored suspension bridge dispenses with the need for foundations to anchor the main cables by anchoring them instead to the bridge deck. The cost and scale of anchorage foundations can be considerable, so this seems to be a sensible approach, since the deck has to be present in any case to carry traffic, and might as well perform a second function. In practice, however, the self-anchored option is rarely, if ever, the best engineering solution. The main cable forces must be exactly balanced by a compression force in the deck, necessitating a much heavier deck than is required in the conventional option. More significantly, the cables cannot be erected until the deck is available to provide their anchorage, which in turn means that the deck must be built using extensive temporary support from below.

Megalomaniac megaprojects like the San Francisco Oakland Bay Bridge aside, this means that self-anchoring is rare, particularly for spans of any significant length. The disadvantages generally outweigh the fairly limited benefits.

Chelsea Bridge was built in 1937 to a design by Rendel, Palmer and Tritton (now High Point Rendel), with the architects George Topham Forrest and E. P. Wheeler. It replaced an earlier and much more ornate wrought iron suspension span designed by Thomas Page and opened in 1858.

The bridge is 213m long, with a 107m main span, matching the span arrangement of the nearby Grosvenor Bridge such that both bridges are easily navigated by river boats. The deck is 25m wide, with the footways cantilevering beyond the suspension cables and their towers. The entire bridge, which is now Grade II Listed, is built of riveted steel.

The main suspension cables consist of 37 locked-coil ropes tied together in a hexagonal arrangement, which is not a system I've seen previously. The deck hangers are clamped to these cables with bolted fittings. The main cables disappear into shrouds at the ends of the deck, so the way in which they are connected to the deck is not visible, which is a shame as I can't quite imagine how it works - the force from each of 37 individual cables has to be transmitted into the end of the main deck girders.

The main towers are in riveted steel box construction, tapering towards the top, where there are exposed saddles. They have something of the shape of Cleopatra's Needle. Most suspension bridges require cross-bracing between the tower legs to provide stability, but the span of Chelsea Bridge is short, and hence the towers aren't tall enough to require this. I think it looks good and wonder what size of suspension bridge renders it impractical.

The towers are hinged at their base, using heavily stiffened rocker bearings, which again is not something normally associated with suspension bridges. On a conventional suspension bridge, the towers have to be stable to support the main cables before the deck is in place, and hence are cantilevered rather than hinged.

I think the largest suspension bridge with hinged towers may be the Florianopolis Bridge, which has a 340m main span. There, the designer, David Steinman, saw the use of hinges as "the most economical and scientific design for suspension bridge towers", because of the reduced bending stresses in the permanent situation, but although Chelsea Bridge adopted the same choice ten years after Florianopolis, it's not a form that has prospered.

From below the deck, its structural form can be seen clearly. The main girders are formed of paired girders, with crossbeams and cantilevers at regular intervals, and extensive lattice-member bracing connecting the other members.

This is the below-stage machinery supporting the dramatic performance above, not really intended for public viewing. It all looks surprisingly well-maintained.

The good level of maintenance is evident throughout the bridge. The paintwork is all good, including the red on the cables and the elements of blue on the parapet infill (an improvement for local Chelsea residents over the previous red-and-white scheme, which reminded them uncomfortably of the colours of rival London football team Arsenal).

The cables and towers are studded with funfair-style lightbulbs, and the main streetlamps are in an unusual arrangement, with the posts carefully set around hanger cables, as can be seen in the photo above right if you look carefully (as always, click on any image for the full-size version).

In his book Cross River Traffic, Chris Roberts describes Chelsea Bridge as "a very striking, if odd, combination of Thunderbird One and seaside pier", which isn't far off the mark.

Further information:

• Google maps / Bing maps
• Wikipedia
• Structurae
• Engineering Timelines
• British Listed Buildings
• Where Thames Smooth Waters Glide (includes images of the previous bridge on the site)
• Civil Engineering Heritage: London (Denis Smith, 2001)
• Cross River Traffic: A History of London's Bridges (Roberts, 2005)
• An Encyclopaedia of Britain's Bridges (McFetrich, 2010)

London Bridges: 8. Grosvenor Railway Bridge

The first of a trio of bridges all within spitting distance of each other is the Grosvenor Bridge, which carries the railway lines out of Victoria station across the River Thames in London.

A bridge was first built here in 1860 as part of the Victoria Station & Pimlico Railway. Designed by John Fowler, the bridge cost £84,000 and carried two railway tracks over four 53.3m river spans (plus a number of approach spans). Masonry-faced brick piers carried segmental wrought iron arches, with six arched ribs in each span.

As the railways rapidly expanded, so did the bridge, with a first widened section completed in 1866 (designed by Sir Charles Fox), adding another 5 tracks on the east side. The structure was widened again in 1907, adding a further 2 tracks on the west side, using mild steel arches.

Due to the cost of maintenance, it was decided in 1958 to replace the bridge, with work taking place between 1963 and 1967, to a design by Freeman Fox (now part of Hyder, and founded by the same Charles Fox mentioned above). The bridge now consists of ten parallel bridge structures supported on common piers. The arch profile and general elevation of the bridge has remained largely unaltered since the original construction, although reconstruction and underpinning of the piers has shortened the spans to 50m each.

Throughout construction of the new bridge, the contractor had to maintain eight live tracks of railway traffic, and two of the four river spans also had to remain open to boat traffic at all times. This led to a very complex construction sequence.

The foundations consisted of a concrete raft (original bridge), four cast iron caissons filled with concrete (the first widening), and a single cast iron caisson filled with concrete (the second widening). These supported piers variously faced in limestone (orginal) and granite (widenings). During the reconstruction, the existing foundations were isolated within a cofferdam, and widened in reinforced concrete. This had to be completed in small sections as access became available.

A temporary steel truss was erected above the bridge, and used both to dismantle the original arches and erect the new arches, lifting the elements onto and off of pontoons. This worked one span and one track at a time, which is why the new bridge is built of several separate decks rather than one continuous structure.

The current arches are two-pinned welded steel box girders, each 1.13m deep and 0.61m wide, with steel plates up to 31mm thick. There are two arch ribs for each bridge deck. Tubular steel spandrel posts carry an orthotropically stiffened steel deck. Further details, construction photographs and technical drawings are available in the book Railway Bridge Construction by F.A.W. Mann.

It's an essentially modern design, but I doubt that any casual observer is aware of it. The retention of the original open-spandrel arch elevations means that the bridge appears at first glance to be an unaltered historic structure.

The visual design of the piers is difficult on a bridge like this, particularly where they have been widened from the original and risk looking very bulky in comparison to the steelwork they carry. They also have to look appropriate at widely varying water levels (the Thames is a tidal river at this point). I think the chess-piece shaped profile works well, particularly the way the pier face curves outwards at the top to meet the arch springings.

The spandrel columns are remarkably slender, but the transparent effect you would see on a simple elevation of the bridge is nowhere apparent, due to the sheer number of columns and the shading effect from the deck.

Further information:

• Google maps / Bing maps
• Structurae (not all details given are correct)
• Wikipedia (similarly, much of the article is incorrect)
• Engineering Timelines
• Where Thames Smooth Waters Glide (includes archive photos and drawing)
• Railway Bridge Construction (Mann, 1972)
• British Railway Bridges and Viaducts (Martin Smith, 1994)
• Civil Engineering Heritage: London (Denis Smith, 2001)
• An Encyclopaedia of Britain's Bridges (McFetrich, 2010)

Footbridge Awards 2011 - technical above 75m span

Right-o, it's time to (briefly) cover the final batch of designs shortlisted for this year's Footbridge 2011 awards, this time the long-span bridges in the technical stream. Thanks once more to Frame and Form for posting all the images, and as on previous posts, you can visit their website to see the pictures, which I won't repeat. You can also now simply click here to see all my posts on the 2011 shortlist.

I've previously commented on four out of the six bridges, as they also featured in the aesthetics category: Center Street Bridge, Des Moines; College Bridge, Kortrijk; Passerelle la Defense, Paris; and Grimburg Harbour, Gelsenkirchen. Of those, the College Bridge and Passerelle la Defense are the technical standouts, I would say.

The other two candidates are the Forthside Bridge, Stirling, and the Kurilpa Bridge, Brisbane. Both are a tangle of masts and cables, and essentially a variation on the classic cable-stayed bridge. Forthside is what is popularly called an "inverted Fink truss", while Kurilpa is a simplified tensegrity structure, but both rely on the successive cantilevering of cables from main support pylons.

I've discussed both Forthside and Kurilpa here before. Both are technically impressive, although to some extent only in how they resolve the problems of their own making, as the architectural concept determines the structural form as much as arising from it.

The winners in each category will be announced on 6th July.

Building-to-building pedestrian bridge contest

This is a little late, but never mind.

DesignbyMany is a website where specific design challenges can be posted to an online community, with the hope that users will discuss options and modify each other's designs rather than merely competing individually. It's a concept I've covered before: see Open sourcing bridge design. The many-minds-are-better-than-one theory is great, but difficult to bring into effect where the individual effort is what is actually incentivised (by ego, even if not by prize money).

They've posted a somewhat abstract contest to design a pedestrian link bridge spanning between two buildings. The location is irrelevant, what they are looking for is a structure which is modular, adaptable, and rapidly deployable. These are not real-world issues, as almost every building link-span is a prototype design adapted to very specific site needs and built as part of the original building construction. But they are potentially thought-provoking, and an interesting conceptual challenge.

The prize is an HP printer, plus being featured on the widely read ArchDaily website, which should make the contest attractive to students and the unemployed in possession and command of some glitzy 3d visualisation software, anyone trying to build a portfolio.

I groaned when I saw that all three judges are architects, two of them specialists in digital design software. At least one, Wilkinson Eyre's Ezra Groskin, has a background in bridge design and presumably an instinct for what might actually be feasible in the real world. Nonetheless, the whole set up is instantly depressing, with nobody present who could really determine whether the designers have tackled the very significant engineering problems which are implicit to the whole challenge. I guess that's not what they're looking for, as is so often the case with similar architectural "design" contests.

I am late to the party, as the challenge was announced on 3rd June, and the deadline for submissions is 26th June. The design community gets to vote for a 2nd prize winner on 3rd July. Full details are available at the contest website.

The first three submissions posted don't immediately fill me with great expectations: one is obsessed with hexagons but showing no visible means of structural support nor any real detail of how it would be assembled; another offers a "bubble bridge", pictured, which although cute is likewise lacking anything which might be described as a structural system.

Only the third, pictured left, seems to offer much promise: the construction of shared atria between buildings, rather than mere footbridges, offering opportunities both in terms of a revitalised working space as well as for energy-related measures such as passive ventilation.

The hyperboloid gridshell structure is not particularly "deployable" for a retro-fitted solution at this scale, but it's imaginative as well as visually attractive.

Footbridge awards 2011 - technical 30m to 75m span

Once again, those kind people at Frame and Form have posted pictures of some of the shortlisted bridges in this years Footbridge 2011 Awards, this time in the medium span / technical category.

I've discussed the Knokke, Paloma and Riverside bridges before, so won't repeat that again.

Glasgow's Tradeston footbridge was the more pragmatic afterthought to a failed bridge design competition (see previous post). The contest had produced an extravagant and unaffordable winner, and the promoter switched to a design-and-build approach to procure the bridge which was eventually built.

I admire its stark aesthetics, and it's always great to see that quality rather than corner-cutting crap can emerge from the D&B process, but I'm not so sure of the bridge's technical merits. The deck is exceptionally slender, but it's hard to see what else on the bridge was a huge technical challenge.

The Peace Bridge, in Tbilisi, Georgia, is a very different creature. The architect, Michele de Lucchi, was personally invited by the President of Georgia to propose designs for a 160m long pedestrian bridge over the River Kura. His three designs were all covered structures, with the as-built organic form being chosen on purely aesthetic grounds over folded-shell and "parachute" alternatives (de Lucchi's paper at the 2010 IABSE Venice Symposium provides full details). What was built differs considerably from the architect's original proposal, chiefly due to the need to provide a more rational structural form for the glazed steel grid-shell.

Clearly, construction of a bridge of this sort is a very real technical achievement. The grid-shell supports the bridge deck on hangers, and its geometry has been simplified by extruding a common parabolic section along its length. However, it is not a structurally efficient means of support, and even if it were, its shape has not been optimised for the loads upon it. Does technical merit for a bridge reside in responding to a difficult architectural challenge, or in establishing an efficient and effective solution to the structural requirements? Both are valid, and the engineering designer has done well, but this bridge still leaves me a little uncomfortable.

The 73m span Robert I Schroeder footbridge in California is a more coventional bridge, but still impressive. It is suspended from twin steel "butterfly" arches, with the thrust into the ground taken by inclined concrete struts. The arches themselves are in the form of triangular space trusses, and the logic of the structural form has been translated into good looking details, particularly the treatment of the parapets.

It's an admirable looking bridge, although certainly less technically exciting than the Knokke footbridge competing in the same category.

Bridges news roundup

The making of an engineering marvel
Spanning 467m, this steel arch bridge in India will be the world's highest railway bridge when complete, at a staggering 359m above the river bed below (321m above water level).

Innovative Washington Bridge Plan Embraced

S-curved suspension footbridge in Connecticut (pictured below) to be part-private funded.

Bridge to breathe life into city
Derry's Peace Bridge opening soon (25th June). Local man has words. Opening to feature specially-written song performed by over 600 schoolchildren.

Carlisle's damaged Millennium Bridge closed
Cable-stayed Irishgate footbridge closed due to corrosion damage.

Peace Bridge misses June opening, now likely to open by year-end
Calgary's troubled footbridge still hoping for completion in 2011, but no final agreement reached yet on the extent of unsatisfactory welds or the repairs required.

Preferred design announced in Sleaford

In March, I discussed proposals to replace a railway level crossing in Sleaford, Lincolnshire, with a new footbridge. At the time, four alternative designs (none of them much good), were being presented to the public to obtain feedback.

I noted at the time that all four options were pretty horrible, but that the cable-stayed design, pictured here was the best of the bad bunch.

The findings of the consultation have now been declared, and even the public prefer this option, giving it a not-especially resounding 34% of their votes. They still haven't been told what any of the options will cost, however, making any kind of consultation at this stage largely futile.

Of perhaps more interest, is that despite a very positive report on the consultation, the overwhelming majority of the feedback received was simply that the public don't want a footbridge at all, indeed they would far prefer to keep the level crossing. Many of the comments are also opposed to the length of ramps proposed, which seem to be partly due to a decision to provide a 5.8m headroom over the railway, a height which may be the current standard but is widely ignored elsewhere.

Plans issued for bridge across Millennium Dome

Yes, the abbreviated description of this project gave me pause for thought when I first saw it at BDOnline. I think my precise reaction was something like: "Huh?"

The 365m wide, 52m tall Millennium Dome, I'm sure, needs little introduction. It's hardly an obstacle - you can just walk round it (or through it, when in its current guise as The O2 it is unoccupied by some event or other).

Planning consent is being sought for the construction of a skywalk right across the Dome, from one side to another, a skywalk which would therefore also be 365m long and 52m high. It will be open only to supervised access, with visitors being escorted across in groups. Apart from the obvious concerns about people wandering off-piste and trailing muddy footprints over the pristine teflon-coated Dome, this is more of an adventure experience than a stroll across a simple footbridge.

The maximum gradient of the skywalk will be some 28.5° (some 1 in 3), and it will support only a single handrail down its centreline, with visitors passing either side, secured by tethers. Access for the disabled will be arranged by specially-trained escorts (probably drawing on techniques that climbing instructors use with disabled people), including a purpose-built wheelchair if required.

Structurally, a rigid-frame solution has been rejected in favour of another tensile membrane, sitting a little way above the Dome's roofing material, and anchored down at its ends. In between, the tension is provided by guys attached to the Dome's existing support masts at intervals.

This slender band of fabric in turn supports a secondary walkway layer made from a non-slip combination of PVC and stainless steel mesh. I imagine that only the sheer height of the Dome allows this to work - a bridge deck at a shallower curve simply couldn't be drawn tight enough against the guy cables to retain sufficient stiffness.

If you have time, you can see the proposals yourself at Greenwich Council's planning website.

The developers hope to establish a tourist attraction to rival the experience of climbing the Sydney Harbour Bridge, and it's certainly something I'd fancy a go at.

Bridges news roundup

Bridge opening draws near
Unique £6.5m swing bridge in Hull is taking shape on site, but they don't have a tenant for the cafe yet. This report focusses on a recent talk from the bridge architect Jonathan McDowell.

Bridge closed over safety concerns
The Shakin' Brig was featured here previously.

Red is for Gantry – Arnside Viaduct
The Rail Engineer reports on the complex piecemeal renewal of a historic railway viaduct.

Ponte della Musica footbridge opens in Italy
British designers responsible for the first bridge at this point on the Tiber for a thousand years.

Peace Bridge could halt free school bus
It's not all good news when a new bridge opens, apparently.