Tintagel Castle Footbridge submitted for Planning Consent

An application has been made for Planning Consent for a new footbridge at Tintagel Castle in Cornwall.

The last time I featured this project was to discuss the six shortlisted competition entries back in December 2015. In March 2016, the winner was announced as Ney and Partners with William Matthews Associates. The scheme is for a new bridge to take visitors onto the Tintagel Castle promontory, a beautiful and deeply historic site. The bridge will provide access for the mobility-impaired for the first time.

You can find the full planning application online, but I've extracted some of the pertinent material to share here.

The bridge gives the appearance of being a very slender arch structure, but in fact it is formed of two giant steel cantilevers, each shaped with a parabolic curve in elevation. In theory, this means that the lower rib carries a constant force when the bridge is subject to a uniform load, allowing for an efficient use of structural steel. In practice, things are never so simple.

The lower and upper ribs each comprise two weathering steel fabricated box girders. These span 66.7m in total. The cantilevers are not quite symmetrical.

The upper ribs are parallel, with the 3.0m wide structure supporting a 2.5m wide walkway. At midspan, these ribs are a mere 175mm deep, impressively slender by any standard. The lower ribs converge towards their foundations, and are also exceptionally small, being only 140mm deep over most of their length.

The structure's strength comes from the depth of the twin cantilevers, which reaches 4.4m near the supports. The upper and lower ribs are connected by what the designers refer to as a "Thomas Telford" detail, for reasons which should be obvious. These spandrel lattices are formed from solid stainless steel bars varying in cross-section from 30mm square to 65mm square.

The structural dimensions illustrate a peculiar talent that Ney and Partners seem to have for exploiting design standards to their absolute limit, and creating structures of astonishing slenderness. The total weight of the steel structure is stated as 66 tonnes, which is quite amazing for this span. It's no surprise given the slenderness to read that the bridge's first natural frequency is 1.6 Hz (well into the vulnerable area for pedestrian excitation).

The bridge deck consists of slates placed on edge in a sand bedding layer, carried on stainless steel pans supported between the primary structural ribs. The pans have drainage scuppers in the soffit, and there is an air gap between the deck pans and the main ribs to ensure the weathering steel can weather properly.

The bridge balustrades consist of stainless steel bars supporting oak handrails, 1.3m high in total.

The bridge foundations are proposed as rock anchors for both the compression (lower rib) and tension (upper rib) elements, with additional rock anchors used to stabilise the exposed cliff faces.

The gap in the middle of the bridge is nominally 42 mm, reducing to 5 mm under maximum temperatures, and increasing to 85 mm under minimum temperatures. There's clearly a degree of controversy to this particular detail, given the risk of a trip hazard or simply the discomfort caused to visitors already made anxious by height and exposure.

At competition stage I observed that significant differential deflections could also be expected when one cantilever was loaded more than the other (by pedestrians or by wind), but the planning submission makes clear that the two cantilevers are in fact connected by shear pins, in a similar manner to a twin-bascule bridge.

I think the poetic idea behind the gap justifies the problems that it creates: the intention is to make intensely apparent the sensation of stepping from the present into the past, of the division between the Tintagel Castle and the mundane world.

The designers have also sought to address the other major objection raised at competition stage, which was to the adoption of weathering steel at an exposed coastal site, where the wind will blow salt spray high above the sea. They have instituted a series of corrosion tests on steel plates exposed at the project site, and the planning submission documents make clear that if these are unsuccessful, the weathering steel will simply be substituted with conventional painted structural steel.

However, there seems to be little acknowledgement of the bimetallic corrosion issue created by the use of so much stainless steel and weathering steel connected together. This combination will tend to lead to accelerated corrosion of the weathering steel at connection points, especially if moisture and salts are present.

Results of the on-site salt spray corrosion tests were due to be completed in June 2017 so it would be very interesting to see the results.

An article in The Guardian focuses on what appears to be increasing opposition to the entire idea of a bridge and captures some of the key issues. If you visit the planning consent website, it's clear there are numerous objectors.

One that's particularly worth reading is from Cornish bard, Bert Biscoe, arguing that whatever the merits of the particular bridge design, they cannot outweigh the damage that will be caused to a site of major archaeological importance. The argument is not about the physical impact of the bridge, but about the very desire of the site's custodian, English Heritage, to increase visitor numbers in such a sensitive site. This is an argument about the merits of preservation over the merits of public access - it is intrinsically anti-populist, but perhaps necessary.

I have been to Tintagel and my initial feeling about the bridge was that the improved accessibility would be very welcome. The promontory is currently accessed via a low-level bridge and a series of awkward steps, which are very difficult for some visitors to traverse. The planning submission notes that some 15% of visitors who buy a ticket for the Castle never actually make it up the existing steps onto the promontory.

However, the bridge is no panacea for this, as there will still be areas which are only accessible via steps or very narrow paths. It's therefore legitimate to consider whether the adverse impacts of such a major intervention are justified by the benefits.

I will be very surprised if the bridge fails this initial planning consent hurdle. However, Scheduled Monument consent will also be required, and I expect opponents of the scheme will petition central government to call in the entire planning application for further review, such is the sensitivity of the site

 I think the designers involved have done an excellent job in addressing the site constraints, within the limits of their brief, and this will be a very interesting project to follow, especially if it proceeds all the way to be built.

Tintagel Castle shortlist announced

The six designs from the shortlisted competitors in English Heritage's £4m Tintagel Castle bridge design competition have been made public. It's clear that they've brought together a very high calibre of design teams, and the designs are all very interesting.

One of the potential problems with a shortlisted competition is that if any of the entrants come up with a design which is in any way unacceptable, significant effort may be wasted and the promoter's pool of options can be drastically reduced. Many of the designs shown here have significant issues to overcome, and it's not clear to me that any of these designs is entirely fit for purpose.

For more details on the designs, see the competition website.

Dietmar Feichtinger Architectes / Terrell

I am strangely reminded of the hallucigenia, a bizarre prehistoric creature which confused many researchers, who initially depicted it upside down. This is something of an upside-down bridge, a steel box-girder installed with a slight upwards arch, and then pulled downwards by a series of stainless steel cables. At first sight, it's as if the bridge is filled with helium and needs to be tied down to prevent it floating away.

The design entry indicates that the cables induce prestress in the bridge deck, although in pure bending terms it is the opposite of a helpful prestress. I can only imagine that the deck is placed into axial compression by virtue of its fixed end points, and this helps counteract normal bending forces. However, it would require a very heavy upper plate to prevent buckling, and has the unfortunate property that the more load is added, the less effective the prestress becomes. I struggle to get my head around the whole idea, and it's not helped by deeply unconvincing cable anchorage details.

The design has a painted steel girder, not ideal for maintenance above such a steep gorge. The parapets are lightweight and open, which will fail to reassure many bridge users, and worse, the deck is an aluminium grille, which will terrify the rest. I find it hard to see how this is an inclusive design suitable for all users, particularly the disabled or infirm.

Marks Barfield Architects / Flint and Neill / J & L Gibbons LLP / Mola

This second design is also a steel box girder bridge, although very different in style. The cross-section is intended to give the impression that the girder is very slender, like the blade of an Arthurian sword, but at heart it's a conventional design, a continuous steel girder on concrete supports.

The supports are "striated" with concrete layers, an hommage to slate and mudstone geology, but are cast as a series of infilled precast shells. This seems a somewhat forced approach, as the precast shell has to be cast in multiple stages only to hide a monolithic core.

The balustrades seem slightly more secure than the previous design, but the most interesting feature of this bridge is that the girders are constructed of roll-bonded structural steel, a material more generally used in cladding and industrial vessels, and which I've not seen used for a bridge like this. Conventional structural steel would be hot-pressed against a phosphor bronze cladding to create a highly corrosion-resistant composite, great for maintenance but at high initial cost. It's an exciting solution, if a little on the unproven side.

Ney and Partners / William Matthews Associates

I think this is both the best presented and the most spectacular entry to the contest. It looks like an arch, but it isn't. Two cantilevers project from either side of the gorge, leaving a tiny gap in the middle. It's highly poetic, yet structurally inefficient compared to a "proper" arch solution.

A slate deck sits on ultra-slender weathering steel plates, slung from the bedrock of Tintagel Castle and propped by weathering steel box girders below. Parapets, and a latticework between the girders, are in stainless steel.

The sense of insecurity inherent in the previous designs is redoubled here by the audacious midspan gap. I wonder how much that gap sways or drops when the bridge is eccentrically loaded by people or wind? The arrangement of the lower arch rib suggests it would be particularly likely to sway in high winds: it's widest at the centre of the span, but that's an arrangement which only makes sense if there's no gap.

All structural elements look impossibly slender at first sight, but I suspect they are not entirely implausible, with the latticework stiffening the lower "arch" box girder against buckling. The real problem with this design is the materials, as connecting stainless steel directly to weathering steel can induce corrosion in the latter, and even at this height, I suspect airborne salt spray would cause serious corrosion to the bridge over time.

Niall McLaughlin Architects / Price and Myers

Here's another design with an interesting use of materials, with a bronze balustrade upon a highly unusual prestressed granite bridge deck.

Prestressed granite is not a new concept: there have been quite a few bridges built in this way at a small scale, but not at the span proposed here. A larger bridge was proposed by German designer Heinz Hossdorf, but never built. The design here owes a debt both to Hossdorf and to Jurg Conzett. It would be built be cantilevering thin granite slices outwards from the supports, assisted by temporary cable support.

It would be an extremely durable solution, but the sheer weight of materials would render the construction phase expensive, certainly compared to some of the other designs. And, as with every design so far, the balustrade design may leave many users feeling exposed.

RFR / Jean Francois Blassel Architecte / EngineersHRW / WSP Parsons Brinckerhoff

Here we have a second granite arch, although of more conventional design - a "traditional" masonry arch bridge, rather than a prestressed structure. Only the arch barrel is granite: the spandrel walls are of an unspecified stone, as are the balusters, which are a series of stone ribs with small gaps between them. The arch backing is a mix of foamed concrete with void formers, the latter to reduce weight.

The bridge would be erected on temporary centering, a very expensive business. The arch has a very low rise for a masonry arch, rendering it highly vulnerable to any ground movement, but there are few locations with such rocky ground to rely upon. The use of materials is inefficient, but if it works structurally, it's the most durable of all the solutions put forward.

I like the fact that the balusters provide a degree of physical and visual shelter, it's the only design which seems to me to really consider the experience of all possible visitors. However, I dislike its visual heaviness, particularly at the mainland end, where as well as being much deeper, the structure is also much wider. There could be considerable cost involved in stabilising the supporting rock slopes.

Wilkinson Eyre / Atelier One

The final design is in some ways the boldest proposal, an uncompromisingly modern structure, all high-tech and angular. It's a lightweight cable truss, with stainless steel struts and ties, and a composite deck comprised of oak and stainless steel elements. The composite construction renders the oak elements of the deck impossible to replace, so we'd have to hope they last as long as the rest of the bridge.

The bridge would be erected either on a scaffold or assembled from a temporary cable framework. The form of the bridge is such that is not functional until it is complete, hence the significant temporary works.

The bridge's main engineering challenge will be to ensure it is stable under pedestrian and wind loading. I wonder whether a less angular structure could have been attempted, and as with most of the designs, I think many users would feel insecure.

For me, none of these designs is the obvious winner, they are all flawed in different ways, and it's a case of choosing the least flawed option. I'd vote for the Ney design on sheer good looks, but would favour the Price & Myers design if it had a more sheltering parapet. It will be very interesting to see who wins, due to be announced in February.

West Country Bridges: 5. Royal Albert Bridge

What is there that I can sensibly say about Isambard Kingdom Brunel's greatest bridge (pictured, above left)?

First, the facts. The bridge was opened in May 1859, four months before Brunel's death. The two main spans of 138.7m are achieved using giant lenticular trusses in wrought iron. The upper chord of the truss is a single tube, oval in cross-section, 5.1m wide and 3.7m high. The lower chord consists of wrought iron chains, with vertical members carrying the loads from the deck into both chords. It's a design with split personalities - as well as the idea of the lenticular truss, you can think of it as a bowstring arch bridge, or as a self-anchored suspension bridge in which the main cable forces are anchored into an overhead strut rather than into the deck.

In this respect, the design was a development of Brunel's 1852 bridge at Chepstow, where again a simplified suspension arrangement relies on tubular strut for its anchorage (although at Chepstow, the strut was circular). Another predecessor was Brunel's 1849 bowstring arch at Windsor.

The Royal Albert Bridge also owed more than a little debt to Brunel's great friend and rival Robert Stephenson. His High Level Bridge in Newcastle was also completed in 1849, and has a series of bowstring arches carrying the roadway, with the railway on a separate deck above the arches. The idea of superimposed systems may have influenced the Royal Albert Bridge, but another Stephenson design was even more relevant, the Britannia Bridge.

Opened in 1850, the two main spans at Britannia were very similar to Royal Albert, at 140m. The same method of erection, by floating out and lifting vertically, was also used on both bridges. The Britannia Bridge (along with a related structure at Conwy) had pioneered the use of riveted wrought iron box girders, although used in a beam arrangement rather than as struts as in the more complex Brunel design. Stephenson had even suggested the use of an oval box girder at Britannia, although William Fairbairn's preference for a rectangular section proved more appropriate.

Brunel's Royal Albert Bridge was a more sophisticated design all round, but not of a type which would see much further use. The first major Warren truss bridge had been built in 1852, and lattice trusses such as the Runcorn Railway Bridge were soon to come into favour. Truss bridges were simpler to assemble than box girder designs, and used significantly less material. Nonetheless, Brunel's lenticular truss certainly didn't stand alone, with several other examples built by von Pauli and Lohse in Germany, and by Lindenthal and the Berlin Iron Bridge Company in America.

None of this is to detract from the Royal Albert Bridge. It may have been a technological dead end, but it remains a remarkable design and a spectacular structure.

I particularly like the texture of the main arch girders, with the riveted plate form very visible. The evidence of handicraft is a welcome contrast to the more impersonal surfaces that modern welded construction produces.

Perhaps less appealing are features which I presume to have resulted from strengthening works over the years. Chief amongst these are the diagonal bracing members immediately below the suspension chain, which aren't visible on older photos, and detract from the clarity of the structural system.

Further information:

• Google maps / Bing maps
• Wikipedia
• Structurae
• Engineering Timelines
• English Heritage
• royalalbertbridge.co.uk
• Royal Albert Bridge (Buxton-Smith, 2007, student paper)
• British Railway Bridges (Walters, 1963)
• British Railway Bridges and Viaducts (Smith, 1994)
• Civil Engineering Heritage: Southern England (Otter, 1994)
• Cornwall's Bridge and Viaduct Heritage (Kentley, 2005)
• An Encyclopaedia of Britain's Bridges (McFetrich, 2010)

West Country Bridges: 4. Tamar Bridge

It's not often that I feature long-span bridges on this blog. The scale and complexity of major bridge projects makes them less easy to encapsulate in a short text. Smaller structures are easier to do justice to. However, in this current series of posts, there are two more bridges that it would be remiss not to include. This post will cover the one on the right, in the picture above.

Opening in 1961, and spanning 335m, Tamar Bridge was the first of three major British suspension bridges to be completed within a few years of each other, its siblings being the Forth Road Bridge (1964, span 1006m), and the Severn Bridge (1966, span 988m). Its nearest significant predecessor was the 101m span Chelsea Bridge, completed in 1937, although the longest spanning suspension bridge in Britain at the time dated back nearly a century, the Clifton Suspension Bridge (1864, span 214m) (I'm not counting the 300m Widnes-Runcorn Transporter Bridge, which is a special case).

By the standards of what had been built in American and elsewhere, the Tamar Bridge was not a major structure. The design, by Mott, Hay and Anderson, was of the old-fashioned stiffened deck-truss type, still the legacy of the Tacoma Narrows failure from two decades before.

However, this conservatism was forced on the designers by the proximity of Brunel's Royal Albert Bridge. Tamar Bridge shared its design and year of opening with the Runcorn-Widnes Bridge, which also had a very similar main span of 330m. At Runcorn, a 314m span suspension bridge was rejected in favour of a steel truss arch design as a result of wind eddies generated by the nearby Runcorn Railway Bridge. At Tamar, the wind tunnel tests showed that a small change in level of the new road bridge would be enough to solve a similar problem.

The Tamar trusses are some 4.9m deep, with a span-to-depth ratio of 68, almost absurdly deep for a suspension bridge even at the time. As built, it had only vertical hangers, but when it was widened at the end of the 1990s, stay cables were added, creating a hybrid form. Unusually, the main suspension cables are locked-coil steel ropes rather than aerially spun cables, with 31 ropes bundled together in each cable.

The design of the widening, by Hyder, involved extending the bridge deck on new cantilevers, which carried traffic during the construction period. The original concrete deck was replaced by a lighter orthotropic steel plate deck, and traffic then returned to its original lanes. The cantilevers now carry local traffic on one side, and a footway / cycleway on the other. The overall result is not just that the bridge is stronger, but also more attractive, as the main trusses are to some extent shadowed by the deck cantilevers.
Further information:

• Google maps / Bing maps
• Wikipedia
• Structurae
• Engineering Timelines
• Pathe film showing construction
• Modern British Bridges (Henry & Jerome, 1965)
• Civil Engineering Heritage: Southern England (Otter, 1994)
• In the Wake of Tacoma: Suspension Bridges and the Quest for Aerodynamic Stability (Scott, 2001)
• The Tamar Bridge (Brown, 2007, student report)
• An Encyclopaedia of Britain's Bridges (McFetrich, 2010)