Kent Bridges: 3. Whatman's Field Downstream Bridge

The last of the trio of millennial footbridges in Maidstone goes by the somewhat cumbersome name of the Whatman's Field Downstream Bridge, so called simply because it is further downstream on the River Medway than the Kent Messenger Millennium Bridge.

Like the Messenger bridge, the Downstream Bridge was built by Balfour Beatty (steelwork fabricated by Fairfield Mabey), and designed by Flint and Neill with Studio Bednarski. It also opened in July 2001, but it's a very different bridge.

On Bednarski's website, it's called the "Blue Art Beam Bridge", and that seems apposite. It consists of a three-span, 75m long single steel box girder, in a very unusual "top hat", "spine beam", or "finback" arrangement. The river could only be closed for 4 hours for the main span lift. In most box girder bridges, the wider flange is at the top, to allow the full width of bridge deck to be carried while allowing the main structural section to be narrower and hence more economic (as well as visually appealing).

There are a few cases where the box girder has been inverted, with a recent example being Gifford's Central Park bridge in Manchester (illustrated in Masterpieces: Bridge Architecture + Design and the subject of a technical paper). This allows the construction depth (i.e. depth from top of pavement or rails to underside of bridge) to be minimised while retaining the advantages of the box girder form, and is a rarely used alternative to a through-truss design. The major disadvantage is that such bridges are often bulky and ungainly in appearance.

So, for the Downstream Bridge to have the wider part of the box section at the bottom seems initially a perverse choice. But the reason may be apparent from the two photographs above. Only the bottom part of the cross-section is highlighted by painting it blue. The upper part is recessed behind the parapet mesh, and a mesh hides the detail from above as well. The intent is to make the bridge deck appear far thinner than it really is, and I think it works well.

The other prominent feature of the bridge deck design is the presence of numerous circular cut-outs in the girder's bottom flange. These are in part a response to the technical approval authority's refusal to accept a fully enclosed box girder (presumably on the grounds that it would be too shallow for internal inspection). They allow for inspection access to all parts of the girder, but are protected against unauthorised entry by the use of grilles. Clearly, they will have made the design more challenging, with the girder effectively having two bottom flanges connected by a series of lateral crossbeams, Vierendeel style, providing the required torsional stiffness.

The bridge certainly needs plenty of torsional stiffness, as its two V-shaped supports are aligned along the bridge centreline, providing no resistance to torsion at all. All the torsion in the bridge is resisted at the end abutments.

The supports are made from tapered tubes (presumably circular tubes welded to flat plates or fabricated boxes), supported by a simple stiffener detail. The choice of V-shaped piers on the bridge centreline is shared with Arup's Gatwick Air Bridge [PDF] and makes me feel a little queasy: the lack of wider supports at the intermediate piers just looks somehow wrong to me, as if the whole thing could topple in the wind.

The bridge parapets use the same stainless steel "Medway" mesh as on the Kent Millennium Messenger Bridge, and like that bridge, it results in a simple and minimalist line to the structure. The photo also shows the grille panels which hide the bottom flange (while unfortunately also trapping debris).

You can also see a slight kink in the vertical alignment, which is unfortunate. I wonder if it's a relic of the need to change the bridge height during the design period. This unhappy tale is told in detail in an audit report [PDF], commissioned to explain the substantial overspend on Maidstone's Millennium River Park project. This noted a £200k overspend on the bridges (relatively small beer), attributable to high tender costs, a major flood which occurred during construction, and the need to raise the planned bridge height by 1.5m in order to mollify river users unhappy with the effect on their navigation rights.

It's nice to see a bridge which is colourful, although blue is perhaps too often the choice on steel bridges. I think the attempt to make the deck look slender has been very successful, and the circular openings below deck help break up the mass of the bridge as well as being functional. As with the other Maidstone footbridges, it's an idiosyncractic design and not one likely to be repeated often elsewhere (if at all). Nonetheless, it's always good to see designers stepping into the unknown, and it's a worthy member of a very interesting trio.

Further information:

• Structurae
• Google maps
• Flint and Neill
• Studio Bednarski
• Steel Design Award

Kent Bridges: 2. Kent Messenger Millennium Bridge

Walk north in Maidstone from Lockmeadow Footbridge along the river Medway, and you'll come to the first of the town's two other millennial footbridges. This pair provide entry across the River Medway into the Whatman Field Park, and are named the Kent Messenger Millennium Bridge and the Whatman Field Downstream Bridge. These other bridges were both designed and built by a single team, as part of the Millennium Commission funded development of Whatman Park, one of the main sections of Maidstone's Millennium Riverside Park. This post will cover the Kent Messenger bridge, with the Downstream bridge to follow later.

In a paper on the two bridges, their architect Cezary Bednarski notes that they were the only millennium bridges in Britain not to be the result of a design contest. I'm not convinced about that, but Maidstone Borough Council certainly got good value, with both bridges being interesting and innovative structures, and both going on to win awards.

At the time the bridges were designed, Bednarski was a director of Studio E Architects, although he started a new firm Studio Bednarski a few months after the Kent Messenger bridge opened in July 2001. His engineering partners for the design were Strasky, Husty and Partners, and Flint and Neill. The main contractor was Balfour Beatty.

I'm not sure what the Kent Messenger bridge cost: Structurae and fib's Guidelines for the design of footbridges both report a figure of £1.76m, which works out at about £5.6k per square metre of deck, but Bednarski's paper states the cost to have been £2.9k per square metre.

The bridge is a very unusual structural form - the world's first cranked stress ribbon bridge. Perhaps it's still the only one. A stress ribbon bridge is essentially a catenary structure which has been stiffened by prestressing against itself. Depending on the specific geometry, it can be thought of as a reverse arch, where the prestress provides a broadly uniform upwards force along the structure's length, locking the structure in place. I've covered one such structure here before, the Punt da Suransuns, in Switzerland.

The stress ribbon bridge's generally low load-bearing capacity, together with the sagging geometry, mean that it's a structural form generally limited to footbridges. Even so, it's rarely an optimal form. What is gained in material economy in the deck (Kent Messenger bridge's 3.1m wide deck is only 0.29m deep) is generally lost in the cost of massive foundations, and the foundations for the Kent Messenger bridge are massive indeed, as can be seen in a diagram at Strasky's website. The sagging geometry is also often hard to reconcile with the need to provide a gentle slope, suitable for access by the mobility-impaired. Keeping the slope shallow enough can result in very high horizontal forces, and still larger foundations.

So, what does "cranked" mean, and why is it so unusual? Essentially, it means that the bridge is kinked in plan. The Kent Messenger bridge is 101.5m long in total, with two spans of 37.5m and 49.5m respectively. At the central support, the deck changes direction in plan by 25°. To make that possible, a large horizontal restraint is called for, as the tension forces in the two spans don't balance.

The nearby Downstream bridge (which I'll cover in the last post in this short series) was also originally proposed as a cranked stressed ribbon, with the resultant force at the "crank" restrained by a tie. It was eventually built in a much different form, but the Kent Messenger bridge retained the crank, with the resultant force restrained by a strut. The strut is angled downwards according to the vector sum of horizontal and vertical reactions, and conveniently doubles as a staircase, a very effective solution.

Unfortunately, it doesn't quite work perfectly under all load cases, and a very slender stainless steel member had to be incorporated below the support position, acting in many cases as a tie to prevent the deck rising, and in some cases as a second strut. It could only have been omitted by providing the staircase with much enhanced bending capacity, but I think it's a shame it was included, as the bridge would be far more impressive without it.

The bridge deck itself consists of 3m long precast concrete segments composite with an in-situ infill deck slab. The segments are supported on a set of four bearing and four prestressing cables cast into the infill.

Drainage grilles are located along the centreline of the bridge (and can be seen on the image on the right), breaking up the structure's monolithic appearance and allowing the river to be seen through them. These alternate with lights embedded in the deck (see below left). These are the same kind of lights you would find on an airport taxiway, a rather idiosyncratic choice.

The bridge parapet incorporates a custom-designed stainless steel mesh, which is now marketed as the "Medway" mesh by Potter and Soar.

As with the Lockmeadow footbridge, the Kent Messenger Millennium Bridge wasn't shown to its best on a dull, overcast day. Given that stress ribbon bridges are often chosen for their lightweight, slender appearance, I was surprised to find this bridge offering an almost brutalist statement. The cranked geometry was clearly an attempt to push at the boundaries of structural design - there are several other forms which would have served as well, perhaps incorporating some of the length eastern approach ramp into the bridge structure. The staircase strut is a great design feature, although it's a shame about the presence of the central tie, as the bridge would have had a far more startling visual tension without it.

The overall geometry aside, it's a plain, functional, rather minimalist structure, and none the worse for any of that. Compared to the tangled cat's cradles of two of its contemporary Millennium Bridges (at Bankside and Hungerford), it deploys its structural ambition to a more self-effacing end. The views off the structure are kept free of cables and struts, and there's little more to see below. I can't say I found it at all an exciting bridge to visit, but I guess that's part of the intention, and it's good to see a strong design that doesn't take its lead from Calatrava.

Further information:

• Structurae
• Google maps
• Kent Messenger Millennium Bridge, Maidstone, UK (C Bednarski, Footbridge 2002)
• Kent Messenger Millennium Bridge, Maidstone (Concrete magazine, Nov/Dec 2003)
• Stress ribbon and cable supported pedestrian bridges
• Footbridges
• Masterpieces: Bridge architecture + design

Bridge criticism 1: Of heads, and parapets

Late last year, I attended a talk by architect Cezary Bednarski, which ran through his experiences in the bridge design competition arena. I've covered some of the issues raised previously, but there's one that I wanted to return to.

A topic that arose repeatedly related to the various occasions when Bednarski had lost a competition and lost it to a design which in his view was a very poor choice. He offered as examples the notorious Krakow footbridge (pictured left, click any image for a larger version), Glasgow, and a competition for temporary bridges in Rome. Other examples of bizarre competition winners include Cambridgeshire Landmark East and River Wear. These mostly offer examples of exotic, ambitious designs where their very structural feasibility should have been in doubt from the start.

There has been ample criticism both of a competition process which frequently delivers designs beyond the budgetary constraints of a project's promoter (e.g. Stratford-upon-Avon, pictured right), or which will saddle them with expensive maintenance costs for years to come (e.g. Trinity Footbridge). But where precisely does the fault lie? With the competition clients, organisers, jurors or the entrants? I actually think at least some of the fault lies elsewhere.

Several of these competition designs were criticised in the press after the winner was announced. However, in most cases serious criticism only appeared after the project had failed (e.g. when the Glasgow bridge scheme, pictured left, went massively over budget). Is there perhaps a failure of the bridge design community as a whole to speak up about designs which are likely to be unusually challenging or high risk?

For several of the competitions mentioned above, there was no specialist landmark bridge engineer involved on the jury, and it is possible that whoever did judge them had no realistic way of benchmarking the likely costs and risks of the designs put in front of them. In many cases, the shortlisted entries are not made public, so there is no possibility for anyone else to assist by making informed comment.

If there's an appropriately knowledgeable and empowered bridge specialist involved in the judging, then in theory the absence of wider input should not matter. But I still wonder whether competitions would benefit generally from greater transparency, and the opportunity for a promoter to receive wider comment on the merits of the entries. One of the aims of this blog is to see whether a space exists for constructive public criticism of the merits of bridge designs.

Of course, that's where the problems begin. The heads, and parapets, of this post's title, and the unwillingness of bridge designers to stick the former above the latter.

Criticism of others' designs is far more common in the architectural field, indeed it's something that's positively encouraged from an early stage through the use of the architectural crit as part of an architect's education. There are very few analogues in structural engineering, although I'll address some that do exist in a future post.

Engineers are traditionally reluctant to publicly criticise the work of their peers. There are several reasons: simple politeness, commercial pressures, legal constraints, etc. All three of these can be found at work here on the Happy Pontist whenever comment is made on the merits of a particular bridge design. Legally, there is the risk of committing libel if criticism goes too far. Commercially, the critic can upset someone who may otherwise be a future client or commercial partner.

In a follow-up post, I'll look in more detail at one: ethics. Could it actually be unethical to criticise designs? That might seem ridiculous, but the engineering institutions promote precisely that view.

When bridge design competitions go bad (and good, sometimes)

I recently went to a very interesting talk in Preston by architect Cezary Bednarski, billed as "River Douglas Bridge - architects and engineers in competition", but which turned out to be a far wider journey through the murky waters of bridge design competitions past and present.

In the process, many interesting points emerged, not least the astonishing statistic that despite Bednarski's exceptional success rate in winning bridge design competitions, only about 1 in 10 of his competition-winning structures have ever been built. The reasons are many - some competitions have little real support to begin with (e.g. Liverpool Cathedral's Glass Bridge); others are the pet projects of politicians and hence quickly dumped when a new regime is voted in (Carlisle's Hadrian's Bridge); others underestimate the funding required to complete an ambitious landmark structure (e.g. Helsinki, Glasgow). It seems that bridge design competitions are great both for getting some publicity and free optioneering for a promoter, and also to allow designers to flex their creative muscles. But they're perhaps not the best way to get a real bridge built.

Bednarski did discuss the River Douglas footbridge competition, for which he had acted as a juror. This was run by RIBA on behalf of REMADE, a local development agency, and sought to restore a river crossing on the line of a long-disused railway. The contest was initially controversial because of RIBA's absurd insistence that teams could only enter if led by an architect. There were 110 entries altogether, whittled down to a shortlist of seven by the jury. The competition was won by Arup and JDA architects, with a combined stress ribbon and arch design.

It was interesting seeing various examples of Studio Bednarski designs, such as the Kelvin Link competition entry, a stressed ribbon supported by an arch (top). Compare this with the winning entry for River Douglas, another stressed ribbon supported by an arch (bottom):




Spot the difference!

We were told that many of the entries to the competition were very poor, and even the shortlisted ones far from perfect. An open competition such as this avoids the predictability of an invitation-only competition, giving newcomers their fair chance to shine. However, it's also far more likely to bring forward designers who lack the knowledge or resources to actually take a bridge through to being built, and the number of entries (and hence low chance of success) puts off experienced designers who do have those resources.

Bednarski showed several entries to the River Douglas competition which brought smiles to our faces: but these were entries to a childrens' competition, colorful, ambitious, and naive. You can find these online at Remade's website.

Another theme which came up repeatedly in Bednarski's competition examples were the many cases where competitions are won by structures which are either stretching the boundaries of feasibility, or simply not feasible at all. Understandably, losing competitors feel somewhat disgruntled when beaten by something like this (I've been in this position myself). But that's a subject to return to another time ...