Salford Meadows Bridge design competition

What amazing timing, for a new bridge design competition to follow so quickly after IABSE's design contest guidelines event!

Salford Council have announced an international bridge design competition for a footbridge over the River Irwell at Salford Meadows. It's being run on their behalf by RIBA's Competitions Office, currently the subject of a RIBA committee of inquiry, so you may imagine that how it is conducted will be watched closely in the architectural press.

The contest is in two stages, with the first open to anyone who can come up with the £50 registration fee. I'm going to bet they get over 100 entries, based on similar past exercises from RIBA, and the scarcity of similar competitions in the UK recently. The designs will be judged anonymously, with the top three each receiving £4,000. A second stage will see the shortlisted trio of designs being developed further, with anonymity removed, before a winner is decided.

Submissions are to be made digitally via the competition website, by 5th September, with the final winner to be announced in late November.

The bridge is intended to increase pedestrian access to the Meadows parkland area, and form a link in a network of walking routes connecting Salford to Manchester city centre. A total bridge length of up to 75m is anticipated, with a significant level difference between the two ends of the bridge posing the main challenge. A feasibility report made available online indicates a cost of between £1m and £1.8m, and reviews various options, such as the one illustrated here, none of which resemble a likely competition design.

It's interesting to read the competition arrangements in the light of the IABSE guidelines, and against what was discussed at last week's workshop. So much is wrong with this contest that it's difficult to know where to begin.

The first question is whether there's sufficient political will, and funding available, to ever build a bridge at this site. The competition brief makes precisely no commitment on this point, indeed there's not even a commitment to offer a design contract to the winner. However, Salford's track record of building "landmark" footbridges is good - I've visited their Trinity Footbridge, Greengate Footbridge, and Spinningfields Footbridge in the past, all of which span the same river. It's also apparent they've been planning further bridges for some time.

To their credit, Salford have done some legwork to confirm that the location for the bridge is appropriate, and that a bridge of some sort is economically feasible. However, the design parameters offered to competitors are skimpy, to say the least. The council seeks an "iconic structure" (don't they all), and notes that no supports should be placed in the river, but the published brief says nothing about required vertical clearance to the river, and acknowledges the river banks have yet to be surveyed. Is the ground suitable for suspension anchorages? Designers should just guess. The end result of all of this is the risk they may accept winning designs which are subsequently shown not to be suitable; or that they reject as unsuitable designs which are perfectly feasible.

In any design contest, the promoters need to consider how the rules and remuneration incentivise entrants, and whether the incentives align with the scheme objectives. As noted above, we can expect large numbers of entries, and to stand any chance of winning, designs need to stand out amongst competitors, to be eye-catching in a way which could impress a largely non-specialist jury (more on that in a moment). This may attract a large share of designs which are less appropriate in terms of maintenance, construction, structural economy etc.

The very low prizemoney and low odds of success means that many experienced bridge design firms simply won't bother entering, especially if they are busy with other, commercially rewarding work. Some of the better designs which could be prepared will therefore never even be submitted.

The low prize money is particularly disappointing because the organisers state that for the winner it will be considered an advance on future fee. This begs the question "what future fee?", as there is no commitment to award any design contract. Worse, it means the winner is less well off than the two runners-up: all three receive the paltry £4,000 as a prize, but the winner is then expected to do a further £4,000 of work for which they cannot charge. Prize money and design fee should always be separate, as otherwise the winner receives no reward for the hard work done in preparing their entry, or to recompense them for the significant risk they have take in allocating their time to the project.

The jury includes noted bridge architect Renato Benedetti, and only one engineer, Urban Vision's Shoaib Mohammad. Urban Vision is a joint venture between Salford Council, Galliford Try and consultant Capita Symonds. One of the recommendations of the IABSE competiton guidelines is to have an expert bridge engineer on the jury, but Mohammad is described variously as a Director for Engineering, Streetworks and Landscape Design (on the Urban Vision website) or as a Highways Director, specialising in highways maintenance (on LinkedIn). The clear concern must be that structurally challenging submissions will not be properly evaluated, leading to greater risks should the project proceed further. While an engineering review will be completed at Stage 2, that's simply too late, if good and bad proposals have been ruled in and out unfairly at Stage 1.

One of the biggest causes of competition failure is a mismatch between the promoter's budget, their aspirations, and their understanding of what the submitted designs may cost. There seems to be nothing in the rules about a proper evaluation of costs for any of the designs, and there is a real probability that large numbers of submissions will depict something well beyond the likely budget. Salford say they are looking for an "iconic" structure, but it's far from clear whether they have the appetite for an "iconic" cost.

An article in the Salford Star reveals the council have a budget of £25-30k for the competition, which gives some idea as to RIBA Competitions costs given that £12k of that is set aside for prizemoney. The Architects Journal notes that Salford have suspended their own standing orders to appoint RIBA as the competition organiser, stating that no other body could run such a contest in the UK. Salford apparently describe them as "an exclusive provider of such services" - the source for the AJ's article appears to be an internal council paper available online.

Quite what other UK architectural competition organisers like Malcolm Reading Consultants or Designed2Win make of that claim is anyone's guess. Only a minority of UK bridge design competitions in recent years have been run by RIBA, so it's a very odd claim indeed.

Despite all these issues, I expect the lack of bridge design contests will make Salford Meadows a very popular competition, and it will be very interesting to follow over the months ahead.

IABSE Guidelines for Design Competitions for Bridges

Since starting this blog, one of the most frequent topics I’ve covered has been that of bridge design competitions. This is largely because of the particular opportunity they provide to develop designs for bridges beyond the ordinary. Many of the entries to such contests are, to put it politely, complete rubbish, but is always interesting to see what happens when designers are forced to be unusually creative, when they have to dress to impress.

If you look back through my old posts, you’ll find many commenting on competitions which ended in failure, typically with no bridge being built, and the efforts of the promoter and of the designers wasted. There are also several cases where competitions have been the source of much controversy within the bridge design community, perhaps most notably in the case of Sunderland’s River Wear Bridge (pictured, right). At the height of their popularity (pre-recession), bridge design competitions seemed to fail to deliver with quite some regularity, leading to an entire conference on the issue (IABSE’s 2007 Henderson Colloquium), and the subsequent development by IABSE of guidelines intended to help clients run more successful competitions.

Last week I attended the UK launch seminar for these guidelines, held at the Institution of Structural Engineers in London. This was an attempt to publicise them more widely, particularly to client bodies who might consider going down this procurement route. In the event, the seminar seemed to have very little client attendance, and to be dominated instead by the usual suspects from the bridge design community. I know at least two UK client bodies who are currently planning such contests, but I suspect most are of the view that the recession has put an end to the flamboyance and frivolity with which bridge competitions unfortunately became associated. If true, that would be a shame, as it was clear from the seminar that competitions could still have much to offer.

The seminar was introduced by one of the members of the IABSE competitions working group, Angus Low. As well as explaining much of the background that I’ve described above, he offered the interesting observation that the running of bridge design competitions goes against the natural order of things.

At school, teachers are presented with a fresh cohort of students every year, and teachers become highly practiced at evaluating the efforts of their unskilled juniors. The opposite is true of bridge design competitions: the set of designers which enters them varies little over time, and thus develops considerable understanding of the evaluation process (and the tactics which best respond to it), while client bodies rarely run more than a single competition. Most clients therefore have only limited understanding of the best process to follow.

To put a rather different slant on Low’s idea, you could consider that until now, there was no guidance available to these competition client virgins, but now it is hoped that the IABSE manual will improve their prospects of securing a fertile design relationship and successfully delivering their hoped-for bridge offspring.

The main part of the seminar comprised four presentations. The first, by Brian Duguid, offered a review of previous bridge design contests in the UK, evaluating their success rate and suggesting common factors which united the more productive ones. By Duguid’s estimate, only about 60% of recent UK bridge design competitions actually resulted in a completed bridge. I think that is pretty good: I can certainly look back on periods in my own career where it seemed every design was destined for a dead end, including many where no competition was involved at all.

Duguid presented a series of examples of failed contests, most of which will be familiar to regular readers of this blog: Poole, Glasgow, Stratford, River Douglas, Sheffield and River Soar (pictured, right). I can easily think of others. He noted more which had been successful, at least in terms of getting built: South Quay, Millennium, Gateshead, Poole (again), Stockton-on-Tees, Bootle, Stirling and Glasgow (again). Personally, I would note that not all of these were success stories for their promoters, with several costing their clients far more than was originally budgeted, the Stockton bridge being the worst offender in this regard.

The presentation listed six factors in competition success: funding and political commitment; design parameters; contest rules; consideration of cost; remuneration; and judging. I was particularly struck by Duguid’s comment that many competition promoters ask entrants for a Lamborghini, despite only having the money for a Fiat.

On that theme, he ended with comments on cost and on economic benefit, revisiting ideas previously presented at Footbridge 2011. Design-and-build was suggested as offering good cost certainty, although my own view is that few contractors have the appetite for risk that would allow the more innovative and exciting designs to be developed. The Foryd Harbour bridge (pictured, left) was offered as a counter-argument to this, but I wonder whether the original budget has been held to on that project, given the highly unusual design.

Duguid drew attention to the client’s attempt to calculate the financial value of “iconicity” for the River Wear Bridge, but my reading of their figures is that it was to some extent a spurious exercise, forced to guess widely in the absence of any real evidence.

The second presentation came from Cezary Bednarski, no stranger to this blog. Bednarski has won ten out of the twenty bridge design competitions he has entered, a remarkably high hit rate, but only three of these have been built (Swansea’s Fabian Way, Cardiff’s Roath Basin (pictured, right), and the Inderhavnen Bridge in Copenhagen, the last of which is on site but not yet complete). These statistics are not a good advertisement for the competition as a form of procurement, but I suspect Bednarski’s choice of contests has included more than a reasonable share of what might be called “vanity competitions”, ones where a client thinks they have a good idea, but where the basic foundations of funding and political commitment referred to by Duguid as a key success factor had never been laid.

Such cases might include the Thames Path footbridge at Lechlade (picutred, left) memorably described by its local opponents as “a yuppy tennis racket from hell”; Hadrian’s Bridge in Carlisle; a bridge in Helsinki; the Portsmouth Causeway Bridge; and the Tamar Bridge at Gunnislake. I haven’t covered any of these bridges here before but they include a number of remarkable designs.

Bednarski also highlighted the notorious Krakow bridge competition, won by a concept which had clearly never been graced by so much as the passing touch of a structural engineer. This is not the only case where the competence of a competition’s judging has been drawn into question (the Glasgow scheme already mentioned is another), and it’s depressing to see the inevitable waste of talent and time involved whenever this happens.

The event’s third presentation came from Martin Knight, who had won Helsinki’s Kruunusillat bridge design competition only a few days previously (pictured, right). It remains to be seen whether that scheme will ever be built (with the client having questioned not only the funding but even the types of traffic it may need to carry), although Knight suggested that it formed part of a chicken-and-egg situation where a landmark bridge design was needed to attract developer interest; and only developer interest could result in the provision of funding.

This was the only presentation to really consider the “why” of bridge design competitions – why should clients procure bridges this way, and why should they do so far more often? Knight noted that in Europe, design contests are far more commonly used than in the UK. I think such contests are to some extent a damaged “brand” in Britain, with complaints over their lack of success extending well beyond the niche of bridge construction. Indeed, concern is sufficiently widespread that the day before the IABSE seminar, RIBA announced an investigation into the performance and processes of their in-house competitions office.

Knight opened by asking the audience to close their eyes and think of a bridge, and posited that the bridge they thought of would typically by historic, and possessed of a strong visual identity whether large or small. A readily recognisable view of the bridge, rather than from the bridge, was the most likely visualisation. Unbuilt bridges, and purely engineering concepts (such as calculations and analysis) were unlikely to come to mind, even for the engineers in the audience.

The presentation seemed to be suggesting that these tendencies were a key reason why bridge design competitions are needed. A sense of place is a key feature in much of the best bridge design. Gateshead and Millau (pictured, left, courtesy chericbaker) were cited as examples, while River Wear was offered as the epitome of a desire for place-making being allowed to overpower all other considerations.

The long normal life span of bridges was noted, and this long-lasting legacy was a key reason why appropriate appearance was vitally important. Knight commented that a bridge may only spend 1 or 2% of its lifetime in design and construction, but 98% being seen and used, which seemed to be a request to allow aesthetics the chance to take precedence over structural simplicity or buildability.

The view was that design competitions allow clients the opportunity to see what can really be done to support their agenda when the floodgates of creative teamwork are opened, whether that agenda is economic development, promotion of sustainability, or simply creating publicity. Here, and elsewhere in the seminar, it was noted that competitions can generate ideas that simply would not arise through a single-designer process, however able they may be.

Returning to the question of “why” design competitions, Knight felt that a positive feature of the process was simply that they brought design higher up the project agenda, allowing the benefits of good design to be better recognised.

The final presentation was an interactive walk through the IABSE guidelines themselves, by Naeem Hussain, who had chaired the IABSE competitions working group. I won’t cover this in detail, as the guidelines are after all freely available for anyone to read online. However, I will say that when I first read the guidelines, my thought was that they were too vague to offer the sort of simple process manual that many clients would like.

I now feel differently: they offer sufficient flexibility to address several different problems, and to satisfy widely varying regulatory requirements around the world. It also seems to me that their key purpose is not to tell clients what to do, but to make clients stop and think, a task which seems to be undertaken far too infrequently. Duguid’s presentation had referred repeatedly to the need to set up a competition process that aligns the incentives to designers with the client’s underlying objectives, and perhaps the guidelines might encourage the more careful thinking which could allow this to happen.

Something I would personally suggest, but which was not covered in the seminar, is that competition rules (and any complex procurement process) should be the subject of scenario-testing, where someone role-plays the bidder and seeks to “game” the system, identifying loopholes, perverse incentives and the like.

Several comments from the floor offered the view that the guidelines are not written in appropriate terminology for clients – some of the language seems to suit the perspective of the competitor. I think this is a little unfair (although it’s notable that there were no clients on the IABSE competitions working group), and certainly far less significant than the challenge of making clients aware of their mere existence. I’m aware of at least two bridge design competitions being planned by UK public sector bodies during 2013, and I wonder whether either body has even heard of the guidelines, let alone be following a process which takes on board their content.

I think the first consideration for those who would like to see better bridge design competitions is to repair the “damaged brand”, which can only happen when prominent contests are organised in a way which learns the lessons of past failure, under conditions conducive to a positive outcome. Only then is there a realistic prospect of promoting competitions as a more frequent tool in the design procurement armoury, as they certainly should be.

Mersey Gateway preferred bidder announced

A preferred bidder has been announced for the £2bn Mersey Gateway scheme in North West England. The Merseylink consortium (Kier, FCC, Samsung) now enters final financial negotiations with the promoter, Halton Borough Council, leading to contract award later this year.

 

While this is good news for anyone who like big bridges and to see people employed welding together lumps of steel and pouring tons of concrete into holes, I am more interested in what it means for the design of the bridge.

The original design was by Gifford (now Ramboll) with Knight Architects, and was used to secure planning consent prior to inviting construction tenders. Their design featured three mono-tower masts supporting the deck via "harp"-layout cable-stays, i.e. an arrangement of parallel cables. This is visually very attractive, but the low slope of cables near the bridge towers mean the deck is not well-supported in these areas. The design also allows for essentially a single plane of cables along the bridge deck centreline (actually, two planes very close to one another), which is visually very open and legible from all perspectives. This is the original design:

For multi-span cable-stayed bridges (with more than two towers), a key challenge is how to stabilise the spans, as they are far less stiff than a bridge with only one or two towers. The original Mersey Gateway design addressed this issue using a stiff steel truss deck, pictured in cross-section below (an excerpt from the planning drawings), which was conveniently deep enough to also accommodate future light rail tracks below the road deck (an idea later dropped from the scheme requirements).

A deck of this type would be lighter than a concrete alternative, and subject to reduced wind load. However, the cost of steel fabrication and site assembly would be high.

I mentioned the bridge in May 2012, noting that the promoter was sensibly varying the planning conditions, to give tendering contractors flexibility to reduce costs. I commented that "the structure which will be built is unlikely to retain either the harp cable layout or the truss deck, although I imagine the single plane of cables has a fair chance of surviving."

It's always nice to be proven right, as this is precisely what has happened in the preferred bidder's design, which I understand is by Flint and Neill. The cable arrangement is replaced with a single-plane "fan"-layout which is structurally much stiffer, and the single plane will be less expensive to install than two closely-spaced planes. This is the proposed design:

In place of the steel truss, the new design uses a conventional post-tensioned concrete box girder, which judging from the video will be cast in-situ in sections. The image below is taken from the project's new publicity video and shows the use of internal steel frames to transfer load between the girder webs and the cable-stays:

I imagine the deck will be shallower overall than the original truss design, which will make up for any heaviness of the solid concrete. I also expect that it makes the transition to approach viaducts very straightforward, as the same box girder cross-section can be applied consistently.

There is one significant visual disadvantage to the preferred bidder's proposal, which comes in the relationship between the tower above and below deck. This is a common problem with large cable-stayed bridges, where a slender tower above deck often gives way to an extremely stocky tower below deck. The Dartford Crossing is a particularly egregious example of how bad this can look. The Millau Viaduct shows how it can be done well.

In the Knight / Gifford design, the truss deck could be pierced, so that the bridge mast and piers form one continuous column passing through the roadway. A similar approach was used on Stonecutters Bridge in Hong Kong, to pick just one example. This is clearly impossible with a concrete box girder, and so the contractor's alternative design features piers below-deck which are much bulkier than the tower above, and appearing doubly so because of the large crossheads used on the two taller piers to provide torsional restraint to the whole deck.

Despite this, I think the promoter's decision to allow a certain range of variation from the original design was clearly the right approach. We will have to wait until later in the year to see whether it has realised the cost savings they expected.

Kruunusillat bridge competition winner announced

The winner has been announced in Helsinki's Kruunusillat light rail bridge design contest: WSP Finland / Knight Architects. There's a video of the winning design on YouTube.

Second prizes have been awarded to entries from Arup  / Amanda Levete Architects, and Pontek Consulting Engineers.

The minutes of the Jury make interesting reading, and readers may also want to look back at my three-part review of the competition entries: 1, 2, 3.

When I get a few spare moments, I'll post some more detailed comments.

Garden bridge proposed in London

Images have been released for a proposed "garden bridge" to span London's River Thames, to be designed by Thomas Heatherwick with Arup. The bridge would cross the Thames near Temple and the South Bank, roughly half way between Waterloo and Blackfriars road bridges. The proposal is supported by London's Mayor, Boris Johnson, as well as by actor Joanna Lumley.

My understanding is that the scheme originated in an unsolicited proposal from Heatherwick Studio to Transport for London, who subsequently appointed an engineering partner via competitive tender to help understand and develop the feasibility of the proposal.

The bridge would form the third of a triumvirate of lavishly expensive central London pedestrian bridges, lying halfway between Hungerford Bridge (£40m) to the west, and the Millennium Bridge (£23m) to the east. The bridge trumps both structures with a reported cost of up to £60m, although it appears neither Transport for London nor the Mayor actually wish to spend any of their own money on it. It shares this pie-in-the-sky nature with the £22m Diamond Jubilee Bridge, a proposal much further west, which recently received planning consent in spite of a complete absence of any funding.

If it goes ahead, the Heatherwick bridge would surely become the most expensive pedestrian bridge ever built, by some margin.

The hope is that a sugar-daddy sponsor may step in and pay for this gigantic concrete flowerpot, as was the case with the ArcelorMittal Orbit, an extravagant folly almost entirely paid for by steel tycoon Lakshmi Mittal, and to some extent the Emirates Air Line cable car, which received the lion's share of its funding from the titular airline. Indeed, the comparison with the Orbit seems particularly apt, as being a spectacular tourist attraction where an artist's peculiar vision has been writ at such gigantic scale as to overpower almost everything around it. I would wonder whether the same sorry aesthetic compromises and pungent brashness that mar the Orbit would find their way into the Heatherwick structure.

There is an element of structural logic to the Heatherwick proposal - it gives the appearance of a prestressed concrete balanced cantilever bridge an entirely reasonable solution for the site and function. Maintenance costs should be low, and the structure can be built with relatively low disruption to river traffic. It will be robust in the event of vessel impact, and invulnerable to wobble.

Heatherwick has some history in bridge design, with Paddington's Rolling Bridge, and an unbuilt proposal for a glass bridge at London's Kings Cross. As with pretty much all of his other work, these combine considerable levels of ingenuity with an open-minded, sparklingly creative approach to design geometry. Heatherwick's training was not that of an architect, but in"three dimensional design", at Manchester Polytechnic and the Royal College of Arts, and this seems to limit his scope when he works with architecture.

Both the bridges mentioned are very clever, very well crafted objects, and looking at these images, I have a sense that this bridge is imagined as an object, or at most a place, but not as part of a wider landscape. Like his other bridges, there is no sense that it responds in any way to its context - it could be plonked down in many other cities without any sense that Heatherwick would design it differently.

The highly articulated fluting of the bridge piers and soffit seems an attempt to break up what would otherwise be large areas of unattractive concrete, but will need constant illumination to retain any visual interest on an overcast, rainy day.

The ends of the bridge must (as with any of the central London bridges) land at a level well above the river bank, to maintain navigational clearances, but the proposed form, handsome in its isolation from what it connects, offers no clues as to how pedestrians will access it from the bankside below. At both ends, there are no pre-existing higher level features in the vicinity, and I wonder what ponderous impact it would have, especially on the south end which is currently occupied by a mix of low-height buildings and open space, one of the few welcome respites from London's overcrowded south river bank.

In short, I'm simply not sure that Heatherwick has the sensitivity to context to allow his vision to dominate such a significant location. He is the British jack-of-all-trades designer with the highest public profile, and it strikes me as a huge shame that nobody else is being offered the opportunity to come up with proposals for the site. The local authority's desire to use name-recognition to draw in sponsorship and obtain public infrastructure at minimal cost is understandable, but shouldn't be an excuse to adopt anything other than the best design solution.

Bridges news roundup

This Beautiful Footbridge Is Inspired by Japanese Hand Fans
If you had to design a bridge to grab the attention of buzz-conscious architecture websites, you might come up with this cute little thing.

‘Prague: Bridge-Building over the Vltava River’ Competition Entry / Juráš Lasovský
If you've ever actually been to Prague, try to imagine this design contest leftover ever actually being built there.

"Bouncy" footbridge set for strengthening
I visited this bridge at Pitlochry last year, and confirmed that it is indeed prone to bounciness, with a resonant frequency of about 1.7-1.8 Hz. For its 100th anniversary, it has been announced that it will receive "some upgrading and strengthening works to accommodate modern loading standards". Hopefully that's not really true - it has survived a century and it's hard to see that modern loads are anything worse than what it has already withstood.

Opladen Bridge Opening Ceremony
Very interesting design for a new footbridge, by Knippers Helbig with Knight Architects.

River Thames Reading footbridge consultation begins
Thin and pointy.

Leaving his mark
Kettuvallam Bridge is P.S. Binoy's "claim to immortality as an architect".

The Golden Gate of Helsinki
A winner in the Kruunusillat bridge design competition is due to be announced this month, although, as this article makes clear, not only is the bridge yet to have any funding committed, the promoter is even considering changing what the bridge carries, to allow highway traffic.

Work resumes on cycle and pedestrian bridge in Hereford
After the unfortunate news that leading UK steel footbridge fabricator Rowecord had gone into administration, it looked like the project might stall, but now progress has resumed. Herefordshire Council has further information.

Pedestrian bridge design unveiled
Designs for a new bridge at Penrith near Sydney, Australia show a garish yellow truss that looks like a jib which has fallen off a tower crane. More images at the Penrith Star.

Controversy over bridge which spans M8 in Glasgow
I have to say, nothing is more tedious than a futile debate about whether a bridge's disability-compliance is satisfied by whether its slope is exactly 1:20 or less. I remember having to meet that requirement on a bridge design once, for a bridge where the ground at either end sloped by more.

Lingzidi Bridge / Rural Urban Framework
Highly architected concrete is not the usual choice for a rural bridge, but may be a robust and durable solution for this bridge in China.

Fictional bridges on Euro banknotes constructed in Rotterdam
Everything that's wrong with post-modernism in one place.

Bowstring arch bridge in Japan
I really don't understand what the designers of this bridge were thinking.

Intersection bridge in Japan
And this one is even more baffling, it really has to be seen ...

Footbridge 2014 call for papers

The ever-popular Footbridge conference has announced a call for papers for its 2014 event. The deadline to submit abstracts is 30th September 2013.

The conference theme is "Footbridges: Past, Present and Future", and papers have been invited addressing the following sub-themes:

1. Historical and heritage structures
2. Dynamic response and structural behaviour
3. Inspirations in footbridge design
4. Planning, design and construction of sustainable footbridges
5. Advances in materials technology for footbridge construction
6. Future directions in footbridge design and construction