01 March 2020

Design unveiled for Sacramento I-Street bridge

The final design has recently been revealed for a new I-Street Bridge in Sacramento, California. Designed by T. Y. Lin, the new lift bridge is expected to cost up to US$210m, with completion expected in 2025.

The design is a 300-foot span lift bridge in the form of an arch. According to bridge architect Noel Shamble "it is a lift bridge and an arch bridge combined together, which is the first of its kind in the world". (Of course, it isn't, at least not by virtue of this particular combination, anyway).

The bridge will supplement an existing trussed swing bridge, built in 1911, which carries a railway through the truss and a roadway on top (picture courtesy Michael Hicks / Wikimedia Commons). It is proposed to retain the old bridge for railway traffic and possibly also pedestrians and cyclists, while demolishing its extensive highway approach viaducts.

The new bridge will carry cars, cyclists, pedestrians and transit vehicles. Approximately $150m of the cost will be funded by federal or state sources, which perhaps explains why Sacramento have decided they can afford such an ambitious design.

At first glance, the design visuals indicate an arch with inclined hangers, from which a truss is suspended. Oddly, the truss is cut short and doesn't run the full length of the span.

The choice of final design had been the subject of an extensive public consultation process, with T.Y. Lin developing a total of four designs, two variants each of two bridge types: a Nielsen-Lohse arch, and a truss span. These had already been whittled down from a wider range of concepts, presumably by virtue of being the cheapest two. Two of the final options are shown here:

My first thought on seeing the final bridge proposal was that the designer had been forced to create some kind of Frankensteinian hybrid of the previous options, a combined truss and arch. This makes little structural sense, as the criss-crossing hangers of the Nielsen-Lohse arch already provide enhanced stiffness, and visually the truss seems to be an afterthought, as in the visual below. It made me wonder whether the outcome was some kind of horrible compromise intended to satisfy as many different views from the public consultation as possible.

Reviewing the images in more detail, it becomes clear that the "truss" elements are not what they initially seem: they are supports to overhead canopies, with giant triangular frames forming part of cantilever canopy brackets. There is no truss.

The bridge project could have turned out very differently. The original forecast cost was from $70-80m. In 2018, the budget was reported as doubling to $172m, following the US Coast Guard's insistence on a span substantially wider than that provided by the existing bridge. At that time, the Cities of Sacramento and West Sacramento expected to contribute only around $3-4m of the total cost, with the rest coming from external government bodies.

This kind of highly-leveraged funding has a chequered history, and it's easy to see the ambitious design struggling to get built if external funding is reduced or if the estimated cost rises further. As yet, there appears to have been no contractor input of any kind, so they'll only know if they have financial issues when construction tenders come in.

T.Y. Lin had been appointed in 2018 following a design competition, with eye-catching proposals such as a solar-power bridge (the arch does not support the deck in this design), or towers with observation decks, or a bridge with gardens on it.

Having won the contest, the garden and solar panels bit the dust when costs of different options were compared. I'm not sure if the observation decks are now excluded as well.

I haven't been able to find the names of the other design contest participants, but some of their designs were shown on Twitter:

Looking further back, the project didn't start out with a design competition. Back in 2014, San Francisco-based bridge architect Donald MacDonald had been appointed to help develop ideas for the new bridge. In 2017, Sacramento writer Rob Turner felt the MacDonald designs (shown below) weren't up to scratch, campaigning via the Sacramento Bee and Sactown Magazine for the design process to be halted, MacDonald to be thrown off the job, and for a design competition to take place instead.

Turner argued that MacDonald's designs were too deferential to the past, "pleasing" rather than aspirational. He illustrated his argument with images of expensive, overblown and largely irrational designs such as the Juscelino Kubitschek BridgeSheikh Zayed Bridge and the unbuilt San Shan Bridge, and praised Calatrava's Sundial Bridge, which had to be privately-funded when costs ballooned well-beyond the original budget. He identified Ipswich's appointment of Foster and Partners to design an iconic bridge as a successful precedent (he could hardly have guessed it would end in ignominious failure as costs rose beyond what could be afforded).

The City of Sacramento proceeded in line with Turner's advice, switching to a design competition, leading ultimately to the newly unveiled design, and a budget now potentially three times what they originally expected. Comparing MacDonald's hand-drawn sketches to the seductive visualisations that came later, it's easy to see why the project client (and perhaps the public) may have changed tack.

It will, as always, be interesting to see how the project fares over the next few years. The proposed structure, despite initial appearances, is broadly rational, with the potential to be a high-quality landmark rather than an inappropriate extravagance. I do wonder, though, whether the promoters have been seduced by a vision well beyond their original requirements, and whether it will get built as currently illustrated.

21 January 2020

Manchester Bridges: 23. Marple Aqueduct

Yes, yes, I know, it's not really in Manchester. Want to make something of it? No? Good.

It's sometimes difficult to imagine how the pre-industrialised landscape would have looked near the end of the 18th century. Some canals had been constructed earlier (e.g. the Bridgewater Canal opened in 1761), but canal mania only really took off in the century's final decade. The Peak Forest Canal, authorised by an Act of Parliament in 1794, was part of that explosion of canal-building.

So far as was possible, canals needed to be level to be economically viable. The Peak Forest Canal connected mines at Dove Holes to Manchester, with two level stretches of water (the Upper and Lower Peak Forest Canals) separated by a 64m tall flight of locks at Marple. Immediately south of the locks, the Lower canal crosses the valley of the River Goyt, and between 1794 and 1800 a huge aqueduct was built here.

It must have been a tremendous endeavour, considering the scale of the stone blocks that make up the finished structure. The enterprise was largely promoted by industrialist Samuel Oldknow, and the engineers were Benjamin Outram and Thomas Brown, and William Broadhead, Bethel Furness and William Anderson were appointed as the contractors. It is claimed to be the tallest masonry aqueduct in the United Kingdom, and over the six years of construction, seven construction workers died.

The aqueduct's most distinctive visual feature is the series of circular openings in the spandrel walls of all three spans. These were presumably intended to reduce weight, and were not a new idea: William Edwards had used them for his fourth and final bridge in Pontypridd in 1756; John Smeaton included them on Coldstream Bridge in 1767, albeit filled with loose stone. No doubt there are other examples. They add visual interest to the bridge but I don't think they lighten its appearance: compare Rennie's Lune Aqueduct, completed in 1796 while the Marple structure was being built, which scarcely looks much heavier.

Tie bars were inserted in the central span in 1860 by engineer Charles Sacre. Use of the canal had declined during the 19th century, with Marple Railway Viaduct being built alongside the aqueduct in 1863.

Decline eventually lead to ruin, with one spandrel wall of the aqueduct partially collapsing in January 1962. British Waterways proposed demolishing the structure and replacing it with a pipeline, but additional funding was found from the local authority to rescue the aqueduct, and in 1966 the restored bridge was declared a Listed Building, Grade I. The canal itself was rescued from dereliction and reopened in 1974.

The aqueduct was altered again in 2018. Until that point in its history, the aqueduct had only one parapet, on the towpath side. The other side had only a very low wall. According to the Canal and River Trust, people had been recorded nearly falling from that edge, and some people were even playing a dangerous game of leaping across the canal to land on the unsecured edge.

It was agreed to add a new parapet, designed by Knight Architects and Arcadis, and manufactured by Bisca. This is a bespoke design in stainless steel, with the top rail left unpainted, and uprights painted matt black. In a nod to the Peak Forest Canal's benefactor Samuel Oldknow, the arrangement of the metalwork evokes the warp and weft of cotton-weaving.

It feels a little odd, having such high-quality metalwork in a place where the ordinary public are remote from it, but it must have been some challenge to navigate the demands of parapet standards and the difficult constraints of modifying such a historic structure.

Viewed along the aqueduct, the new parapet gives an impression of solidity, while viewed looking across, it is much more transparent. Engineers will note that the location where the woven bars intersect is the weak point. I did wonder if the design is not a little over-done when something more minimal may have sufficed, but I didn't find it too objectionable.

Further information:

12 January 2020

Derbyshire Bridges: 1. Cathedral Green Footbridge

As with my last post, the title of this post is not an indication to expect lots of posts about bridges in Derbyshire, it's just a placeholder for the future. If you have suggestions of interesting bridges to visit in Derbyshire, let me know via the comments.

Derby's Cathedral Green footbridge was opened in 2009, following a design competition held in 2007. The design, by Whitbybird (team members now dispersed into Move Architecture and Ramboll), was for a moveable cable-stayed pedestrian bridge. Mechanical engineering design was undertaken by M G Bennett and Associates Ltd (team members now in Eadon Consulting).

Spanning the River Derwent, the bridge is around 57m long with a 22m tall central mast. As well as creating a new river crossing, the intention was to contribute to revitalising the Cathedral Green open space.

The bridge deck is cranked in plan, with an 19m back-span and 38m main-span, supported by stays from a tilted mast. The back span crosses a historic mill race. The reason the bridge is moveable is not for navigation, but to allow the bridge to be moved clear of the main river in times of flood.

In this arrangement, the layout is intended to allow pedestrians to continue to use both riverside walkways, an arrangement that is best understood either from the aerial view on Google Maps, or from this video:

The ability to retain the riverside walkway in an open position seems slightly unnecessary, as the mill race is covered over and hardly an obstacle any more. When I visited, the riverside path was also closed in both directions for construction work.

The asymmetry of the bridge led to the mechanical engineering being relatively unusual. The bridge rotates about a central pintle bearing, but its main supports are a wheel under the central area, and a second wheel at the rear of the back-span, which resists uplift and runs in a curved track. A hydraulic motor drives a pinion against a rack at the rear of the back-span when the bridge is required to rotate.

A further pair of wheel-bearings lock the other-end of the main span in place, the load in these being highly dependent on the thermal state of the bridge. It's interesting to think while crossing the bridge that it is so carefully balanced on just a few individual wheels.

The site looked so neglected and overgrown when I visited that I doubted the bridge still opened, but it seems that it does. Indeed, the impression of neglect was considerable, as will be obvious from some of my photos. The benefits of a stainless steel parapet are clear, although the mesh infill has been severely dented in several places, and adorned with love-locks.

So many "designer" bridges of the millennium years share the same conditions of grime and disrepair now. It was easier for aspirational local government bodies to secure capital funding for ambitious new infrastructure than to provide the funding for their proper maintenance.

This bridge is a case in point. I visited on a grey, overcast day at a time when the city centre was largely devoid of activity. It took a feat of imagination to see how this bridge may have looked soon after opening, well-used and glowing in the sunshine.

The bridge design is impressive and well-considered (there's a technical paper referenced below which is well worth reading if you can get hold of it), but on the day I visited it felt very much like seeing an expensive sports car covered in dust and with insulting messages finger-painted in the dirt.

Further information:

09 January 2020

Staffordshire Bridges: 1. Ferry Bridge, Stapenhill

Don't get too excited by the title of this post: I only visited one bridge in Staffordshire recently, so the title is just in expectation that I may return and find others in the future. If you'd like to suggest any bridges in the area which I should visit, please respond via the comments to this post.

Historically, the River Trent formed the boundary between the counties of Derbyshire and Staffordshire, and a ferry served passengers between the town of Burton and the village of Stapenhill. In 1865, plans were laid for a bridge to replace the ferry, but nothing came of it until 1888, when Michael Arthur Bass, the 1st Baron Burton, agreed to fund the new crossing. His bridge was formally opened on 3rd April, the following year. In 1891, it was extended with an 81-span iron viaduct across the river's extensive flood plain (pictured).

The main river crossing is a suspension bridge with a main span of 120 feet and side spans each of 60 feet. It was built by local engineering firm Thornewill and Warham, who contracted Edward William Ives for its design. Ives, in turn, obtained the assistance of Alfred Andrew Langley. Suspension bridges were hardly new in the late 19th century, but Ives and Langley came up with something that departed significantly from what might then have been "normal" practice.

It's an unusual structure in two main ways. It is a self-anchored suspension bridge, where the main suspension "cables" are anchored not into the ground, but to the ends of the deck. There can't be many such bridges in the United Kingdom; the other examples that come to mind are Chelsea Bridge, built in 1937, and Derry's Peace Bridge, completed in 2011.

Ground anchorages are normal for suspension bridges because they allow the suspension cable or chain to be erected first, and the bridge deck to be assembled in sections afterwards. This minimises work within the river, to whatever is needed to pull the cable or chain across (and that can be done in stages) and to erect small deck sections. A self-anchored suspension bridge requires the bridge deck to be built first, usually necessitating extensive temporary works in the river. The choice here might have been down to poor foundation conditions within the river flood plain.

The suspension cables are also neither chains nor wire cables, but formed from riveted wrought iron plates. Bridges of this type are exceedingly rare, and the only other one I've visited is the 1910 Grunwaldzki Bridge in Wrocław, Poland.

The Stapenhill Bridge uses three layers of plates, one layer being jointed at each hanger position, so the strength is equal to two plates throughout. Judging from the bridge inventory at Bridgemeister, UK suspension bridges of this period were mostly built using wire cables, although some were still constructed using iron eyebars (e.g. Hammersmith Bridge, 1887). Perhaps the choice of riveted plates was driven by Thornewill and Warham's capabilities.

The suspension elements are supported on cast-iron towers. Although these are impressively ornate, they are are considerably diminished compared to the original structure - some historic photographs can be found on the Burton-upon-Trent local history website. Those show taller cappings both on the towers and at the ends of the bridge.

The bridge deck is supported from lattice trusses on each edge. These pass through the tower portals, with the result that the suspension "cables" are inclined inwards. This geometry imposes transverse bending on the lattice trusses, which is not the most efficient arrangement. The hanger bars are wrought-iron rods, except at midspan where there is a fixed connection between the chains and the edge trusses.

The deck itself consists of wrought iron transverse lattice cross-beams (through which service pipes are threaded), supporting timber flooring. New parapets have been added inboard of the main edge trusses: I guess this was part of the major refurbishment in 2016, with works designed by Inertia Consulting.

I'm not sure I could describe this as a beautiful bridge, but it is characterful, and good-looking in its refurbished condition. When I visited, the bridge and its approach viaduct were very well used, and hopefully it has a long future ahead.

Further information:

04 January 2020

Some recent books about bridges

I have a couple of posts on recent bridge visits in preparation, but meanwhile here's a quick round-up of a few books about bridges that have recently arrived at Pontist Towers ...

From Brycgstow to Bristol in 45 Bridges by Jeff Lucas and Thilo Gross (Bristol Books, 144pp, ISBN 978-1-90944-618-2, 2019) is a catalogue of all the bridges spanning Bristol's main waterways (at least, those that can be crossed by foot), presented in the order of a possible walking tour (albeit quite a long walk). The city is mentioned as Brycgstow in the Anglo-Saxon Chronicle, translated as "place by the bridge", sited at the confluence of the Rivers Frome and Avon. Over the years, it expanded as a port, especially after the creation in the early 19th century of the New Cut to divert the main river, turning the remainder into the "Floating Harbour".

Inevitably, a profusion of bridges resulted, and Jeff Lucas shares their stories with his own photographs in this book. It's a general interest book, so more of a travel guide or social history than a book on architecture or engineering, which is fine. The idea to write the book arose from an article by mathematician Thilo Gross applying the Königsberg Bridge Problem to Bristol, and a chapter by Gross explains this topological network puzzle in more detail.

Thomas Telford's remarkable bridge over the Menai Straits was opened in 1826, so to find an excuse for a bicentennial history, Menai Suspension Bridge: The First 200 Years (Menai Heritage, 206pp, ISBN 978-0-9932351-3-9, 2019), the author Bob Daimond has had to date events to the laying of the first stone, in August 1819. Spanning 176m, this was the longest bridge in the world when completed, a tremendous achievement given the state of engineering knowledge at the time.

Daimond's book is a definitive history of Telford's masterpiece, and very well illustrated with extracts from archive drawings, photographs etc. It discusses in detail the planning, testing, design and construction of the bridge, and its subsequent history including storm-induced failures, alterations and eventual reconstruction in the mid-20th century. As a history of engineering it is exemplary; my only complaint would be that it has little to say beyond that, on the bridge's cultural status, on the people who use it, and on its place in the wider history of suspension bridges.

Bridges by David Ross (Amber Books, 224pp, ISBN 978-1-78274-576-1, 2018) is essentially just a coffee-table photo book, a collection of photographs (with short descriptive text), arranged chronologically. The bridges are from all around the world and the photos are from a variety of photographers, so there's no special theme or style. Nonetheless, I found it a very enjoyable book. The photos are very well-chosen, and well presented, often across two pages. There are plenty of familiar bridges, plus quite a few that are less well-known, or were to me, anyway. A real effort has been made to span the globe, and the result is a fine reminder of the variety and ingenuity that bridge-builders have brought to their art over many centuries.

Ann-Mary Paterson is the great-grand-niece of William and Murdoch Paterson, two of the engineers responsible for construction of various railway lines radiating from Inverness in Scotland in the late 19th century. Her 2017 book, Spanning the Gaps: Highland Railway Bridges and Viaducts (Highland Railway Society, 96pp, ISBN 978-0-9927311-1-3; my copy was purchased from Old School Beauly) describes the history of the Highland Railways, with a focus on the structures that carried traffic through often quite difficult terrain.

The book is very well illustrated, with a mixture of historic and modern photographs, and several historic drawings. There are some informative photographs of construction, and some following various disasters, such as the 1989 collapse of the Ness Viaduct. There are some fascinating and impressive bridges along these railway routes: Culloden Viaduct, William Fairbairn's box girder bridges across the Rivers Findhorn and Spey; the timber Aultnaslanach Viaduct; Findhorn Viaduct; swing bridges over the Caledonian Canal; ornate castellated viaducts at Blair Atholl and elsewhere; and many more.

23 December 2019

London Bridges: 54. Chiswick Park Footbridge

It has been a while since I've visited and reported on a recently-built bridge (May, since you ask, and before that, August 2018), so this visit to see Chiswick Park footbridge in London felt long overdue.

According to the Ian Visits blog, the bridge itself is also long overdue, with planning applications dating back to 2003. It forms part of a walking route connecting Chiswick business park to Chiswick Park tube station. The bridge finally opened to the public in January this year. It was designed by Expedition Engineering and Useful Studio, with the steelwork constructed by Severfield.

I believe it's only the second network arch bridge to be built in the UK, and the first such pedestrian bridge. The design didn't start life as a network arch structure, and has been through a lengthy evolution to get to what was eventually built.

Originally, proposals were for a truss bridge of some form, as per the 2003 and 2006 planning applications shown here:

The bridge's three spans were largely determined by clearances to road and rail routes below, and in these early designs it was indicated that the truss span over the railway would be fully enclosed to prevent risks from vandalism.

In the 2012 application (top diagram in the image above), the design had become three steel bowstring arches, each of increasing span and height from west to east, with vertical hangers supporting the deck. Tall mesh parapets were indicated above the railway, eliminating the need for full enclosure.

The overall form of the bridge changed very little thereafter. The shape of the arches was the result of a form-finding exercise, to maximise the visual slenderness of the arch. I'm not clear how that will have worked, as the critical bending arrangement for an arch of this sort is usually with only half the span loaded with pedestrians.

That scenario is often more onerous in design than the full span loading which produces the greatest axial load in the arch. In a conventional bowstring arch, the stiffness of either the arch or the deck (or both) is required to resist this half-span bending.

In any event, during design development the bowstring arch was found to perform badly under dynamic pedestrian loading, and was amended to a network arch in the 2015 planning application (middle diagram above).

This is many times stiffer than the previous design, raising the bridge's vulnerable natural frequencies, and eliminating or mitigating the dynamic problems. Bending moments in the arch due to asymmetrical loading arrangements are also greatly reduced.

I believe the previous design incorporated a concrete deck - adoption of the network arch also allowed a lighter all-steel deck to be used, minimising the weight required and making craneage of the spans into place easier.

The final change is indicated in the 2017 planning application (bottom diagram above), and indicates that the design team had failed in their desire to persuade Network Rail that a mesh parapet would be sufficient above their railway line.

The railway authority is never noted for its flexibility when there is a rulebook that can consulted, so the final introduction of a solid (imperforate) parapet screen above the railway tracks is unsurprising. It is at least largely disguised by being hidden behind the facing mesh.

The bridge's spans are 37.0m, 40.7m and 44.4m, totalling 122.1m. The arches and deck are connected integrally to the two intermediate piers, with bearings allowing thermal articulation at each end. All the structural steelwork is weathering steel, with stainless steel parapets and hanger cables. The decking is floored in timber planks.

The articulation is interesting, as conventional wisdom would be that arches of this type should sit on bearings at all points, allowing the tie girder connnecting the ends of each arch span to expand freely. This allows it to take up a full tension balancing the compression in the arch, and allowing the hanger network to interact efficiently with the main steelwork.

In this instance, the V-shaped piers are sufficiently flexible longitudinally that they will offer only limited restraint to the arch thrust, and the network on the central arch will still work although I'd guess with slightly reduced effectiveness.

The most striking aspect of the bridge is the effort that has been expended to make its main elements slender, with cruciform sections for the arch and piers, and a simple stiffened steel plate for the deck. In addition to being slender, all parts are visible for inspection and maintenance, unlikely the closed box sections often seen in footbridges.

What is also very much apparent is an impressive attention to detail. Wherever possible, connections are kept simple, with welding used extensively instead of bolting. The edges of the deck are made clear and sharp in profile, and the hanger and parapet connections are well-detailed and as minimal as possible.

It's the sort of bridge that any designer would be proud of, and especially impressive given that this is predominantly a structure used to get rapidly from A to B, rather than a destination in its own right. it has been shortlisted for an IStructE award, been a finalist in the CE Awards, and won two ICE awards. I'm a little surprised that it hasn't been more widely rewarded, to be honest.

The bridge is not completely without its flaws. Site constraints mean that although at its western end the bridge connects directly with a podium deck level in the business park, at the eastern end the approach is via steps and a lift.

I imagine mobility-impaired users are crossing their fingers in the hope that the lift will be better maintained than is often the case. It is at least attractively detailed in keeping with the rest of the bridge.

Also on the eastern approach I noticed a sign that recommends no cycling, perhaps inevitable given the steps and lift, but also advises users to "Walk With Care" due to gaps in the decking.

The timber decking is visually attractive, but perhaps some users with high heels have found it a problem. In any event, I suspect problems with the decking won't end there.

The timber slats are raised above the bridge deck, the upper surface of which is a flat steel plate. Rainwater drains through the slats, and flows along the deck (which I am told is waterproofed), before spilling straight to the ground at the ends via cut-outs in the deck plate. The whole arrangement is an inspection and maintenance liability - nobody will lift the decking to properly inspect underneath, and it's easy to imagine dirt and detritus leading to trapped water over time.

Despite these oddities, the Chiswick Park Footbridge is a very impressive feat of design and construction and well worth a visit.

Further information: