German Bridges: 9. Oberbaum Bridge, Berlin

This is the last of the bridges I visited in Berlin in September, and it's going to be short and only a little sweet, as I only visited at night, and very briefly. It's Berlin's most notable bridge, so clearly I'll have to cover it more thoroughly next time I get a chance to visit.

I'll let you read about the bridge at the links below, I'll only comment briefly on the photos.

The bridge carries light rail on its upper level and a highway and footways on the lower deck. The upper central span was rebuilt in 1996 to a design by Santiago Calatrava, and I think it's one of Calatrava's best designs: constrained by working so closely with such a historic bridge, he was obliged to deliver something more modest than usual, and it's a beautifully elegant span which sits well amidst the much more lavish architecture of the historic bridge.

The arch itself seems impossibly thin, although this is a trick of the design, which uses triangular cross-sections for the arch ribs and main deck to emphasise slenderness. There are struts at each end of the bridge supporting the upper level (presumably reducing loads on the historic viaducts), but you generally don't notice them.

Seen up close, the new span is a very modern, almost industrial structure, but seen from further away it appears fragile. As with several other bridges over the River Spree, it was built as a result of German reunification, and may be politically symbolic.

Further information:

• Google maps
• Wikipedia
• Structurae
• Ponton's Brücken
• Berlin.de tourism
• Berlin.de bridgeworks
• Calatrava Bridges (Tzonis and Donedei, 2005)
• Berliner Brücken (Thiemann and Desczyk, 2012)

Yorkshire Bridges: 15. Park Square Bridge, Sheffield

Park Square Bridge is one of several bridges built for the introduction of Sheffield's Supertram system, completed in 1994/5. This tram system's three branch lines converge in a delta junction just to the west of Park Square, and this bridge carries the western branch above Park Square roundabout and up onto Commercial Street (hence the bridge's other name, Commercial Street Bridge).

Alongside other major Supertram structures, the bridge was designed by Sheffield City Council's in-house team, Design and Building Services. That seems barely imaginable today, it's hard to think of a UK local authority with the capability to design such a major structure any more.

The bridge spans 75m (with two smaller approach spans making a total length of over 100m), and was built by Balfour Beatty, with steelwork by Watson Steel (now part of Severfield).

The main bridge span is a bowstring arch. Inclined steel arch ribs are braced overhead and tied with steel girders, which form the edge members in a ladder-deck arrangement. The bridge deck is a 250mm thick reinforced concrete slab composite with the ladder-deck crossbeams. The deck carries a non-structural concrete slab, containing the tram rails, drainage channels and stray current protection system. The deck is suspended from the arch by 60mm diameter steel hangers.

Although the three spans are each simply-supported, the structural deck slab is continuous, to avoid leaking joints, and a paper on the bridge design states that there are "concrete hinges" in the slab above the piers.

The superstructure sits on reinforced concrete piers and abutments, with concrete blockwork cladding, and these are supported on piles down to bedrock. The design language is a little post-modern, I think, with an elaboration of detail and a type of mock-classical architecture that seems at odds with the high-tech or modernist sensibility of most bridges of recent decades.

The bridge span was driven by the highway layout and the presence of a large culvert below ground. However, Supertram planners were also keen to have a "gateway" structure at this prominent position. A cable-stayed alternative was considered but rejected. The choice of a bowstring arch allowed a low construction depth to be employed, minimising the length of approach ramps.

The designers sought to maximise maintainability by avoiding closed structural sections, so the arch rib is a channel section with the open face downwards (similar to another bowstring arch the other side of the Pennines, in Salford).

According to the paper, "careful treatment" was given by the designers to the parapets and to the anti-climb blisters on the arches. The latter are pretty horrible looking, but not enormously so given the scale and clarity of the rest of the bridge.

The arch ribs are pin-ended, which I find really quite odd. Are there other modern bowstring bridges with this detail? The design paper offers no explanation for why this choice was made. It isn't visually incongruous, but I think hinges should generally be avoided in main members of bridges unless there's a very good reason.

Behind the hinges, there are thrust blocks comprising a group of vertical steel stiffening ribs, which transfer the arch thrust into the end node of the tie girders. As you can see in the photograph, they double as a handy waste receptacle.

On the whole, I think this a well-designed bridge, generally with clarity of form and well-proportioned. It is, however, let down by some of the detailing, and not only the impromptu waste-bins, the lumpen parapets or the blisters.

There is a pastel colour scheme used to distinguish between a hierarchy of components, both the hanger assemblies and also the connection of these to the tie girders, which for my taste has been over-elaborated. The use of this number of pale colours is not very successful, as they are simply impossible to distinguish under most lighting conditions. A smaller number of complementary colours would have been enough.

The bridge was the winner of an ICE (Yorkshire) Award in 1994/1995, and Commended in the National Steel Design Awards.

Further information:

• Google maps
• Wikipedia
• Structurae
• LUSAS
• Commercial Street Bridge, Sheffield, UK (Wilson and Jones, First International Conference on Arch Bridges, 1995)

French Bridges: 18. Pont de Bercy, Paris

Ok, this is the penultimate bridge I'll cover in this series of posts on the bridges of Paris. There are many more fabulous bridges in this city, but I'm sticking to those where I have some reasonable photos.

The Pont de Bercy crosses the River Seine to the south of Paris city centre, near the gargantuan Ministry of Finance building and the horrible multi-sports stadium.

As with so many Paris bridges, there is a bit of history.

The first bridge here was a toll bridge which replaced a ferry in 1832. This was a three-span chain suspension bridge designed by Bayard de la Vingtrie and Fortune de Vergès, who have also been credited with the 1828 suspension bridge at the site which is now the Pont d'Arcole (although I've also seen that bridge credited to the better known Marc Seguin). Several images of the bridge are available online.

In 1864, the suspension bridge was replaced by a stone bridge, designed by Edmond Jules Féline-Romany, which is at the heart of the structure seen today. This bridge was widened in 1904, and an arcade built on top to support a Metro line. The bridge in these various stages of development can again be seen online, and my photo on the left shows a view which has not changed since the 1904 widening. The arcade gave the bridge something of the appearance of the Pont du Gard.

Curiously, the bridge has 5 shallow masonry arches, however, sources from the date of its construction refer only to 3 arches. I haven't found any reference to the bridge being lengthened at any stage in its history.

In 1986 a decision was taken to widen the bridge again, this time symmetrically so that multiple lanes of road traffic could pass on both sides of the Metro arcade, not just to one side. A design competition was held, and the winning design essentially replicated the original facade on the outside of a series of new reinforced concrete arches. The stone facade is an exact match for the opposite side of the bridge, but is in new masonry. The widening was completed in 1992.

If you look closely underneath the bridge arches, you can see the concrete section, the widening for the Metro arcade, and the original stone arch barrel. Above pier level, there is a gap between the reinforced concrete and the original masonry, ensuring that the new structure is structurally independent.

The arcade is an unusual feature, and ensures this is an exceptionally well provided bridge for different forms of transport - there are two wide roadways, footways at the edges, the metro line, and a cycle path running beneath the central arcade.

At the end of the bridge, the Metro line is carried on a metal truss structure over adjacent roadways. As can be seen in the final two photos below, this leads to an interesting contrast: the main arcade consists of parallel stone arches supporting a metal bridge deck, whereas the end sections consist of parallel metal trusses supporting brick jack arch deck construction. The relationship of metal and stone is reversed.

This is a fascinating bridge, which I passed by quite quickly but which I expect could repay considerably closer attention.

Further information:

• Google maps
• Wikipedia
• Structurae
• Bridge of the Week
• Planete TP
• Soundlandscapes Blog
• The Glow of Paris: The Bridges of Paris at Night (Zuercher, 2015)
• Bridges of Paris (Saint James, 2015)

Tyneside Bridges: 8. Queen Elizabeth II Metro Bridge

Heading further west along the Tyne, we came to a group of three bridges which are considerably less well known than those in the immediate centre of Newcastle and Gateshead.

Designed by WA Fairhurst and Partners, and built by Cleveland Bridge, the Queen Elizabeth II Metro Bridge opened in 1981. It was built to carry the Tyne and Wear Metro, the first major modern light rail system in the UK. With a main span of 168m, I think it's probably the longest spanning light rail bridge in the country. It's also the longest span over the Tyne (the Tyne Bridge is 162m, while Redheugh Bridge reaches 160m).

In form, the bridge is a three-span continuous through truss girder bridge, although it was constructed as two balanced cantilevers. It's a striking, impressive structure, but I don't find it beautiful.

Perhaps its most interesting feature is the paint scheme. The bridge was not originally blue, but was repainted in 2006 in its present white and blue colours. At the same time, 140 lights were installed as part of artist Nayan Kulkarni's Nocturne artwork, which projects changing colour patterns onto the bridge at night.

Whatever it may look like at night, the effect is interesting in the day time. The "inner" parts of the truss members are painted white, and the effect is to make the truss members appear more slender than they really are. At first I found the effect quite disorientating.

Further information:

• Google maps / Bing maps
• Wikipedia
• Structurae
• Bridges on the Tyne
• British Railway Bridges and Viaducts (Smith, 1994)
• Civil Engineering Heritage: Northern England (Rennison, 1996)
• Crossing the Tyne (Manders and Potts, 2001)
• An Encyclopaedia of Britain's Bridges (McFetrich, 2010)

Tyneside Bridges: 3. Byker Metro Bridge

The third and most recent high-level bridge to cross the Ouseburn Valley is the Byker Metro Viaduct, completed in 1982 for the Tyne and Wear Metro.

It is 815m long and curves in between the Ouseburn Viaduct (1839) and Byker Bridge (1878).

I only had time to visit its western end, which spans the valley. Much of the viaduct, at its eastern end, runs at about 6-7m above ground level, and is visually less imposing.

The bridge was the first glued segmental precast prestressed bridge to be built in Britain. The main spans are 69m long, with haunched concrete box girders supported on twin-stem piers. The bridge's appearance was considered significant enough for the design to be presented to the Royal Fine Art Commission in 1975.

The form of construction lends itself to a site where there was highly restricted access for plant. The concrete segments were precast off site, and winched down the valley slopes on temporary tracks. The length of each segment was just narrow enough to allow it to pass through an opening in the bridge piers, allowing the segments to be manoeuvred along a very narrow construction strip. They were then lifted and erected using the balanced cantilever method. Each segment was bonded using epoxy glue, and stressed in place with short length bars. Further prestressing was added in later stages.

The segments were "match-cast" i.e. each one was cast against its preceding segment, to ensure a perfect fit. Sawteeth keys in the webs, still visible in the completed structure, help align the units during erection. In practice, this method requires very careful measurement and control, as errors have the potential to accumulate over multiple segments. This is exactly what happened with the Byker Metro Viaduct, with a significant twist detected as more segments were added. Special segments had to be cast to correct the twist.

The finished bridge is aesthetically striking, and I think very successful. The plain finishes to the concrete deck contrast with a vertically ribbed finish on the piers and precast parapets. The ribs are nicely detailed, splaying out as the piers widen out towards their base. Horizontal feature grooves have been included on the piers to disguise construction joints.

The splayed piers both give an impression of considerable stability (required to resist centrifugal loads from the metro vehicles), but also make what are quite substantial concrete sections look quite elegant.

I think this is easily one of the most impressive and well thought out bridges on this scale to have been built in Britain.

Further information:

• Google maps / Bing maps
• Structurae
• Engineering Timelines
• Northern Architecture
• Tyne and Wear Metro: Byker Viaduct (Smyth, Benaim and Hancock, Proc. ICE, 1980)
• British Railway Bridges and Viaducts (Smith, 1994)
• A Celebration of Bridges between the Tweed and Tees (Hartley and Brown, 1995)
• Civil Engineering Heritage: Northern England (Rennison, 1996)
• Engineering Architecture (Benaim, Ingenia, 2002)
• An Encyclopaedia of Britain's Bridges (McFetrich, 2010)

Rotterdam Bridges: 1. Erasmus Bridge

I managed to find time on a recent trip to Rotterdam to visit a few of the city’s most spectacular bridges, which I’ll discuss over this and the next few posts.

As with many modern city bridges, the Erasmus Bridge serves both a practical and a symbolic purpose. Prior to its construction, transport connections across the river were limited, and the new bridge made generous provision: two single-lane highways, two tramways, two cycleways, and two pedestrian walkways. However, it was also required to satisfy the needs of civic pride, to be the most visible symbol of Rotterdam’s post-war reconstruction and growing economic success. It had to be a landmark both in purely visible terms, signposting the city centre from far and wide, and also as a technological achievement.

The bridge design was proposed by an architect, Ben van Berkel of UN Studio, directly inspired by Calatrava’s Alamillo Bridge in Seville. It was initially hoped that the bridge could be built in the same manner, without back-stays. The backwards-leaning inclination of the inverted-Y-shaped pylon allows the tower to act as a counterweight to the main deck. With a main span of 284m and a tower height of 139m, it proved impossible to make the bridge work without backstays, with even a balance of dead load alone requiring a significantly more substantial pylon.

The cranked arrangement of the pylon is the bridge’s signature feature, seldom replicated (I can think of one obvious example). It apparently provoked controversy amongst engineers not used to being subservient to architects and unhappy with the significant increases in cost required to build such a structure.

It was put to me by a friend that the bridge is not structurally logical, but this is unfair. The inclined pylon does to some extent balance the main span, reducing loads on the back stays significantly, and the crank is a coherent response to the vertical cluster of anchorages high on the mast. The only obvious enhancement in terms of how it distributes its forces would be for the upper part of the pylon to be curved, a way of reducing tower bending under spread cable forces which has been adopted elsewhere by Calatrava.

Certainly, the temporary propping required during bridge construction will have added to its cost, but I really find it hard to imagine that a more conventional vertical pylon would have looked as satisfactory.

A key element which makes less structural sense is the treatment of the deck girders. In the main span, the steel plate deck sits on transverse ladder beams, which span between and cantilever beyond two primary steel box girders. This allows the deck to appear reasonably slender in elevation, and is probably less expensive overall than allowing the entire deck to span between massive edge girders.

The back span is very different, as it does span between two massive edge girders, these girders forming the distinctive “legs” to the tower, and giving it a kind of abstract resemblance to a person kneeling (I am not sure if this was a conscious intention).

This arrangement has no structural rationale, as it would be more logical to continue the recessed girders. The result is the need to transfer the axial forces in the front span girders into the back-span girders (and pylon foundations) via substantial transfer steelwork. The “legs” are also far larger, at a maximum of 12m deep, than is required to actually carry the back span, with the result that they are largely comprised of fascia elements, with the actual structural girder being much shallower.

Behind the legs, there is a bascule span, designed to provide a shipping clearance of 50m. This is claimed to be the longest single bascule leaf in the world, making it an important structure in its own right, although it is only a small part of the overall structure.

Shortly after the bridge opened, the cables were observed to vibrate under certain combinations of rain and wind. The problem has since been solved by the provision of a hydraulic damper at the foot of each cable.

Close up, the pylon has been detailed largely for architectural effect, with a variety of kinked plate surfaces presumably intended to catch the light and avoid the plainness present on many other cable-stayed bridges. I think it looks good from pretty much almost every angle. There is a slightly odd "lipped" element on the main span face of the pylon, but I think it does need this sort of treatment to avoid looking quite dull.

One issue with cable-stayed bridges at this scale is that the cable sag can get quite significant. This is irrelevant from most perspectives, but disconcerting when you actually look directly along the cables, where the sag leads to the impression that the cables aren't properly tensioned. However, I suspect hardly anyone other than the curious bridge engineer will ever notice this.

Although the deck, cables and pylon are the primary expressive elements of the Erasmus Bridge, I was particularly impressed by the treatment of the substructure, the piers, abutments, and associated ramps and staircases at the north end of the bridge in particular. These take the shape-making desire visible in the steelwork even further in concrete.

There are a couple of twisted pier columns, and the north river pier is in the form of a monolithic "V" with some attractive chiselling. The ramps and staircase which lead from below the bridge up to deck level are also very nicely sculpted. These are all areas where the attention to detail of the architect has paid dividends.

On the whole, I find it hard to see why there's much controversy about this bridge. The design is well-detailed and visually appropriate, given everything the bridge was asked to do. I think it's a fine bridge that deserves to be more highly regarded.

Further information:

• Google maps / Bing maps
• Wikipedia
• Structurae
• Architecture in Rotterdam
• Designing the Erasmus Bridge, Rotterdam (Reusink and Kuijpers, Structural Engineering International, 1998)
• Numerical modelling of rain-wind-induced vibration: Erasmus Bridge, Rotterdam (Geurts, Vrouwenvelder, van Staalduinen and Reusink, Structural Engineering International, 1998)
• 30 Bridges (Wells, 2002)
• UN Studio Erasmus Bridge (Gannon, 2003)
• A critical analysis of the Erasmus Bridge (Hewett, University of Bath Student Conference, 2008)
• The Erasmus Bridge: Success factors according to those involved in the project (de Jong and Annema, European Transport Conference, 2010)