Wrocław's Bridges: 14. Zwierzyniecki Bridge

The most striking feature of Most Zwierzyniecki is the overhead bracing which connects its twin truss arches.

The bridge was completed in 1897 (replacing a two span timber bridge) but according to Wiesław Jurewicz, it was modified in 1910 "due to the hazardous vibration of the arch girders". That may account for the presence of just so much overhead bracing, which functions both as X-bracing in plan, and portal bracing in cross-section.

The bridge was designed by Karl Klimm, A Fruhwirth, and Richard Plüddemann. It spans approximately 60m and is 12.5m wide between the truss centrelines, 21.8m wide in total. It carries a highway and tramway across a side channel of the River Oder, in the vicinity of Wrocław's Zoo and Centennial Hall. An earlier span was reportedly called the Paßbrücke because it formed a checkpoint at the city limites.

The main arch trusses have more than a hint of the later Hell Gate Bridge or Sydney Harbour Bridge about them. The upper chord is essentially a stiffening rather than primary load-bearing element, terminating in mid-air vertically above the lower-chord springing. This is disguised by the presence of decorative sandstone obelisks at each corner, just as the same effect is disguised by monumental abutments on the two larger and better know spans.

The yellow-gold colouring on the bridge's metalwork, and the intricacies of the truss and overhead bracing, put me in mind of a gilded cage, where the structure is as much as a shelter as a means of support for the floor.

You couldn't build a bridge quite like this today. Quite apart from the cost of the heavily laced metalwork, the hanger bars are unprotected against impact from traffic. While it's acceptable to design a bridge for the possibility of loss of a single hanger, this bridge clearly suffers from the potential for multiple hangers to be lost in a single incident.

Technicalities aside, I found Zwierzyniecki Bridge to be a lovely bridge, with a great deal of antique charm.

Further information:

• Google maps
• Structurae
• Wikipedia (Polish)
• Wikipedia (English)
• Wratislaveae Amici (includes pictures of the previous bridge at this site, and the current bridge under construction)
• The best known bridges in Wrocław
• Wrocław Mosty - Bridges - Brücke, Wiesław Jurewicz (undated book, ISBN 978-83-926759-5-2)

Wrocław's Bridges: 13. Frog Footbridge

The Kładka Bielarska (or Kładka Żabia) connects Bielarska Island to the main river bank. There has been a suspension footbridge here since 1975, although it was largely reconstructed in 2002, including extensive reinforcement of the foundations with micro-piles. Before that, a truss bridge occupied the site. The bridge was designed by Kazimierz Gałajda, with the reconstruction attributed to Joseph Rabiega.

The bridge spans 56.5m, with pylons 10.5m high. The main suspension cables are 46mm spiral strand ropes. Both the pylons and the main deck beams are made from 457mm diameter steel tubes.

The pylons consist of portal frames, with the crossbar and legs each comprising twin tubes, "stitched" together with cross-stiffeners. These are ungainly enough, but made worse by the addition of frog-like protrusions at the top which carry lighting units. Presumably these give the bridge its "Frog Bridge" nickname.

Sadly, the paired tubes are not unique to this bridge - I've seen them on a photo of another Polish footbridge. Overall, the styleless reliance on tubular steel in combination with bold colouring is emblematic of an undesirable trend which seems to have informed a great deal of Polish footbridge design.

The bridge evidently suffered from considerable vibration prior to its reconstruction, and tuned mass dampers were added to the midspan and quarter span points. These are large dampers, and quite easily visible in the first photo of the bridge above.

Further information:

• Google maps
• Structurae
• Wikipedia (Polish)
• Wratislaveae Amici
• Footbridges in Wroclaw, Poland, Biliszczuk, Radiega & Tadla, Footbridge 2008
• Redukcja drgan kladek dla pieszych z wykorzystaniem tlumikow masowych, Dalmer, Fiebig & Rabiega, 2004 (Polish, explains the addition of the tuned mass dampers to the bridge)

Wrocław's Bridges: 12. Malt Footbridge

Not far from Kładka Piaskowa, another footbridge can be found which was built at the same time (2003), linking Słodowa island to the main riverbank. Kładka Słodowa is a steel bowstring arch bridge, with inclined hangers forming a triangulated system (this is stiffer than using vertical hangers, but is prone to load-reversal in the hangers if they are not sufficiently pre-tensioned). The designers were Research & Design Office Mosty-Wrocław with ISBA architects.

The arch spans 48m, and is 15.6m tall. It consists of two tubes, 508mm in diameter, and parabolic in profile, intersecting at a high level. Some form of connection is essential to brace the arches against buckling, but the choice to intersect in this way is essentially an arbitrary one.

The bridge deck consists of a cellular steel box girder with a curved soffit plate, similar to the nearby Sand Footbridge, and which picks up reflected light from the river in a similar manner. The deck is paved in blue-grey sandstone slabs.

The bridge elements were fabricated in a shipyard and brought to site by boat for final assembly. The arch was assembled lying on its side, with a temporary bowstring for support, before being lifted by crane into the vertical position. Three deck sections were then erected one by one using temporary vertical strands for support.

This is a reasonably straightforward and attractive design. The view of the cables gets a bit confused because of the variation in angles seen from anywhere other than side-on to the bridge. As with the Sand Footbridge, the olive-lemon green colour is not unattractive, especially amongst trees, and the deck looks slender.

I'm not sure why they went for a different parapet here - mesh infill panels rather than horizontal wires - and as at its sister bridge, the parapets look quite utilitarian.

It's interesting to see one of the alternative options that was considered for the site, an asymmetric cable-stay bridge without any back-stays supporting the pylon (shown below). This would clearly have been more expensive, both in terms of the superstructure and the foundations.

Further information:

• Google maps
• Structurae
• Wikipedia (Polish)
• Wratislaveae Amici
• Examples of newly built footbridges in Poland, Biliszczuk, Berger, Machelski, Wegrzyniak, Onysyk & Prabucki, Footbridge 2002
• Footbridges on the River Odra's Islands in Wroclaw, Styrylska, Boniecki, Biliszczuk, Barcik, Rudze & Stempin, Footbridge 2005
• Footbridges in Wroclaw, Poland, Biliszczuk, Radiega & Tadla, Footbridge 2008

Wrocław's Bridges: 11. Sand Footbridge

Most of the bridges in Wrocław that I've covered so far in this series have been historic in nature. The next two or three, however, are all relatively contemporary designs.

Kładka Piaskowa was built in 2003, and is a three-span pedestrian bridge (spans 4.5m, 45m, 4.5m), designed by Research & Design Office Mosty-Wrocław with ISBA architects. The deck is 3.19m wide and 1.06m deep, and comprises a steel box girder with a curved soffit. The deck is painted a lemon-olive colour, while the steel parapet post are painted light grey. The deck surfacing consists of Iroko hardwood planks.

Essentially, it operates as a fixed-ended beam, which allows the relatively large span-to-depth ratio to succeed. The deck is of a constant depth, but is lightly arched. The photo on the left illustrates the support system. The transition to the much shallower approach ramps looks as good as can be expected for something which is always very awkward.

The bridge was erected by "pull-launching", with its front end supported on a barge while the whole deck was towed across the river from one bank.

What's especially pleasing about this footbridge is its undemonstrative nature - there is no attempt to be unnecessarily showy, unlike some Polish footbridges. The low profile was a deliberate attempt not to interfere with historic riverbank views. The parapets are somewhat utilitarian, but not so much as to ruin the design.

The curved underside of the deck picks up shimmering reflections from the water below, and enhances the impression of slenderness. The colour seems initially a little odd, but it is muted rather than garish.

Further information:

• Google maps
• Structurae
• Wikipedia (Polish)
• Wratislaveae Amici
• Footbridges on the River Odra's Islands in Wrocław, Styrylska, Boniecki, Biliszczuk, Barcik, Rudze & Stempin, Footbridge 2005
• Footbridges in Wrocław, Poland, Biliszczuk, Radiega & Tadla, Footbridge 2008

Wrocław's Bridges: 10. Saint Clare Bridge

What's that? You think I have included Most Świętej Klary in this series solely so I can feature a photo of a nun on a bridge? Ok, I confess.

This appears to be a two-span timber truss bridge, although there are steel members visible below as well.

Further information:

• Google maps
• Wikipedia (Polish)
• Wratislaveae Amici

Wrocław's Bridges: 9. Słodowa Bridge

I'm including Most Słodowy simply for completeness, and because it is the only road crossing onto the Słodowa island.

There are a series of islands at this point in Wrocław, within the boundaries of the River Oder, and for most of the town's history, access to them has been limited. The construction of two footbridges (both of which I'll come to soon) in recent years has greatly improved accessibility.

The present structure is a temporary Bailey bridge, with a separate pedestrian walkway to one side. It spans only about 8m. From what I can determine, this must be relatively recent, as most of the information on the internet and linked below refers to a steel beam structure which is clearly no longer there.

Further information:

• Google maps
• Wikipedia (Polish)
• Wratislaveae Amici

Wrocław's Bridges: 8. Mill Bridges

The two Mosty Młyńskie originally spanned over the streams for two watermills, named Maria and Feliks. The latter mill has been demolished, but the former is still present, although disused.

As with Sand Bridge, there were bridges here for several centuries. The current steel parabolic arch trusses date from 1885, and are clearly a more developed design than the Sand Bridge. The general form of the truss is such as to follow the typical bending moment diagram for a simply-supported span, making a more efficient use of material than in a more uniform truss. The web members are also more open than in the lattice truss form which was previously prevalent.

The two bridges span 28m and 36m respectively, and are of slightly different heights. The slightly flattened top to the trusses is characteristic of the so-called Schwedler Truss. I presume it was done this way to cater for the variability in live load positioning, responding to the envelope of load effects rather than simply uniform loading over the entire span.

The northern of the two spans is currently being refurbished, and the bridge is closed to its normal roadway and tramway traffic.

In their current form, I am not greatly enamoured of the bridges. They are very honest structures, but as Schwedler himself is reported to have believed, the flattened arch form is not the most attractive.

Further information:

• Google maps
• Structurae
• Wikipedia (Polish)
• The best known bridges in Wrocław
• Wratislaveae Amici (includes historic photos of the bridges and their timber predecessor, and images taken during the present and previous refurbishments)
• Wrocław Mosty - Bridges - Brücke, Wiesław Jurewicz (undated book, ISBN 978-83-926759-5-2)