Australian Bridges: 8. Rex Creek Bridge, Mossman

For the last in this series of posts from "down under", I travelled to scenic Mossman Gorge, home of Rex Creek Bridge. Mossman Gorge is part of a World Heritage status rainforest zone, a traditional home to the Indigenous Kuku Yalanii people, and a very popular tourist destination.

The Gorge is accessed via a series of elevated walkways, intended to carefully control visitor access and impact. The trails cross Rex Creek via this 40m long suspension footbridge.

A bridge was first built here in 1986. Constructed by army engineers, this was a catenary suspension bridge, with a timber deck hung from the main cables via a series of steel u-frames.

In 2010, inspections identified defects in the bridge, and a decision was taken to replace it with a new bridge, at a cost of $450,000. The present bridge is a more conventional suspension bridge, although certainly with a number of oddities.

The new bridge also had its own issues with defects, and was closed from July to August 2012 when cracks were found in the parapet posts. Looking at the parapet design, it's not hard to see why they would be vulnerable, as the parapets are in short section lengths with gaps between them.

As well as rendering the parapets more vulnerable to damage, this has implications for the stiffness of the bridge, and unsurprisingly it is very vibration-prone. Depending on the visitor's perspective, this is either a bit of a thrill, or a serious difficulty. For my part, I think it's a significant design failing, making it difficult or unpleasant for some visitors to proceed further into the Mossman Gorge area. It would easily have been prevented or reduced by using the parapets as stiffening members.

The bridge deck is suspended via hangers from twin suspension cables on each side. The hangers pass through a connecting clamp,, which prevents sliding along the main cables. The hanger connection itself is just a simple threaded nut.

The main cables are connected to steel framed towers. A beefy crossbar at the top houses connections for the cables on either side, and is bolted onto a Y-frame portal.

It's an ugly arrangement, and I think part of a general philosophy amongst Australian bridge designers which favours robustness and pragmatism over beauty. It possibly derives from an American rather than European tradition, a hang-over from a frontier mentality.

However, this is a bridge which had to be assembled in a difficult location, and having selected steel for reasons of strength, the bridge needed to be transportable in small selections which could be lifted over the forest via helicopter.

The "industrial" detailing continues to all parts of the bridge. The anchorage at one end comprises a cross-braced frame, picking up the forces from the main cables and carrying them back into the ground via compression struts. I can only guess that the issue here is that the ground is falling steeply away from the bridge, and that anchoring the cables directly into the ground at their normal angle would have been impossible.

At the other end of the bridge, there's a more conventional anchorage arrangement, with box frames taking the force from the twin suspension cables into a single threaded rod anchored directly into the ground (I don't know whether there's a concrete anchorage or whether this is simply a rock anchor).

Overall, I like this bridge a lot less than Lloyd's Bridge (see my previous post). Lloyd's Bridge was somewhat ramshackle and wobble-prone, but it was locally funded and is not a major tourist site. The Rex Creek bridge is crossed by hundreds of visitors every day, and it would have added little or no cost to make it stiffer and more visually attractive.

It begs the question of the extent to which visual appeal is necessary in this setting - the bridge is not a destination in its own right, merely a means to an end. However, that's the same philosophy used to justify ugly bridges everywhere. I think the best that can be said of the bridge is that it is an authentic representative of Australian bridge culture.

This concludes my posts on Australian bridges, until a further visit. Next stop, Berlin, for the Footbridge 2017 conference later this week, which looks like being as stimulating an event as ever.

Further information:

• Google maps

Australian Bridges: 7. Lloyd's Bridge, Yungaburra

The last three bridges are all located in Australia's largest city, Sydney. The next two are from further north, in Queensland.

Head inland from the north Queensland coast to find the delightful historic town of Yungaburra. One of the attractions here is the walking track along Peterson's Creek, from where it's possible to see platypus and tree-kangaroos (although not when the Happy Pontist visited).

In 2008, local man Lloyd Abell provided funding for a new bridge to connect the walking trail across the Creek, a structure now and hopefully forever known as Lloyd's Bridge. There's a commemorative plaque on one of the bridge masts, and an explanatory sign nearby.

The suspension footbridge spans 30m over the creek. A sign makes clear that the maximum load is 10 persons - this is not a bridge suitable for the absurd 5 kPa of normal footbridge design, nor should it be.

The bridge comprises a deck of longitudinal planks supported on timber cross-bearers. These are suspended via 8mm hangers from the 16mm main cables. A cable in a rough plastic sheath is clamped to the hangers to form the handrail and to hold up mesh infill balustrades.

The main cables are anchored into turpentine poles forming the bridge towers, and then into concrete anchorage blocks.

Some people may wonder why I'm covering such a seemingly minor structure and non-notable structure here. Well, firstly, I just have a soft spot for small-span pedestrian suspension bridges. Secondly, I think they illustrate how much variety is possible even with a seemingly small niche. If you look at the minor details on this and similar bridges (how the main cables attach to the tower, or how the hangers attach to the main cables), you'll find every one is different, and I think there's endless fascination in seeing how others have solved the same problems in such a variety of different ways.

The bridge lacks stiffness and moves considerably under load, but the mesh balustrades effectively damp vibration. The bridge creaks very noticeably when used, the level of noise being proportional to the success of the damping, and therefore reassuring, to a structural engineer at least.

It's a pragmatic structure, blessed with a minimum of design, and well done.

  

  

Further information:

• Google maps
• Weekend Notes

Australian Bridges: 6. Parsley Bay Bridge, Sydney

This charming suspension footbridge is tucked away in a quiet eastern Sydney suburb, little known to most of the city's residents let alone anyone else.

According to a history published online by the local municipal council, plans for a bridge across the beautiful Parsley Bay were first discussed in 1906, with the structure in place by 1910.

The bridge was designed by local town clerk and engineer Edwin Sautelle, and cost the tidy sum of £500 to build. Reportedly, it was built to improve access to ferries via the nearby Point Seymour.

The bridge is predominantly a timber structure, with the towers, deck and parapet rails all timber. The other parapet elements are in metalwork.

The towers are A-frame in form, which provides good stability in the longitudinal direction, but the bridge is evidently less stable laterally. Tie-back cables have been installed on all four corners, presumably to reduce lateral sway.

The bridge appears to have changed very little over its lifetime, judging from old photographs, although an image in Pictorial History Eastern Suburbs indicates that lamps or ornaments once sat above the bridge pylons. The same image doesn't appear to show the tie-back cables, although these can be seen in a postcard from circa 1930.

The other main change over time appears to have been the steady growth of surrounding vegetation.

The bridge was repainted and repaired in September 2003, and a further refurbishment was completed by GPM Constructions within the last couple of years.

The bridge's main asset is its beautiful setting. The Parsley Bay reserve features a fine sandy beach with water protected by shark netting, as well as a small patch of rainforest towards its rear. Views from the beach and from the bridge are both very attractive.

It seems to be a popular spot for bridge-jumping, although signs on the bridge make clear this is prohibited.

Further information:

• Google maps
• Structurae
• Bridgemeister
• Weekend Notes
• History of Parsley Bay Reserve
• Australia for Everyone

Australian Bridges: 5. Napoleon Bridge, Sydney

There is plenty of development going on in Sydney, with a prominent site being the Barangaroo area, which sits along the harbour-side north-west of the central business development, and north-east of Darling Harbour.

The southern end of the site has already seen a number of new buildings completed, while the northern end is home to Barangaroo Reserve, a public park with fine views. Between the two, work continues, and will include the Wilkinson Eyre-designed Crown Sydney Hotel.

Barangaroo is separated from the city centre by a number of streets and particularly the elevated Western Distributor Highway. Finding a way through can be like negotiating a concrete maze.

The new Napoleon Bridge, designed by Wilkinson Eyre and Arup, provides one point of connectivity. It's a covered footbridge spanning the busy Sussex Street. It opened in late 2015, and received an ASI Steel Excellence Award in 2016.

The bridge takes its name from adjacent Napoleon Street, which in turn owes its name to the former presence here of Frenchman Francois Girard, who was at various times soldier, teacher, convict, baker, miller, merchant and farmer.

Napoleon Bridge connects at two different levels: at road level to the west and at an elevated level to the east. The level difference is addressed through two escalators and a staircase at the west end.

The span of the bridge over the highway is a slightly odd structure, comprising two steel edge girders connected by chevron-shaped crossbeams, which support a ribbed floor. The edge girders are shaped to resist a maximum bending in the middle, rising visibly above the floor level on either side.

The roof and walls which shelter the walkway are supported on a series of steel portal frames, arranged so that they are perpendicular to the upper surface of the edge girders, rather than vertically. The outline of the portals is crisp and clear.

The ends of the deck girders are quite shallow, and noticeably shallower than the western approach span, which houses the staircase and escalators. There's an awkward piece added to the girder steelwork to visually address the difference in depth of the two edge elements - I don't think it works very well.

The walkway roof continues horizontally above the staircase area, creating a yawning open-ended atrium. I think this is visually effective, a funnel-like portal which announces arrival into the Barangaroo area.

I like the way that it leans out, giving it a visual presence from side alleyways and sense of dynamism. This is a bridge for city-dwellers briskly on the move, not a bridge for flaneurs or ponderers.

One oddity to the roof structure is that it is not fully enclosed, being partially open on the north edge. I'm not sure why this is, but it begs the question as to why this bridge is covered at all.

The answer, I think, can only lie in the Australian obsession with vandalism risks, as most other footbridges above their highways seem to be "adorned" with massive tall anti-vandalism fences. These are normally a hugely disfiguring feature, so it's good to see the issue dealt with in a much more integrated manner here.

Napoleon Bridge sits in a very difficult site, hemmed in by tall buildings and surrounded by street clutter. The temptation for a designer in this setting is to opt entirely for modesty, to avoid adding further to a visually overwhelming environment. I think it's to this bridge's credit that it combines an appropriate degree of restraint with just about the right amount of excess presence.

There are details that can be picked at, such as the awkward structural junction above the support piers, or the disappointingly small extent to which the edge girders are visible above the floor seen from inside the bridge.

Overall, however, it's an appropriate and well-considered design.

Further information:

• Google maps
• Speirs and Major
• City Walk Bridge, Barangaroo, Sydney

Australian Bridges: 4. Pyrmont Bridge, Sydney

My next few posts will cover a handful of bridges in Australia.

The best known bridge in Sydney is clearly the Harbour Bridge, but it's not the only historic engineering landmark in the city.

The Pyrmont Bridge is designated a National Engineering Landmark by the Institution of Engineers of Australia. It is a bridge rich in history and technical interest, and I can only scratch the surface here. I'd particularly recommend the original paper on the bridge in the ICE Proceedings (1907) and a series of articles in The Engineer (1917), the latter of which include excruciating detail in the form of description, drawings and photographs. There is an excellent history of the bridge in the document proposing the bridge as an engineering landmark, all links can be found at the end of this blog post.

A timber toll bridge crossed the Darling Harbour, dating from 1858, operating at a lower level than the present structure and incorporating a swing span. The replacement bridge was completed in 1902 to a design by Percy Allan, and was built at a higher level. Allan designed the bridge for the local public works department after a design competition had been held and all 41 entries had been rejected.

It was built as a highway bridge, being closed to highway traffic in 1981 following the construction of new highway viaducts nearby. The bridge was partially refurbished in the 1980s, reopening as a pedestrian boulevard in 1988. The bridge carried the Sydney Monorail from 1988 to 2013. The paper by Trueman linked below describes the refurbishment work.

The bridge is generous in proportion, originally accommodating a 40-foot wide carriageway and two 7-foot wide footways. The two central spans comprise cantilevering steel truss girders supported via an array of roller bearings on a central caisson pier. The steel was supplied from Belgium, with Australia lacking any significant steel industry at the time. However, all the approach spans are built as a variant on Howe trusses using Australian ironbark timber, primarily to save on construction cost, with the total price being roughly half what an all-steel bridge would have cost.

When built, the bridge was immediately recognised for its engineering significance. The opening spans were larger in area than the vast majority of moveable bridges built previously, and it was thought to be a pioneer in the use of electric power for its operation. The bridge is operated from a single control cabin at the centre of the opening section, with power supplied by cables running below the harbour bed. The cabin appears largely unaltered, although it was relocated to the edge of the bridge when the monorail was added.

The main perception at deck level is of generosity of space. It's unlikely a pedestrian promenade this wide would ever be built as new, it's purely a legacy of the bridge's historic use as a road bridge. Now there is space for banners, for people to admire the views of the harbour to either side, and for walkers, cyclists and roller-skaters to coexist reasonably happily.

Seen from below, the bridge appears in good condition, although I understand it is subject to an ongoing maintenance programme (BIM-empowered). When it was converted for pedestrian use, an extensive study into its defects and repair requirements was made. The engineers proposed the use of preservative treatment for the timber, but this was not immediately undertaken due to cost. The bridge was reassessed and only those repairs strictly necessary for safety reasons were completed: other damaged timber was left to continue to deteriorate. During a subsequent ten-year maintenance programme, timber preservatives were installed, both diffused into the timber core and applied as a barrier to the timber surface.

The timber trusses appear to have been overpainted at some stage, a largely cosmetic treatment for such a structure, I would think. The various connections are clearly visible. I believe some of these were altered during the 1980s restoration in order to locate connecting bolts in less damaged parts of the timber.

The west end of the bridge appears largely unaltered, with impressive stone approach structure. The east end has been more significantly altered, presumably because the bridge was cut short when the Western Distributor Highway was built. There are now escalators down to quayside level, and a cable-stayed footbridge provides a higher-level extension for those wishing to continue over the highway.

Overall, it's great to see that a bridge which was originally built as a cut-price alternative to other designs has survived so well, and I don't think engineers at the time of construction would have expected it to last so well. It has survived a variety of major changes in use, and its continued value as a key link in the Darling Harbour surrounds should hopefully ensure it survives for a long time to come.

Further information:

• Google maps
• Wikipedia
• Structurae
• NSW State Heritage Inventory
• Pyrmont Bridge, Darling Harbour
• Sydney Architecture (includes several historic and construction photos)
• The New Pyrmont Bridge (Sydney Morning Herald, 1902)
• The Pyrmont Bridge, Sydney, New South Wales, Proceedings of the Institution of Civil Engineers (Allan, 1907) (and Discussion)
• Pyrmont Electric Swing Bridge (The Engineer, 27 Jan 1917, 2 Feb 1917, 9 Feb 1917, 16 Feb 1917)
• Spanning Two Centuries: Historic Bridges of Australia (O'Connor, 1985)
• Pyrmont Bridge - Construction and Restoration (Trueman, National Conference on Engineering Heritage, 1988)
• Proposal to Landmark the Pyrmont Bridge, Darling Harbour, as a National Engineering Landmark (Institution of Engineers of Australia, 1991, this includes copies of several other key papers regarding the bridge)
• Management of an engineering landmark bridge: Pyrmont bridge, Darling Harbour, Sydney (Higgins, Bridge Management 2, 1993)
• The cabin man on Pyrmont Bridge (Sydney Morning Herald, 2014)
• Pyrmont Bridge (Kishan, ICE Australia, 2014, essay as part of the Bridge Ahoy! tour)