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