01 September 2017

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.

  

  



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1 comment:

stone cutting said...

Your comments here chime with my feelings about similar bridges in New Zealand and actually, around UK if you look. A bridge that does the job for minimum cost. Perfectly fit for purpose for many years at least. As you imply, the opportunity for "design" is almost unlimited. Start with design for min material cost, min erection cost, min labour.

Hanger cable connections are always fascinating. Combine friction and support requirements or keep separate. Make a flexible joint or specials. Worry about centering the loads, or ignore the problem or even work out the moment and show it is OK.

These also make very good first design projects for students as the statics is first year stuff and can actually be taught sufficiently in a one hour intro to the project.