Construct a straw bridge - middle years
One of the projects is to work as part of a team to construct a single-span straw bridge using plastic drinking straws. The bridge is to span a distance of 40cm, with no supporting pillars to the ground in between the ends of the span and be approximately 10cm wide. The bridge also needs to be strong enough to support a load, like a book, without collapsing.
To begin, here is some interesting information about bridges.
We take bridges very much for granted in our daily lives. We regularly cross bridges of various types and lengths without even thinking about it. But imagine how much harder travel would have been for the early explorers and settlers of Australia without bridges, particularly when they needed to cross larger streams and rivers. For example, it is documented that the explorers Hume and Hovell, travelled through New South Wales and Victoria towards South Australia in 1824, improvised with tarpaulins wrapped first around a dray, and later around a structure made of poles, in order to ferry their party and its supplies across the Murrumbidgee and then the Murray River. They were severely hampered by the lack of bridges!
The earliest history of bridges is not recorded, but it is likely that they would have been fallen trees across streams, across which people could walk. Later would have come simple ropes (perhaps woven from vines) that would have allowed people to climb across, and from this would have come more complex bridges made up of many ropes fixed together, eventually with a decking across which people could walk.
However rope bridges are not long-lasting, and not suitable for heavier vehicular traffic. So the next technological development in bridge design occurred in the form of stone arch bridges. These date from Roman times, some two thousand years ago. For their structural strength they depend on the shape of the arch. In a simple arch bridge, the weight of the bridge (and the load crossing it) is transmitted through the arch to the walls (the banks of the stream or river), so the arch does not collapse. In order to cross a wider span, numerous arches could be linked together, with the ends of each arch supported by a post or pillar. Many very large and long bridges and viaducts were constructed in this way in Roman times, and some even survive to this day.
Stone arch bridges continued to be built for many centuries. There are some classic examples of these still surviving from early Australian convict times, particularly in Tasmania. However the emergence of structural steel as a building material during the industrial age allowed the design and construction of bridges on a scale never before contemplated. Some of these involved massive engineering projects. The early citizens of Sydney and San Francisco could never have imagined that their harbours would be spanned by huge steel bridges. Even some eighty years after its construction, the Sydney Harbour Bridge remains a great Australian icon and engineering achievement.
There are numerous variations in the design of such large bridges, including cable suspension bridges, steel arch bridges, cantilever bridges, and truss bridges. These different designs will not be discussed further here as they are part of the research activity that follows.
If you would like to construct a straw bridge as an EngQuest project, why not ask your teacher?