Two pronged Cable-stayed Bridge

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Two pronged Cable-stayed Bridge

As I've gone through my civil engineering degree, I've learnt more and more about how bridges are designed. At the end of 2016, my third year of uni, I was itching to apply what I'd learnt and build a model of a bridge out of knex. This is a cable-stayed bridge with a length of 2.8 metres (straight distance, not including curves). Two angled beams (called prongs) are the main support structures besides the abutments. the ends of the bridge are attached to abutments on a knex 'cliff face'

A cable-stayed bridge is a type of suspension bridge where the bridge deck is supported directly from a vertical beam at a mid- or interval span. Cables are the tension members and are balanced on each side of the beam and connect to the bridge deck. This means that these cables are pulled straight in tension unlike most suspension bridges where there is a sag curve in the main cable. Notable examples of a cable-stayed bridge are the Anzac Bridge in Sydney, Australia and the Milau Viaduct in France.

I particularly wanted to play with balance here which is why I made the bridge deck curved and aimed to have angled vertical beams. 

At the time, all my regular knex was packed away, hence my decision to build it entirely out of micro knex. I think in the end this ended up being beneficial as it was a lot lighter and I was also able to use the shorter rod lengths to my advantage. The difficulty arose, however, in that the structure was more flexible. This made it trickier to keep the bridge deck flat but ended up working pretty well in the end.