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I am working on a suspension bridge with my sons and have encountered two problems.

Our technique:

  • 8x6 plates for the roadway
  • 1x8 bricks (with side-holes) underneath each side of the roadway to support the roadway, connect the plates, and link the Technic beams to the suspension rope.
  • Technic beams of varying lengths (2- to 9-studs) which hang from the suspension rope.
  • Twine, as the suspension rope, to connect the technic beams to the bases at either end of the bridge.

Our Dimensions:

  • 6-studs wide
  • 69 studs long.

Our problems:

  1. The twine works, but is ugly and doesn't seem like proper Lego technique - is there a more appropriate (i.e., LEGO-approved) material?
  2. The Technic beams all lean as if pointing towards the center of a sphere; I added a second 1x2 brick (with hole in the side) which allowed me to pin the Technic beam in place but that seems bulky and is very inelegant. How do I construct a non-leaning, but more elegant solution?
  3. I suppose that it would also be helpful to know whether there is a ratio for tower-height-to length that is most appropriate.
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Do you have any particular dimensions in mind? –  Kramii Nov 16 '13 at 19:27
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Added dimensions to my request. –  SammyB Nov 19 '13 at 0:47
    
LEGO do supply a black thread in a number of their sets - usually for cranes/lifts. –  Zhaph - Ben Duguid Nov 20 '13 at 11:28
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2 Answers

I've used white nylon rope / string for my Lego bridges. When I think of twine I think of the tannish, brownish, rough, fraying type of string. The nylon stuff is cleaner and in my humble opinion appears sharper. A Lego purist would not allow any non-Lego element in a creation, but when it comes to suspension bridges, there are very few options. I've viewed the use of plates (typically 1 X 6 , 8 , 10, etc.) for the main cables and suspenders, but this takes away from the cable feel and appears more like an eyebar suspension system - which were used before the invention of wire cables, but no major suspension bridge built in the past 140 years uses eyebars.

As for your issue with the leaning Technic beams... I'm unable to visualize the problem. Could the Technic beams be leaning due to the pull of the twine? That is if the twine is not going straight vertically to the maincables above then there may be a pull on the Technic beams away from true with that of the plates.

A good tower to span ratio seems to be 1:10 and that is tower above the roadway as what is below the roadway is irrelevant, with ship clearance the contributing factor there. Example... the Akashi Kaikyo Bridge in Japan has towers that are ~975 feet high. The roadway at mid-span is ~320 feet above the water. That makes for the tower above deck of ~655 feet. The span of the AKB is 6,532 feet. This is close to the 1:10 ratio. The Golden Gate is a bit different with towers ~750 feet high and a deck height of ~220 feet. That makes 530 feet of tower above deck and the span is 4,200 feet. Now we're down to 1:8. The difference is the steel in the Akashi Kaikyo Bridge's cable is much stronger due to advances in the 50+ years since the Golden Gate and thus shorter towers could then be built. The shorter the towers the more tension on the cables. Had the steel in the main cables stayed the same strength, then the towers for the AKB would then had to have been at least 150 feet higher.

Here is a link to photos of two lego bridges I've built: http://www.flickr.com/photos/suspensionstayed/sets/72157610808577323/

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A team in Perth (Australia) built a pure Lego suspension bridge that is quite long. All the links I can find are to youtube videos which is not very useful (here and here, for example).

This media story has the photo I've copied below

enter image description here

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