Simple Suspension Bridge Model

While cleaning out my storage, I found a model of my old suspension bridge model from summer school many summers back.  I looked at its shoddy engineering and said to myself, I think I could make this better now that I am more experienced.  This is my journey to make a new and improved version of my old suspension bridge model from scratch using the same materials.

For thousands of years, man has had to travel across vast chasms containing either dense forests, rocky terrain, or roaring rapids.  The most primitive bridges were designed and built to span these depths to make the lives of humans drier, safer, and more efficient.  The specific type of bridge I chose to base my model off of, the suspension bridge, has been around for a few hundreds of years. The most primitive being the ones designed and built by Tibetan, Thangtong Gyalpo, in 1433 (read more about the roots at this link).

There are many types of other bridges, ranging from the simple beam bridge consisting of essentially a plank to the complex truss and arch bridges.  All these use beams placed at strategic angles and combinations.  The suspension bridge however, relies on a combination of steel cables that run along the length of the bridge and steel suspenders that connect the bridge deck to the cables above.  This whole system is held up with towers that have foundations anchored deep into the ground, often to bedrock.

This guide details the highlights of my process in making my very own suspension bridge out of simple, cheap materials.  Hope you enjoy it!

 The properties of a suspension bridge are pretty straightforward.  The decking is held up with the suspenders that are in constant tension (steel cable performs exceptionally well with tension and not compression like concrete) attached to the cables that are also in a fixed state of tension.  These two are held up by the towers (also known as the pylons) that are typically made of a concrete/ stone combination that fairs well in compression or steel structures that contain cross bracing to add stability and rigidity. 

One of the major benefits of suspension bridges include the flexibility of these structures, making them ideal for earthquake-prone locations.  However, this flexibility comes at a disadvantage in that it might vibrate more violently in strong winds.  This downside is most characterized by the Tacomas Narrow Bridge which collapsed in 1940.  In this example, the wind was able to oscillate the suspension bridge at its natural frequency, leading to oscillations of higher and higher amplitude (a property known as mechanical resonance [read more about it here]) eventually leading to the collapse of the bridge.  Here is a video of the bridge and its collapse: http://youtu.be/xox9BVSu7Ok.

By mentioning the Tacomas Narrow Bridge collapse, I do not mean to scare you to believe that suspension bridges are unsafe.  I just want to bring to attention that this is a property of these types of bridges.  Know that many bridges have been either retrofitted with stiffeners or designed in wind tunnels to make sure that the bridge's natural frequency is far from the known wind speeds that would occur in the area.  Therefore, these bridges are very safe.  

Suspension bridge diagram (here)

Suspension bridges are prevalent throughout the world, some famous examples include the following:
-Golden Gate Bridge Image (here)
-Akashi Bridge (here)
-Brooklyn Bridge (here)

To make the suspension bridge, I used the following materials and tools:

~Materials
-Cardboard
-Popsicle Sticks (24 for a bridge with 1 deck, 36 for 2 decks, etc)
-String
-Scotch Tape

~Tools
-Hobby Knife or Box cutter
-Scissor
-Ruler
-Pen/Pencil
-Sand Paper
-Elmer's Glue

For each pylon, you will need 12 Popsicle sticks.

Make two I beams, two L beams.

And then combine them with each other and two separate Popsicle sticks.

To make the deck for the bridge, start out with a relatively medium sized cardboard.  My piece was large enough to fit two decks.  Depending on how many decks you want your bridge to have, you can make more or less. 

Measure out the deck, mark the holes from the intersections to where you will punch holes for the suspension cables to go through, cut the piece.

Tip: if you are using a knife like I am to punch the holes, be sure to clean them by either making them larger than needed or by forcing the material aside such that there is a distinct hole with a pen or pencil.  This is to ensure that the string can easily be threaded through the holes and will make the process a lot easier to make the decks.  

Your lengths of string can vary depending on the distance from pylon to pylon, your total span of the bridge, and your height you want the decks to be above the ground.  These are some distances I used, for reference.  Explore with different lengths to see which bridge looks nicer or more realistic.  

This part of the process is placing the string you determined from the previous step into the holes of the deck.  

There are many ways to do this step.  Just remember that if you have 5 total suspenders on each side of the deck, you'll need to make the 4 holes on each corner of the deck have longer loops than the one in the middle since the cable droops slightly, like a catenary, similar to the curve of a parabola with a positive a coefficient (y=ax2+bx+c). 

Above is a look into how I started the process. 

These are a few images and close ups of my completed model.  Remember that this is a very open ended project with no real set guidelines on the lengths and distances just like suspension bridges in real life. 

There are many ways this bridge can be built and I am curious to seeing what different spins members of this community have to put on this build.  I hope you learned something new or interesting in my guide or at the very least, enjoyed it.

Thank you for viewing this Instructable and happy building!

Special thanks to my friend Jonathan, for reviewing my ible and giving great suggestions.