Introduction: 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!

Step 1: Design + Examples

 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)




Step 2: Stuff Needed to Build It and Make It Happen

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


Step 3: Make the Pylons

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.

Step 4: Making the Deck(s)

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.  

Step 5: String

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.  

Step 6: Stringing the Deck(s)

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. 

Step 7: Bringing It Together

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. 


Step 8: Finale

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.  

Comments

author
KerryG16 made it!(author)2016-04-08

we made a similar one for my daughter's project following your pictures.It's amazing how much measuring goes into this seemingly basic project. New respect for civil engineers, cannot imagine building one of this in large scale.

author
viraj+karande made it!(author)2015-09-24

I want to make model by using news paper which can sustain to specific load...could you please help me with this

author
rehan90 made it!(author)2015-09-15

It's nice but it can't bear load
I want to build a bridge. Which can bear Max load

author
Anu+Susan made it!(author)2014-09-23

Awesome Work!!!

author
flinstifer made it!(author)2013-12-18

I found this really helpful

author
jamesabt007 made it!(author)2014-01-09

Thank you!

author
PKM made it!(author)2013-10-19

I like the look of your pylons, this is certainly a neat way to build a model bridge. Two engineering facts:

1) On this kind of bridge, the suspender cables are meant to be vertical. This means they are pulling straight up on the deck so there is no sideways tension in the deck. You might have to tie the loops to the main cables to make them stay in the right place.

2) If you hold a piece of string from both ends and let it hang downwards with nothing attached to it, it will create the shape that the finished bridge's cable should have- you can then copy this shape to a piece of paper or cardboard and use it to figure out how long the suspender cables should be. This shape is also the shape of an ideal arch bridge upside down- I used this technique to help a friend build a brick arch for a pizza oven :)

Technically it isn't a parabola, it's a catenary, but they look very similar and a parabola is a good approximation to a catenary at this scale.

author
jamesabt007 made it!(author)2014-01-09

Thank you for your guidance, I edited it to make it more accurate but kept the parabola reference for those who might not know what catenary is. That is really awesome how you were able to use this type of math to help your friend with building the arch. :)

author
flinstifer made it!(author)2013-12-18

maybe I can look up what the heck a parabola and a catenary is Master of science *smirk*

author
artworker made it!(author)2013-10-18

Nice engineering thing you got here! Awesome project!

author
jamesabt007 made it!(author)2013-10-18

Thank you, I am a fan of your work!

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Bio: In my free time, I enjoy building/modifying/dismantling anything I can get my hands on. I am currently an engineering student at UCLA and ... More »
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