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
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
-Popsicle Sticks (24 for a bridge with 1 deck, 36 for 2 decks, etc)
-Hobby Knife or Box cutter
Step 3: Make the Pylons
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)
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
Step 6: Stringing the Deck(s)
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
Step 8: Finale
Thank you for viewing this Instructable and happy building!
Special thanks to my friend Jonathan, for reviewing my ible and giving great suggestions.