Pedestrian Suspension Bridge Model

My name is Evan, I'm a 17 year old senior from Aurora, CO. There are thousands of hiking trails in Colorado, but there's also an abundance of small streams and rivers that crisscross those trails, and oftentimes the only way to cross these streams are rocks or flimsy wooden planks laid across the gaps. These methods, while simple, raise the risk of injury and limit the accessibility of these beautiful trails. My bridge design does not have a single location where it would be installed, but because of its modularity it could be used across many different environments and sizes of streams without any modifications to the design or installation process. Hopefully the installation of this bridge would foster a connection with nature by making these hiking trails safer and more accessible to the elderly and small children, among other groups.

Supplies:

23/32in Pine Ply

Pocket Hole Screws

3D Printer

ABS Filament

Spray Foam

Spackle

Acrylic Paint

Moss

White Glue

Sand

Steel Cable

Eye Bolts

Turnbuckles

Tools:

3D Printer

Table Saw

Circular Saw

Jigsaw

Pocket Hole Screw Jig

Tap & Die

All parts are designed in Fusion 360, and several of Fusion's specialty features were very helpful to create the organic feel that I was aiming for. Both the generative design and Automated Modelling features were very helpful. The design is intended to be 3D printed as a final production method on a large scale printer, and it was designed to accommodate the limits of additive manufacturing. It is designed to be printed on its side, which limits the amount of support material necessary and orients the layer lines parallel to the direction of bending under load, maximizing strength. The knurled pattern increases traction and helps prevent becoming a slippery surface in the rain. The rope bridge design and natural shapes are supposed to feel at home in a forest, and as such one of the goals of this project was to make installation as simple and noninvasive as possible, so as to not disrupt the natural environment. The four supporting posts will have to be poured in concrete, but the cables themselves will rely on large ground anchors, which can be set deeply in the ground without disrupting plant life.

Simulation:

Fusion's simulation feature helped me determine the ideal material for the bridge, and test how the materials would test under load at different temperatures. These simulation led me to choose polycarbonate as the material of choice, because of it's better heat resistance compared to ABS. While polycarb is hard to print, additives like carbon fiber can be used to reduce warping and slightly increase strength. As well as simple installation, another focus of this design is is simplicity in repair. Each track for the cable is bolted in place so that damaged bridge sections can be removed and new ones installed without removing the other sections or the cable. These bolts are threaded into captive nuts that are inserted during printing, another freedom that additive manufacturing affords. Originally I thought that adding rebar across the length of each bridge section would be necessary to stiffen it up, but the simulations showed only marginal improvements to the safety factor (from 6.188 to 6.752) and so the rebar was deemed unnecessary.

Disclaimer: Using hardware store materials for a diorama is not advisable, I would absolutely recommend buying products for model making. However, I didn't. The wooden base is made of pine ply, measuring 49.5" L X 15.5" W X 6" H. Ply is roughed out with a circular saw and trimmed with a table saw, and the slope is made with a jig saw. The whole thing is assembled with pocket hole screws. The four posts(made of 1/2in PEX pipe), are tapped and bolted in place through the bottom of the wood. Then Spray foam is applied to create the slope of the landscape. It is trimmed to shape with a box cutter and spackle is applied to smooth out the shape. Then the spackle is painted, and the sand and moss is glued on. The bridge is printed entirely out of ABS, which took me about a week of 24/7 operation and took about 2.5 kilos of filament. Next the PEX pipe is tapped and eye bolts are inserted and the turnbuckles are bolted on, then the cable is run through the bridge sections and attached to the turnbuckles, where it can be tensioned.

I'm happy with this project, it was my first time using F360 simulations and I thought they were super helpful. The spackle cracked a lot as it dried and more cracks appeared slowly in the weeks after it was applied, it's definitely the wrong material for the job. If I did it again I would use a modelling putty, and buying paints for modelling, but overall I'm happy with the end product.