Design a Pedestrian Bridge With Fusion 360

I'm Ronan Tolla, a 17-year-old student and Livermore High School in Livermore, California. I will be a senior for the 2023-2024 school year. This instructable is intended for the Make It Bridge student design challenge.

In this project, I use Fusion 360 to design a pedestrian bridge that crosses a gap between an isolated area of my town and its center. This project will walk you through the process I used to create my bridge design and teach you how to do it yourself. I will show you everything you need to know about designing a bridge that brings people together, connects humans with nature, creates robust social infrastructure, and brings accessibility and belonging to your town.

For this project, you'll need the following things:

• Fusion 360 license and a computer to run the software (students get a free education license!)
• 3D Printer
• Assortment of paintbrushes
• Assortment of spray paints and acrylic paints (the colors are up to you)
• Super glue and epoxy
• A plywood, foam, or plastic sheet

When you are choosing a location, look for a location in your community where there is a gap between two places that would benefit from connection. It could be as small as a creek or as big as a highway. Go to the location and snap pictures, take measurements, and think critically about how a bridge could fit into this location.

In my town, Livermore, the interstate highway runs between the majority of the town and an unfortunately separated area called Spring Town (not actually its own town). Spring Town is connected to the rest of Livermore by two busy bridges that cross the interstate, but these bridges are intended only for cars - they don't have sidewalks so there is no way you can walk to the rest of the town. It is impossible to get to a friend's house, walk downtown, or get groceries without using a car. Thus, I knew this would be the perfect location to add a pedestrian bridge to connect spring town to the rest of Livermore.

I went to the location and took reference pictures for my bridge design and decided on the optimal spot for the bridge to be positioned. I then used google earth to take some approximate measurements of the area for sizing the bridge. I determined I'd need a 256m straight section with a 70m diagonal section.

Google around and look at some interesting designs of pedestrian bridges. Think about how your bridge can stand out and be exciting but still fit into the surrounding area and blend with its surroundings. Consider the ways your bridge will be functional, incorporate nature, and successfully connect people. Make a list of all the design elements you want your bridge to incorporate.

I found these images of pedestrian bridge designs that resonated with me. Looking at these images allowed me to better visualize how my bridge would be laid out. I then made the following list of ideas I would incorporate into my design:

• Intriguing design, but not too bold and futuristic as the bridge will be in a semi-suburban area
• Must incorporate elements of nature into the design to connect people to the environment
• Mostly white so the bridge doesn't heat up
• Curves and arches for both style and structural integrity
• Wide enough for groups of people to walk comfortably in both directions
• Lights to illuminate the bridge at night
• Must be able to built from real materials
• Must use standardized, real-world sizes
• Reasonable material and construction cost
• No stairs or steps so the bridge can be used by people in wheelchairs and the elderly
• Must be safe and structurally sound

Layout:

• Import a satellite image of the area you are designing your bridge for as a canvas, use your measurements from step 1 to scale it to real-world dimensions.
• Sketch the general path of your bridge. Make it practical, but also interesting. Don't forget to leave space on either end of the bridge where lead on-ramps will go. I made my bridge slightly curved simply so it is more visually appealing.

Support Structure:

• Start by doing some research into bridge support structures. Many bridges use suspension, vertical foundation supports, and arches. I took inspiration from the Chiagco 41st pedestrian bridge, which uses metal piping braced with an internal triangular pattern.
• Create the main framework of the bridge first, then make the vertical supports to hold it in the air. My framework is a long set of welded metal pipes, braced together with a triangular pattern. I used concrete block supports, similar to those you'd see on any overpass. I made heavy use of the pipe and pattern tools here to save time. Also, make sure your bridge is the right height and width - if it's going over a road it should be at least 16ft high according to the Department of Transportation.
• Now model the lead on ramps on both edges of your bridge. I used a slightly different framework structure here, including the use of smaller piping connected to a central beam.
• Throughout this step, make sure to always check your angles. The angles will change how the load is distributed, how easy it is to walk up the lead on ramps, how sturdy your triangles are, etc. There are online calculators and references you can look up to check the range of angles you can use for different aspects of any bridge.

Walkway Surface Interface:

• To support the walkway surface that your bridge will have, you need a way to interface it with the support structure. This structure can vary depending on what your walkway surface is.
• I knew that I would be modeling precast concrete as my walkway surface, so I modeled my interface using steel beams and an interlayed thick wire mesh.

Walkway Surface:

• Model the surface for your pedestrians to walk on. I chose to use 30cm thick, rebar-enforced precast concrete as my surface so it was simple to model.
• You could use any number of common bridge walkway surfaces like specialty tiling, metal grating, or wood beams.

Main Feature:

• Initially, the current design of the bridge would be a bit bland if I just made a simple walkway over the support source. Instead, I created too large circular areas in the bridge, both above bridge locations where the bridge travels over grass. Here, I carved holes in the bridge where large trees can be planted that will grow up through the bridge. This helps bring people closer to nature.
• In your design, I recommended adding some more interesting main features to the shape of your bridge like my trees. Smaller trees and plants in any capacity are always good design elements to incorporate.
• Don't forget to support your new areas of the bridge properly. I modeled more large metal tubing with triangular lattices, held up by metal poles attached to the ground. I used the same precast concrete and wire mesh for the walkway surface that's used on the rest of the bridge. I also created a large concrete cylinder that the trees will be planted in, with plenty of space for their roots to grow underground.

Railings / Cage:

• You need to create a structure on top of your bridge to of course stop people from falling off the bridge. Depending on the style and location of your bridge, you might want to consider a cage or a railing. In my design, I used both railings and cages. I created a cage structure using arches and wire rope (A type, right regular lay, single layer) for the parts of my bridge that go over the freeway. The arches are made of 8-inch C-Channel steel, welded directly to the bridge support structure. I used railings for the parts of the bridge that go over grassy areas, with the posts being 8-inch metal tubes attached together with the same wire rope used for the cage.
• When modeling these structures, safety is a top priority. You need to have designs that will keep your pedestrians safe.
• Aesthetics matter here too. When modeling your bridge, always think about how someone (someone who hasn't been staring at your CAD model for hours) will think about the shape, colors, patterns, and forms on your bridge. I put spaces for lights in all the railing posts and paid attention to the shape of the cage arches. The cages looked very plain to me, so I created many organically shaped holes in the cages that would be outlined in thin-walled metal tubes. The tubes would be welded to the cage arches, and the cage cables would attach to the inside of these pipes.

Now it's time to bring your bridge to life!

Central Areas:

• Model all the decorations and ornamentation that will draw attention and detail to the main features of your bridge.
• For the two central areas of my bridge, the areas with the large trees, I added lots of detail as I wanted people to be attracted to a peaceful and comfortable space to sit, eat, and enjoy. First, I created circular wood benches that sit around the base of the trees, with painted blue highlights. Then I created beautiful oak wood patterns that surround the trees. I used many 1/4-inch weather-rated oak boards, cut into trapezoids, patterned around the tree, and bolted into the precast concrete.
• Next, I added 8-inch diameter steel tube light posts, featuring 5000-lumen LEDs (that's around the brightness of a standard residential streetlight) to illuminate the whole area at night. I also put less powerful LEDS inside the railing posts to make sure the edge of the bridge is clear to any pedestrians, and of course, because it will look incredible at night. I added many plant pots for small trees to rim the railings of these central areas. In between the plant pots are sets of concrete benches. These benches can be easily created via a simple concrete mold and rebar.
• When you model your central areas, keep practicality in mind. For example, if your bridge is small and just goes over a creek, it probably won't need large lamp posts, just some smaller LEDs embedded in the railings.

The Rest of the Bridge:

• For me, this step refers to the caged areas around the rest of the bridge. For you, it could be a different area. Here, you want to keep the design of your bridge coherent. Make sure your auxiliary details on all parts of your bridge follow the style you created for the central areas.
• For light, I added long, white LED string strips to two of the upper cables on the cage structure. This will look fantastic at night and provides great, clear illumination for anyone walking along the bridge.
• In your bridge, you'll need a method to connect your bridge users with nature in these areas too. I modeled a series of concrete planter boxes along the edges of the cage walls. The planter boxes will all be open to public use, just like how many communities have public gardens where you can have an assigned lot to grow your own plants. Thus, the bridge also features a complete water system with drip watering that connects to all of these planter boxes, each with an individual flow and timing control system. If I built the bridge in real life, I would contact the local nonprofit Fertile Groundworks to see if they would be interested in managing these garden beds (They grow food for the homeless and impoverished, so these garden beds would be a great way to get more public involvement and produce more food.)

Color:

• This step is simple, just use the appearance tool to set the colors of all the parts of your bridge. I made my bridge mostly white to prevent the metal from becoming too hot to touch, as Livermore gets scorching hot in the summer (in fact it's 102F while I'm writing this).

Here's a simple step! Use the render workspace to create some great images of your beautiful pedestrian bridge. If you ever want to show your bridge to someone who could create it, perhaps propose the design to your city council, this step is important as good images of your bridge will help convince them that they should build it. Try to get your renders to show multiple intriguing angles of your design, be artistic with it.

Splitting the Model:

• If your bridge is as big as mine, you can't print the whole thing at once. Choose how small you want to scale your bridge compared to the original, then you'll have to split it into pieces to fit in the 3D printer. This is as simple as extruding a few rectangles with the width of your printer tolerance (probably around 0.4mm) through your bridge. Be smart with the placement of these rectangle splits, you want to cut through the least amount of features.
• If you have any fine details that would be difficult to print, you can split them in the same manner and print them separately. The same thing goes for parts that need to be different colors from the main structure.

3D Printing Settings:

• This can depend on how your bridge is actually designed, but the settings I used should be good for most. Because the bridge doesn't need to be structurally sound as a display prototype, you can use 2 perimeters with 15% lighting infill. Lighting infill will print much quicker and save your filament. Also, enable a brim on this print. For all your other settings, just use whatever you normally do to get good prints.
• Print the models with PLA, in whatever color your bridge is primarily. My bridge is mostly white, so I used white PLA.

Print Cleaning:

• Cut off the brim from your print and remove all the supports. Use an exact knife to cut away any other imperfections.

Disclaimer: Unfortunately, I only have these images of the assembly process, not the final product, as my 4-year-old brother knocked the model off the table when I was 70% finished. Don't make the same mistake I did and leave your model within reach of young children.

Assembly:

• Arrange all the parts you printed on a table. You'll need a base to glue them all too. I used a thin piece of plywood that I spray-painted blue.
• Begin by using epoxy to glue the center segments to the base and to each other, then work out from there. Once all of your base pieces are finished, use superglue to attach any small detail parts to your model, however, if the details are different colors it is better to glue them after you paint them.

Painting:

• First, if you have any pieces that are solid colors different from the base structure color, spray paint them. Then do detailed painting on the small features. Make sure you do enough coats so you can't see the filament color.
• You can now glue any remaining pieces onto the main bridge structure.
• Do all the necessary detailing and painting on all of the bridge pieces. For me, this meant painting lights, garden beds, and the bottom of the metal pole supports.

This project taught me a lot about the process of large, structural design. I learned new elements of Fusion I normally leave untouched. I learned about using industry standards, common construction practices, and standardized parts. I also go much better at design workflow in general.

Through the design process, I ended up with a fantastic bridge model that would connect two parts of town, promote a relationship between people and nature, strengthen civic life, and increase general accessibility. Plus, the bridge looks awesome.

I hope you learned something from this project too. Enjoy your bridge!