Design a Creek Bridge

I'm Thyme Tolla, a Livermore High School student in Livermore, California. I am going to be in my sophomore year when school starts again in August. This instructable is made for the Make It Bridge Student Design Contest.

In this instructable, I design a pedestrian bridge that crosses a creek near my house. I will explain how I did it and I'll help you (the reader) through the process of designing a bridge for situations like mine.

How is the bridge helping?

The pedestrian bridge will help many residents of spring town (a subsection of Livermore) by bridging a separated neighborhood to the rest of the spring town area. The bridge will be located in Altamont Creek Park, crossing Altamont Creek.

Currently, if you live on the far side of the creek and you need to get to the nearby Elementary schools or Middle school, you have to walk all the way up and back down the creek. This is almost a mile of extra walking for students who just need to get to school every day. For this reason, many parents simply drive their students instead. Thus, creating a bridge over this Altamont Creek will allow students to quickly get to school. Furthermore, the bridge allows all residents on the far side of the creek to easily access the park. The bridge even helps the environment by stopping the daily emissions of cars taking a roundabout route to the schools across the creek.

For the residents on the nearside of the creek, the bridge will help connect them to nature as the Altamont Hills (a popular hiking/walking destination) are just on the other side of the creek. Plus, people on both sides of the creek can now easily visit their friends via a short walk.

You only need 4 things for this project:

• Fusion 360 license
• Computer to run Fusion 360
• Creek
• Tape measure

Find a creek or waterway in your community that needs a pedestrian bridge to connect two separate areas. I chose this stream, Altamont Creek, for the reasons outlined in my introduction.

Measure the length of the creek with your tape measure. Mine is about 15ft wide, with one side being 2ft higher than the other. Take reference pictures of the creek to look at later.

Note: In the image, the creek is very dry and small, but during Winter months the creek will fill up to where you couldn't simply jump across the low areas.

Google some images of pedestrian bridges over waterways, creeks, rivers, etc. Find ones that inspire you. Here are a few that I found interesting and well-designed. The last image, the Olentangy River bridge, was my main inspiration.

After looking at these images, I decided my bridge would need these elements:

• Wood design features
• Light colors and earth tones
• No over-complicated design elements
• Lighting for the night
• Archs for support
• Modern look, but rustic feel
• An overall welcoming aesthetic

Model the creek that your bridge is going to be built over. Use your measurements from step one. The design of this doesn't need to be elegant, you are just using it as a reference for designing the rest of the creek.

I modeled a simple area shown here. I just made the two edges of the creek with some flat water in between.

Create two arches to support your bridge. They could be overhanging like mine, on the sides of the bridge, or below the walkway.

Inspired by the Olentangy River bridge, I made two wooden crossing arches that would help to suspend the base structure of the bridge. The fixtures that attach the wood to the concrete stand-offs are made of plain carbon steel, the industry standard, and would be connected via internal bolts sealed in the concrete. The joint between the two arches is also made of the same plain carbon steel. The wood itself is treated redwood, as its better suited for outdoor environments.

Design a walkway support structure for your bridge. Make sure your design is robust and sturdy, as this is what will support people. You'll need a method to evenly distribute the weight.

I created a diamond-shaped pipe pattern using thin plain carbon steel pipes. The diamond pattern will help evenly distribute the load when the bridge is being used. The pipes are edged by curved concrete segments with lips for boards to sit on. The concrete segments have a simple design as they are meant to be precast in a mold and enforced with rebar. Rather than having one large piece the concrete is split into many smaller segments to account for thermal expansion and earthquake resilience. I calculated the optimal spacing between the pipes and concrete to space dimension this. If an earthquake occurs, they can move independently of each other instead of cracking the concrete. Smaller segments also make the bridge easier to construct, as they can be pushed onto the pipes as the pipe pattern is welded together.

Model the main walkway of the bridge. Add a form of railings, but be creative with it, don't just copy mine. Make sure you account for how rain will fall on your bridge, don't create a walkway surface where it will pool. Also, account for how you will ensure your bridge is sturdy enough to hold people on the walkway surface.

I used wood-treated oak wood boards for the surface of the walkway, attached via dome-head concrete bolts. They have spaces in between so that the rain can drain. At each end of the bridge, I created a curved metal plate that's buried into the ground so the surface leading onto the walkway stays smooth. For the railings, I went with a simple design. I modified the concrete sidings to fit metal posts that will attach to long, treated redwood beams. I'm planning to add lights under these in a future step.

Use rope wire to strengthen the design of your bridge (In construction terminology, those metal cables are called "rope wire"). This usually means attaching wire between the walkway and structural elements like supports and arches.

I attached 8 lines of Warrington braided wire rope from my overhanging arches to the sides of the bridge. These lines will distribute the weight of the bridge across the two arches, pulling them down. Arches are extremely strong, so this setup allows the bridge to be incredibly resilient to downward force. To attach the cables, I designed two adjustable cable mounts: one for the wood arches, and one for the concrete bridge. The one for the arches mounts into the slot in the wood framing. The other one is bolted directly into the concrete siding.

Add some lights to your bridge for the dark! Make sure the walkway is well-illuminated. Then make your creek look a little nicer. Nothing fancy is needed, but you can make it look a bit better if you want.

I added a lot of simple lights under the railings so people know where to walk if it's late. I then added 4 brighter lights to the underside of the arches. I also redid the creek and used some real water textures.

Get some great images of your completed design in the render workspace. Go for some at night and some during the day. Try to get shots that show off the detail of the bridge and its full scale at the same time. This step is especially important if you ever want to show off your project, especially if you were to actually make the bridge.

These were the best renders that I got. I made sure to capture some with detail and some of the entire bridge.

Congrats! You made it to the end! I hope you learned something through the bridge-designing process I walked you through.

I learned a ton about modeling with Fusion 360 in this project. I'd never used the render workspace before and I've never designed something large and structural like a bridge. I am planning on sending this design to the city council of Livermore to convince them to build a bridge over Altamont Creek, even if it's not mine, because the community really needs it. Hopefully, they accept it and I can aid in the process of building a new bridge over the creek.

Enjoy your bridge design! Good luck on the contest everyone!