A Self Supporting Connection

My name is William Gergen. I just graduated from Pewaukee High School and will be attending UW-Madison in the fall for Engineering Physics. I live in Pewaukee, Wisconsin which is a small lakeside village with a beach and Main St. with shopping and restaurants. Behind the shops, there is a river that flows from Pewaukee Lake called the Pewaukee River. On either side of the river, there is parkland and trails. The river quickly branches out into a small offshoot that divides my community's two green park areas. There is a covered bridge that was built in the past five years by our Pewaukee River Partnership. On the other side, there is a broken boardwalk that partially spans the gap but falls short. I chose to design my bridge to replace the current boardwalk to help create a more friendly green space for people, support local businesses, and provide a new connection for a walking path.

For this project, you will need if you are starting from scratch and would like to build the bridge:

• AutoDesk Inventor
• AutoDesk AutoCAD
• 2- 2”x8”x8’ Douglas Fir Lumber 
• 4- 2”x8”x12’ Douglas Fir Lumber
• 4- 2”x8”x17’ Douglas Fir Lumber
• 8- 2”x8”x87” Douglas Fir Lumber
• 77- 2”x6”x6’ Douglas Fir Lumber
• 12- 4”x4”x42” Douglas Fir Lumber
• 4- 2”x6”x75” Douglas Fir Lumber
• 2- 2”x6”x108” Douglas Fir Lumber
• 4- 2”x6”x79.5” Douglas Fir Lumber
• 4- 1”-8 X4.5” Grade 8 Hex Head Bolt
• 4- 1”-8 X5.5” Grade 8 Hex Head Bolt
• 2- 1”-8 X7” Grade 8 Hex Head Bolt
• 4- 1”-8 X8.5” Grade 8 Hex Head Bolt
• 14- 1”-8 Grade 8 Nylon Insert Lock Nut
• 28- 1” Notched Wood Washer
• 80- 1” ID 1.5L Spacer
• 1kg Red PLA
• 3D Printing Utility

Waukesha County, where Pewaukee is located, has an interactive map of the area for property lines that allows homeowners to measure and locate their land. I utilized this tool to get accurate measurements to work off of to start designing the bridge. The bridge location is within the hundred-year flood plain in Wisconsin which means that the bridge has to be above a certain height to be within accordance with the ordinance with the Wisconsin DNR to allow proper water flow within the area. This height restriction makes the majority of the bridge that goes over the river taller than 5 feet.

I started off with the idea of the bridge being self-sustaining and having a lighted pathway made with environmentally friendly materials. The bridge initially would generate power for the lights with a hydroelectric turbine as that would not need batteries since the water constantly flows. But unfortunately, due to the restrictions of the flood plain, this option was not feasible. The alternative I went with was solar panels along the railing to allow the bridge to charge battery banks to power lights for the bridge all night. 

I also wanted to keep architecturally with the idea of self-sustainability so I wanted to design a self-supporting bridge that would negate the need for concrete mid-supports. I found that Leonardo Da Vinci's self-sustaining bridge would be a perfect fit for this type of design. This design transfers all of the forces through the interconnected beams to the two base beams allowing for all of the weight to be maintained at a point.  The design arches are high enough above the water to allow to fit within the DNR’s regulations. 

For material, I wanted to go with pressure-treated wood from the Douglas Fir tree which grows in Wisconsin. This tree grows at a rapid rate and is very strong when the wood is at a young age allowing for the trees to be grown in mass to be a very sustainable construction material. The pressure-treated part of the wood allows for longevity with no maintenance. The wood should be able to hold up to the wear and tear of foot traffic.

To start designing, I hand drew a sketch of the bridge's overall design with some rough dimensions to help give me an idea of the scale of the bridge before beginning to model.

I designed this model in AutoDesk Inventor by starting off CADing each component of the bridge. I started with the supporting beams of the bridge and the hardware to interconnect them. I added the files into an assembly file and constrained them to one another allowing the base of my bridge to come to life. For the hardware, I used hex head bolts as that is what is used in the industry within this application, and chose the stainless steel option for the material type so the bolts would not rust over time. The washers are wood washers which allows them to grab into the wood making the bridge much more sturdy.

From there, I used the mirror tool to create the other half of the supporting structure. I designed one of the baseboards and used the rectangular pattern tool to create the individual planks. I finished the model by adding on the railings.

As I wanted a lighted pathway for this project I used AutoCAD to create a circuit diagram that can be used throughout the bridge to allow for lights to be added at each railing support.

The model would not be complete if there was no way to bring this model into reality, so I created a Bill of Materials along with a set of drawings to allow for this bridge to be built.

I used PrusaSlicer to prep the models from an STL to g-code for my 3d printing utility to be able to create a scale model of the project.

Now you got your blueprints and a list of materials there is nothing stopping you from building this exact bridge. Or you can take my journey as a guild and adapt it to your own problem within your community. First, observe. See what is wrong and you can help fix it. Next research. Find inspiration and learn the constraints of your environment. Sketch it up. Make a rough idea of what you need to get started modeling. Model. Design with whatever method works best for you. Whether it is Fusion 360’s timeline, rivets full libraries or Inventor’s exact file organization get CADing! Make drawings. Allow your models to become reality with blueprints of any kind. From drawings for carpenters or STLs for 3D printing, the world is your oyster. Get started and have fun!