Hi, I'm Shovan, a rising senior from the Bay Area. I designed this bridge to address the lack of pedestrian walkways in some areas of the Bay. The decision to design this bridge was prompted by the Make It Bridge challenge. I opted to use Autodesk Inventor for designing the bridge, due to my familiarity with the software. The main goal of the bridge is to create a scenic pedestrian route over the bay without interfering with pre-existing commerce. Additional effort was taken to promote sustainability through the use of solar panels while also making the bridge more hospitable. Finally, the center of the bridge features an observation panel allowing travelers to take in the entire bay with little obstruction.

Supplies

Autodesk Inventor(and a computer capable of running the software)

Google Earth

Step 1: Backrgound

Last year, a plan was announced by San Francisco to turn an unused stretch of shore into the India Basin Shoreline Park, a massive 140 million dollar project that promised a great waterfront park. As I read through the proposed plans, I noticed that right across the bay from the proposed project was Shoreline Park, a pre-existing and popular park in Alameda, another city across the bay. I saw an opportunity there for a bridge to connect the two parks to create a scenic hiking route/trail through the bay.

Pictured above is an artist's rendition of the India Basin Park, as well as a Google Earth image demonstrating what the path of the bridge may look like.

Step 2: Scaling and Logistics

Before stepping into using CAD, it was important to establish the dimensions of the bridge as well as any limitations imposed by the bridge's dimensions. The bridge measured 6.2 miles according to Google Earth. This was scaled down by a factor of 3300(turning it into a distance of 10 feet) while designing the bridge in CAD to make it easier to work on. However, it also came with the question of the best way to support the bridge. There are various types of bridges, a few of those bridges were considered with their merits discussed below along with links to references.

Suspension Bridges:

These bridges are formed through the use of tower(s) that connect to the bridge deck via cable. Although these bridges are aesthetically pleasing, the longest suspension bridge only measures under 1.5 miles. The bridge's construction cost rises increases as the length of the bridge grows, as the support needed for the towers and cables increases. As such, this type of bridge didn't stand out as a particularly feasible one to implement in the real world. An iconic example of this bridge is the Golden Gate Bridge, north of the proposed location for this bridge.

Cantilevered Bridges:

As the name implies, these bridges are formed using beams that are only supported on one end(i.e. cantilevered). These bridges can be more cost-effective despite a long length and was definitely considered for the bridge's design. However, this style of bridge is difficult to design and maintain, due to the calculations needed to balance the bridge's cantilevered components. Additionally, the bridge performs poorly during earthquakes(due to their weight), which are a common occurrence in California. As such, the bridge became a less viable option in terms of structural soundness and feasibility.

Truss Bridges:

Truss bridges sustain themselves through the use of vertical and horizontal members which form trusses throughout the bridge. These trusses help the bridge remain structurally sound. They are often employed with cantilevered bridges to help reduce the strain on the cantilevered beams. They are well used as an additional form of support for bridges, and naturally provide overhead protection. As such, they were incorporated into the final bridge's design.

Box Girder Bridges:

Box Girder bridges use girders(support beams often used in construction) to create the main beams of a bridge. They are supported by box shaped columns and offer the ability to traverse long ranges cost-effectively. Since they are defined by their support of the bridge's underside, many modifications can be made to the bridge's upper structure for both aesthetics and functionality. A well known example of this bridge is the San Mateo Hayward Bridge, south of this bridge's proposed location. The bridge traverses over 7 miles and carries a highway, demonstrating this type of bridge's ability to sustain great weight over long distances. With this in mind, the bridge's main structure heavily utilizes this style of bridge. Note that the image above depicts box girder bridges with the longest span, which is different than length. Span is the distance between two towers/supports, rather than the end-to-end distance of the bridge.

Note: Although these are classified as distinct bridges, many components of bridges can be mixed and matched. For instance, the box girder supports can be used on a suspension bridge. Box girder bridges themselves have cantilevered components, just over shorter distances. As such, the bridge I designed is a mix of these bridges to get their best qualities. There are also many other types and subtypes of bridges, but these are the ones I looked into the deepest and felt like the ones with the clearest distinct qualities. The table is taken from the following source.

The final design settled on using box girders to anchor the bridge to the bed of the bay and using truss structures to provide overhead protection and overall rigidness for the bridge.

Step 3: Starting Design

Following the research from the previous step, I settled on a bridge design and reference bridges to model off of. With that I began designing in Inventor. With larger models, I usually start with a master sketch and then use derives to break up the sketch into different components. This provides a few benefits: it enables a more modular design process with each part in its own file, it is easy to re-integrate into one assembly, and the master sketch allows you to easily reference other aspects of the geometry without much hassle, as well as have different parts be unaffected by geometry changes in other parts.

The master sketch begins with a Side profile sketch, since a lot of the geometry of the bridge is most complicated from the side. Other views are depicted: the front face of the girders under the bridge, and the top-down view(featuring the observation deck and the solar panels covering the walkway. Other decorative and structural components have their sketches also stored in this part, which basically houses all the geometries for the bridge.

Another key characteristic of is establishing the scale. Since the bridge is scaled down by 3300, the overall length is 10 feet. The rest of the bridge was based off this initial scale.

Additionally, some useful Workplanes(such as in the center of the bridge, the top of the walkway, the ground plane, etc.) are defined in this part. This is especially helpful for mirroring solids and other features since you don't need to keep redefining these planes, instead just deriving them in from the master sketch.

Definitions:

Sketch = a 2d drawing/diagram that can depict the geometry of a part, including its dimensions.

Workplane = a plane that can be defined in a variety of ways(offset from another plane, normal to a plane, etc) and is useful for manipulating solid features and extrusions through mirrors and patterns.

Step 4: Girders and Ramp

Beginning from the ground up, the girders were one of the first things I designed. The girders' primary requirements were to be able to bear the weight of the bridge and to be high enough off the seabed that cargo ship can pass underneath. As such, each girder is in reality over 230 feet(which is more than the height distance from the seabed to the underside of the Golden Gate bridge). Additionally, the gap between each girder is 250 ft, allowing for even the widest cargo ships to pass between the girders with no need to disrupt the flow of foot traffic when ships pass underneath. The girders were created and spaced by creating one base component from the derived master sketch. Then using the pattern tool, I created enough of them along the entire bridge to provide uniform support. Additionally, the side front profile of the bridge is an arch, this is to help alleviate the tensile stress on the concrete that the girders are made of. Concrete has a high compressive strength but low tensile strength, arches utilize compressive forces and are better suited for use with concrete compared to other side profiles(e.g. a solid block).

The other lower structure on the bridge is the walkway, which is a gentle ramp up to the main bridge. A ramp was chosen because it would be an accessible option, and special care was taken to follow the regulatory values regarding the slope of the ramp(a wheelchair accessible ramp should have a slope of less than 12:1).

Step 5: Main Deck and Lower Support

The next step was to create the upper truss structure. This was created by deriving the side profile from the master sketch and using the Extrude tool on the upper truss. The truss structure utilizes triangular patterns to maximize the strength of the structure without extra material costs. The triangle pattern was generated by using the Rectangular Pattern tool in the master sketch which made it easy to make several uniformly spaced triangles quickly. The inner corners of the triangles were quickly rounded using the Fillet tool's select solid option, mostly for aesthetic purposes. This type of truss lacks any vertical members, and is known as a Warren truss structure. The lack of vertical members is made up for by the relatively thick diagonal members. After making one side of the upper truss, it was mirrored over to the other side using the Mirror Solids tool.

The other truss structure featured is the lower support. The piece was made by creating a solid block and then hollowing out the interior of the block using the Shell tool. From there, the Rib tool was used to select the lines created from in the master sketch's side profile to create the triangular structure. Since this truss is taking a lot more weight than the first one, it's diagonal members are closer to vertical to help with the increased weight from above. However, at this point, the lower truss required several very wide and thick beams to form the diagonal members of the truss, which would make sourcing the build materials harder. To resolve this, a extrusion was used to cut out the center of the entire structure, creating a hollowed out core with ribbing on the sides which serves as the lower truss.

Something to note for both of these trusses is that their triangles are not to scale. The actual number of triangles would be much more, but also unneeded to depict in the CAD and would also slow down the software immensely on older computers, hence the decision to show a less accurate truss structure.

Step 6: Observation Deck

The Observation deck was initially formed with a simple extrusion from the top view of the master sketch. However, after further analysis of the deck's shape and size, especially in terms of how much force the deck might exert on the bridge when it just "slots" into the bridge, I decided to design supports for the deck. The supports use arches again to help spread the load of the deck over more of the bridge's frame and also affix to a larger plate that would be bolted to the lower truss, helping prevent the strain of the deck from weighing too heavily on any on point of the bridge. The supports were made using extrusions and cuts. After that, they were hollowed with a shell, and then reinforced with ribs, similar to how the lower truss was made. These ribs are more simplistic, simply running vertically and horizontally. Afterwards, the support solids were replicated via the Mirror tool. Using the Mirror tool is especially helpful here, since the supports underwent a lot of iteration. Only having to change the original body to get the rest to automatically update can save a lot of time and make your workflow more efficient.

Step 7: Solar Panels and Spirals

The solar panel was relatively simple to design, it just involves simple extrusions from the master sketch combined with some coloring and cuts to create a protective bump around the actual panels. They serve a dual purpose of both shading the bridge from the sun and helping to generate power(e.g. powering lights at night or supplying the surrounding area with excess energy). After creating one set of the panel, it can be mirrored over a midplane to create the second panel.

The last part of the bridge is mostly aesthetic, a group of spiraling metal pieces that join at the top and sit on the observation deck. These spirals can use a simple cloth covering to help provide shade and shelter on the observation deck, as well as form the railing at the edge of the deck. The creation of this spiraling structure was relatively complex for me, as it used the unfamiliar Coil tool. The Coil tool allowed me to create a set of spiraling cylindrical tubes. I then used a taper of -30 degrees to force the coils to come to a point in the center. Afterwards, I used the height feature to set the max height of the coils. After putting some effort to offsetting the spirals to avoid collisions with the rest of the bridge, I extruded out a large pole at the center to anchor the spirals to. At the top, a hemisphere was constructed by using half a revolution of a semicircle. Note that this spiraling structure will likely have to be much shorter in future iterations of the design, since it pushes the limits of how tall a building can be when accounting for the scale factor for the entire bridge.

Step 8: Assembly and Rendering

The individual parts are then re-integrated with each other in an assembly file, which is where another benefit to the master sketch workflow appears. Since the master sketch is the basis of every part, every solid feature is in the same "3d space". For example, if the truss is 5 inches away from the origin in the x-axis in its own file, then when derived into another part file, it'll also be 5 inches away from the origin. What this means is that when placing parts into the assembly we can use the ground to origin option to automatically constrain each part to a point in space. Since each part is in the position where it is supposed to be relative to the other parts(thanks to the master sketch being a shared base sketch between all of them), the entire assembly is fully constrained easily. At the same time, the modular design workflow allows me to easily modify and iterate on individual components without worrying about having to put the entire assembly back together.

In terms of rendering, I am pretty inexperienced, but used a white presentation surface along with ground reflections to create a rendered image. The rendered image is the title image of this Instructable, but the unrendered finished product is pictured above.

Step 9: Conclusion

This bridge will help connect people across the Bay through a one of a kind, water based, trail route, and would be one of the longest bridges in the Bay(if not the world). In the future, this bridge could likely be expanded to accommodate a highway to help relieve the immense traffic buildup that often occurs in SF while still retaining a pedestrian footpath. This was definitely a very interesting experience, especially the deep dive I took into architecture and bridge building in order to properly understand how to design a bridge. It is very different from the stuff I usually work on and forced me to be more meticulous in my work as I knew I had to document everything afterwards. Hopefully there are some design workflows and techniques that you found helpful in this guide, or at the very least you learned something interesting about bridges.