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Initial Design Phase + Planning
Initial Design Phase + Planning
Our group modeled the initial design for our bridge using Fusion 360. We decided to go with the Truss bridge design as it is a strong, formidable design, and easy to build. As for the amount of Trusses, 4 Truss structures were used as the bridge needed to span 3 feet and to conserve materials while using simple measurements, the whole bridge spans 4 feet long, with one Truss being 12 inches long. As a group, we concluded that creating smaller Trusses left more room for error and inaccuracies to occur in measuring and gluing, which is why we decided to use larger Trusses. The legs were designed such that it would conserve planks, each leg consisting of 3 T-shaped structures to provide support. Our group resulted in modifying this CAD design during the building process of the bridge in an effort to use leftover resources where possible.
CAD Design
CAD Hollow View
Bottom View of Supports
3D Printed PLA of the CAD design
Base Construction
Base Construction
The base of the bridge consisted of 3 layers: 16 planks glued horizontally next to each other, balsa wood cross sections for interior strength, and four rows of planks laid along the longer side. This was one of the modifications we made from our initial CAD design after receiving advice from Mr. Irvine and our TA to include some additional form of support so that our bridge wouldn't collapse in the middle.
Bottom Layer
For the first layer of the base, our group measured and cut 16 3 in. by 12 in. planks and glued them together, creating a 4 foot long board. Laying the planks vertically for the first layer and horizontally for the third layer lessened the amount of force each plank would feel at the spots they were glued in. This way, the pressure isn't directly on the glued sections, rather it is dispersed across the base. One layer of glue was applied to each section to conserve materials. This process took longer than anticipated as we wanted to start off slowly to ensure we were not wasting materials and making accurate marks, incisions, and attachments.
Grace, Yolanda, and Laasya measuring the 12 in planks and placing them into a 4 foot layout
Middle Layer
The middle layer of the base followed a series of balsa wood X shaped designs intended to make our base thicker and add more structural support. This layer connected the planks in the first layer diagonally in both directions. Our group accurately measured and cut the balsa wood strips accordingly using measurements that were calculated from using the 30-60-90 triangle ratio. With the base as 12 inches, the vertical leg would be 2(12), or 24 inches, and the hypotenuse as 12 sqrt(3), or approximately 10.39 inches. Following the diagram we drew on the white board, one group member would be cutting the strips while another was placing the strips in place so that a third person could glue them down. This system was very efficient and the middle layer was completed within an hour.
In this picture our group is in the middle of cutting, placing, and gluing the balsa wood sticks in a cross racing configuration.
Overview of the cross racing pattern and the measurements for the balsa wood sticks for their respective triangle segment.
Top Layer
The top layer for the base simply consisted of placing the planks down horizontally. We used the full length of the planks plus a smaller 12 in. piece per row to fully cover the 4 foot long bridge. The top and bottom planks were where the most of the planks were used in our design as they were the widest piece of wood available, allowing for a strong base to be built without using an excessive amount of glue. With the base complete, our group moved onto planning out how to construct the Truss structures.
Full image of all 3 of the layers of the base. The 16 planks are placed at the bottom followed by the X pattern, topped off with the horizontal plank layer
Truss Mounting
Truss Mounting
Each Truss structure spans 12 inches long and roughly 10.4 inches tall. For extra strength and durability, the trusses were constructed 3 balsa sticks thick. The measurements for the truss structures were determined using 30-60-90 ratios, with 12 in. segments for the sides of the structure and a 10.4 in. beam running vertically along the Truss. Our group built one Truss and used a beam to connect another Truss structure directly across the side. Once all 8 Truss structures were mounted, we used leftover balsa wood sticks to create X crosses between the trusses.
Once the 8 Truss structures were built, both sides of the bridge were joined by vertical beams attached from opposing tips of the trusses, with additional cross bracing added with the spare balsa wood strips we had leftover.
In this picture, 4 Truss structures are mounted on our base and we used wooden blocks and a water bottle to support the structures as the hot glue dried so it wouldn't dry in a crooked or tilted orientation that would disrupt our structure.
This is a picture of the Truss structure measurements using 30-60-90 ratios and a drawing of one full side of the bridge. Our group also kept a count of how many glue sticks and large planks we were using to maintain awareness of the materials used.
Leg Supports
Leg Supports
Our initial idea was to make three T-shaped planks, each 1 foot tall, on both sides of the bridge. The legs were positioned such that each T section would act as its own individual support while also adding modular support with its 2 other counterparts. However, the T structures by themselves were flimsy, so we decided to add 3 beams: 8.6 cm long, 2 balsa wood sticks wide on both sides of each of the T-shaped planks. Creating these triangular structures added more support and durability to the vertically glued plank so that the balsa wood sticks would redistribute the force if the bridge shakes left or right. Since triangles are the most formidable and strongest structure for bridge construction, we formed 6 triangles on each leg with the leftover balsa wood.
Final Bridge
Final Bridge
Fully completed bridge
Bridge Testing 7/3/24
IMG_2575.MOVFull Video of Bridge Testing: Successfully carried 4 bricks across
Takeaways and Improvements
Takeaways and Improvements
Overall, our group was successful in creating a bridge that was capable of carrying 4 bricks across using balsa wood sticks, wooden planks, and hot glue. The project trained us to carefully plan out every step prior to building to ensure materials would not be wasted. Our group began with researching different bridge designs, making drawings, and noting important concepts before moving on to the building process. We implemented principles of Truss structures used in civil engineering and applied our knowledge of force distribution, compression, and tension through force vectors. We incorporated symmetry, cross-bracing, and structural analysis throughout the construction process. If we were to create another bridge, we would make multiple improvements to the supports, base, and Trusses. Our group conserved materials while building the entire bridge which resulted in us having an abundance of balsa wood sticks left. Knowing we would have such a surplus amount, we would use more sticks in our design early on. These sticks could be used to create a thicker, stronger middle layer for our base and perhaps add another Truss layer to our pre-existing structures. Additionally, we would redesign the leg supports, integrating cross-bracing and more balsa wood sticks to create a stronger, sturdier design. We would also take our time to accurately measure, cut, and glue sticks as all of these inaccuracies pile up which eventually contributed to some beam connections being off-centered. Symmetry is key when building a Truss bridge, and we would heavily prioritize this principle in bridge design.
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