Designing the Bridge

Objective and Constraints

The objective of the project was to design, model, simulate, and construct a bridge using the information that we were taught about material engineering and civil engineering. To test the strength of the bridge, a cart with a variable number of bricks on the top would be rolled back and forth across the bridge.

In the construction process, we had both dimension constraints and material constraints:

Dimension Constraints
- The span, or length, of the bridge must be at least 4 feet
- The bridge must be able to support itself over a 3 foot gap
- All components of the bridge structure must be at least 1 foot in the air
Material Constraints
- 150 balsa wood strips (1/8” x 1/8” x 36”)
- 15 balsa wood planks (1/8” x 3” x 36”)
- 15 hot glue sticks

Initial Design Process

The first constraints we considered were the materials. As a group, we explored the strength and properties of the materials, and how those factors would affect our bridge's structural integrity. The materials we had were made of balsa wood, which is an extremely soft wood that as low compressive and torsional strength.

Given the constraint in the number of materials that we had, we decided that a simpler design would be beneficial. Taking the decision in the consideration, we decided that the bridge should have as many trusses as possible, as that would provide the most amount of strength given the space and material constrains that we had.

Our initial design composed of two layers of trusses, and lower, larger layer which sat directly upon two pillars. The higher, smaller, layer of the truss is the roadway, where the cart will move. The two layers of trusses are meant to provide as much strength and support for the bridge considering the more simple design that we chose.

TinkerCAD and Design Changes

While putting the design that we created into TinkerCAD, we decided that we needed to make some changes to the design. Firstly, we decided to remove the upper layer of trusses, and have the roadway go through the bottom layer of the truss. We felt that the upper layer was an excess of material that we could not need, and that the benefit of the increased structural integrity that the layer provided was outweighed by its extra weight and added complexity.

To compensate for the loss in structural integrity, we decided to add trusses in the support pillars, as this would be a more efficient use of material and would be more beneficial than another layer of trusses above the roadway.

Modeling Software

To model the bridge, we used Onshape. We found Onshape much easier to use than ModelSmart3D, and one of our members had some experinece using Onshape in the bast, so we decided it was the better software to use for the situation.

Unfortunately, in Onshape, some of the details could not be modeled due to time constraints that we had. Because we needed to the bridge to be built within a day, we needed to finish the modeling process quite quickly. Thus, the truss in the pillars and the truss from connecting the two trusses on either side of the bridge was not modeled.

Bridge Scaled Drawing

Below is a scaled drawing of a bridge. The scaled drawing was taken from Onshape, and so the truss on the top of the bridge and the trusses on the pillars are missing. The two front views are in the 1:12 scale and the isometric view is in the 1:10 scale.

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