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Virtual Design
Virtual Design
Just as we did in Tinkercad, we wanted to create a virtual model of our bridge and simulate the forces that would act on the bridge (as if the forces were the bricks). When we made this model, we first had to learn how to use the program, which proved to be simple after some practice. Since it was a 3D model, we had to work with x, y, and z planes as well as be able to place joints and members exactly where they needed to go on the plane depending on the measurements we drew out in our design (drawing can be found in the Physical Bridge section of the website). We also had to account for the fact that ModelSmart doesn't have wide wooden planks like we did in real life, so we had to understand that error would be probable in the simulation.
This design helped us because it gave us a baseline for how we would make our physical bridge as well as a good idea of what order would work best for building it. When we made this model in ModelSmart, it was extremely time consuming to connect each and every member and joint individually. So, instead of connecting the bridge piece by piece, we decided to build our bridge in sections. Not only did this make our process more efficient, but the extra sticks from each individual section made our bridge stronger.
Fig. 1: Side view of our bridge.
Fig. 3: Top view of our bridge.
Fig. 2: Front view of our bridge. As you will see in our physical bridge, this model doesn't have as many supports at the base of the bridge (where the car drives). However, with the realization that balsa wood is extremely flimsy and bricks would carry a lot of force especially the farther they get from the supports, we made our base stronger.
Virtual Testing
Virtual Testing
bridge video 15.movVideo 1: Our virtual bridge with a load of 15 N downwards. Our bridge failed with this amount of force.
bridge video 5.movVideo 2: Our virtual bridge with a load of 5 N downwards. Our bridge failed with this amount of force.
succesful bridge video.movVideo 3: Our virtual bridge with a load of 1 N downwards. Our bridge held with this amount of force.
It was extremely concerning when our virtual bridge couldn't hold under 15 or even 5 newtons of force. However, this gave us key pieces of information for our physical bridge. As shown in videos 1 and 2, our pressure points were throughout the superstructure and legs of our bridge. So, for our bridge to hold more bricks, we needed to add more support to these weak spots. This was considered when we actually started building our bridge. For example, we added planks of wood to our legs on the corners, added groups of three sticks of wood to the the legs to create right triangles for our trusses as well as equilateral triangles, and we added strips of wood across our superstructure to also create right triangles. This made our bridge stronger and more durable especially when some trusses started to break from the stress of the bricks. The extra support allowed for the stress to still be distributed enough that the bridge wouldn't break.
Second Virtual Testing
Second Virtual Testing
Fig. 4: The individual forces acting on the bridge and the pressure points coming from specific members of the bridge. As you can see, just as it showed in ModelSmart, our bridge was weakest in the legs and superstructure, as well as the understructure. This was reflected in our test when the left leg gave out.
The red lines are where compression forces are present. The blue lines are where tension forces are present. The lighter lines and white lines are where little to no forces are present. In the table, if the force is negative, it's compression. If it's positive, it's tension. The greater the number, the greater the force. These forces are split into x and y forces.
Within each truss or joint, there is either a reaction, displacement, and/or truss force, depending on the truss or joint. The reaction forces, as shown in figure 8, are the forces on the support joints (ex. 1, 5, 26). The displacement forces, as shown in figures 8-10, are forces on the joints that aren't the support joints (ex. 13, 32, 29). These forces show where the stress is distributed amongst each joint. The truss forces, as shown in figures 5-7, are forces on each individual member of the bridge (ex. 72, 19, 11).
Finally, due to the simplicity of the program, we weren't able to make our bridge exactly like our design. This made our simulation slightly inaccurate.
Fig. 5: Table of forces for truss forces.
Fig. 8: Table of forces for reaction forces and displacement forces.
Fig. 6: Second section of table of forces for truss forces.
Fig. 9: Second section of table of forces for displacement forces.
Fig. 7: Third section of table of forces for truss forces.
Fig. 10: Third section of table of forces for displacement forces.
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