ENGR 100: Bridge Design Project
Design a structure to span a given distance while supporting a maximum load using a minimum of materials.
2. Idea generation and component test
3. Prototype bridge
4. Final bridge
5. Presentation and report
Both your prototype bridge and final bridge are limited to the following materials for construction:
Each team needs to design and build a prototype and a final bridge which span a distance of 16 in. and hold a minimum weight of 15 lbs.
The dimensional constraints are shown in Figure 1:
(Side View) (Front View)
Figure 1. Required dimensions for the bridge design.
Each bridge will be tested using the testing apparatus in Mechanical Engineering Bldg. All bridges must accommodate a 2" x 6" plate accessible from the top for loading purposes. The 2" x 6" plate is used to resemble a big truck riding on the bridge. A dowel connecting to the testing machine will be pushing down on the 2" x 6" plate, thus pushing against the bridge, to resemble the weight of the truck. The bridge will be sitting on a jig to resemble the support from the terrain or the piers. The maximum loads carrying by the bridge will be recorded. After the maximum load is reached, the loading capacity of the bridge will decrease, and the test can come to a stop.
Figure 2. Loading Procedure
The grade for the bridge project comes from the following assignments:
Bridge report, bridge presentation, bridge performance, memo writing, and force analysis
To evaluate the performance of your bridge, scores index are used to evaluate the performance of bridges.
Score index = Load at Failure(lbs) / Weight of Bridge(lbs) (Eq. 1)
Teams should strive for outstanding performance and originality.
There are several types of bridges people usually build in the world of engineering. The followings only show you one type you usually see in this class. For other types of bridges, check How stuffs work or PBS Building BIG and learn about the engineering behind them.
Here is the truss type of bridge. (If you don't understand it, you might want to check here to get some background information.)
Figure 3. Examples of Truss Bridges
Trusses work by converting bending of the whole truss into tension and compression of its individual members. To visualize this transfer, imagine taking the whole truss structure in your hands and bending it into a U -shape (the same deformed shape that a heavy truck in the center of a flimsy bridge would cause). Next imagine that the whole truss is wrapped in rubber skin so it looks like a large rubber box. As you bend the box, wrinkles will appear on the top of the box, and the rubber on the bottom will be stretched. The rubber gives an indication of where tension and compression are occurring.
Thus by inspection, the truss members on the top will go into compression (squeezed together) and the members on the bottom with go into tension (stretched apart) as shown in Figure 4. The members in the middle of the truss will also either be in tension or compression, but require force analysis (engineering statics) to figure out. Because truss members are pinned at the joints causing the bending moment to equal zero (equilibrium), all members experience tension or compression, not bending!
(Compression on Top)
(Tension on Bottom)
Figure 4. Truss Analysis