ENGR 100: Bridge Design Project

Project Objective:

Design a structure to span a given distance while supporting a maximum load using a minimum of materials.

 

Project flow:

1. Project definition

• Make sure you understand what you were asked to do and the time/material constraints.

2. Idea generation and component test

• Construct the idea of your bridge from your own creativity and existing models.
• Learn about the material you were given and its capability.
• Build a small part of your idea and try.

3. Prototype bridge

• Build a full scale prototype and see how it performs.

4. Final bridge

• Identify problems in your prototype bridge.
• Fix, improve, and build a final bridge.

5. Presentation and report

• Tell the whole class what you have learned from this bridge project.

 

Project constraints:

Materials:

Both your prototype bridge and final bridge are limited to the following materials for construction: 

• 100 sticks of wood tongue depressors (6 in)
• Glue guns and glue sticks or wood glue
• string (unlimited)

Geometry:

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.

• The length of the bridge should not exceed 19 inches. 
• The width of the bridge should be at least 2.5 inches but not more than 3.5 inches. 
• The height of the bridge should not exceed 8 inches above or 2.5 inches below "ground level".
• There must be a clearly identifiable and realistic “deck” or passage running from one end to another, to represent the road surface of a real bridge.
• There has to be at least 3 sticks in the width of the road deck. A flat or curved road surface are acceptable. Other road surfaces with abrupt changes in the height will be regarded as not-realistic since it will be difficult for a vehicle to drive through it. Thus, the height change of the “deck” has to be less than 4 inches.
• The testing machine will apply load on the road deck. See testing procedure for details.

 

Testing Procedure:

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

 

Project Grading:

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.

 

Engineering Fundamentals:

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