Week 2: Bridge

Overview

Using balsawood and glue, we built a 4 ft bridge that can withstand the weight of multiple bricks and vigorous shaking from a shake table.

Design Process:

To start the design process, our group built a model bridge made of index cards. After testing, we discovered that adding more support to the bridge allowed it to hold more weight. Using this information, we brainstormed idea's on how to make a bridge that could support as much weight as possible. We also used 3D modeling and bridge designing programs visualize our ideas. We decided on a slightly modified Pratt Truss design, using an X design with two diagonal members, instead of one (see bridge image below). Once we began building, we progressed quickly. About 2/3 the way through the building process, we were told that we couldn't use tape on our bridge, which caused a setback for our team. We managed to remove most of the tape, but some of the tape on our bridge was already hot glued. The tape doesn't provide any structural integrity to our bridge, but remains simply because it would be impossible to remove without the destruction of our bridge. Once we overcame the tape issue, we continued, only using hot glue to connect our pieces. We finally finished the main part of our bridge, added the legs, and tested our bridge. When we tested out bridge, we were able to put all bricks on top of the bridge. The legs did fail under all the weight as we predicted.

Civil engineering and building science:

Civil engineering focuses largely on the design and construction of infrastructural structures such as bridges, roads, tunnels, water treatment plants, and water supply. Though this is the primary focus of civil engineering, there are several subsets including structural, geotechnical, water resources, and transportation engineering. Each has a slightly different, more specific focus, but still is widely related to infrastructure and public well-being. Civil engineering is extremely important and can be seen in nearly every facet of our everyday lives from irrigation to transportation, civil engineering is the base of much of our societal function.

More on Civil Engineering from our research paper: Exploration of the Structural Design Process

Bridge Designer

Using Bridge Designer, we were able to create a model bridge. Using a Pratt Through Truss design, our model bridge is able to successfully complete the load test, while also meeting the following requirements:

- Compression force/strength ratio cannot exceed .40

- Tension force/strength ratio cannot exceed .45

- 15% of bridge of members hollow tubes (4/29 hollow, or 14%)

Below and to the right are images and calculations for the model bridge.

The calculations show the force that is being applied to each joint of the bridge. The sketch and the numbers next to the calculations correspond, showing the force on that individual joint.

Model 3D

Using Model Smart 3D, our group was able to simulate how the bridge would work by using forces on the bridge which would simulate the bricks being placed on the bridge. The bridge was made as close as possible to the final result so that there was a safety on how much weight the bridge was guaranteed to hold.

TinkerCad

This is our TinkerCad bridge (image on right). We modeled it after the Modelsmart3D design. We also used this design to make a 3D printed bridge.

Index Card Bridge

The index card bridge was an introduction to bridge structures and the weight balancing. The first part of the index card bridge was to test one index card spanned across the two wood blocks and observe the result of 10 nuts placed in the middle. With those 10, the index card had bent inwards and fell through the gap due to lack of support. The next few steps had us add some supports made out of strips from index cards in the shapes of triangles, seen in the middle of the top image, which then reinforced the center of the card. This support strengthened the middle and made the card bend a little less from the weight of the nuts, but still had drastic bend. We then added a surrounding to the triangle, seen in the bottom picture, which added a lot of support and reduced the bend dramatically. Finally we added crossbeams going across the width of the card on each side which stiffened the whole structure, removing almost all bend, leaving only natural bend from the card being paper.

My Movie 9.mp3

ANSWERS TO DISCUSSION BOARD

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