Articulating Balsa Wood Bridge
Project designed and manufactured by Nate Harris
Contact: nathar275@gmail.com
Engineering Report
Fall Presentation Video
Video 1.1: Fall Presentation
Winter Presentation Video
Video 1.2: Winter Presentation
Images
Image 1.3: Bridge and Articulation Tower
Project Introduction
The purpose of this project was to design and manufacture a bridge made only with balsa wood and wood glue. The bridge must span a 400mm gap, be able to support a 20 kg load, and weigh less than 85g. The bridge must also be able to articulate such that an object could pass underneath the bridge that otherwise not be able to when the bridge is in the horizontal position. Bridges are an essential part of infrastructure, and although this project is only a small, model sized bridge, understanding the engineering concepts to design, analyze, and manufacture a functional bridge is crucial to engineering.
Project Requirements
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32mm by 25mm block must traverse the bridge in under 10 seconds.
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Midpoint of road deck must be 140mm above horizontal resting position when articulated.
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Abutments are 60mm wide and made of steel and are 400 mm apart.
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Bridge must be longer than 400mm to rest on abutments.
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38mm wide solid balsa road deck.
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10 gram mass on articulation component allowing gap for printer paper to slide under bridge
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Supports load between 18.9 to 20 kg.
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Weight of bridge must be 85 grams or less (with articulation hard components removed)
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Max vertical deflection of 25 mm
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Ascend/descend of bridge must be done with push of button/lever.
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Articulation from fully closed to fully open must be done in under 60 seconds.
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Bridge must be only made from 2 materials: balsa wood and wood glue (excluding articulation components)
Results
The bridge successfully passed three of the four tests, as well as meeting the dimensional requirements of the project.
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The bridge was able to successfully articulate to an average of 155 mm, exceeding the requirement of 140 mm by 15 mm. The bridge was also able to articulate open in an average of 0.97 seconds, which was well under the 60 second requirement.
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The bridge was not able to lift enough under a 10 gram load so a sheet of printer paper could slide under the bridge. It took the bridge 15 grams to allow a gap large enough for the paper. One way the could've been improved would've been to replace the cotton cord used for articulation with a cord that was more rigid, since the cotton cord sagged before the bridge was able to lift.
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The bridge was also able to support the required load of 18.9 to 20 kg. The bridge reached a maximum load of 190.02 N (19.4 kg). The bridge displaced 1.66 mm under that load, which was well within the allowed 25 mm of deflection. The bridge also didn't experience any permanent deformation after being subjected to the load.
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The final weight of the bridge and articulation tower was 79.5 grams, which was below the maximum weight of 85 grams by 5.5 grams.
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The results are accurate and precise. Tape measures were used to measure the height of the bridge. The articulation time was determined by examining the footage of the articulation test to the nearest hundredth of a second. A scale was used to weigh the masses, and the Instron gave accurate data for the force and displacement.
Image 5.1: Articulation Test Data Sheet
Image 5.2: Tensile Test Data Sheet
Image 5.3: Force vs Displacement Graph