Embedded Files
Unit Overview
In this unit, students utilized the epilog laser cutters alongside CorelDraw to construct a chipboard bridge. To begin, each group assembled a bridge template, including a cardboard sheet, reference diagrams, and parchment paper---all were secured using masking/clear tape. After creating the template, each group reviewed the basics of CorelDraw, designing and cutting gusset plates, diagonal/bottom chords, and vertical/supporting tubes. All pieces were then attached with super glue. Nearing the end of the unit, each group designed additional chords and tubes to be tested under compression and/or tension.
1. Blueprint
Link to textbook: https://drive.google.com/file/d/1U-qYN7UlromiVJHNW5YgojewDLzHpwhw/view?usp=drive_web&authuser=0
Materials:
1 18 " x 24" sheet of cardboard
Approx. 20 " x 26 " sheet of wax paper (slightly larger than the cardboard)
Masking/clear tape
Before assembling, we referenced the West Point bridge textbook and constructed the cardboard template, which we would later use to assemble our components. After taping the six blueprint sheets to the cardboard, I applied a layer of parchment paper and secured it to the back of the board with masking tape.
Bridge template
Gusset plate template
2. Processing Parts
Designing the Gusset Plates
Materials:
Several sheets of 0.022 chipboard
Masking/clear tape
Sharpie
Tools/Apps used:
Epilog laser cutter
CorelDraw
Google Drive
For the gusset plates, I first downloaded the provided file (taken from Google classroom) into Corel Draw. The original file size was too small and did not fit the template, so I changed the scale factor from 100% to 110%. Additionally, I opened the object manager and disabled printing for the layer containing the original image. After saving and uploading the modified file to the eng. proj. google drive, I re-downloaded it on the laser cutter computers and opened it in CorelDraw. I placed a new sheet of chipboard into the laser cutters and used the jog function to manually focus the laser cutter; I reoriented the laser back to the origin once I was done. Because I was using the big laser cutter, I didn't have to include specific print preferences---instead, on the Epilog application, I imported cardstock as the material and separated the job by color, indicating vector and scoring (red = vector, yellow = scoring). I clicked "print" on the application, bringing it to the laser cutter, and ran the job. Before proceeding, I compared these plates to the template to verify that they were the correct size. I had to print two copies of this file in total. Kerf did not significantly impact this project as the value is too small.
SVG file of gusset plates in CorelDraw file (post-modification; sized up 110%)
Bridge Plates.MOVLaser cutting the gusset plates
Final gusset plates on the template
Designing the Chords, Verticals, Floor Beams, and Struts
How I designed the project file: To design each tube, I first calculated the full dimensions (ex. 38 mm x 12 mm) and used the rectangle tool to create a box. Next, accounting for all of the scored flaps, I used the two-point line tool to draw the scoring lines (6 mm or 10 mm apart typically). I colored the scored lines red and set the entire file to hairline. Finally, using the rectangular tool, I designed a series of 4mm bars of varying length. I saved the .svg file and uploaded it to the eng. proj. google drive. Similar to the gusset plates, I selected "group by color," setting black to vector and red to scoring. After placing a sheet of chip-board into the laser cutter, I manually focused it again. I hit "print" on and began the job.
Table of all truss components
Tube members in CorelDraw
IMG_1749.MOVTimelapse of laser cutting the tubes
4 mm bar members in CorelDraw
Cutting Strips.movTimelapse of laser cutting the bars
3. Assembling the Bridge
Materials:
Super glue
Box cutter
A pair of scissors
Main Truss Assembly: To begin assembling the trusses, I followed the textbook, taping the gusset plates to their proper location on the cardboard template. I designated each piece with their appropriate label. After they were secured, I folded the laser cut tubes along the scored lines and applied super glue to the gluing flap. I folded the tube as instructed by the manual, pressing the flap to the inside of the tube to hold it together. For the main assembling, I first glued down the bars, connecting the gusset plates. Because the original design only contained one size for the bars, I used scissors to trim them accordingly. To glue the top chords, I laid them out on the template, used a sharpie to draw the diagonal line, before cutting them with a box cutter. Finally, I glued down the vertical tubes and repeated the same process with the other half. One of the main issues I encountered during this process was imprecision with the gluing; because the super glue dried quickly, certain tubes were glued unevenly, which could have impacted the cohesiveness of the bridge and its compressive/tensile tolerance during the testing. Additionally, if some components were glued incorrectly, I would have to manually pull them apart/off, leaving residual glue or damaging the component itself. In the future, I would look through the textbook more thoroughly to get a better understanding of the process and the final result.
Lateral Bracing: After taping down the plates, I trimmed and glued the bars, gluing in their appropriate location. Finally, I added the 6 mm x 6 mm struts. The main issue that occurred during this process was the positioning of the struts. When I connected the two trusses, I found that some of the struts were positioned too high up.
Main truss construction progress, 8/31
Finished-half of the trusses
Final completed bridge
4. Laser Cutting Structural Members
Materials:
1 sheet of 0.022 chipboard
Super glue
Tools/Apps used:
Epilog laser cutter
CorelDraw
Google Drive
To create the structural members, which we would later use in testing, my partner and I split up different parts of the file to design in CorelDraw (for instance, I designed the 3 cm x 3 cm squares, the bars of varying widths, etc.). We uploaded the file to Google Drive, brought it into the Epilog application and ran the job on the big laser cutter.
Above contains the table of all strutural members. We cut replicas of all the bridge components to determine the theoretical load that the bridge can handle.
SVG file of all strutural members in CorelDraw, labeled according to the type of member
LaserCuttingWrappingBars.MOVTimelapse of laser cutting the structural members
All 6 mm x 10 mm and 10 mm x 10 mm tube members (5 cm, 10 cm, 16 cm)
All bar members (4 mm, 6 mm, 8 mm)
5. Testing Structural Members
Materials/Tools Used:
Bridge lever (testing machine)
1 bucket
2 clamps
metric scale
sand
ruler
We first measured the distance from the fulcrum to the notch (constant) and from the fulcrum to the component. These would eventually be substituted as L2 and L1 in the equation, respectively. Next, securing a component between the lever and the base, I slowly added sand into the bucket until the piece broke. I measured the mass of the bucket and sand and recorded the values in a spreadsheet.
Diagram of testing set-up
Set-up for compressive test
Set-up for tensile test
IMG-1100.movVideo of tension test (credits to Landon Broadwell)
My Movie 36.movVideo of compressive test (credits of Landon Broadwell)
Additional Images of Testing
Testing the 10 x 10 mm tube (5 cm)
Testing the 6 mm wide bar (20 cm)
Testing the 4 mm wide bar (20 cm)
Testing the 6 x 10 mm tube (16 cm)
Problems Encountered and Solutions
A major issue I encountered while testing was the placement of the longer tubes. For the compression tests, I had initially positioned all of the components 30 cm away from the fulcrum. However, I later realized that a longer distance would naturally require much more load than a shorter distance. Thus, about half-way through, I changed the distance from 30 cm to 5 cm. Although, this change was taken into account when calculating the compressive forces, it still resulted in many outliers.
Early test with the tube 30 cm away from the fulcrum
On the left contains the equations used to calculate the tensile/compressive strength. Weight was determined by multiplying the mass (kg) by g, 9.81 m/s^2.
T = Tension force
W = Weight of bucket and sand in N
L1 = Length from notch to fulcrum
L2 = Length from fulcrum to piece being measured
C = Compression force
Observations and Trends within the Data:
The cross-section of the bars was directly proportional to the amount of tensile strength it could handle; as it increased, the tensile strength increased as well
For the tubes, the shorter and more hollow components tended to handle more compressive strength, whereas longer pieces gave out much more easily. For instance, the 5 cm 10 x 10 mm tube could handle significantly more load than the 16 cm 10 x 10 mm tube.
Both the bars and tubes tended to break near the top/bottom, either crumpling or snapping.
A potential weak point on the bridge would be near its longest top chord, as it can not handle a lot of compression. The same can be said with any particularly long compressive member.
Tensile Strength (N) vs. Member width (mm) graph, provided by West Point; similar to our observations, cross-sectional area and the resulting tensile strength are directly proportional
Compressive Strength (N) vs. Length (cm) graph, provided by West Point; similar to our observations, member length and the resulting compressive strength are indirectly proportional
6. Bridge Testing
Materials:
6 2x4 lego blocks
Fully assembled chipboard bridge
5 kg of books (start with 3.5 kg, then 1.5 kg)
2 tables, approx. 60 cm apart
Process: For bridge testing, we placed 6 2x4 lego blocks on top of the six middle joints (I, J, K and their reflected joints, I', J', and K'). We first placed two books, with a mass of 3.5 kg, directly on top of the six lego blocks--this ensured that the load was strictly on top of these joints and not touching any other members. If the bridge could hold it for 3 seconds, we proceeded to add the last 3 books with a mass of 1.5 kg.
Bridge with six legos on top of the six joints
the bridge carrying the 3.5 kg mass of books
Bridge Testing.movVideo of bridge testing (credits to Griffin Orsinger for testing our bridge)
Bottom of the bridge after collapse - only tensile members were ruptured
Side of the bridge after collapse
Observations and Points of Failure:
After placing the last 1.5 kg on top of the bridge, tensile members BC, CD, FG, C'D', and F'G' snapped and broke near the joints.
There was a slight downward arch on the bridge afterwards (caused by the bridge "folding" during collapse)
Compressive member HJ experienced a strain, denting around joint I
Potential fixes/adjustments to prevent future failure:
Tensile member D'E' snapped before testing, which was likely the cause of bridge failure; I determined this because all points of failure involved tensile members; for the rest of the bridge, besides minor dents, there were no other points of rupture
In the future, I would pay more attention to these members during construction, ensuring that each component was secured to the joint before proceeding.
Another adjustment could be increasing the cross-sectional area of these members, as bar width and tensile strength are directly proportional
Although the bridge failed under 5 kg, I noticed that vertical member BH (labeled 0 on the bridge diagram) was unaffected and did not experience any load under 3.5 kg.
While testing others' bridge, we were able to shift BH around with ease
7. Final Conclusion
Tools and processes used:
Using CorelDraw to design .svgs for laser cutting
CorelDraw
Large Epilog laser cutter
Assembling process
Bridge component testing process
Bridge lever/testing machine
Bridge testing process
What was made: In this project, we constructed the West Point bridge using chipboard, documented theoretical loads for individual components, before we tested the actual max load for the bridge. We designed our bridge by using a template, and we laser cut all of our components (after originally designing them in CorelDraw)
What I learned about civil engineering from this project: I learned how to use Newton's third law to calculate the Method of joints on our bridges, as well as grasped how different elements of the tensile/compressive members can affect their respective strengths.
Why it is important to account for both compression and tension when designing a bridge: Whereas tensile forces pull and stretch materials, compressive forces push the material down or inward; if one exceeds the other, the bridge will not be flexible enough, and as a result, it will collapse. It is crucial to account for both in order to maintain the balance of the bridge.
Improvements that could be made to the design: Increasing the cross-sectional area of the tensile members using shorter compression members could all be potential improvements to the design. From our individual component testing, we found that both decreasing the length of the compression members and increasing the width of tensile members resulted in an increase in the maximum load they could handle.
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