Welcome back to my blog. This post is for the first Action Project of the STEAM class Urban Planning. In this first unit, Load, we have been studying bridges and how they can support different loads. We looked at the four main bridge types: beam, truss, arch, and suspension. Each of these bridge types has its own means of supporting loads. Truss bridges for example can be helpful to support a distributed load. It does this by distributing the forces of tension and compression into its trusses. Suspension bridges distribute the forces into tension in the cables that transfer into compression at the piers. For math in this unit, we learned about vectors, forces, force body diagrams, and Newton's Laws of Motion. For Field Experiences, our class walked down to the Chicago Riverwalk and looked at the bridges that connect the city's North and South sides. Chicago's bridges use the truss design and act as drawbridges. Another FE was a visit by Drew Valentine from GMB Architecture + Engineering. Drew is a structural engineer who has worked on many projects such as learning environments and athletics buildings. For this AP, we have been asked to build a bridge out of popsicle sticks that can span a 1-foot gap and hold at least 10 pounds. The bridge would have to use some truss design of our choosing and be only made from popsicle sticks and glue. With limited materials and a fixed plan, it was important to create a truss bridge that would use its materials effectively and hold as much weight as possible. I hope you enjoy reading about my bridge.
Our inspiration comes from the Warren truss bridge design. Warren trusses have a pattern of equilateral triangles that span the length of the bridge. Because Warren trusses are good for supporting a distributed load, we hoped this design would be the best fit for the testing that would come after its construction. Below is a Warren truss bridge that helped us plan our own.
Structurae
Before designing our bridge, we had to create a few sketches. One that would show the tension and compression forces acting on it, and a second scale drawing of the bridge that was a digital blueprint.
Sketch 1: Forces, GS, 2022
Sketch 2: Blueprint, GS, 2022
The last step before the building was to create a labeled drawing of the bridge we planned to make. For this drawing, we made sure to include where the top chord, bottom chord, and other important sections.
Labeled Sketch, GS, 2022
Construction: The bridge is 18.5 inches long and is placed 35 inches above the ground. The angles used in this bridge come from the trusses and because of the warren truss design, each triangle should be equilateral or have 60-degree angles. After its assembly, the bridge’s trusses do not all equal 60 degrees but the intention was to have equilateral trusses. All 50 popsicle sticks were used for the bridge.
Bridge 1, GS, 2022
Because each triangle is equilateral, all sides are 4.5 inches long and each angle is 60 degrees. As shown in the image, to confirm the angles I used the law of sines and the law of cosines. For the law of sines, I solved for angle A and using the law’s formula, it equaled 60 degrees. For the law of cosines, I solved for angle C and I used the formula to reach 60 degrees again.
Calculations 1, GS, 2022
A few more calculations can be done with the bridge such as determining how much potential energy it had before falling, the kinetic energy it has when hitting the ground, and the velocity before hitting the ground. Using the potential energy formula, mass times gravity times height, the potential energy of our bridge before it falls would be 8,506.4 Joules. If the bridge were to fall, the velocity would be 26.2 inches per second and the kinetic energy is also 8,506.4 Joules. These two energies are the same because due to the Law of Conservation of Energy, energy cannot be created or destroyed, only converted from one form to another. The potential energy is converted into kinetic energy because it has the energy of the potential to fall and the energy while it is in motion.
Calculations 2, GS, 2022
This bridge is designed in line with Sustainable Development Goal #11: Make cities and human settlements inclusive, safe, resilient, and sustainable. Our bridge is meant to serve as a possible replacement for the i90 overpass in Chicago near Stewart Ave. and 28th Place. This overpass has shown its wear over the years and now displays rust and unsafe bridge conditions. Target 11.2 of SDG11 states “By 2030, provide access to safe, affordable, accessible and sustainable transport systems for all, improving road safety, notably by expanding public transport, with special attention to the needs of those in vulnerable situations, women, children, persons with disabilities and older persons.” For this situation improving road safety is very important because the overpass is used frequently and preventing road disasters is the top priority.
Deficient Overpass, GS, 2022
We decided on creating a Warren Truss bridge because they can spread the load evenly across several different members. With its equilateral triangular structure that is connected to span the length of the bridge, it has the strength to withstand more heavy loads. Our teacher Hasnain provided some feedback on making the bridge longer so it could make the 1-foot gap. We adapted our bridge with this feedback. We added our remaining sticks to elongate the bridge by the bottom deck. We also added struts and sway bracing to make it ready for a distributed load. Drew from GMB was also able to provide feedback on the day he visited and recommended where to place our final sticks to increase support.
Bridge 2, GS, 2022
As a team, we struggled with the idea of having the restraints of 50 popsicle sticks and no extra materials. Our team worked through this obstacle by making the most of those sticks. The Warren Truss bridge had a simple but effective design that didn't require too many sticks and is still quite sturdy. The bridge did get slightly lopsided because of the angles of the triangles but we overcame that by choosing where to put our remaining sticks to fill in some gaps such as adding some along the floor beams for extra support.
After building our bridges we tested them out as a class. The video attached below is of our bridge being tested. The process for adding weight was using a block of wood on the deck of the bridge to suspend a bucket that weights could be put into. Our weights were textbooks and the bridge held 24.6 pounds before it collapsed. Excuse my yelling at the end of the video, it started to get very intense.
Popsicle Stick Bridge Testing
Thank you for reading and watching. I hope you found this project as interesting as I did and perhaps got excited to see the bridge collapse at the end of the video. I had a lot of fun with this unit and I am very proud of how the bridge turned out. All of my classmates had their own interesting bridges that held more weight than you would think or less than you had hoped for. I didn't expect the bridge to be able to hold 24.6 pounds so it definitely surprised me. I am very thankful for the feedback given by Drew and Hasnain to make the bridge the best it could be in the time frame. Thank you again for reading and watching and I hope to see you in the next post.
Sources:
Boon, Garrett. "Truss Series: Warren Truss." Garrett's Bridges: Resources to Help You Build a Model Bridge, 3 Dec. 2021, https://www.garrettsbridges.com/design/warren-truss/. Accessed 15 Apr. 2022
Gurstelle, William. "Make a Warren Truss Bridge with Popsicle Sticks." Make:, https://makezine.com/projects/make-warren-truss-bridge-popsicle-sticks/. Accessed 15 Apr. 2022
"Make cities and human settlements inclusive, safe, resilient and sustainable." United Nations, https://sdgs.un.org/goals/goal11. Accessed 15 Apr. 2022
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