These are the supporting files for the Learning Blade Bridge Design Experiment.
This Maker Quest gives students practice designing and testing basic bridges. The model files in this lesson can be used to assemble a variety of bridge types, and can also serve as the beginnings of customized 3D bridge designs. Once the students have designed, printed, and assembled their bridges, they will measure them for rigidity, load bearing strength, and the economy of materials used.
The first few STL files are for the spans and pins. We've included single pins and spans, as well as groups maximized for ~ 100mm x 100mm printing area. We've also included model files for a span and pin sliced into sections so students may more easily modify them into their own designs.
Overview and Background
Have you ever considered how massive construction projects are possible? Modern buildings roadways, bridges, and other structures can be enormous, yet most are usually constructed from much simpler and smaller components.
This exercise gives students practice designing a bridge structure using simple 3D printed spans and connecting pins. The model files in this lesson provide a basic starting point for simple bridge construction. However, the main purpose of the exercise is to encourage students to go beyond a simple, triangular truss bridge design, and create their own unique solutions.
Lesson Plan and Activity
Modular Bridge Design
Students should identify and download the bidge component files to their computer.
Once the files are ready to print, students should import them into the 3D slicing software. The individual components of the bridge are designed to print within a 110 mm x 100 mm x 100 mm print volume. The components could be scaled, but any changes in scale could affect the fit of the components.
Once the slicing software has prepared the printing instructions, all of the bridge components should be printed and carefully removed from the printing surface. Make sure to print enough bridge spans and pins to build the desired bridge design.
The bridge pins are designed to join up to six spans, with the ends of the pins fitting through the holes in the spans.
Students should attach spans to each side of the pins so the bridge has 2 sides, and a passage for a roadway between them.
After construction, bridges can be tested for rigidity and the economy of materials used.
Testing Bridge Rigidity - Strain Under Stress
Begin this test by weighing the bridge itself and recording it on the included worksheet. Next, set up a gap for the bridge to cross. Place two desks, tables, or stacks of books a specific distance apart. A gap of at least 12 inches is recommended. Once the gap is setup, place the bridge across the gap with no additional weight on it.
Each bridge will flex and deform once a load is placed on it. This deformation is expressed as the amount of Strain under Stress. Stress is typically expressed in pounds per square inch (psi) or kg/mm2. To calculate a bridge’s stress, first calculate the bridge load area. Measure the length of the bridge from the two support points that touch the base just outside the gap. Also, measure the average width of the bridge. Record those measurements on the included worksheet and use them to calculate the bridge load area.
Next, turn the yard stick on its edge and place it across the gap next to the bridge. Find the middle point of the bridge, and use the ruler or tape measure to measure the distance from the bottom of the yard stick to the bottom of the bridge. Record this distance on the bridge testing worksheet.
Next, add a weight to the center of the bridge. The weight can either be placed atop the bridge, or possibly hung beneath the bridge if you have string, tape, or hooks to securely attach it to the bridge.
Once the weight is attached, place the yard stick on its edge and across the gap beside the bridge exactly as you did in the measurement with no weight. Find the middle point of the bridge, and use the ruler or tape measure to measure the distance from the bottom of the yard stick to the bottom of the bridge. Record this distance on the bridge testing worksheet.
If you have multiple weights, you can continue adding weights and measuring the amount of sagging they produce. There is space on your worksheet to record and graph multiple measurements.
Bridge Testing Worksheet
Calculating the Economy of Materials
Engineers have many ways to calculate the economy of materials and how cost-efficient projects will be. In this experiment, we’ll use a simple ratio, based on the weight of the bridge, to determine materials economy.
To calculate the economy of material, take the bridge’s Strain amount under the greatest weight tested, and divide it by the weight of the bridge (do not include the load’s weight). This result tells the amount of material (by weight) required for the bridge to bear the strain of a particular load.
Students could design and 3D print additional structural elements for their bridge design. These elements could be functional, decorative, or a mix of both. For example, in its basic design, the bridge has no actual road or railway between the spans. Students could model and print a series of roadway sections that attach to the bridge’s pins.
Students could also design a way to more easily attach a hanging weight below the bridge. The standard 3D pin and span files could be modified with such an attachment.
This basic design could also be modified into a suspension bridge design with two towers made of the existing spans, connected by a series of shortened or lengthened spans and pins to form suspension cables.
By modifying the standard bridge spans, students could design different types of bridges.
Students could also modify the standard bridge pins with hooks, or other changes to increase the functionality of their bridges.
Teachers can facilitate class discussion about the Bridge Design project by asking the following questions:
Two major characteristics of bridge designs are their rigidity and load capacity. What conditions would require different bridges to have different load capacities? What conditions would require bridges to have different levels or rigidity?
The basic bridge design in this project used a truss bridge with triangular bracings. What are the advantages of using triangles to brace a structure? In addition to bridges, what other types of structures benefit from triangular bracing?
Suspension bridges are another common bridge design, but unlike simpler truss or span-type bridges, suspension bridges use a series of towers and cables to distribute and bear a bridge’s load. In what conditions would a suspension bridge be preferable to a truss or span-type bridge?
A 3D printer with PLA, or another printer-compatible filament
The 3D model files for this experiment
3D modeling application
3D slicing software
Weights (at least 5 lbs)
Desks or stacks of books (a gap for the bridge to cross)
A yard or meter stick
A ruler or tape measure