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Description
Students will design and build a truss structure in a group of four. They will compete against other groups and receive rankings based on a performance metric. This performance metric will factor in strength of the structure, cost of materials, and environmental impact. Each student in the group must come up with their own unique design for either a truss bridge or truss roof, which should be analyzed using the method of joints. All analysis should be completed by hand. Each student must identify the forces in each truss member and identify which members are in tension, compression, or experiencing zero force. Afterwards, the group must come together to decide on the pros and cons of each design and choose one to build. A short write up will be required to justify the design decisions. Lastly, students will construct the bridge using the provided materials to test the strength of their bridge and the accuracy of the calculations. Students must provide a written comparison and discussion of predictions and results, along with an explanation for discrepancies.
Engineering Topic Covered
A truss is a structure composed of thin, two-force members jointed together at their ends. These truss members will either experience tension, compression, or zero force. Two important assumptions that we make when designing and analyzing a truss are as follows:
All loading is applied at the joints
The members are jointed together by smooth pins
To analyze a truss structure, we assume the system is in equilibrium. The two methods used for analysis are the method of joints and the method of sections.
Learning Objective
Students will demonstrate the ability to make simplifying assumptions in constructing models of trusses
Students will demonstrate the ability to apply the method of joints or the method of sections to determine internal forces in members of statically determinate trusses
Students will demonstrate the ability to isolate portions of a structure and to draw their appropriate free body diagram and solve equilibrium equations
Students will be introduced to the process of design
Procedure for Design Challenge
Each student in the group must come up with their own unique design, which should be analyzed by hand using the method of joints.
Each student must identify which members are in tension, compression, or experiencing zero force.
The students come together to discuss the pros and cons of each of their designs with their group of four and choose one to build.
Students must document and justify their design decisions.
The group constructs the one bridge using the materials provided.
The competition: The bridge is to be supported by the acrylic pin and rocker support and elevated between two chairs or desks. Then, using a lanyard clip or carabiner, a string must be attached to the specified loading location as illustrated in the image of the design space. A hanging mass should be used to represent the point load. (Ideally, this step can be implemented in person during a small competition event with all students who chose to participate in the challenge)
The minimum loading requirement of 5 lbs is applied first. If the structure does not fail, gradually increase the load in increments of 0.5 lbs until failure to find the maximum load capacity.
Students must calculate their figure of merit and submit to the instructor to determine the winner.
Lastly, students will complete a write up comparing their predictions and results and discussing reasons for any discrepancies.
Results from Testing:
Results from Testing:
The results from the Truss Design Challenge tests proved that the materials chosen are able to satisfy the desired functional requirements. Both the lollipop sticks and the wooden dowels are able to break and demonstrate failure, and the cardboard gusset connection is secure enough to allow for visualization of truss member deformation. The wooden dowels are stronger in compression and able to withstand heavier loading, which prompts us to assign more favorable values to the lollipop stick for the other design parameters, including cost and environmental impact score. Incorporating these tradeoffs and having different advantages to each material enables this challenge to be open-ended and helps incentivize students to be more creative in their designs.
Analyzing the Strength of Truss Members
In order to logically scale the cost and environmental score values for each truss member, several loading tests were conducted to assess the strength of the materials. After obtaining that information, we were able to figure out appropriate values to assign so that the challenge would be more complicated and require tradeoffs in certain aspects. Below is the results of the analysis:
a. Truss structure using only wooden dowels
The lateral bracing rod broke at about 18 lbs for a point load test (a hanging mass from one of the rods), so we tested with a distributed load to better assess the compressive and tensile strength of the wooden dowels. The maximum distributed load at failure was 29.4 lbs. Member AB in Figure # was the first to break due to compression. This compressive force was calculated to be 16.97 lbs.
Physical implementation of truss structure (a). Multiple members broke but the far left wooden dowel was the first to break from compression. Broken sticks are indicated by the purple arrows.
b. Truss structure using only lollipop sticks
The maximum distributed load carried for this test was 13.2 lbs. After using the method of joints to solve for the forces in all members, the maximum compressive force experienced by the failed lollipop stick was determined to be 7.62 lbs. The broken physical structure can be observed below.
Physical implementation of truss structure (b). Multiple members broke but the far left wooden dowel was the first to break from compression (indicated by the purple arrow).
c. Truss structure using combination of both
For this test, wooden dowels and lollipop sticks were used for the members experiencing compression and tension, respectively. Failure occurred at 22.4 lbs using a distributed load on top of the structure. By modeling this experiment as two equal point loads of 11.2 lbs the maximum compressive force experienced by the broken wooden dowel was calculated to be 12.9 lbs. The forces in the members were calculated using the method of joints. The figure below shows the physical structure after failure. The members that failed match with the predictions of the analysis.
Physical implementation of truss structure (c). The far left wooden dowels were the source of failure. Broken sticks are indicated by the purple arrows.
2. Material Analysis for Connectors
Several different materials were tested to assess their feasibility as a connector between truss members. Below is a comprehensive list of all of the conducted tests, along with the reasons that they are not suitable for this application.
a. Smooth foam spheres (with glue)
These smooth foam spheres were too weak, even with the addition of hot glue. The source of failure was due to a weak connection and we were unable to see any deformation of the truss members.
b. Rough foam spheres (no glue)
This structure quickly collapsed and was unable to support any hanging load above 1.8 lbs.
c. Foam core k’nex inspired connector
This connection was slightly stronger than the rough foam balls without glue, but still failed relatively quickly. It could not support a hanging mass over 2.0lbs. The truss member sticks were not very secure since the foam core was fairly thin.
d. Foam core circles
Using wooden dowels and lollipop sticks
This connector performed well when we initially tested it and we were hopeful about this option. However, when we constructed a slightly larger structure and applied a hanging load, we observed the members that were experiencing tension were being pulled out of the connectors for loadings above 1.2 lbs.
d. Foam core circles
Using wooden dowels and string
Swapping out the lollipop sticks for the string solved the issue of the truss members that were popping out of the foam core circles from being in tension, however, instead of members compressing or buckling, sticks in compression would just lodge further into the foam core and compromise the stability of the structure
e. Rough foam spheres (with glue)
This connection was very strong and deformation of truss members was easily observed. This would be a viable option for the challenge, however, we decided to go with cardboard gussets to minimize cost and environmental impact. Foam spheres would have contributed ~$200 to the cost of this challenge and polystyrene manufacturing and waste is very harmful to the environment.
f. Ring terminal connectors
The metal rings on these terminal connectors were weak and bent fairly easily. As soon as they would bend, the structure would become unstable and collapse sideways. Even with reinforced lateral bracing, this problem still persisted.
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