ABSTRACTThe purpose of this project was to investigate and take part in the engineering design process through the research, design, and execution of spaghetti bridges under given constraints; each bridge was assessed on its strength-to-weight ratio under direct load. MAK Bridge Solutions first bridge ended up testing better than their second by a ratio of 1.33, and although their second bridge ended up taking 6th out of 8th place, each member was able to take away knowledge of the engineering design process.

INTRODUCTIONThe need for proposals for higher strength-to-weight ratio bridges are rising, especially considering recent catastrophic bridge failures like the Interstate 35W bridge collapse in Minneapolis in 2007, with the NTSB (National Transportation Safety Board) citing a design flaw a triangle corner failing as the primary cause.1 By adhering to the engineering design process, new bridge design solutions may be realized with scale models of such bridges made out of fettuccine pasta and household glue. By analyzing the way these models handle weight and pressure, teams may form better ideas in regards to how real bridge designs can be tailored to hold the maximum amount of weight while weighing the least amount. BACKGROUNDThe users of these new bridge designs would be the general public, as bridges are an archetypal example of a civil good. Although this project involves simply constructing a model of such a bridge under given constraints and then optimizing it through the engineering design process, the ideal end-result would be to discover innovative improvements to current, public-use bridge designs so as to lessen the chance of future bridge failures.

Due to the inherent complexity of materials science, several assumptions were made throughout the engineering design process, such as assigning consistent properties to each strand of fettuccini pasta (penne and linguini, in this case), as well as the consistency of the glue (traditional hot glue).

Design constraints included a maximum mass (fettuccini and glue) of 0.5 pounds (227 grams) of the bridge, a bridge span of at least 300mm, and a minimum height of 50mm. In addition, although not specified directly, the team placed a maximum width of the bridge to 3 inches, as that was maximum width of the end supports. Any brand of pasta was allowed, but only non-toxic, non-flammable, non-epoxy glue was allowed. The bridge was not allowed to encroach upon the minimum clearing zone of 8mm during the actual testing. Safety was observed at all times; no sharp edges, broken parts, hazardous materials, etc. were used/implemented. Care was taken while handling the hot glue and box cutter and during final testing, safety goggles were worn.

Relevant research included distinguishing between the many available, established truss designs for bridges, as well as an in-depth analysis of how bridges actually work.2 The research team investigated the properties of differing brands/types of pasta with different combinations of glue, and actual bridges both used in the past and in use today3 to determine the optimum design given the constraints and objective maximizing the strength-to-weight ratio specified below..For consistency and simplicity, each team was instructed to use the imperial system unit of weight, the ounce.

METHODOLOGY:Following the research phase, an initial proposal for the project was developed, and then acted upon based on a design sketch (see appendix A-1) agreed upon by the team members. With materials obtained from home and convenience stores such as Wollastons, construction of the first draft took place in Snell Engineering Center room 368 of Northeastern University. A post analysis of both the teams bridge as well as others consisting of photos, videos, and intergroup discussion allowed for improvements to the current design. Following the design, construction, and testing of the second and final bridge, the team members contributed to a written report detailing the steps of the engineering design process. The table beginning on the next page details the aforementioned tasks, self-assigned deadlines, assumed member responsibilities, and completion status.

TaskMaterials RequiredDateStatusWho

Project ProposalComputer and Project Notes9/15CompleteAll

Material ObtainingBox Cutter, 3 Boxes of Linguini, one box each of whole grain and gluten free penne, Glue Gun with glue, other types of glue for experimentation (gorilla, superglue), newspaper, ruler, design specs, computers, and calculator9/16CompleteArnold & Michael

Massing LinguiniLinguini, Electric Balance9/15CompleteMichael

Design DrawingPaper, Pencil, Linguini Mass Values9/16CompleteArnold & Karan

First Bridge ConstructionMaterials outlined in Material Obtaining task9/17CompleteAll

First Bridge TestBridge Prototype9/19CompleteAll

Final Bridge Box Cutter, Three Boxes of Linguini, Hot Glue Gun, Revised Design Sketch9/24CompleteAll

Final testFinal Bridge9/25CompleteAll

Written ReportComputer, Project Notes, Project Proposal10/2CompleteAll

Note: All constructions and brainstorming sessions will take place in the designated student room (SN 368) of the Snell Engineering Center. Research and work on the proposal and written report was conducted together, as well as on each members own time.

ALTERNATIVE SOLUTIONSAn alternative solution that the engineering team thought of was a variant of the truss bridge design with a triangular prism build ultimately implemented. Following the testing and intergroup discussions with other teams, a discussion over whether or not to pursue the same design or add another main support for the second draft of the bridge took place. Reasons for considering the rectangular prism build over that of the triangular prism included a greater distribution of force during testing (due to the increased surface area of the velcro with the bridge), which would allow for greater strength. However, more materials would need to be used, resulting in a greater mass and lower strength-to-weight ratio, as well as decreased stability as a result of shifting from a triangular cross-section to a rectangular one. In the end, the team collectively decided that the structural integrity of the penne casing with linguini would be sufficient enough to counter a large amount of weight distributed over a smaller surface area.

FINAL DESIGN SOLUTION & RESULTSThe teams first bridge had a mass of 7.90 oz, just under the 8.0 oz limit, but was able to hold past the available amount of weights that the supervisor provided (29 lbs). As a result, the teams final design emphasized a more conservative design so as to cut down on mass. These cuts included decreasing the height of the bridge from 12cm to around 6cm (Appendix A) Increasing the bridges height, while it would certainly decrease the bridges deflection under testing, would of course be accompanied by both an increase in mass and instability. Because post-test analysis of the first testing indicated negligible deflection (as compared to other teams whose bridges were shorter), the team collectively decided that decreasing the height to around 7cm would maximize the bridges strength-to-weight ratio while being tall enough to abide by the 8mm maximum deflection allowed.

Furthermore, the team paid greater attention to which pieces/structures of the bridge would be under either tension or compression based on a free-body diagram (Appendix B). It was determined that the trusses holding the two main structural pieces on the bottom of the bridge together would be under compression; simple experimentation with the available materials tension-to-mass properties confirmed that using two pieces of linguini for these trusses instead of six pieces encased by whole-grain penne would decrease the end mass of the bridge while maintaining its proper function (Appendix B).

As the team was rushed on time for the construction of the first bridge, craftsmanship became another focus for the final bridge. The trusses of the first bridge came out asymmetric, but in the second, came out symmetric. Aesthetics wasnt of highest priority, yet the correlation between that trait and function was a clear one, so the team emphasized glue conservation for the second bridge.

Contrary to team intention, the initial prototype outperformed the final product. The final bridge design employed two less supports at the ends of the bridge as well as less triangle trusses (which through research was established as the strongest type of truss2). While this was a minor flaw in the design, poorer performance could also be attributed to the fact that the trusses of linguini were intentionally not shielded by penne (to conserve on mass) like they were in the original design; the tension tolerability of the linguini was overestimated. That, finally coupled with human error in the manifestation of a poor hot glue job (i.e., not enough) on the top joint, can be blamed for the final bridge failure.

The table below lists the monetary costs incurred:ItemUnit#Total

Standard Box Cutter$10.991$10.99

Elmers Superglue (3 pack of 2g each)$2.391$2.39

Elmers Washable School Glue (polyvinyl acetate, 4 fl oz)$1.691$1.69

Elmers Glue-all max (0.5 fl oz)$3.391$3.39

Prince Linguine (1 pound)$1.993$5.97

Gorilla Glue (2 fl oz)$7.991$7.99

Barilla whole grain penne (13.25 oz)$2.491$2.49

Sam Mills Pasta doro Gluten Free (16 oz)$2.591$2.59

DualMelt Glue sticks (pack of 24)$12.291$12.29

Surebonder Glue gun$9.991$9.99

Newspaper, ruler, computers, calculators, and paper for design specsFree-0

Total (not inc. tax)NA-$59.78

CONCLUSIONS & RECOMMENDATIONSThe primary purposes of the project wasnt so much on the actual mechanics or foundations of civil engineering as on the engineering design process itself. Consequently, though the first draft outperformed the second by a factor of 1.33 on the scoring criteria, the team was able to experience firsthand the dynamics of working in a group on a strict time budget, skills that lecture cant pass on. The functionality of technologies such as AutoCAD was demonstrated first-hand, extending its use to outside preconfigured scenarios. Finally, all team members bettered their technical writing skills with the assignment of the proposal and the formal written report.

The design was sufficient, as it was backed by both the physics of free-body diagrams (Appendix B) and by standing bridges in use today. However, the implementation of the design - from shoddy craftsmanship to incorrect analyses of the properties of penne and linguini - would clearly not suffice. Consequently, for further engineering design projects, the MAK Bridge Solutions will undergo a more intensive, quantitative approach to materials testing, and raise craftsmanship standards. Tools to aid the process include designing a jig for the particular design and assigning job roles based off of comparative advantage.

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