Balsa Wood Bridge
Abstract
Faculty at Central Washington University proposed a challenge to mechanical engineering students that could be accomplished in an in-home setting. The goal was to create a balsa wood bridge, weighing no more than 85 grams, that can support a load over an open span and raise above its resting position by means of a mechanical system. To produce a successful solution to the problem, a vertical lift bridge was created consisting of two lifting towers and a load bearing bridge. Using equations of static equilibrium and strength of materials, the required width for each member was determined. Project requirements were accounted for with multiple supporting analysis on various parts in the design. The contents of each structure were limited strictly to balsa wood and glue, by evaluating both the tensile and shearing strength of each, stress concentrations were identified and mitigated. The construction of the device was achieved by setting up multiple fixtures to manufacture and join each component of the assembly. Testing is achieved with multiple nondestructive procedures first, followed by a final load bearing test. The device when tested can successfully raise and lower through use of the articulating components. When at rest the device can support the static testing load of 20kg and meets the weight requirement at 84.4 grams.
Balsa Wood Bridge Design Information
This years projects were limited to at home resources due to the circumstances involving the global pandemic. As one of the solutions, building an articulating balsa wood bridge capable of supporting nearly 20 kg, and weighing no more than 85 grams was proposed as the challenge.
Preliminary Drawings
These are the original sketches done to determine which would be most practical for the specified criteria. The focus of this exercise was to decide on a truss pattern that could realistically be made at home while still supporting the project's requirements
Summary of Analysis
The strength of the selected truss was determined with static equilibrium equations. By applying the maximum load to the center of the truss, the reactions in each support member was found. By identifying load concentrations throughout each truss design, the analysis of which design would distribute the weight more evenly could be evaluated. After testing two variations of the same truss design with different diagonal support angles, it was clear that maximizing this angle will provide more support for vertical loading.
Construction
The techniques used to manufacture each part revolves around simple fixturing. The purpose of this approach is to create an accurate repeatable means of fixing the part in place as cuts are made. The image shown to the left is a fixture used to cut 1/4" and 3/8" square stock for the upper and lower long sections of the bridge. For the remainder of the parts, more fixtures will be developed.
Project schedule
The construction and assembly of the device have been completed. The stages of testing and evaluating the project are underway and will proceed with formal use of testing procedures and data sheets. Two of the three tests have been completed while the approach to the final load bearing test is still being developed. As depicted in the gantt chart below, the plan to preform evaluations will be completed by the first week of May.
Budget
The funding for the project will be provided by the student leading the design. Labor cots for cutting and assembling the wood pieces will not be included in total cost. The Balsa wood and Glue will be purchased through amazon, while the lifting components will be sourced from Etsy. The necessity for three different shapes of wood, pulleys, and string brings the estimated total for this project to be around $165, this includes the cost for shipping and taxes. The rates of balsa wood far outweigh the cost of other components included in the full assembly. This value is higher than if the components could be purchased to order. After the Bridge is assembled, waste and excess material is expected.
Testing
After going through the first test, aspects of performance were compared with the predicted results. The parameters of the articulating test included three different recordings and required the system to smoothly raise and lower without intervention. To properly evaluate the function of the design the maximum height, time at rest in raised position, and force required to lift one end of the bridge was be recorded, the basic requirements for each are 140 mm, 10 seconds, and 10 grams respectively. The second test was intended to evaluate the dimensional accuracy and precision of the design. Although its crucial that the bridge must fall within the parameters to be considered successful, the extent of the second test is not any more complicated than taking a few measurements and ensuring pieces are consistent. The third test will consist of a single load bearing measurement on the bridge, all other testing must be completed before this is attempted in the event that the bridge failed under pressure. By attaching a hanging apparatus to the road deck, a five gallon bucket will be attached underneath. The weight will be reached by slowly adding sand, once all is added, the bridge must freely support the weight.
Results
After preforming evaluations on driving aspects of the project, the success of the bridge could be accurately depicted. Throughout the testing process, two specific parameters were put in question. The height of the bridge from the abutments and the amount of force required to activate the lifting system failed to meet their intended marks. The height of the road deck exceeded what was required by .5mm. This value should indicate a failure however the initial parameters of the project allow for a 1mm deviation. The excessive force required to activate the lifting system is a function of how each spool was assembled. If each component of the spool was manufactured with more precision, the amount of force required would be closer to the design value. All other parameters for the design of the bridge were met. The structure weighs less than 85g, cleans an open span of 400mm, can smoothly articulate to allow passage beneath, and supports the static load of 20kg. Even though one of the requirements was not met, the overall result of this design has proven to be successful.
