Boomilever Project

Team “Truss Me”

Amanda Resha
Ridge Coffman

ENGR 2110-01

Engineering Statics

November 24, 2014

Table of Contents

3 – Report 3 – Introduction 3 – Literary Review 3 – Frist Design 4 – Second Design 4 – Final Design 5 – Conclusion
6 – References
7 – Appendix

Introduction: The purpose of the Boomilever Project is to build a cantilevered truss that is light-weight while still able to support 15 kilograms. With this project it serves to reinforce the cumulative concepts that have been taught throughout this semester of Engineering Statics. It has been necessary to research designs, types of wood, and types of adhesive that will create a final result that is consistent with the desired result. Literature Review: The design concept behind a cantilever truss consists of a series of triangles joined together. The strength of these individual triangles, when combined, creates a very strong final result. Triangles are a very important part of many types of construction as they are the “only geometric figure that cannot be pulled or pushed out of shape without actually changing the length of one of its sides” (Bridges - Trusses, n.d.). As well as the advantage of the added strength, truss also do this without much additional weight. These concepts combined allow for trusses to be a very efficient way of adding strength. A cantilevered beam is what of which is supported on one end (usually a fixed support) and end is left hanging on the other end (Hibbeler, 2014). Cantilevered beams are able to support a load on the free-end. Cantilever trusses are popular in bridge and building architecture and possess many different uses. A cantilever truss with only a fixed end and a free end supporting a load and the negative y direction will translate the load across the supporting cantilever member and will thus have a positive y reaction at the fixed support since the sum of all reactions in a truss must equal zero (Hibbeler, 2014). Often times in a cantilever truss several zero-force members may be added in (Hibbeler, 2014). While these may often be zero-force members, they are not unnecessary. These members are used to keep the main cantilever member from bending. It makes sense that cantilever feature in a bridge must remain rigid while vehicles and pedestrians are commuting over it. One example of a cantilevered bridge is the Forth Railway Bridge in Scotland, UK. This bridge was completed in 1889 and is one the earliest examples of a cantilevered railway bridge (The Earthquake Engineering Online Archive, n.d.). First Design: The process of designing the Boomilever evolved with each design. The first was the most complex of the designs. The initial design was two triangles with a height of 20 centimeters and a width of 40 centimeters. Within the main frame, 5 supports, spaced eight centimeters apart, were added. Additional diagonal supports were added as well. The triangles would have been spaced approximately 5 centimeters apart and connected with strips of balsa wood. Upon analysis if this design it was discovered that the internal support members were all zero force members. Zero-force members are supports in a design that are neither in tension or compression, as the members have no force going through them. The only use of these members in the design would have been to prevent members from flexing in the truss; however, as weight is to be considered and flexing would be minimal, the zero-force members were removed upon re-design of the boomilever. After studying the behavior of the wood and future designs the conclusion was drawn that, if the boomilever were to fail, it would be approximately mid-way on the piece of balsa wood supporting the hypotenuse. (See pages 7 and 10 for design and calculations) Second Design: After the analysis on the first design brought to light the zero for members, it was decided that these members should be removed. In order to combat the flex of such a simple design the decision was made to double the thickness. This design would have the same outer frame of two triangles with a height of 20 centimeters and a width of 40 centimeters. Instead of being spaced five centimeters apart the wood be bonded directly together using Gorilla Glue. Balsa wood, a very lightweight wood with properties similar to a dense foam, was used for the main supporting member. Basswood, a lightweight yet firm wood, was used for the horizontal member. The final design added a triple layer sheet of balsa wood to rear side of the truss. This balsa sheet was added for the purpose of mounting and supporting the main structure. Additionally, a five centimeter square frame of bass wood was added at the end of the boomilever to place the loading block for testing. Once this design was implemented two key issues were discovered. First, due to the properties of the bass wood and balsa wood it was obvious that the placement needed to be reversed. The bass wood should be used for the hypotenuse and the balsa wood should be used for the horizontal member. Secondly, the five centimeter square frame attached to the main frame was not strong enough to support the needed load. This piece would be the most likely point of failure. (See pages 8 and 10 for design and calculations)

Final Design: The information gained from the second design and subsequent failure led to the third and final design. The third design consisted of design elements from both the first and second designs. There was a return to the original 3 dimensional design of the 2 triangles spaced five centimeters and attached with balsa wood. This decision was made due mainly to the need to support the loading block for the 15 kilogram load. Leaning on the second design, it was decided to continue forward with no internal supports. The internal supports will only add weight with minimal usefulness to the structure. In order to attach the boomilever to the support wall, a twenty-centimeter sheet of balsa wood with a thickness of approximately one half of a centimeter would be fixed to the vertical members of the boomilever. In order to keep the overall weight of the design low, a rectangular section of the aforementioned balsa wood sheet was removed. This piece was not needed as it only helped to distribute the load onto the support wall, which the two rigid vertical members were more than capable of doing. A hole of diameter one-quarter of an inch was drilled into the balsa wood sheet in the approximate middle. This would place the boomilever being connected to the support wall at approximately seventeen and one-half centimeters from the bottom of the vertical members. On review of the boomilever report guidelines, it was discovered that while the boomilever had a horizontal length of forty-centimeters, that the testing load had to be applied at forty-five centimeters from the mounting hole. While a slight inconvenience, an overall simple addition would need to be carried out. A small square rigid body with area of approximately five-centimeters by seven-centimeters would need to be added to extend the boomilever to the correct length. Upon visual and tactile inspection, the boomilever appeared to be fairly rigid with structurally sound connections between each of the separate members. At this point I would expect the boomilever to fail somewhere in the two and one-half by five-centimeter area in-between the mounting hole and the top of the vertical member. Additionally, the connections of the new addition have a potential for failure as they weren’t figured into the original design. (See pages 9 and 10 for design and calculations)

Conclusion: In conclusion, this project required the use on all the knowledge that was gained throughout this semester of Engineering Statics. One major piece of knowledge that was gained is that when the main supporting member has the same slope throughout and vertical supports are added the vertical support will be zero force members. While many designs were attempted and discarded, there is a strong confidence in the final design and execution. This confidence comes from the research, successes and failures along the way.

References
Bridges - Trusses. (n.d.). Retrieved from Science Encyclopedia: http://science.jrank.org/pages/1031/Bridges-Trusses.html
Hibbeler, R. (2014). Statics and Mechanics of Materials. River: Pearson Prentice Hall.
The Earthquake Engineering Online Archive. (n.d.). Retrieved from NISEE e-Library: http://nisee.berkeley.edu/elibrary/Image/GoddenD28

Appendix

(First Design)

(Second Design)

(Final Design)
Calculations

First Design:

Second Design:

Third Design: