STATICS DESIGN PROJECT 2008 Drs. Norman and Thompson OBJECTIVES 1. With a partner, design and build a truss to carry the maximum load per unit mass. 2. Predict the load-carrying capacity of your truss. DESIGN CONSTRAINTS AND RULES The truss building kit consists of 200 wooden splits, approximately 5.4" long by 0.2" wide by 0.04" thick; One 1.25-fluid-oz bottle of Elmer=s Glue-All No additional construction materials may be used. The truss will consist of two identical, vertical, two-dimensional (plane) trusses. The trusses should rest on nodes spaced precisely 16" apart (the actual gap will be slightly less than that). The contact surface between your truss and the supports should be horizontal only. The depth of the truss below the level of the truss supports cannot exceed 4". Its maximum height above the level of truss supports cannot exceed 10". The deck of the truss must be within one vertical inch of the level of the supports. A 7"-long by 3"-wide by 2"-high Avehicle@ with a wheel base of 5" must be able to pass through the truss along its deck and rest at the center of the deck. It will be the means whereby the load is applied to your truss. You must have two joints on your truss, centered, and spaced 5" apart where this load will be applied. Your truss must be a minimum of 3.5" wide. Since the truss is wider than the Avehicle@ you must supply cross beams to support the vehicle. These cross beams must be supported by the nodes of your two parallel trusses, with no supports in between your trusses. Your design must be a statically determinate simple truss. Do not over-constrain. This means: a. All joints must occur at the ends of the members (although lap joints may be used). b. Lap joints at truss joints, may not overlap more than the width of two sticks. c. Lap joints not at truss joints may overlap by any amount. d. Box-shaped cells should not have two members crossing diagonally in the truss plane, because this is an indeterminate configuration. e. No gussets are allowed at joints. f. Since the members joining the two plane trusses are ideally zero-force members, you may use diagonal members in addition to horizontal members to connect the two plane trusses. Since you will only receive one kit, double-check your design before beginning fabrication! Splints may be cut, split, or ganged to create members of various length or cross section. 1
TESTING PROCEDURES The student team will place their truss on the testing machine. The platen will then be lowered at a constant (slow) rate to apply the load to the truss. The instantaneous load will be measured and recorded by appropriate instrumentation. When the truss cracks or fails, or the load drops off and it appears that the maximum load has been achieved, testing will cease. The platen will be raised and the student team will return the truss to the table. The failure load will be recorded on the tally sheet and displayed. The trusses will remain on display for at least one day. DESIGN PROCESS Phase I -Preliminary Design - Evaluate three very distinct simple truss configurations (examples: Pratt, K, or Baltimore) of your choice. Use the computer program found at www.jhu.edu/virtlab/bridge/truss.htm to evaluate your trusses. You can easily use the online tool to modify pin locations for each concept. Use the truss criteria consisting of a span of 16@. In order to model a 100-lb load on your truss, apply two 25-lb loads spaced 5@ apart centered over the truss to check the loads in the members. From the three designs, pick one that you feel is the best choice for this project. Write your Phase I Design Report. Phase II - Detailed Design - From your preliminary design work, you will have selected one type of truss that you feel will be the best for this competition. In this step, you will perform a detailed design process using this type of truss. At the end of this design step, you will have the final design which you will build for the competition. Again, using the computer program found at www.jhu.edu/virtlab/bridge/truss.htm develop the best design of your truss for the competition. While you should primarily use the type of truss selected in your Preliminary Design phase, you may find that your final design may be a combination of truss types. Also, you may switch truss types if you feel this is warranted. You are given data in this handout concerning the maximum tensile load (stress) that the sticks can take. You are also given data on buckling of members in compression. Use this information to design your truss. While designing your truss, consider the length of each truss member along with the force it supports, and allocate the materials to various truss members as efficiently as possible. Do not forget to save some sticks to join your two plane trusses together. Some truss members may need two or more layers of sticks; others may need only one. This design process is an iterative process that might involve a lot of iterations before you have what you feel is the ideal design. Once you have settled upon your final design, identify which tensile member and which compressive member of the truss are most likely to fail first. Assuming the precise locations of your joints, use the methods of sections or joints to confirm the forces in the tensile and compressive members selected. Then, determine the maximum load your truss can withstand. Write your Phase II Design Report. Phase III - Construction - Upon submission of your Phase II Design Report you will be given your sticks and glue for construction. REPORT REQUIREMENTS 1. Phase I - Preliminary Design Report. Prepare a two page report. In this report include a snapshot of your three trusses with the forces shown (consider using Fullshot, Screen Capture, or the AConvert to PDF@ button on the web page to do this). Also, include your choice for type of truss to develop in your
detailed design. Justify this decision. Due Date: Friday, 15 October, 2008, 9:00 AM. 2. Phase II - Detailed Design Report a. Give a short description of the thought process you followed in producing your design. b. Prepare a CAD drawing showing top, front, and right side views. c. Document and annotate the calculations you did to design your truss. Include the member force analysis and member failure analysis. This failure analysis would include that performed use the Johns-Hopkins truss program and your method-of-sections and method-of-joints analysis of your critical members. d. Draw a separate full-page CAD drawing of the front view labeling each member. Denote which member you predict will fail first. e. Fill out the cover page supplied with the predicted values and locations and failure modes along with your name(s). Due Date: Friday, 27 October, 2008, 9:00 AM. 3. Phase III - Construction Your truss must be submitted by Thursday 6 November, 2008, 4:00 PM. If you changed your design from that submitted in your Detailed Design Report, you must submit an addendum to that report detailing the design changes and a new analysis using the Johns-Hopkins Truss Design program. Also, complete a new cover page. FINAL ADVICE THE COMPETITION WILL TAKE PLACE AT 3 PM FRIDAY, NOVEMBER 7. This project is supposed to be fun! It is not a major portion of your course grade, so don=t spend an excessive amount of time on it. Figure 1. Apparatus
Tensile Failure of Wood Stick Statics Truss Competition Loading Configuration and Dimensions Breaking and Buckling of Truss Members The force needed to break a member in pure tension is calculated from P = S u A where A is the cross-sectional area and Su = 8680 lb/in 2 is the approximate ultimate strength of the wood. The actual breaking strength of the members is found from experiments performed in 1994. The results are attached. Note that the reported results are for sticks that had a cross-sectional area of 0.012 in 2 whereas the sticks in the current project have an area of 0.0082 in 2. Therefore the experimental results need to be scaled DOWN: Failure of P tensile failure = P tensile experiment * 0.70 Failure of joints is complicated and is found from experimental data form 1994 tests. Note that the reported results are for sticks that measured 0.245 inches wide whereas the sticks in the current project measure ~0.200 inches wide. Therefore the experimental results need to be scaled DOWN: P lap joint failure = P lap experiment * 0.80 Buckling Failure If a member pinned at both ends is in pure compression, the member can fail either in ultimate compression or buckling. Buckling generally occurs for members pinned at both ends. The force cannot exceed the critical buckling load which is found from: 4
Tension Tests on Sticks with crosssection dimensions of approximately 0.050 X 0.240 Sticks Alone ¼ Lap ½ Lap ¾ Lap 5