STATICS DESIGN PROJECT 2006 Drs. Dewhurst and Thompson

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1 STATICS DESIGN PROJECT 2006 Drs. Dewhurst 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 overall length of the truss can not exceed 21". The base will rest on supports that will be 16" apart (so each truss should be nearly 18"). The height of the truss= deck above its base cannot exceed 1/2" while the depth below its base cannot exceed 6". A 7"-long by 3"-wide by 2"-high Avehicle@ with a wheel base of 5" must be able to 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. The contact surface between your truss and the supports should be horizontal only. Therefore, your deck members should extend up to an inch beyond the truss joint at the supports. Since you will only receive one kit, double-check your design before beginning fabrication! Splint may be cut, split, or ganged to create members of various length or cross section. Your design must be a statically determinate simple truss. Do not overconstrain. 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. 1

2 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: inverted Pratt, K, or Baltimore) of your choice. Use the computer program found at 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@. Apply two 50-lb loads spaced 5@ apart centered over the truss to check the loads in the members. In the computer model you are modeling a single truss. In the competition, you will have two parallel trusses. Therefore, the loads in the program are twice the loads a single, plane truss will see. From the three designs, pick one that you feel is the best choice for this project. Write your Phase I Design Report. Due Date: Friday, 27 October, 2006, 9:00 AM. 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 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 maximimum load your truss can withstand. To keep track of the number of sticks and the total length of sticks you are using, you might want to set up a spreadsheet (e.g. in Excel). This could be accomplished by entering the coordinates of each node, and letting the spreadsheet calculate the distance between nodes (plus overlap). Then, you could enter how many sticks are used to construct that member. Finally, the total length of sticks used could be calculated. This step is not required, but it is recommended. Write your Phase II Design Report. Due Date: Friday, 3 November, 2006, 9:00 AM. 2

3 Phase III - Construction - Upon submission of your Phase II Design Report you will be given your sticks and glue for construction. Due Date: Friday, 10 November, 2006, 9:00 AM. 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, 27 October, 2006, 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, 3 November, 2006, 9:00 AM. 3. Phase III - Construction Your truss must be submitted by 9:00 AM, Friday, 10 November, 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 THE SAME DAY. This project is suppose to be fun! It is not a major portion of your course grade, so don=t spend an excessive amount of time on it.

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5 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 The results are attached. Note that the reported results are for sticks that had a cross-sectional area of in 2 whereas the sticks in the current project have an area of in 2. Therefore the experimental results need to be scaled DOWN: Failure of Joints 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 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: 5

6 Tension Tests on Sticks with crosssection dimensions of approximately X Sticks Alone ¼ Lap Joints ½ Lap Joints ¾ Lap Joints 6