Name ______Date ______

You will design and build a model through,truss bridge from balsa wood that will span a gap of 12 inches. The bridge will be load tested to evaluate how close you are to an efficiency rating of 2000. You will submit ashort technical report. (2 photos and a Data and Discussion)

Procedure: (Document each part of your engineering design process in your log with a title, and date) There will be periodic design log checks during this project. If you are absent, it is required for you to make up the missing time.

_____1. Complete a “Design Brief” in your log with a labeled diagram of the test apparatus.

_____2. Read the entire project handout and packet to complete a list of 20“Specifications” in your log.

_____3. Complete the “Bridge Science-Based Objectives” in your log. Use the handout to help you think.

_____4. Type in “Balsa Bridge” into Google Images to see different ideas for Balsa truss bridges. Look at how

people treated members under compression compared to how they designed members under tension.

_____5. Brainstorm ideas, and make “Thumbnail Sketches of 6 different ideas in your log.

_____6. Select one idea that will be efficient and meet the specifications and make a rough sketch of it in your

log. Color Compression members red, tension members blue, violet for both compression and tension.

_____7. In your packet of information, research the type of joints that are possible for balsa wood beams. Some are better for compression, some are better for tension and some better under shear stress.On your sketch, indicate the type of joint you choose for each joint on the bridge. Use a key to help identify these. Lamination can double the cross section of a member or part of a member, or can be used to create gusset plates on particular joints. Decide if you will laminate any of the beams, parts of beams or joints and identify the location of laminations on your sketch and add it into your key.

_____ 7. In your log, make a bullet point list of “Design Features you have selected for your bridge so it will be strong and efficient.

_____ 8. Make a full-size three-view set of orthographic engineering drawings-top, side and end views. The side view is placed directly above the top and end views. Use a 1:1 scale. Draw an information box. Use proper dimensioning.

_____9.Calculate the total length of balsa required to build your design and how much you expect it to weigh based on the average weight of a strip of balsa. In your log create a Bill of Materials” similar to Figure 61.

_____ 10. Inspect and select your balsa wood strips to store in your bin. Note that not all balsa is cut from the same part of the tree, and so each differs in mass, strength and elasticity. These properties may be useful in increasing efficiency depending on whether a member is under tension or compression.

____ 11. Place your engineering drawing under a piece of wax paper on top of a foam construction board. Measure and cut your balsa strips to create the members of your bridge. It is extremely important to make accurate measurements, so the bridge will be symmetrical and all the joints fit together tightly. It is better to cut a balsa strip a bit too long, and trim off extra to get the right fit, than to make it too short and have a gap at the joint. Adding extra glue around a loose joint will not strengthen a loose joint.

____ 12.Learn joint making and pinning techniques. Construct your bridge using the joints you had planned. Use pins, clamps, and string to hold joints together. Safety note: Use extreme caution when using craft knives or other cutting tools. Always cut on a hard surface and away from the body.

____13. Conduct a bridge inspection to check all connections. Use a ruler to check to make sure your bridge meets the specifications. Meeting the specifications is an important part of your grade for this project. Weigh your bridge and check that it meets the mass limit of 18 grams. You may have to remove parts if it does not meet the weight limitation.

____ 14. Take a photograph of your finished bridge. Check your photo before load testing.

____ 15. Just before load testing, the teacher will weigh your bridge and you should record its structural massin your log. The teacher will load test the bridge by suspending a container from a block inserted onto the deck of the bridge and adding sand to the container at a slow steady rate, until either an audible cracking sound together with visual evidence indicates failure of some structural member or glue joint of the bridge, or until the reference point on the deck at the centre of the span has lowered by more than 1.0 cm. All decisions of the teacher are final. The critical load, which is the total mass of the test structure, container, and container contents shall be recorded in your log. Calculate the efficiency of your bridge and record it in your log. Efficiency = Critical load (kg)/structural mass (kg). While the bridge is being tested, observe carefully and write observations of how the failure occurred and its location. Take an after -photo, especially focusing onmaking sure to show the point or points of failure.

_____ 16. For the Data part of the “Data and Discussion”, in your log create a data table of qualitative and quantitative results for all the groups. The table will contain, each bridge, its structural mass, critical load, efficiency, statistical analysis, design features of each bridge andyour qualitative observations about how it failed.

_____ 17. Write a Discussion in your log that analyzes and addresses the following:

  1. Patterns and trends you notice in the data.
  2. In your opinion, what made the bridge or bridges with the highest ratings most efficient?
  3. In your opinion, what made the bridge or bridges with the lowest ratings least efficient?
  4. What were the tradeoffs that you encountered in designing your bridge?
  5. Was your bridge in the top 25 %, middle 25% or bottom 25%. What features contributed to this result and why?
  6. How should your bridge design be modified in the next prototype?

_____ 18. Type up and submit your Data & Discussion, with labeled Before and After photos of your bridge

In your After-photo, label the point or points of failure.

Assessment Criteria:

  1. The lab safety rules, must be observed at all times. Safety glasses must be worn when testing the bridge.
  2. Materials and equipment must be used appropriately and not wastefully. Your workstations must left clean.
  3. Any project time that is missed due to your absence from class must be made up in order to receive credit.
  4. While working in your team, stay on task. Do not chat with people outside your group. Work together efficiently and share the work equitably.
  5. Though you are working collaboratively, your ideas must be your own in your individual written work.
  6. The bridge will be assessed for workmanship and whether it meets the specifications.
  7. The bridge will be assessed for its workmanship. (for example, joints, symmetry)
  8. The bridge will be evaluated for its efficiency. [top 25% (A), middle 50% (B) or bottom 25% (C).]
  9. Your log is to be handed in to receive credit for this project.
  10. Your Data and Discussion must be typed and handed in to receive credit for this project.

Bridge Science Based Objectives Name ______Date ______

Directions: Use this page to answer the questions. Make a list of science-based objectives in your log. Use this handout to think.

Question / Answers
  1. In a truss, Where are the struts and what type of stress is in them? Where are the ties and what kind of stress is in them?

2. What kind of stress is in the top cord and what type of stress is on the bottom cord?
3. What are the different types of loads: dead and live, static and dynamic- and which type or types will your bridge carry?
4. How many main trusses will your bridge have? Why can a truss bridge be built longer than a Girder or simple beam bridge?
  1. What is equilibrium and how does it apply to your bridge design?

  1. What are compression, tension, shear and torsion stress and where will they likely occur on your bridge?

  1. How does cross sectional size affect members in tension?

  1. How does the length affect members in compression?

9. How does cross section size affect members in compression?
10. What is lamination? For what kind of members can it be useful and why?
What is the trade-off in using lamination?
  1. How does length affect members in compression?

12. What is efficiency and how will it be measured?
13. What are three ways a bridge can be made efficient while carrying its load?
14. What will happen to the symmetry of the bridge and the tightness of the joints if members that are supposed to be identical in size are not due to measurement and cutting errors?
15. What is the difference between joints that connect the side grain of the wood, and joints that use the end-grain of the wood?
16. Are there any particular joints recommended under either compression or tension or other types of stress?

Bridge Project Performance Assessment Name ______

Team Members: ______Date ______

Criteria /

Description

/ Rating: 4 = excellent,
3.5 = good, 3 = fair, 2.5 needs improvement 2 = poor, 1= attempted / Wt. / points earned
Safety / Safety procedures were always followed without reminders. / 4 3 2 1 0 / 2
Cleanup / Materials and equipment was always put away. / 4 3 2 1 0 / 2
Collaboration / The group members always listened, and supported each other’s ideas and efforts. The group dynamics encouraged each person’s equal participation in the work. / 4 3 2 1 0 / 2
Personal Responsibility / The individual made excellent use of time during project remaining focused on the project. Any project time that was missed due to absence and/or lateness was made up. / 4 3 2 1 0 / 4
prototypes / The group met completion deadlines for their bridge. / 4 3 2 1 0 / 3
Device Design / The device was constructed within the product specifications. The device showed the application of creative and scientific thinking / 4 3 2 1 0 / 3
Device Workmanship / The device was symmetrical, and had no loose joints. Extra glue was not used instead of careful measurement and cutting. Joints were planned to match the particular location. / 4 3 2 1 0 / 3
Device Performance / The bridge had an efficiency rating of least 2000, which meant it was strong and light. / 4 3 2 1 0 / 3
Engineering Log Quality / Entries in the Engineering log were complete, and developed with sufficient detail and substance. Log entries were titled and dated. / 4 3 2 1 0 / 3

Comments:

Bridge Short Technical Report Assessment Name ______Date______

( handed in on ______On Time: _____) Technical Report Grade ______

Criteria /

Description

/ Rating: 4 = excellent,
3.5 = good 3 = fair 2-1 = effort / Wt / Total
Title Page /

The title page includes a descriptive title, name, course, and date.

/ 4 3 2 1 0 / 1
Photos / The prototype design and results of load testing are represented by Before and After photos with labels that reflect computer graphic skills. The photos are properly sized and have titles and Figure numbers. The Before photo is clearly labeled and keyed with dimensions and design features. The After photo shows the point or points of failure, which is labeled. / 4 3 2 1 0 / 2
Technical Writing / Report is a finished piece of writing that has been through all the steps of revising, proofreading and editing; Each paragraph addresses a single topic and has a single primary focus stated in a topic statement, accompanied by supporting detail sentences; Paragraphs flow from one to the next. Report is written in past tense; The writing is precise, factual, objective, unemotional and unbiased. The meaning of statements is clear and specific. / 4 3 2 1 0 / 2
Data: / The report contains both quantitative data and qualitative observations on the technical performance of each group’s prototype in a table. Data includes Structural mass, Critical load, Efficiency, Statistical Analysis, Design Features and Qualitative observations for each groups prototype. Data is organized in tabular form and is easy to understand; Measurements include headings and units;
Discussion of the data focuses on comparing the average performance of the prototypes to each other and the project objective. Major changes and reasons are explained and analyzed in the discussions. / 4 3 2 1 0 / 3
Discussion: / Discussion of the data focuses on analyzing the results of the load testing for all the bridges. The discussion should identify trends in the efficiency results and design features of the bridges. To compare prototypes, numerical language should be used. / 4 3 2 1 0 / 3
Engineering Drawings: / Orthographic drawings consist of three views, neatly drawn, with straight lines. Drawings are a 1:1 scale, properly dimensioned, and clearly show the types of joints used. Drawings include an information box with name, scale, and date. / 4 3 2 1 0 / 3

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