APPL 160 Design Project

A. Oldenburg

Fall 2008

Due Date: Dec. 2 for everything

 

I could have you build a bridge, but some of you may have done that before. So, it will be tug-of-war! You will build a frame on a horizontal platform designed to accept a horizontal load applied 4 inches above the platform. You will write a report explaining the reasoning behind your design and showing calculations of the forces and moments on the members of your frame. Your frame will then be used to "tug" against an opponent's frame with increasing force, until one of the frames breaks apart. On the final day of class, we will have an elimination match to see whose frame survives -- there can be only one!

 

 

How it will be graded: Don't worry, only a small portion of your project grade will depend on how well your frame stands up in competition. Your grade will be comprised of the following:

 

Written Report -- 70 points

The written report should be composed of:

1)      Introduction. The basic idea behind your design and hypothesis about how it should work. Why do you think it is better than other designs? You may choose a more creative design that you are not certain will be better -- this is a valid (and highly encouraged) approach. This part of your report should be 1-2 pages, typewritten, and may include citations.

2)      Calculations of your design. For a given tugging force F, what are the forces and moments on each of the members in your design? Do you have zero force elements? I expect to see diagrams and equations here. This should be as many pages as you need, and does not need to be typewritten.

3)      Optimization. How do modifications of your design change the forces F? This may be difficult, but pick at least 2 aspects of your design (for example, an angle that a joint is at, or the length of a certain element), and show how changing that aspect will change the forces and moments in your frame. Motivate what values are actually best for your frame's structural integrity. Alternately, you may compare your design with 2 other significantly different designs. This part should be as many pages as you need, and does not need to be typewritten.

4)      Conclusions. Did you learn something from your calculations that was unexpected? Did you find that your frame should have a certain shape to be optimal? This part should be 1-2 pages, typewritten.

Additional credit will be given if you can account for "real world" factors that make a difference in the design, like the cross-sectional shape of the beams.

 

Execution -- 30 points

I will be taking photographs of every frame before the competition in order to gauge how well you built it. It will then be graded on the following criteria:

1)      Functionality. Did the frame fall within the design criteria (does it fit the rail and accept the load correctly)?

2)      Unfair Advantage. Was there evidence of the use of excessive materials and/or excessive glue? Did you follow the rules?

3)      Precision. Is the frame built carefully and not slapped together at the last minute?

4)      Competition. How well did the design stand up in competition?

 

Grades: This project is worth either 20% or 25% of your grade, whichever is more favorable to you, while the final exam will be worth the additional 25% or 20% (so the sum total of the final project + exam will be 45% of your grade). I see the project as a way of evaluating your creative skills and having your semester grade be based less upon being able to work quickly on a test.

 

Design Criteria:

 

Materials: You will be given

-         3 strips of balsa wood

-         6 rectangular blocks of balsa wood = "joiners"

-         1 bottle of all-purpose Elmer's glue

-         1 platform with a hook

 

The platform you get will look something like this:

 

 

which has two pencil lines along the side. These markings are telling you to avoid building your frame along the edge of the platform because it has to be able to slide into a rail like this:

 

 

So the pencil lines mark a safe distance away from the rail overhang. Your frame must fit within these lines, however, it may overhang the platform in the front or back, and it also may overhang the pencil lines a vertical distance at least 3/4 inch above the surface of the platform (to allow enough clearance for the rails).

 

You will build your frame only out of the glue and balsa wood materials provided. We have found that the all-purpose glue is extremely strong and unlikely to fail before the balsa wood does. You are only allowed to use glue where you are joining pieces together. The use of excessive glue outside of the joining areas is forbidden! If the glue oozes, just do your best to wipe the excess away.

 

You can use scissors to cut the balsa wood strips to whatever shapes you need, but be warned, you only have a limited amount of material to work with. (If you make a mistake and want a new strip, I can exchange an unbroken strip for your broken one). The rectangular blocks are helpful for providing enough surface area to join your frame to the platform. See this example:

 

 

You can see that each rectangular block here is used to connect strips of balsa wood to the platform. In this example:

 

 

two of the rectangular blocks in the center have been used for support of the load. This brings us to the final design criterion: how your frame must accommodate the load strip, which is how it will "tug"on your opponent. The load strip looks like this:

 

 

This strip will be placed vertically into your frame like this:

 

or this:

 

and the tug-of-war rope will then be connected to the hook in the exact center of the load strip.

 

The load strip is 1 inch wide x 8 inches tall x 1 1/2 inches thick when you account for the hook. (The wood itself is only 1/2 inch thick but the hook extends out of the center). Plan carefully to make sure the load strip can fit snugly against your load-bearing elements, yet, allow for passage of the hook in the center, and the rope that will ultimately be connected to your opponent. The load strip must be able to sit vertically on your frame and also must be able to stand right on top of the platform so that the hook is exactly 4 inches off the platform surface (so the tug-of-war is not occurring as some oblique angle).

 

You may come by my office to vet your design and see any of the pieces that will be used in the competition.

 

Other tips:

 

Review of Design Criteria (see text above for details):

 

  1. The frame must be connected to the platform within the pencil lines.
  2. The frame must not overhang the pencil lines less than 3/4 inch above the surface of the platform.
  3. Only provided materials may be used in construction of the frame.
  4. Glue is only to be used where pieces join together, excessive glue must be wiped off.
  5. The frame must accommodate the load strip which is 1 inch x 8 inches x 1 1/2 inches, and allow it to stand vertically and immediately on top of the platform, when under a load.
  6. The frame must also accommodate the tug-of-war rope in the center of the load strip without bending it. This rope may have knots in it making it effectively 3/8 inch thick.

 

Competition Day:

 

Right now the plan is for the judges (myself and a designee) to gradually apply tension to the opposite cables until one of the frames breaks. We may modify this to include a force probe if possible, to record the maximum rope tension achieved before failure. An elimination playoff will be performed until only one frame is left standing!