Introduction: Investigation: K'nex Bridges

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Hey All!!

Today, I present to you my latest Instructable: An investigation! In this Instructable, you will be guided on designing, developing and making your own bridges using K'nex. Stumbling across the Scientific Method Contest, I wondered if there was a way to link the world of K'nexing in with Science. This is the final product that I came up with, which I am very pleased with. :-)

The objective of this investigation is to find the best design for a bridge, that is able to hold the most weight possible. So the question for this Investigation is:

What is the best design for a bridge made from K'nex, able to hold the largest amount of weight?

I hope you enjoy this Instructable as I had making it!

Also, this is (correct me if I am wrong), the first K'nex investigation on Instructables! :D

This is for the Science Method Contest, and so I would really appreciate a vote - it would make my day!

Anyways, lets proceed with the investigation, beginning with the hypothesis on step 2, and designing on the next step! :-)

Step 1: The Variables, Regulations and Beginning Design Ideas

Firstly, lets begin the investigation by starting designing, what could be bridge designs for your K'nex bridges for the test. However, lets start by stating the different variable as well as the regulations for this test.

There are three different types of variables for tests in general. These are Control, Independent and Dependant. Below are are the different variables, categorised between the three. If you think of any more variables, feel free to comment.

Control Variables

The Control Variables (the variables we keep the same) of this investigation are:

  • The blocks are kept the same
  • The blocks are identical to each other
  • The blocks are kept the same distance away for each test (25cm)
  • The weights are put in the middle of the bridge
  • The same types of weights are used

Independent Variable

The Independent Variable (the variable we change) of this investigation is:

  • The bridges
Dependant Variable

The final variable, the Dependant Variable (the variable we measure) of this investigation is:
  •  The maximum weight the bridge can hold before it collapses.
Now with all the variables stated, here are the regulations (rules) for this investigation


The bridge should be....
  1. Made from K'nex
  2. At least 30cm in length
  3. Able to support its own weight without collapsing
Here are some questions that you should think about before beginning your design:
  • How can you make your bridge really strong? Jot as many ideas as you can down on paper
  • How can you prevent your bridge from sagging in the centre?
  • How can you prevent the bridge breaking under its own weight?
Once thinking about these ideas, it is finally time to begin designing your bridges! Above are 2 designs of 2 that I made. This is how you should try to present your ideas as they will be easy to develop. These designs are rough and so a ruler is not required. Be sure to use a pencil in case you wish to erase anything :-) 

For a new design, you should start fresh on a blank piece of paper and make sure that you have a sharp pencil when sketching.

You done that already!?! Wow, in that case, proceed to the next step, where you will be thinking of a Hypothesis and developing your designs further! :D

Step 2: Developing Ideas and Hypothesis

In this step, you will be developing your ideas that you sketched in step 1 into scale drawings and designs with colour. This will help you in the building process of your bridges. Below one example of a scale I used for one of my bridges. In order to do a scale for your bridge, you will need to have a set number of squares for each rod. For my one I used 4 squares for a blue rod, 1 square for a purple connector and the hypotenuse was the yellow rod.

For this part, you will need to use a pencil (like in step 1) a ruler (for straight lines) and ideally some graph paper for neatness and precision (this is all shown in picture 2). This will make it much easier when making your bridges!

At this stage of the investigation, you should also come up with an hypothesis (what you think is going to happen). You must also give a reason why you think this.

My hypothesis is that I think bridge D will be the strongest to hold the most weight since it is supported from all four sides, although it is the longest (meaning it may sag more in the middle).

After you have finished with this part, and have your scales and developed drawings of your bridges, it is finally time to build! :D Yay!!!

Step 3: Begin Building: Part 1

Finally!!!!! It is time to take out your K'nex, and begin tinkering away at your ideas. In total, I made eight bridges for my investigation, but it is entirly up to you how many bridges you wish to test! :D

While building, you should use your designs for reference to help you build (picture 3). This will make it far easier to make. You should test particular aspects of your bridge in this step, to ensure it is strong! :-) 

When you have got the basic structure for your bridge, move on to the next step (part 2).

Step 4: Building: Part 2, Finalising Bridges

In this step, you will be finishing off the bridges and seeing how well they match there designs you did previously. As shown below, you should finish off all your bridges and line them up as shown in picture 2.

You should do steps 1-4 for each of your bridges. After this, you should see if they have similarities with your plans that you did. :D

After you have finished your bridges, onto step 5! :-)

Step 5: Drawing a Table of Results

Next, you will be doing a Table of Results. This will be what you will use to record your results. You should have 3 tests for each bridge (in order to increase reliability). You should then have an average column, which you will work out after the test. For each bridge, you should use a letter to represent them.

After you done this, yes, we will be preparing the test. :D

Step 6: Preparing for Testing

In this step, preparations for the test will need to be made. These are making sure that the blocks that the bridge will rest on are the same length away each time (25cm). They must also be parallel with each other.

To make this a fair test, this should be done before testing each bridge in the next step. You will also need to make sure that each bridge is at least 30cm in length (as mentioned in step 1). If not, try to extend your bridge so it reaches this requirement.

And finally, the moment you have been waiting for has arrived....testing!!! :D :D :D

Step 7: Testing and Recording Results!

Congratulations if you have made it this far, as now we are testing! Follow the pictures on how to test correctly. Since I did not have any level blocks, I used 2 of my identical K'nex boxes (except the colour of them).

Prepare a range of weights to use to test your bridges as shown in picture 2. You should record your results in your table after each bridge that you test so that you don't forget at later stages of the investigation! :-)

You should apply weights one by one until the bridge collapses and record the results. It is ideal to do this on a hard surface as material like carpet, it is likely that the bridge will/may leave marks and go down.

After testing, you will be calculating the averages in the next step.

Step 8: Calculating the Averages

In this step, as you may have guessed from the title :) we are calculating the averages. To do this, you add up all the values for one bridge and divide by 3. I used a calculator for this for the most accurate results.

After this, you should have a complete table of results.

Step 9: Graphing the Results

Lastly, you will be inputting your results into a graph. Since I did the table by hand, to do the graph, I used Microsoft Excel. Here you should reflect on your results and think of improvements. If you do think of improvements, feel free to test your bridges again as I have. You can see my improved results compared in my second graph.

Remember that the independent variable always goes on the X-axis and the dependant variable goes on the Y-axis. You should use the averages of the tests for each bridge. :D

After this, go to the next step. :-)

Step 10: Conclusion

Finally in this step you will be writing a conclusion to draw an end to your investigation. Here is mine:

In conclusion to which bridge is the best design to hold the largest amount of weight, I found out from my results, that bridge D was the strongest. This proves that my hypothesis was correct. This might be because it was the only bridge supported from all sides and areas as well as being the most compact. In the investigation,one thing that went well was all the bridges had unique designs and therefore the results were varied. In the future, if this investigation was ever to be repeated, I would have more bridges to show a wider range of results. I would also test the bridges more than three times in order to increase reliability of the test. I could increase the accuracy of this test by checking my averages and results more than once. Thus, this answers the question:
What is the best design for a bridge made from K'nex, able to hold the largest amount of weight?
The best design for a bridge is one that uses the idea and structure of triangles. This is because triangles are the only polygon that cannot be 'squished' without changing the length of one or more sides. An example is that the angles are held constant. This is also the same for real life bridges.

Some real life famous bridges that use this structure are
Since triangles have been proven to be the strongest structure for a bridge, a famous worldwide attraction, the Eiffel Tower in Paris, France, is also a structure that consists of triangles.

Step 11: Ending Statement

Thank you for reading through this Instructable! :D I hope you enjoyed it as much as I had making this instructable.

If you did enjoy it, please vote for it! I appreciate it deeply :-)

Thank you again!!! :D

Also, be sure to leave a comment. I would love to know your designs, what you think, and how well your investigation went.

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