Break It

You’ve come far in the model bridge building process. The last step is to break it into as many pieces as possible. But before you go blowing your bridge to smithereens, take a little time to plan how to get the most out of breaking your bridge.

Ways to Break Your Bridge

There are many different ways to test your bridge, but they generally fall into a few categories:

  1. Hanging Bucket Method
  2. Weights on Top Method
  3. Human Weight Method

There are pros and cons to each method, and we have an entire page, How to Test Your Model Bridge, dedicated to that. The main thing to remember is that the method you choose probably will affect how much your bridge will hold. If you don’t have a choice in the loading method, then read up on how to maximize each type.

Are you in the mood for a laugh? Check out our Bridge Builder Blues music video. It is a humorous look at the Science Olympiad bridge building experience.

Videotape Testing the Bridge

One other thing you can do to help you analyze your bridge is to record it breaking on video. Most smartphone cameras will do a decent job if they can be held still. IF possible, record from several different angles to get the full picture. Playing the video back in slow motion might give you some clues as to why the bridge started to fail.

Protect Your Eyes

Always wear proper eye protection when you are testing a bridge. Always. You never know when the bridge will explode and send wood fragments into your eyes.

Evaluation

You might be looking for sheer strength, the efficiency, aesthetics, or a combination to evaluate your bridge. At Garrett’s Bridges, we value the efficiency score the most when it comes to model bridges. This is a simply calculation that looks like this:

Mass Held
_______________ = Efficiency Score
Mass of bridge

Make sure that you use the same unit of force when dividing the mass held by the mass of the bridge. Most times on this website you will see the mass of the bridge in grams, but the weight held in pounds. Before calculating the efficiency, we must convert pounds to kilograms.

The efficiency score does not tell the complete story, however. One of our commentators once said that the length of the bridge should be included somewhere in the calculation. This makes sense, because a longer bridge is inherently less efficient than a shorter one. That is something to keep in mind as you come across efficiency scores.

Use the Bridge Designer from JHU to Anaylze

One of the very first things to do after your bridge breaks is to plug the design into the free online program, Bridge Designer. You might have already done this during the Design It stage, but now you can use the weight that the bridge actually held as the total load to see exactly how much force each member was holding when it broke.

Here is an example from version 1 of our Short Pratt Truss Bridge:

The image on the right shows the forces at the time of failure. The program from JHU only allows load increments of 5 units, therefore it may not be possible to get the exact number. You can get very close most of the time.

What can you do with this information? If you are able to identify the first point of failure of your bridge, either from the video or remains, you can then combine this information with the Bridge Designer and see exactly how much force was required to break that point. If you know that the left-most member on the Short Pratt bridge broke first, then you know it was able to hold 57 pounds. If it was the joint that broke, then you know how much surface area is needed to hold 57 pounds. You can use this knowledge to either write a report or to refine your next bridge design.

Using this Knowledge

We took the analysis of Version 1, and reinforced our Pratt Truss Bridge to get a much more efficient Version 2.

Common Points of Failure

We’ve seen bridges fail in three different ways:

  1. 1 or more members failing
  2. Joint failure
  3. Instability

The first two are pretty straightforward, but the third is more complex. Instability can come from different things. It can come from the method of testing. For instance, having a person stand or walk on a bridge can create instability from the weight not being evenly distributed and changing constantly. You can see this pretty clearly from our 5 foot long popsicle stick bridge. This bridge was able to hold 220 pounds in the form of free weights placed carefully on the bridge. It was then able to hold Garrett (160lbs) walking across. When Brad (175lbs) tried to stand on it, the bridge collapsed. How could a bridge that was known to hold 220 pounds without a problem fail under a significantly lighter load?

Static and Dynamic Loads

We define a static load in a model bridge as one that is applied in a controlled manner. An example of this would be a load cell machine slowing applying a force on the top of a bridge. What makes this “static” is that the rate of increase is smooth and that the load is always being applied to the same place. A dynamic load is where there is variance in the rate of loading or in the area of loading. If you have ever tried to balance while standing on a model bridge, you will instantly realize that you are constantly shifting your weight. This changes where the load is applied on the bridge.

Adding weights to the top of a bridge, such as books or free weights, is a mix of static and dynamic. The mix is determined by how carefully you place each weight. If you plop down a 40 pounds weight on top of the bridge, it might have time to accelerate in the air increasing the force. If you carefully and slowly place each weight you are minimizing the dynamic aspect. Pouring sand into a bucket hanging below the bridge is similar. If you dump a lot of sand at once into the bucket, it accelerates before it hits the bottom thus increasing the force. Also, if the bucket is swinging, it can change how the force is pulling on the bridge.

Dynamic Loads Break Model Bridges Quickly

The more dynamic a load is, the less weight your bridge will be able to hold. It is important to minimize the affects of dynamic loading, and we give lots of ideas on our Testing Tips page.

What’s Next

This page is an overview of the testing and evaluation process for model bridges. Lots of links have been included throughout for further reading, but there are even more articles related to bridge design.
To access those, go to the Testing and Evaluation Article List
If you have something to add, share your wisdom here
You might be interested in participating in model bridge contests and sharing photos and videos of your own bridges

You can also download the full 5 Steps to Building a Model Bridge ebook for free here:
5 Steps to Building a Model Bridge


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