Stand on a Bridge

The method you use to test your model bridge will make or break it. Okay, most of the time the bridge breaks no matter what. How much weight your bridge holds before collapsing is partly determined by how you test the bridge. Whether you have a popsicle stick bridge, balsa bridge, toothpick bridge or other type of model bridge, these testing tips will give you a jump start and help you make an informed decision on how to test your bridge.

How to Test Your Model Bridge

The method you use to test your model bridge will make or break it. Okay, most of the time the bridge breaks no matter what. How much weight your bridge holds before collapsing is partly determined by how you test the bridge. Whether you have a popsicle stick bridge, balsa bridge, toothpick bridge or other type of model bridge, these testing tips will give you a jump start and help you make an informed decision on how to test your bridge.

Methods of Testing Bridges

I’ve used and seen a lot of different ways to load model bridges. A lot of people do not want to buy lots of fancy equipment, so they use only what they have available to them. This makes for a lot of creative ideas for testing bridges. I’ll outline some of the ways I’ve seen that work the best.

Testing the Warren

Hanging Bucket Method

#1 – Hanging Bucket

This is a classic method, and is used for Science Olympiad competitions. The bridge rest on two elevated supports, which could be two tables with a small space between them or one table with a hole cut in it. A loading block is placed either on top of the bridge or inside the bridge and a bucket is suspended below using a eye-bolt, S hook, and some chain. The bucket is then filled up with weight (typically sand, water, or free weights) until the bridge breaks or the maximum load is reached.

Read more about the hanging bucket method

Weights on Top

Weights on Top

#2 – Weight on Top

If you do not have access to a hanging bucket system or want to keep the bridge lower to the ground, you can simply load the bridge from the top. This does not work well for arched bridges, as they typically do not have a flat surface to put weights on. Many teachers host a classroom competition to see how many textbooks each bridge can hold. The textbooks are stacked on top of the bridge.

You can use books, free weights (from a weight set), or a bucket filled with weight to load your bridge from the top. Be careful when the bridge does break because if you have a tall pile of weights, whatever they are, they will come tumbling down with a lot of force. Sometimes I have placed cameras around the bridge filming the destruction, and the cameras have come close to being smashed by sliding weights.

Stand on a Bridge

Stand on a Bridge

#3 – Stand on It

Standing on your bridge is perhaps the ultimate testing method. I love this because if the bridge holds, you were able to create a fully functioning bridge. This is why I am separating the standing method from the other “weight on top” methods. Standing on your bridge gives the best sense of fulfillment and moves your bridge from simply a “model bridge” to a real bridge, even if it is constructed from only balsa wood or popsicle sticks.

However, there are some situations where the human weight method is not appropriate. These cases are usually when you are given a maximum load the bridge should support, which is less than your weight. Also, standing on a bridge is a more dynamic loading than placing weights on top or loading a bucket with sand. It is a lot harder for you to stand still and load the bridge evenly, which causes the bridge to be stressed more in some parts than others. Standing on the bridge can also be dangerous, depending on how high off the ground your bridge is. The lower the better.

Machine Loading

Machine Loading

#4 – Machines

Machines usually make things easier. They can definitely help make testing a model bridge quicker and smoother, while providing an accurate measurement of the weight held. Machines are generally very consistent in how they load bridges, which allows you to not worry about loading error causing premature failure. Also, I think machines let you just sit back and enjoy the pride of your life being crushed, so why not make the most of it? Since you have your hands free, pull out a camera and snap some shots or record a video.

General Loading Suggestions

Efficiency

My time in the Science Olympiad taught me to be very efficient during the testing process. The longer a model bridge has to hold weight, the greater chance that it will fail early. We were given 10 minutes to set up and test our bridge. I spent most of the time setting up the loading block and as little time as possible actually pouring sand. As soon as I starting pouring sand, I didn’t stop or delay at all.

Record the Event

While most handheld cameras do not capture frames quickly enough to really see what is going on, you can still get a good idea of how your bridge failed from watching a video. I recommend always taking a video of the testing. The more angles you can get, the better. I now try to set up three cameras: one broadside view, one looking into the bridge, and one from a higher angle.

What is the Best Way to Test a Bridge?

I’ve shown a lot of different methods, and you want to know which one is the best to use for your bridge. The answer is: it depends. As I mentioned earlier, a machine is probably the most steady and consistent method, but they sure are expensive. I grew up using the hanging bucket method in Science Olympiad, and I can use that method very well and get very consistent results. However, because that takes a while to set up, I am moving to using free weights placed on top of the bridge. This is the method that works the best for me with the equipment that I currently have. You will have to figure out how much time/money you want to invest into testing your bridge and choose a method.

If I have missed anything, or you would like to share your experiences with one of these methods, please leave a comment below.

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5 Responses to “How to Test Your Model Bridge”

  1. Chris - May 24, 2013 at 1:45 am

    Is load testing on the top of the truss accurate? Shouldn’t the load be place where the actual live load (traffic) be happening? so at the bottom of the two trusses? I ask because I’m helping with my son’s class where they’re building bridges and will be testing soon, the teacher said in the past they’ve put a cord over top of the bridge and tied a bucket to the cord below the bridge. The other concern I have with their method is that they build the road surface with stringers and then build two trusses which are then surface glued to the road deck. It seems to me that this is two separate structures, the two should be more integrated. Thoughts? Thanks

    • Garrett Boon - June 2, 2013 at 5:18 pm

      Chris,

      Testing with the load on the top of the bridge does not change the forces on the bridge significantly. You can see this if you plug in a design into the Bridge Designer and change the load from the top to the bottom.

      Ideally the roadbed would sit on top of the bottom chords of the truss.

  2. Mark - June 5, 2013 at 10:08 am

    I disagree with this and Chris is right to be concerned. With the design used in your bucket testing photo, when tested with the load on top, the sides of the bridge may fall over as it is not braced for a sideways load. We discovered this when my son decided to stand on his bridge the third time and of course both sides went over. Luckily, some more hot glue repaired it.

    If it is cross braced then of course it will work, but as my son pointed out, “But Dad, how will the cars drive along it”

    • Garrett Boon - June 6, 2013 at 2:57 pm

      Mark,

      There is another issue here. Standing on a bridge is completely different than loading with a bucket. Try as we may, it is pretty much impossible to weight both sides (trusses) of the bridge equally when a person is standing on the top of the bridge. This is much more of a dynamic load which will cause the bridge to fail earlier and perhaps in a different way than it would have with a bucket. This unbalanced load will often cause the bridge to fall over sideways as you described. Cross bracing does help, but cross bracing helps no matter how the bridge is loaded.

      If you look at real life truss bridges, you will notice that most have cross bracing close to the top of the bridge. They still allow clearance for cars and trucks, but they do bring the cross bracing down below the top chords. Every little bit helps.

  3. joe - February 5, 2014 at 10:10 pm

    Where would you buy a machine tester from, and generally how much are they?

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