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.

#1 – Hanging Bucket

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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

#2 – Weight on Top

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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.

#3 – Stand on It

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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.

#4 – Machines

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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.

Model Covered Bridge

Garrett-
Jobs well done – interesting site and information!
We built a 1/8″ scale model bridge just like the bridges we build for production. We successfully loaded it with 345 pounds with the weight being distributed all the way across the middle. Do you know of a formula where we can figure out that weight capacity for our full scale bridge? We have contacted 3 different engineers in our area who don’t have access to such information.

Thank you for your time and any helpful resources. (I cannot make the Johns Hopkins nodes and members work….?)

Garrett’s Note
Their website was http://www.rainbowbridgeco.com but it seems the website is no longer available.

How to Build a Model Bridge

Building a model bridge is one of the most rewarding hobbies I have done in my entire life . I have learned a ton about working with wood and tools. I have also learned the basics in bridge engineering, and have a greater appreciation for real bridges. Model bridge building has also given me a originative outlet that I can share with others. Not only does it give me something fun to do, I can show off my creations and prove their abilities.

To start out building a model bridge, you have to decide what materials to use. Frequently I use Balsa or Basswood. These two woods are common, and you can find them in most any hobby store. You can also use popsicle sticks, toothpicks, paper, cardboard, and tons of other things. Often I choose to make a balsa wood bridge because they are versatile and fun to construct. What materials you choose will depend on the goal you have for your bridge.

Most competitions tell you what materials you can use, and limit your choice. If you want to build this model bridge to exhibit on your shelf, then you can use materials that have more aesthetic appeal. However, if you are building a bridge to test it to failure, in order to see how much weight it will hold, then you want to use efficient materials.

Once you decide what materials to use, you need to start designing your bridge. Decide how long you want the bridge to be. Are you trying to set the world record for the longest popsicle stick bridge, or are you going for an easy project? You also need to pick out a truss design for your bridge. There are three standard trusses used in engineering, the Warren, Pratt, and Howe. After you pick a design, I find it helpful to draw it to scale on graph paper.

After you have designed your bridge, you need to start construction. Make sure you have all the tools and materials you will need, such as glue, saws or exacto knives, clamps, jigs, wax paper, and such. The glue you choose is important to the strength of your bridge. I recommend Cyanoacrylate Glue. Use your drawing on graph paper as blueprints to build your bridge on. Consider covering your blueprints with wax paper to help keep glue from ruining your drawing. You can do this step in two parts. Make one side of the bridge first, and then remove it from your blueprint. Repeat for the other side, and then join the two sides together to make a bridge.

Now that you have finished your bridge, you can enjoy it. If you are going to test it, remember to wear safety goggles. If you are going to display the bridge, or use it for a model train set, you are good to go.

First attempt at a DIY model bridge

It’s taken me just under 48 hours to get this far. Although I wouldn’t say its completely finished.. it’s a good as. I might just polish it off with a lick of paint and other aesthetically pleasing type things.

As you can see, I’ve modeled it on the Pratt Truss. Although, I didn’t do any “real” plans, just rough drawings (no force/load drawings).

Let me know what you think!

Cheers, 

Joe.

The Model Railroader’s Guide to Bridges, Trestles & Tunnels

Are you interested in building bridges for your model railway set? This book is a great resource, and is cheap from Amazon. This guide is rated 5 stars and will help you create bridges and tunnels for your layout. It will teach you how to build prototypes of many real life train bridges. What is neat about this book is that it gives you the history of the bridge you will be building. It is always fun to know the story behind the structure you choose to add to your set.

Model Bridge Kit

This kit is actually one of my first introductions to building model bridges. My mother bought this for me one year, thinking that I might like to build a model bridge kit. I loved it, and kept the suspension bridge for a long time on my desk. Unfortunately I lost it somewhere along the years.

This bridge building kit from Amazon actually has three different bridges. It comes with all the supplies you need to build a suspension bridge, a truss bridge, and an arch bridge. All of these combined make this kit an excellent gift idea, or something you can build just for fun. The package also contains an informational booklet which talks a lot about how bridges work, and is really quite good. I remember referring to this booklet time and again when trying to figure out the different forces that act on bridges.

If you are looking for a good bridge building project, this could be perfect. The kit is not expensive, and contains a lot for the price. I definitely recommend this kit. The Kit is called “Our Amazing Bridges Architecture Kit” and can be on Amazon.

Model Bridge Truss Design Software

No Longer Available

I am always on the look out for good software to help design model bridges. My latest find, Dr. Software, is a series of programs that allow you to customize a design, put a load on it, and see what happens. Dr. Truss comes in 2D and 3D options. You can also get Dr. Beam, which allows you to word with a beam and not a truss. The pictures in this post are from Dr. Truss 2D.

The Dr. Software is very comprehensive and in-depth. You can do a lot of different kinds of analysis on your truss design. One of the coolest features is that all your modifications are shown in real time. Even while you are dragging a joint to a new location the program shows you the changes in the forces and moments of inertia. I think that the Dr. Software can be a serious help to the serious model builder who wants to better understand the physics of truss design. Check out their website:

Video: Build A Model Bridge

This is the absolute best video I have seen about building model bridges. Roy Coleman, the founder of the International Bridge Building Contest, gives a very informative presentation in this video. The presentation is over an hour long. He shows how to build a bridge from scratch. He is gearing his video towards the Chicago Regional Contest in particular. The concepts, however, apply to virtually all model bridge building projects. This is a great companion video to my 5 Steps to Building a Model Bridge ebook. There are a lot of sweet little tricks in this video. Highly Recommended.

Here is an outline I made for the presentation:

  • History of Bridges
  • Nature
  • Greek
  • Roman
  • Types of Bridges
  • Arch
  • Da Vinci Bridge
  • Truss Bridge
  • Suspension Bridge
  • Cable Stay Bridge
  • Moveable Bridges
  • Tacoma Narrows Bridge
  • For A Contest
  • Read the Rules!
  • Design Within an Envelope
  • Why use Triangles
  • Compression and Tension
  • Building: Wood and Glue
  • Joints
  • Bending Wood
  • Construction Techniques
  • Check Bridge Against Rules
  • Weigh Bridge
  • Testing a Bridge
  • Different Techniques
  • Safety
  • Efficiency

What do you think about this presentation? Is there anything you disagree with?

Important Link in Video:

1997-2006 Academic Years – Mechanics: Bridges

Model Bridge Learning Kits

Model Bridge Design is the place where model bridge builders of all ages come to learn. Glean valuable tips and tricks from award winning builders for designing and building model bridges of various kinds, including Balsa wood, Basswood, popsicle stick and toothpick bridges. Whether you are building for fun or competition, a weekend project or school assignment, you will find something you can use here.

5 Steps to Building a Model Bridge

This ebook, 5 Steps to Building a Model Bridge, answers the most common questions people have about building bridges. In the ebook I outline the steps of building a model bridge, and then walk people through the actual construction of a bridge. The ebook contains many photos and illustrations, as well as 25 bonus tips.

Download Now

Download the ebook
Downloads: 4789 (application/pdf)

Support the Cause

I am giving out this ebook for free. There is no obligation for you to donate, but please consider helping the cause. You can donate as much or as little as you want. Even as little as $5 helps me to know that you find the ebook helpful. Any donations will go to my time making this website better and so I can continue to make updates to this ebook. The payment is processed through PayPal’s secure servers.





Here is a condensed version of the table of contents:

#1: Know the rules!

  • Be able to define in your own words what the bridge must accomplish
  • Do not get disqualified

#2: Design the bridge

  • Design the bridge around the loading points
  • Choose a truss to use
  • Draw the bridge to scale

#3: Gather Materials

  • Wood
  • Tools
  • Workspace

#4: Build the bridge

  • Step One
  • Step Two
  • Double check for leaning

#5: Testing and Evaluation

  • Testing Procedures
  • Evaluation Procedures

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