Design It

Know the Rules

You must start the entire process by clearly defining what your bridge is trying to accomplish. Depending on the type of project you have, you might have been given a set of rules or guidelines your bridge must follow. If so, read these very carefully. There are two reasons for this. The first, is that you don’t want to get disqualified by simply not following the guidelines, even if it is just a little thing you missed. Secondly, by understanding exactly what your goal is you can custom design a bridge to meet these goals. This will help you create a more efficient bridge, a stronger bridge, lighter, or however your bridge is going to be scored.

Some of the main things you want to pay attention to are minimum and maximum dimensions, materials allowed, and how the bridge is to be scored. If you are building a bridge on your own, be creative and come up with a set of goals to give yourself. Create a challenge, maybe something you don’t think is possible at the moment. Then see if you can make it happen. You will probably be surprised at how well little bridges can preform when designed and built well.

Minimum and Maximum Dimensions

How long, wide, and tall does your bridge need to be? Generally speaking, do not make your bridge bigger in any direction than it absolutely needs to be to support itself. If you have to span 12 inches, don’t make your bridge 15 inches. You only want enough extra to support the bridge. In this case, no longer than 13 inches, which gives you 1/2 inch on each side. Doesn’t sound like much, but it is plenty of extra. Any more will simply add extra weight, which will reduce the efficiency of the bridge.

Design Concepts

The #1 question to ask as you consider your bridge design is  “Where is the the bridge going to be loaded?” This could be a single point in the center of the bridge, it could be spread along the entire span, or it could be an unknown location. The key here is to structure your bridge so that the load is spread out to the entire truss, and that the loading area is reinforced appropriately. Answering this question will help you decide what type of bridge to build.

Types of Bridge Designs

There are different types of bridges, including:

  • Beam
  • Arch
  • Truss
  • Cantilever
  • Suspension/Cable Stayed

Each of these types of bridges has its place in both real and model bridge building. Listed in order of complexity, a beam bridge is the simplest to create while a suspension bridge requires a lot more work. Garrett’s Bridges focuses on truss bridges, but we cover the basics for each of these bridge types.

One of the best tools to help analyse a truss bridge is a java applet from Johns Hopkins University. It is available for free online, but it requires java installed on your computer. Read a tutorial for the Bridge Designer from JHU.

Draw Your Bridge to Scale

After you decide what type of bridge to build, and have picked a design, the next step is to draw it out on paper. Alternatively, if you have access to a CAD program, you could plug the design in and print it out for you. A 100% scale drawing will make your life much easier when you are ready to build the bridge. If you are building multiple bridges, I suggest that you make sure to label and date your drawings.

Type of Joints

Before you actually draw your bridge out on paper, you need to decide the type of joint to use. There are four types that we use with model bridges, and you can read more about each on the Bridge Joints page.

  • Lap Joint
  • Butt Joint
  • Notched Joint
  • Gusset

What’s Next?

This page is an overview of the design process for building 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 Design Article List
If you have something to add, share your wisdom here
If you are looking for pre-designed bridge plans or kits, check out our store

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

Go Forth and Build

Once you are done with the material on this page, you are ready to move on to Building Your Bridge.

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