Design It

Step 1 of 3: Designing Your Bridge

Design It is the first of three main steps in my model bridge building process. The other two steps are Build It and Break It.

In this section, you will learn the basics of bridge design and how engineers use these concepts in real life bridges. You will learn about the history of bridges and some of the important figures who helped invent new ideas and designs. You will learn how to take your specific design constraints and apply these concepts to maximize the effectiveness and efficiency of your bridge.

Let’s jump in.

A: Preparation

A1: Define the Goal

You must start the entire process by clearly defining what your bridge is trying to accomplish. Here are some example goals:

  • To build a bridge that is 1 meter long and is able to hold 50kg without breaking.
  • To build the lightest bridge that spans 12 inches that can hold at least 12 textbooks.

Depending on the type of project you have, your goal might be given to you by a teacher, coach, or through written instructions. But if you are building the bridge on your own, you still need a goal. Create a challenge for yourself, maybe something you don’t think is possible at the moment. You will probably be surprised at how well little bridges perform when designed and built well.

A clear understanding of the goal is very important to keep in mind as you go through the rest of the design and building process. You will want to continually double check what you are doing is meeting/heading towards your goal. You don’t want to get sidetracked by less important things. In addition, a clear understanding of what you are trying to do will help you 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.

Closely tied to the goal are your constraints, the guidelines under which you must meet your goal.

A2: Understand the Constraints

Just as important as defining your goal is clearly defining your guidelines/rules. You don’t want to get disqualified or a worse grade by simply not following the guidelines. Sometimes just a small detail can make all the difference.

A bridge, interestingly, is simply a way to solve a problem. An engineer, therefore, is simply a problem solver. Often the problem is “how to get from here to there”, but the solution is made complicated by restrictions in budget, social/political demands, and materials available.

Real life engineers spend time understanding the area in which the bridge will be built. They want to know how much traffic will cross the bridge, and if this traffic is small cars or semi trucks. They start with a budget, which will dictate the materials and construction methods they can use. Your job as a model bridge builder is more simple, but the same concepts apply.

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.

Here are some examples of basic constraints building on the two examples given above:

  • To build a bridge that is 1 meter long and is able to hold 50kg without breaking using just 200, unmodified popsicle sticks and Elmer’s White Glue.
  • To build the lightest bridge that spans 12 inches that can hold at least 12 textbooks with balsa wood, string, and glue.

I’d go so far as to say to memorize your guidelines or rules. It really helps a lot to have these lurking in the back of your mind as you continue on building the bridge.

Ask yourself these questions:

  • How long, wide, and tall does your bridge need to be?
  • Where is my bridge going to be loaded?
  • Do I have to allow traffic across (a matchbox car for example)?
  • Am I limited in what materials (wood, glue, etc) I can use?

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 long. 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. This many not sound like much, but it is plenty of extra. Any more will simply add extra weight, which will reduce the efficiency of the bridge.

The point (or points) where your bridge will be loaded is a key detail which will affect how you design the bridge. Since the purpose of a bridge design is simply to transfer the load from the loading point to the ground, the structure of the bridge can change significantly depending on this detail. Is your bridge loaded from the top or the bottom? Are you using a bucket full of sand, placing weights or books on top of the bridge, or trying to stand on the bridge yourself?

Now we are ready to go into force/load transfer ideas. This is the essence of bridge types and truss designs.

B: Design Concepts

Bridge design is such a big topic, I’ve decided to break it down into smaller sections and will send you to multiple pages on my website. Here is an intro:

B1: Types of Bridges and 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. I’ve listed them in order of complexity. A beam bridge is the simplest to create while a suspension bridge requires a lot more work. I’ve highlighted truss bridges because these are super common in model bridge building, and I spend a lot more time focusing on these.

Learn about the basics for each of the bridge types listed above.
Dive deep into Truss Bridges.
What are the forces that act on a bridge?

You can download a free program called the Bridge Designer which allows you to create a bridge and see how it responds to a truck driving over it. The goal of the program is to design the cheapest bridge for any given situation, and you can choose from several different scenarios. I have spent a lot of time using this program, and not only is it fun, it is very helpful in learning how bridges work.

I highly recommend you check out the Examples on my website to see what other ideas have been tried and what you can learn from them.

B2: Customize Your Design to Your Scenario

By now you have learned more about the bridge types, and a lot about truss design. If you haven’t, go back and check out those two sections mentioned above. Now we will talk about how to take a design and apply it to common model bridge situations.

I grew up participating in the National Science Olympiad structure competitions. In fact, this is how my love of bridges started. If your school has a Science Olympiad team, I would highly recommend checking it out.

Sometimes, in this competition, we learned not to use common bridge designs. In fact, our designs started looking nothing like traditional bridges. Our goal was often to build the absolute lightest bridge that could hold a specified amount of weight. This goal, combined with the given rules, got us thinking about maximum efficiency in a structure, and we started thinking outside the box. We tailored every part of the bridge to exactly fit how the bridge was going to be loaded.

This type of thinking will help you problem solve like an engineer.

How can you implement these ideas?

Simply put, your model bridge has to to transfer the load, from wherever it must be supported from, through your bridge and eventually onto solid ground (or the table) that the bridge sits on. Most commonly, the load is centered over a gap. That is the scenario we will run with here. If yours is different, you will need to use your problem solving skills to transfer the concepts.

Take a look at this diagram. You can clearly see there is a span over which the force must be supported. On the left, the force is high and on the right the force is low.

Now, in this next diagram, you see two typical bridge designs (the Warren Truss).

Now, in this third diagram, you see two non-standard designs which have been tailored to fit this exact and unique situation.

The bridge in this last diagram most likely does not look like a bridge you have seen in real life. This is because it probably wouldn’t work in real life, as a real bridge must support a moving force instead of one concentrated force. However, we can use this simplicity in loading model bridges to our advantage when you want to push the envelope of efficiency.

My challenge for you is to think about the simplest way to transfer your load to the ground. Don’t think in terms of a “bridge”, but think outside the box. Sometimes the best/simplest way is by using a standard truss design. But in some cases you can leverage simplicity to your advantage.

You are almost ready to take all these ideas and come up with a design for your model bridge. But I want to touch on a couple last points before you start.

C: Finishing Touches to Your Design Plan

C1: Materials and Joints

You should have already defined or figured out what materials you can use. However, often there is still room for you to make some decisions about your materials. If so, here are some things to think about.

Wood.
Wood is amazing. It has been used in bridge building for millennia. It is also the most common material for model bridges. Balsa and Basswood, popsicle sticks, and paper are pretty common for us. When you are designing your bridge keep these things in mind:

  • Wood, (materials in general), is strongest when loaded in its widest dimension (Link 1 Link 2)
  • Balsa wood is finicky and inconsistent. Learn how to choose the best Balsa wood.
  • Often the weakest part of a wood bridge is the joints
  • Wood grain plays a part in joint strength

There are four types that we use with model bridges, and you can read more about each on my Bridge Joints page.

  • Lap Joint
  • Butt Joint
  • Notched Joint
  • Gusset

C2: Draw Your Bridge to Scale

The next step is to draw your design out on paper. Ideally you will draw a 100% scale rendition, and this will make your life much easier when you are ready to build the bridge. Alternatively, if you have access to a CAD program, you could plug the design in and print it from there. I’ve found having that scale drawing to be incredible helpful in making sure there are no surprises when you start building, which you are almost ready to do.

If you are building multiple bridges, I suggest that you make sure to label and date your drawings.

Graph paper works best, and you can tape multiple sheets together if you have a long bridge. I would make drawings from three ang

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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