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.

Overall, I have been very impressed with the quality and precision of the cuts that come from Specialized Balsa. I have also ordered from other online companies, and bought from local hobby stores but Specialized Balsa is the best. They also have limited pre-weighted sticks of balsa for competition purposes, which is great. One thing that I really thought was neat is that the balsa sheets come with a sticker with the weight on them precise to a 1/100 of a gram. Very professional.

• It saves money
• It saves time
• Helps you get familiar with the wood
• Increases your skills as a builder

Video Tutorial

Video is about 4 minutes long.

I am not responsible for any injury caused by someone using this technique. I recommend that students cut strips of wood only under adult supervision.

The Tools

Here is a list of the tools you will need:

• Table
• Cutting board
• Metal Straight Edge
• Cutting Tool (Exacto or razor)
• Ruler with no extra metal before the beginning of the marks

You will also need the sheets of wood. I recommend buying sheets of Balsa at Specialized Balsa. The quality of the wood sheets is important. Learn more about Balsa wood.

The Process

Begin by placing your sheet of wood on the cutting surface. Place your straight edge on top of the sheet. Now place a C-clamp at each end of the sheet of wood, but do not
tighten.

Take your ruler and measure one end of the sheet to the desired width. Lightly tighten the clamp on that end. Now measure the other end but firmly tighten the clamp. Go back and re-measure the first end to double check that it is still good. Make sure both clamps are fairly tight. You don’t want to over tighten.

Now take out your Exacto knife and make a light cut along the straight edge. Be sure to keep the blade perfectly vertical and keep the blade firmly against the straight edge. You do not have to push hard for this first cut, since it is simply a guide for additional cuts.

Now make additional cuts along the straight edge until you have cut all the way through.

Sometimes the first strip of wood from a sheet will not be the same width all the way along, and may need to be scrapped. With practice, you will be able to make very fine cuts. Eventually, the quality of your cuts will be better than what you can buy at a hobby store.

Where to Buy Balsa

You can find Balsa wood at most any local Hobby store. In fact, I would recommend looking local first before buying online. Micheals, Hobby Lobby, and Hobby Town USA all stock Balsa wood. You may not be able to find what you want, but it is worth a try.

The reason I say it is better to go to a local store before buying online, is because you can actually see the wood.
You can look and see what type of grain it has, make sure it is straight, and possibly weigh the wood yourself. Nothing compares to actually getting to see the wood before you buy. But, the second best thing is to buy online.

There are numerous places online to buy Balsa wood, but not all are equal. Jake Zimmer runs a great business at Specialized Balsa. Specialized Balsa is the place I recommend buying Balsa from. The quality of the cuts is excellent, and they offer pre-weighed Balsa sticks. The shopping cart is slightly unconventional, but works great once you are used to it.

For other online stores, see this list:
Online Stores that Sell Balsa Wood

My favorite article about Balsa wood was taken down, but I found a copy of it at Archive.org.

Balsa or Basswood?

I have heard a lot of people claim that nobody should use Balsa. They claim that Basswood is the way to go. I question that claim, and want data to back it up. Now, I do know that you can build a very efficient bridge with Basswood, but I also know you can do the same with Balsa.

Balsa and Basswood are completely different woods. I don’t think it it possible to claim that one is better than the other, without considering all the possible situations. Read my observations about the differences between Balsa and Basswood.

Glue for Balsa

Is there a particular glue that works wonders with Balsa wood? I don’t know. I have heard that people have had great success using plain old wood glue with Balsa.

Here is a hypothesis of mine. Since Balsa is such a porous material, it can take every advantage of glues that expand when they dry, such as Gorilla glue or Probond. I know both those glues work well on most anything, but I believe they have the potential work even better gluing Balsa wood.

This isn’t a discussion about glues, but I want to point this out. Both Gorilla glue and Probond are heavy, and because of that many people don’t want to use them. But keep in mind that you don’t have to use hardly any of the glue, and you still have a very strong joint.

Conclusion? Nah, I’ll leave that up to you. Here is my page discussing glues.

Using Low Density Wood

Here I quote “rjm” from SciOly:

Contest-grade balsa generally refers to model airplane applications. You don’t get enough strength from it to be useful in structures carrying loads. You’re better off using a denser wood with a smaller cross-section. Balsa increases in strength as the density increases, and the increase in strength is not a linear proportion. The strength to weight ratio gets better with higher density. The only drawback to high density is that, to achieve a light weight, the cross section dimensions get too small to work with. The legs and braces need enough surface area for a glue bond, and compression chords need a large enough cross section to resist buckling. You will have to experiment with densities to find an optimum balance. I’d suggest that you weigh out every piece of wood you own, sort it, and build a number of structures with identical geometry and varying densities. Record your observations as the structures fail. I think that you’ll find that the contest-grade balsa that you mentioned (5.4 lb/ft^3) is far too light. Don’t pay extra for it.

Bob Monetza
Grand Haven, MI

Balsa Wood Grain

Not all pieces of Balsa wood are equal. Besides having different densities (strength), Balsa wood also has different grain structures. What is a grain structure? Take a look at these pictures. Click for a closer view.

B grain Balsa wood lies somewhere in between A and C.

A grain Balsa wood has long fibers across the entire length of the sheet. Pieces like this are the best type for tension members. You should make sure that the grain goes straight across the piece of wood, and doesn’t go on an angle. C grain Balsa wood is better for compression members. C grain is more stiff than A grain.

It is easy to tell the difference between the grain types when you are looking a full sheet of Balsa wood. But it isn’t so easy with a small square or rectangle piece. That is another reason to cut your own strips of balsa wood.

Where to Buy Balsa

You can find Balsa wood at most any local Hobby store. In fact, I would recommend looking local first before buying online. Michaels, Hobby Lobby, and Hobby Town USA all stock Balsa wood. You may not be able to find what you want, but it is worth a try.

The reason I say it is better to go to a local store before buying online, is because you can actually see the wood. You can look and see what type of grain it has, make sure it is straight, and possibly weigh the wood yourself. Nothing compares to actually getting to see the wood before you buy. But, the second best thing is to buy online. Balsa Wood Inc is an online store that sells Balsa wood. You can start by checking out this website because they have a great selection of Balsa wood sticks, sheets, and blocks.

For other online stores, see this list: Online Stores that Sell Balsa Wood

The ideal piece of wood has a high stiffness to weight ratio. It really does you no good to have a very heavy piece of wood that also happens to be very stiff. For almost every competition I have been in, one needs to build light while keeping strength.

Another interesting number is the stiffness coefficient of your wood. Basically this is a comparison number. Generally accepted comparisons are:

A stiffness coefficient of:
<90 = poor
90-100 = average
110 = good
>120 = super

The calculations of the stiffness coefficient are slightly complicated, so I will not explain them here. Thankfully, brilliant people have done the work for you and created programs that will make the calculations automatically. You can find a calculator you can download to your computer.

Here are some more good sites about Balsa stiffness:

• Balsa Stiffness
• Stiffness Test

Testing the Stiffness

The accepted method of testing the stiffness of wood is to take one end of a balsa sheet, and push it down onto a scale. You stop pushing when the sheet when the numbers on the scale stop going up. I have made a video tutorial so you can watch this process:

Now you can do your own stiffness testing. Have fun

Buying Balsa Wood Sheets

Balsa Sheets from Midwest Products
Midwest Products has a wide range of Balsa wood sheets available at amazon. You can buy their sheets in lots of sizes and in different amounts.

I have a pot, or basin rather, that is about 18″ long and 12″ deep. I fill it with about two cups of water (I have found that more water just takes longer to start boiling, and I usually don’t boil all of it anyway) and stick it on a stove burner. Currently I am using a stove that has a glass top and the burners are underneath, but in the past I have used one that had burners above the top of the stove.

I place any wood I am steaming on top of the basin, I usually have at least one or two pieces that are longer then the basin and some that are shorter, but the shorter ones lay on the longer pieces so that nothing is actually touching the water at this time, for the goal was to steam and not soak the wood. I turn the burner on high, and wait for the water to start boiling. I also place a shallow cookie sheet on top, to hold in the steam.

I have never steamed my wood for over a half hour, perhaps I am too impatient. But with this amount of time the wood is very bendable.

Beforehand, I have prepared two “molds” that consist of two sheets of wood, with a form of nails hammered into each, to mold the steamed wood. I have taken to making the mold a half inch or so taller then what I want the actual height of my arch, because the wood always bends a little less then the form afterward. You may want to use more or less extra height depending on the overall height of your arch.

This form is fairly easy to make, I just set the height and length of what I want with nails, and then bend a stick of wood (usually 1/16″ by 3/16″) and hold it in place by clamping the wood to the nails with clothespins. I then add as many additional nails to the form as I see fit, along the arch of the wood.

After steaming the wood I clamp the pieces to the form and wait another thirty minutes to an hour for it to dry. Again I may be impatient, but it works for me. I then apply glue to the wood, and wait 4-5 hours for it to set (that is the amount of time my glue takes to set). I then do a little sanding, and finish building my bridge. So far, I have built an 8.5 gram bridge (as of Jan/19/04) that held 16k and broke. However, the testing apparatus was not ideal, in the fact that I forgot to bring an S hook to the place I was testing, and had to make do with something else, which I believe may have unevenly applied the load to my bridge. But then again, that may not have been the case.

I usually make my arches into the shape of an L beam, but have used a T beam in the past. I have never used an I beam, though that would most definitely be stronger but heavier. It is worth experimenting with.

Balsa is cheaper and more available than basswood. This means you can buy more wood, and build more bridges for the same amount of money. By building more, you will gain valuable experience and improve your skills. When it is all said and done, you may be able to build a better balsa bridge.

Balsa is stiff, while basswood will bend.That’s right, you can bend basswood more than balsa. That is good if you are building an arch bridge. But what if you don’t want your bridge to bend? Balsa will give you a stiffer piece than basswood for the same size. However, I have seen balsa bend a lot under heavy load without breaking. I am not sure why it was able to bend so much, but it did.

You can get more cross-section for the same mass with balsa. In other words, you can have a bigger piece of balsa for the same weight of basswood. In compression members, larger and lighter pieces will actually hold more than smaller, heavier ones.

Basswood won’t rip off at the joints as much as balsa. Sometimes the face grain of balsa will tear away at the joint. Basically, basswood holds itself together better than balsa.

Balsa comes in a wide range of densities. You don’t always know how strong the wood is that you are getting. Basswood usually comes in a much smaller range, which means you can be more consistent with basswood. That problem can be fixed if you weigh each piece of wood before gluing it to the bridge. That way, you know exactly what is going on your bridge and can keep accurate records.

Balsa is less likely to be the same strength throughout its entire length. If you buy a piece 24 inches long, it may be stronger at one end then the other. This just complicates things, making it harder for you. Basswood is usually uniform in strength.

Balsa will sand easier, but basswood won’t crush.You can squeeze a piece of balsa, and totally deform it. A “squeezed” piece of balsa is weaker, because the internal atomic structure is messed up. You need to be more careful when working with balsa.

Balsa changes weight with changes in humidity more than basswood. Many times I have had a balsa bridge lose weight after setting it in the sun for a couple of hours. Of course, the opposite is true. Balsa bridges will gain weight after humid days. Basswood does not change so much with the weather.

For the same mass, basswood pieces must be smaller. Usually, this means that a basswood joint will have less surface area for glue than a balsa joint. That means you may have to use stronger glue with bass, which might add weight.

So you see, there are many factors that play into which wood is better. Basswood may be better in some places, and balsa in others. I encourage you to experiment with both woods. Specialized Balsa is the company I recommend for buying balsa.

Don’t forget about the other woods.

Birch, Spruce, Aspen, Pine, and others are all alternatives to balsa and basswood. But I guarantee that you won’t be able to find them like you can balsa. They will cost more money. But don’t immediately rule them out, you may want to play around with them, and check out their qualities. Let me know what you find out.

Resources

1. Interesting Facts About Balsa
2. Balsa Wood