The answer to that question is not simple and probably is not going to be what you expect. The truth is, I cannot answer the question. There are too many variables that are not being defined.

Example:
Is the the Pratt truss or the Warren truss stronger? Actually, the answer is up to you. You can make the Warren truss stronger. Or you can make the Pratt stronger. It depends simply on the strength of the wood you use. You can use 2×4′s to build the Warren, and toothpicks to build the Pratt. Obviously the Warren is going to be stronger in this case.

A Better Question

I think though, many people are trying to ask whether or not one of these trusses has an inherent advantage over the others. In my mind, none of them do. If you look at my truss design page, you will see that each truss spreads a load out differently, but none with an apparent advantage. Conclusion: no bridge design has an inherent strength over another.

However, this conclusion only applies to this general setting. It may be that in a specific situation one bridge design would be better suited than another. It is up to you to examine how each truss works and decide which one to use.

• 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

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 $1 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

If what you’re building is a home then general contracting is something to try to learn as much about as possible before hiring new home builders to work on your home. The process of building a home is something that still might involve the use of a remodeling contractor so that’s something to keep in mind.

Basic Design:

There are two basic types of boomilevers. First is the tension boomilever, where the tension chord is longer than the compression chord.

Second there is the compression boomilever, which is the opposite of the tension.

It is generally accepted that the tension boomilever is inherently better than the compression. This is because the strength of wood in compression decreases per length unit the longer the piece of wood is. However, wood holds the same strength in tension no matter the length of the piece. In the compression boomilever, the compression chord is longer than in the tension boomilever.

Height:

The “taller” the boom, the less force is put onto the top and bottom chords. This allows you to decrease the weight of these chords. However, the taller the boomilever, the longer the truss pieces must be, which increases the weight. The goal is to find a balance. My suggestion is to not go any shorter than 3-4 inches. I do not, however, believe you need to use the entire 20cm. Plug your design into the Bridge Designer and see how the numbers change as you increase the height of the boomilever.

Truss:

Most boomilevers, unlike the two demo examples, have what is called a truss. There are three standard trusses, the Warren, Pratt, and Howe. Each of these trusses have different ways of spreading out the load, and it is worth playing with the bridge designer to see how each of these work. Remember the KISS principle, and keep the truss simple.

The basic idea behind a truss goes back to the tension verses compression boomilevers. A piece of wood loses strength as the length increases. Thus the truss serves to divide the compression chord up into smaller sections, greatly increasing the amount of load before buckling. Basically the truss keeps the compression chord from bending and twisting. As a starting place, I would try and break the bottom chord into sections 2-3 inches long.

Base:

Most boomilever bases I have seen through the years are way overbuilt. Anybody should be able to make a base under a gram, and less than .5 of a gram if you are good. Obviously, you will not be able to achieve this if you are using both bolts to attach your boomilever to the testing wall. I honestly cannot see any reason to use both bolts. If you are worried about stability, I say look at the bridges of the past couple years. Most were no wider than 5cm and had no problem with stability. You will cut a lot of weight by only using one bolt, not only on the base but also with the lateral bracing.

Lateral Bracing:

Don’t let the term turn you off. Lateral bracing is simply a horizontal “truss” connecting the compression chords of your boomilever. Like any normal truss, there are various designs you can use. And the goal is the same for lateral bracing as is a normal truss, to keep the compression chords from bending and twisting.

You generally do not need a lateral truss connecting the tension chords of your boomilever. The tension chords do not bend, and only twist under very extreme force. If anything, you can use a couple perpendicular pieces to connect them.

Glue:

Glue weight can be a major weight factor, but it doesn’t have to be. You can easily use 1-2 grams of glue (or less) on a 10-gram boomilever. A lot of people use CA glue, or ambroid. I use an interesting glue called Weldbond found at Ace
Hardware. Find a glue you can use well and stick with it. Make sure not to use too much.

Testing:

It is generally a good idea to test your boomilever before competition. Of course, you never test unless you have enough time to build another one before competition. Testing a boomilever does not weaken the structure in most cases. The key to know if your boomilever is damaged is to listen while you test. If you hear any pops, cracks, or groans then the boom is probably in need of repair. However, if you do not hear anything, your boomilever should be as good as new.

One method is to test the boomilever to the point where you like the efficiency score and stop without breaking the boomilever. I definitely think it is a good idea to test some boomilevers to failure, but you can use this method if you are nearing a competition. Testing to failure lets you know where the structure is weak, and gives you the knowledge to make the boomilever better. Typically, the weak spot on boomilevers is the connection to the base.

Boomilever is a fun event; make sure you don’t get caught up in trying to win so hard that you stop enjoying it. Build a lot, test a lot, and document everything.

A healthy knowledge of science can help you to understand many physics concepts better.

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

Lateral bracing is the term I use to refer to any pieces on a bridge that help keep the top chord from bending horizontally. In the figure before, lateral bracing is red:

Why is lateral bracing so important?

As you may have read on this website that the shorter a piece of wood, the more compression it can hold. Lateral bracing serves to break the top chord into smaller sections, giving it more strength. Simply put, lateral bracing is the “truss” that goes on top of your bridge.

Why only on top?

The bottom member of a bridge is in tension, and is not going to bend or twist. Wood has the same tensile strength no matter how long it is. The top chord will want to bend and twist, and needs support to keep it straight.

Why an X?

The reason many builders choose X’s is because they make triangles, which resist deforming. Of course, it doesn’t have to be X’s. You could use half X’s in a zigzag pattern, just take away half of the pieces in the figure. Or you could use straight pieces, circles, or any combination. However, straight pieces would just make a rectangle, which won’t really help.

There is a place and time when you can use straight pieces for lateral bracing instead of X’s. That idea is illustrated by the Fernbank Bridge. Notice that this bridge uses an L-beam for the top chord. And a large L-beam at that. The key lies in that fact; the straight pieces across the top had a very large surface area joining them to the top chord. This automatically made them stiff, and able to resist deforming. This kind of made a triangle, so to speak.

How much lateral bracing?

The amount of lateral bracing you need to use is based on the ratio of dimensions of your top chord. You want to use only just enough and not overdue it in order to get the best efficiency. However, there is a danger of not using enough. As a general rule of thumb, I try and use the same of amount of bracing joints as I have truss joints to the top chord. You can see this idea in the example pictures.

There is another thing to consider. Let’s say your top chord is 5 units wide and 5 units tall (a square) That beam is going to be equally hard to bend in any direction. However, if your top chord is 2 units wide and 8 units tall, even though it has the same total mass as before, you will need to use more lateral bracing.

There comes a point when it is no longer useful to add lateral bracing. If your top chord is only 1 unit wide, and 9 units tall, your top beam is going to bend and twist like a slippery snake. There isn’t a lot you can do about it except increase the width.

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.

h3>How Changing Height Affects a Bridge

Control Bridge

Now I will show you another bridge, with the same design. The only difference is that this bridge is 4 inches tall, one inch taller than before.

1 inch taller

What does this mean to you?

As you saw in the example bridges, by increasing the height of the bridge you decrease the load on the top (and bottom) chord. A decrease in load means you can make it smaller. Smaller means lighter in this case. However, there is a catch.

Look back at the two examples. By increasing the height, the load decreased on the top and bottom chords but remained the same on the middle “truss members”. Why is this a drawback? By increasing the height, the middle members have to become longer. That inherently adds weight. However, bridge builders have another problem when pieces become longer. The amount a piece of wood can support in compression before buckling decreases with length. So that means the middle members will have to be made stronger the taller the bridge is. Adding strength usually means adding weight.

So we have a draw

By increasing the height of a bridge, you can make the top and bottom chords lighter. But at the same time, the middle truss members have to be made heavier. The goal is to find a balance. I believe that there is an optimal height for a model bridge in every situation. The trick is to find that height, by balancing out these two factors. I was once told that the optimal height for an arch bridge is 1/6th the length of the span. That means if your bridge is 6 inches long, it should be 1 inch tall. My experience has confirmed that this is a very good starting place for an arch bridge. And I believe the same ratio of height to length can be applied to regular truss bridges.

The Optimal Height

Interestingly, the Golden Gate Bridge uses the 1/6th ratio almost exactly. Perhaps you can do some studying on other real bridges and see if they follow this idea also.

Data taken from the Bridge Designer.

2. Go easy with the glue bottle. As a general rule of thumb, if you can see it than you are using too much.

3. Keep your hands clean! Oils and grease from your skin can ruin your glue joints.

4. Perfect practice makes perfect. The more bridges you build, the better your construction skills will be.

5. Keep your bridge from twisting by using lateral bracing.

6. An L-beam is more efficient than a square, but harder to build.

7. Balsa wood comes in a wide range of densities and stiffness. Weigh each piece that you buy.

8. It is cheaper to buy Balsa in sheets and cut your own wood strips.

9. It’s still true, measure twice and cut once.

10. Keep a log of every bridge you build. Record notes and dimensions; you won’t remember later on.

11. Try to videotape testing your bridge. You may get a clue on what failed first.

12. Always keep safety in mind when using sharp tools. Most mistakes are made when you aren’t paying attention.

13. By cutting a piece in half, you more than double its strength in compression.

14. Good lighting when working will help you perfect those little details.

15. Always test your bridge before taking it to a competition, but leave enough time to build another.

16. Draw out your bridge on graph paper to make sure that it is symmetrical. I prefer the 11″ x 17″ graph paper.

17. Different trusses have different ways of spreading out the load.

18. Wood has about the same strength in tension, no matter how long it is.

19. CA glue is a fairly strong, light, fast-drying glue used by many builders.

20. Balsa wood sands very easily. Be careful not to sand off too much.

21. You can mix wood glue with water to cut down on weight. Doing this also helps the glue to seep into the wood, creating a stronger joint.

22. Remember to close your glue bottle when you are done using it.

23. Basswood will bend easier than Balsa wood. Try steaming or soaking your wood to help it bend.

24. Use Lap joints whenever possible to get the best strength.

25. What you want to look for in glue: drying time, price, weight, and strength.

These bridge building tips will give you a head start when you start designing and building your model bridges. These tips come from my years of experience starting from my time in the Science Olympiad competition and continuing beyond building for fun. Disclaimer: these tips are my own opinion based out of my experience. Other builders might have different views and we might not agree. I encourage you to try things out on your own and decide for yourself what is the best way to build a bridge. Who knows, I could have been wrong about something.

I don’t use CA glue, or any other super glue. I have had lots of trouble with normal super glue sticking. I do know that CA is a whole lot better than regular super glue though. However, there are no stores around where I live that sell CA glue. It is fairly expensive to order it over the internet.

I am currently using a glue called Weld Bond. It looks a lot like Elmer’s white glue, but dries completely clear and not just transparent. It is also a lot stronger. I glued several nickels together, and cannot get them apart. I had to throw them away. It actually dries pretty quickly, less than 10 minutes to get mostly hard.The only thing is, if you mess up, as in glue something in the wrong spot and have to pull it off (you realize you mess up right away), the glue has already dried enough that you won’t get a good joint anymore. You have to sand off the old glue and try again.