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.

I wrote an ebook, 5 Steps to Building a Model Bridge PDF to answer the most common questions people have about building bridges. In the ebook I show all 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.

The 2nd edition has been downloaded 18622 times since November, 2008.

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.

The concept of stiffness is one I have been delving into this year. The stiffness of the wood you use directly translates into the strength of your bridge. Density no longer matters much, and is almost not worth calculating unless you can find a relationship between density and stiffness. I have not been able to yet. There are too many other factors.

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 online stiffness calculator and a calculator you can download to your computer.

Here are some more good sites about Balsa stiffness:

• Balsa Stiffness
• SFA Forum
• 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

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.

A grain:

C grain:

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.

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

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.

The type of joint you use on your bridge can drastically change its strength. These pictures are not of glued joints, I simply laid sticks together. On a bridge, you want to make sure that the joints are clean and there are no gaps between the wood. Glue does not work well as a filler, the two pieces of wood should have no gaps.

Lap Joint:

The lap joint is one of the strongest, and you should use it whenever you can. It helps members in compression to resist bending. The lap joint has a potential weakness, however. Depending on the type of glue you use, the joint is only as strong as the face of the wood. If your glue soaks into the wood then this will not be a problem. The face of Balsa wood is typically not strong, and tears easily. You can also help avoid tearing by making sure your lap joints have plenty of surface area for the glue.

End Joint:

The end joint is not a very strong joint, especially for tension members. In tension, the two pieces of wood will just pull right away from each other. In compression, this joint will allow the piece to bend in a perfect arc. The lap joint holds the piece stiff, which does help it to hold more.

Notched Joint:

The notched joint gives more strength than the end joint, but less than the lap joint. And if the notch is a little too big, it creates a weakness in the notched member. It is also more difficult to build, which makes it not very common.

Gussets:

Sometimes it is impossible to avoid using an end joint on your bridge. But you can add a gusset to get all the benefits of a lap joint. In fact, you can make two gussets to create the strongest joint possible.

Typically gussets are thin pieces of wood, and not as thick as in these photos. Again, I was simply throwing together some scrap sticks of wood to get these pictures.

Hey everybody once again it’s me, First Timer. I thought the one thing that would make this website even better would be some vocabulary on the parts of bridges. Here are a few terms that might help if you ever need to describe different aspects of a truss or bridge:

Arch: A structure that is curved and carries weight in a vertical manner primarily by using x-axis compression.

Beam: Horizontal structures that hold a vertical weight while not bending. Girders are multiple beams placed together and are usually the foundation of a truss.

Bridge: A structure used in aiding humans and animals in transportation over gaps, rivers, etc.

Column: The part of a bridge that connects the footing to the bottom of the bridge’s deck.

Deck: The surface of a truss or bridge that people or things drive and walk across.

Fixed Arch: A structure that is permanently in a single area/position.

Footing: The part of a bridge that is under ground level.

Member: A part in a structure, most especially a truss.

Portal: The open ends on a Through Truss, a.k.a the entrance.

Span: The length in between the inner edges of two of the “legs” of a structure.

Strut: A member that is compressive.

Substructure: Bridge parts below deck.

Superstructure: Bridge parts deck and above.

Suspenders: Tension members on the cable from the main cable to the deck of a suspension bridge.

Tie: Tension member of a truss.

Tower: A large frame holding the cables of a suspension bridge.

Truss: A stronger form of a beam or girder made with a web of members.

I hope this helps you build and describe your bridges for your classmates, students, and/or professors. Happy building everybody! If there is any other term that you don’t understand, post a comment and I will attempt to define it for you.

What you want to look for in glue:

• Drying time
• Price
• Weight
• Strength

CA Glue. This is a very common glue used for a variety of projects. It dries in seconds, like super glue. It is also widely available. Numbers of hobby stores and online companies sell it. However, it is not cheap. CA glue can be bought in three types: thin, medium, and thick. The thin stuff is very light, but not especially strong. The thicker stuff is heavier, but stronger. I have known people to build extremely efficient bridges using CA glue.

White Elmer’s glue. This glue can be bought anywhere, and is relatively cheap. It usually dries in 30 minutes, which is somewhat long for building bridges. Also, it is not very light or strong. I have come to the conclusion that white Elmer’s glue is not a good choice for model bridge building.

Titebond Wood Glue bonds balsa wood very well. It also can be found in most hardware stores. However, it is fairly heavy. Many people mix water with the glue to cut down on weight. It dries fairly quickly, less than 10 minutes in small amounts. Elmer’s wood glue also provides a strong joint. The reason I don’t use this glue is because I don’t think that it is the best. But it is still pretty good.

Other wood glues. There are many other wood glues out there. One of the major brands is Titebond. It acts very much like Elmer’s wood glue. It is strong, heavy, and widely available. All glues will dry faster when you use small amounts. Mixing it with water also makes it dry faster.

Epoxy. The first glue I ever used in building model bridges was 30-minute epoxy. I liked it because it was very strong and could be bought at many stores. Epoxy comes in several different “setting times”. You can get 90 second, 5 minute, 15 minute, 30 minute, and up to 4 hour epoxy. The bad thing about epoxy is that you have to mix it. I had trouble mixing it correctly in small amounts. I ended up wasting a lot of glue mixing a lot of it at once. Also, epoxy is not very light. An interesting feature about epoxy, it heats up when curing. Sometimes it can get quite hot.

Special Glues:

Probond. This is polyurethane based glue and is super strong. It is a cousin of Gorilla glue. It is not light, but you don’t have to use very much of it. However, the only place I know of that sells it is a local Ace Hardware store. It is also very expensive, which is why I don’t use it much anymore. It takes at least 4 hours to set, which is a really long time to wait. I still keep a bottle of it on hand, because it works for about anything.

Weldbond. This is the glue I have switched to, after seeing it at good ol’ Ace. It is lighter than Probond, almost as strong, and a whole lot cheaper. It looks a lot like white Elmer’s glue, but dries completely clear. It sets in no more than 30 minutes, but often in less than 10.

If the type of glue you use is not listed, shoot me an email telling me about it at webmaster@garrettsbridges.com

Chart Comparing Glues

Obviously this chart is biased to my own experience with these glues. Please share your thoughts and experiences by leaving a comment.

Additional Resources:

• Woodworking Guide: Wood Glue
• Minimizing Weight While Gluing

• 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

I came up with the idea when I was looking through my closet for my scale for a new bridge (about to become Second Timer) and stumbled across a box of K-NEX. I took them out and started messing around with basic trusses to glue to a Poster board I had to turn in. Then I thought why not just make full bridges and weigh them to find out the best truss for a situation. You can also make towers with those things to some their good for any situation. I built a K-Truss and it held my entire brother (80 lbs), a stool (25 lbs), and a piece of plywood (.5 lbs).

I know its K-NEX not balsa, but still it’s always satisfactory to run out of materials to put on a bridge. It’s also fast to do it took me 25 minutes to build that K-Truss and look at how well it did. Also more recently I built a Warren Truss and it held 40 kilograms and weighed .1 of a kilogram which puts efficiency at 400. And for a plastic child’s toys I think that’s pretty good. So go ahead and try it. You can “Truss-t” me!