25 Bridge Building Tips
Wednesday, November 29th, 2006
1. Humidity affects the weight of your bridge. Keep your bridge in a closed container with a few
grains of rice.
2. Go easy with the glue bottle. As a general rule of thumb, if you can see it than you are using
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
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
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
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
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.
Lateral What? Exactly. Perhaps the most important aspect of your bridge and you aren’t sure
what it is.
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 Fernbankbank 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.
Common trusses used in engineering:
The following figures are shown under a load. The numbers represent percentage, where the total
load = 100. The numbers are rounded to the nearest 5. For absolute numbers, visit the Bridge
Red represents tension, blue represents compression, and green is no load.
On this Warren truss, each of the down arrows represents 50% of the load. Notice how the
two middle pieces have no load. Notice the load is quite a bit less on the ends of the top and
bottom chord than in the middle. When you build for efficiency, keep in mind that for a bridge
loaded in the center, the ends can be smaller than the middle.
Now examine the same Warren truss, but with added vertical members:
The added vertical members serve to break the top chord into smaller segments, making it
stronger. However, the percentage of the load has increased both on top and bottom. Also
notice where the green members are now.
I want you to notice the difference between the Pratt and Howe trusses under a load. The Pratt
has bigger numbers on both the top and bottom chord, but its middle compression members are
shorter, and hold less than those on the Howe. But the load is still concentrated in the middle,
and gets less further to the ends on both.
It is a trade off between the Pratt and Howe. For the Pratt, you’d have to use bigger top and
bottom chords, while on the Howe you’d have to use bigger compression members.
Forces that Act on Bridges
Sunday, February 20th, 2005
Compression is a pushing (compressing) force. You can take the middle a straw and squeeze it
between two fingers, the straw flattens. However, if you hold each end of a straw and push
together it is harder to make the straw compress, or flatten. The shorter a piece of wood is, the
more compression it can hold. The longer a piece of wood is, the less compression it can hold.
Tension is pulling. It would be hard to break a straw if you held both ends and pulled
apart. Wood also has the ability to resist tension. Tension may be applied parallel to
the grain of the wood, but should be avoided perpendicular to the grain. Wood is very strong
in tension parallel to the grain, but extremely weak in tension perpendicular to the grain.
Torsion is twisting. When you wring out a cloth, you are applying torsion to the cloth. If you
take a stick pretzel, twist one end, and hold the other end still, it will break very
easily. If you do that with a baseball bat, it will not break. However, if you take a piece
of licorice and apply torsion to it, the licorice will twist around several times before it
breaks. Each of these materials has a different way of responding to torsion. Bridge
designers must watch for torsion and try to reduce it as much as possible.
Shear is a interesting force. It happens when there are two opposing forces acting on the
same point. If you hold a piece of wood with both hands next to each other, and push up with
one hand and down with the other, you are applying shear to that piece of wood. Shear usually
occurs horizontally, and not vertically.
Wednesday, April 27th, 2005
The type of joint you use on your bridge can drastically change its strength.
The lap joint is one of the strongest, and you should use it whenever you can. It strengthens
compression members because it adds stiffness. The lap joint has one drawback, however. The
joint is only as strong as the face of the wood. That means that only one part of your piece of
wood is in contact with each other. The face of Balsa wood is typically not
strong, and tears easily. So make sure your lap joints have plenty of surface area for the glue.
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
The notched joint gives more strength than the end joint, but less than the lap joint. It is more
difficult to build, so it is not very common.
If you have to use an end joint, it is a good idea to add a gusset to make it stronger. The gusset
creats a lap joint, which is strong in both tension and compression. Usually I try to make each
part of the gusset the same length. If there is more glued to one piece, the one with less surface
area for the glue will pull away first anyway.