I would recommend that you double your truss members by putting them on the inside as well as on the outside of your top and bottom chords.

Also, if it is allowed, you might want to consider mixing a little water with your wood glue. I know it sounds weird, but it can actually strengthen your joints by thinning the glue just a little bit. Thinner glue can penetrate into the wood better. I’ve heard that a 30% water 70% glue mix works well.

Make sure you have adequate lateral bracing and that your bridge is built so that it is perfectly vertical, and the sides are parallel to each other.

Balsa bridges tend to break for two reasons, a member in compression fails, or the joints pull out. Typically the face of balsa wood is not very strong, which is why you want your glue to go deep into the wood.

How can I scale down the Fernbank Project-style bridge?

Firstly your site is great. very helpful in the amount of info given in respect to truss design.
Our task is to create a bridge 920mm long, 200mm wide and no more than 40mm deep which will secured at one end to our testing rig, a pillar in the center 60mm in length(not part of design) will help support the middle. this is all done as a load will be placed at the opposite end to which it is secured to the rig. what im asking is which would be the best design where a load is applied to one end which is freestanding?

I’d probably advise a Pratt truss, or any other truss design where you can keep the compression members short.

Firstly, your site is amazing and chock full of very useful information. I appreciate the effort you put into helping educate others out there.

I was recently assigned a project to design a bridge only of straws. The shortest acceptable straw length is 2″, bridge width max is 1″, and minimum length is 14″. From what I’ve seen on your site, the bridge constructions, joints, etc, involve the use of wood.

My question is how the techniques you’ve illustrated can be translated to construction with straws. In other words, when using a material much more prone to buckling, would certain joint types, or trusses serve me better.

Again I appreciate the extensive effort you’ve put into your work, and look forward to hearing your option.

Best regards,
David

I will look around to see where to u/l photos & give you a couple photos of our effort. Thanks again for this website & your advice.

Once we get this built, I will endeavor to send you a photo & let you know how it turned out. Thanks again for the great site & your enthusiasm for the topic.

Here’s my disclaimer. I might be wrong. I’ve got a lot of experience to base my thoughts on, but that doesn’t mean I’ve tested absolutely everything in every scenario.

1. I don’t like gluing to Balsa wood end grain, which is what you would have to do in a end joint. So you have a face grain joint vs end grain joint. Does a lap joint add extra force to the bridge as a whole? Maybe. A lap joint makes a compression member bend in a more complex shape than just an arc. I’ve seen it look more like a shallow S. This does make the compression member more stiff, which in turn would hold more force. However, it might add more torsion force to the entire bridge, I’m not sure if this would be a major factor. I’ve actually not looked for this before.

The last couple years I’ve been building exclusively with popsicle sticks, which require lap joints. End joints are not a good option, obviously because of the thin width of a popsicle stick.

2. Pieces of wood are stronger in compression the shorter they are. However, the shorter your truss is the more force is put on the top and bottom chords. Thus, a shorter bridges is not always better. You have to find a balance. I typically start with a 1/6 height to length ratio and work from there.

Balsa bridges are tricky because balsa wood is so variable. The best builders in the Science Olympiad competition that I knew would spend most of their time sorting and picking out the best wood, which involves knowing the density/stiffness of each stick.