Tags: bridge joints, end joint, gusset, joints, lap joints, tips
1,748 views in 2012
Bridge Joints
The type of joint you use on your bridge can drastically change its strength. There are three basic types of joints, the Lap Joint, End Joint, and Notched Joint. To increase the strength of an end or notched joint you can add a Gusset. Learn about each type of joint and when to use it on a model bridge in this article.
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:
-
- Lap Joint
-
- Lap Joint 2
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:
-
- End Joint 2
-
- 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:
-
- Notched Joint 2
-
- 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:
-
- Gusset Joint 2
-
- Gusset Joint
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.
-
- Double Gusset 2
-
- Double Gusset
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.
What did you mean when he said “Yes, the sandwich would be stronger than just 1 lap joint. Two chords at the top would not be overkill, but two chords at the bottom might be. This is because the top chord is in compression and the bottom chord is in tension.”
How do you get the wood to bend?
Just need to know, will this work with towers because I’m making one and I think that the pressure is distributed differnetly. Also is it possible for you to post a picture where a lap joint was actually incorporated into the structure.
I was wondering if i was to sandwich the legs between to two chords at the top and bottom, glue and pin my joints, would that make my bridge stronger than if i just did 1 cord and a lap conection? would the two chords at the top and bottom be over kill? I’m trying to beat the record (80 lbs hanging at center) in my strutures class in college. all the legs, cords and braceing have to be 3/16 x 1/2″ balsa. i’m thinking of going with the warren truss. we are allow to use pin, glue and gusset conections
Yes, the sandwich would be stronger than just 1 lap joint. Two chords at the top would not be overkill, but two chords at the bottom might be. This is because the top chord is in compression and the bottom chord is in tension.
I’d also consider making your top chord into a T shape as well. If you have your sandwich as described, glue another 3/16 x 1/2″ stick flat on top of the top chord. That will greatly increase its lateral strength.
Do you have a weight limit for your bridge? How is it going to be loaded?
Thank you for getting back to me. No, we don’t have a limit on the wieght, However the project must meet a 25% or less load capacity percentage. The bridge will be loaded from the center of the roadway deck with a hanging hook. wieghts will be added till it breaks. I like your idea of the t shape on top, however we’re only allowed to use 2 pieces of wood. I’m now thinking of using 2 pieces in a t shape for the top cord and sandwich the legs on the bottom cord.I want to hold a 100 lbs.(the record on this project is 80lbs) what do you think?
So you are only allowed to use two pieces of wood for the top chord. Hmm. That takes out the easy way to make your bridge stronger. I’d say to go ahead with your idea.
If a lap joint tears the face of balsawood, would it have similar effects on toothpicks? I’m making a toothpick bridge and need to know whether or not to use lap joints, end joints, or possibly gusset.
I haven’t done a lot with toothpick bridges, but it seems to me that lap joints would be the way to go. I’d think that you would want as much overlap in the joints as you can get because the surface area of toothpicks is not much to begin with. I also think that a toothpick would generally be of a higher density than Balsa. That would mean the face ripping off is less of an issue. Granted, I don’t think that toothpicks are high quality wood and there is probably a mixture of densities.
Thanks
hey, is it possible to make a double gusset with a notched joint instead of an end joint? would that be stronger or weaker?
Eric, I’d say that if you are going to do a double gusset, then do not weaken your other members by cutting out a notch.
thanks. just wanted to be sure. this will help out my project
yo,thanks man this really helped in my bridge contest and this yo site u’ve got man is the bomb but it would be nice if u would put something in this on an arch bridge
I have to build a spaghetti bridge for my physics class at school. Would this information also work on a spaghetti bridge? Also, what kind of glue would you recommend?