Tags: bridge joints, end joint, gusset, joints, lap joints, tips
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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:
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- Lap Joint
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- 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:
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- End Joint 2
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- 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:
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- Notched Joint 2
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- 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:
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- Gusset Joint 2
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- 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.
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- Double Gusset 2
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- 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.
Thanks for your inf about joints, but i want to explain me about all joints regarding the stractures, bridges…………Etc and type of bridges.
I am waiting for your kind notice.
Thanks,
is there a special glue for balsa wood
Please see my Glue Tips page:
http://www.garrettsbridges.com/building/gluetips/
Can you PLEASE report on the Miter Joint BEFORE Friday (2 days from now) I need it for a report im writing and i can NOT use wikipedia!
Salam Garret…those ideas really opens my mind, my vision wider on producing a better bridge for my statics subject…we in UNITEN are only allowed to use ice cream stick or could be said as popsicle..anyway do you have any link on designing a bridge..it will be great though
i was wondering whether lap joints work well with basswood and gorilla glue as you mentioned balsa has a high chance of getting torn up or the glue not soaking in???
I highly discourage using a notch joint. As one can see from the picture, it clearly degrades the integrity of the wood that’s been notched. Once a tension is put on the joint, the notched wood will break easily.
this info could be mo0re helpful if you showed bridges that you have made with these joints and where you have used them other than that you have helped alot
Thanks for the idea, I will see about incorporating this.
Thank you soo much for all your posts and infomation!! I’ll be competing at state finals in a few weeks, and your infomation has been the most useful I’ve found so far in my research! Thanks heaps! (again)
Howdy! I was wondering if there was a good joint for tension members. I noticed that most of the joints were recommended for compression members, so should I just use a lap or gusset (if needed) joint for tension members?
And what size wood do you suggest for lateral bracing?
Thanks a lot!
Wow… I see you have updated the site so it is a little more understanding
This helps.. Thanks 
Thanks a bunch, the photos help a lot.
I do have one more question, though. I want to put x-shaped supports on my bridge (it’s an elevated bridge, just for class but with all the SciOly requirements), but I don’t know how best to join them. I’ve heard of something, I think it’s called a half-lap joint, that I think might work best. You cut half of the wood off the part of the strip that will be the actual joint, and then connect the two pieces at those joint parts. There isn’t any part of the wood sticking out, and they are very connected, I think, as far as how the weight/force is distributed, which I figured would be good. But is there a better way to make an x?
Thanks guys for the comments. I will work on getting better pictures soon.
Oh and I forgot to say—I don’t know whether my inferior glue (cheap glue gun glue, because it’s all I have) will work for a lap joint. There is no issue with craftsmanship or with time, but I don’t want to use a lap joint if, because of my glue, it will not be the strongest possible joint.
Also, the gusset image is confusing—it doesn’t really show me how I could construct a gusset if necessary.
Thanks for all of the information on this site, though, it’s been amazingly helpful.
Can you show a picture of a real Lap Joint because the diagram is a bit difficult to understand?
I’m a little confused too. This is good information, but I can’t say this gives me enough to go on. A picture (and a more detailed description) would be very nice.
I googled it but it doesn’t give me the specifics that I can usually find on this site.
well ya know that the lap joint is two overlapin peices. well think of a T but with the stick of the t to lay over the top and the overlap does not excede the edge of the top so you will have a T but with the stick over laping t.
What is a Gusset?