They were only glued together at the ends, per specified rules, although we learned later there was an unwritten rule that you could not stack pieces to make a thicker member, so we were penalized slightly..

This next pic shows you the setup for the test: a 5 gal bucket is placed on the bridge & you slowly pour sand in until the bridge breaks. Max weight of 20kg. (my suggestion to dump a 50# bag all at once was not accepted)

We were unaware that the footing for the bridge would be 2″X2″s, & as fate would have it, they hit right in the middle of our trusses. You can see how it is flexing in between trusses along the bottom chord. This was ultimately the failure point. Otherwise, the bridge actually stayed very straight underneath the load.

Here you can see it holding up well:

In the specs for the bridges, the organizers did not specify a minimum height requirement, & 1 of the other teams took this to the extreme & built basically a pallet:

It was a great learning experience for us & has encouraged me to start a club next year where we do little beyond build bridges & towers!

Garrett Adds
Thanks for sharing! It is too bad about the unknown rule about doubling up the sticks. Your lateral bracing on the top and support for the bucket look really good. Another thing that probably would have helped is to have lateral bracing on the ends of the bridge as you look through it. This would stabilize it and help keep it from leaning or falling over sideways. You mentioned that the bridge failed because the supports (2x2s) caused a lot of shear pressure on your bottom chords.

That beam bridge is interesting, and it reminds me of a conversation I had recently with an engineer. He was saying that in real life they try to design bridges so that the mode of failure is gradual, and not sudden. It is hard to do this with model bridges, as most often they look pretty good and before you know it they just explode. But that beam bridge has enough flexibility built into it that you can definitely tell when it is having problems way before it actually breaks. Flexibility is the only plus to using hot glue. I’ve seen a bridge hold up a lot more weight simply because it did not break at the joints due to the flex in the glue than if it had used a harder glue, such as wood glue or white glue.

Here is a picture of the Hell Gate model:

frame work started using steel bars welded together

Garrett’s Thoughts
This is an excellent piece of work. Thank you for sending in this photo of the progress you have made so far. I am definitely looking forward to seeing the finished product. What are your plans for it once it is completed? What inspired you to start building this model bridge?

My 1st brigde

Thanks again for all the great info and advice. This project only has to hold 10 lbs. extra credit if it has a 25% or less load capacity percentage. The record in the class on this project is 80 lbs… I’m planning to donate 1$ for every 10 lbs this bridge holds to your web site.

Model Covered Bridge

Thank you for your time and any helpful resources. (I cannot make the Johns
Hopkins nodes and members work….?)

Rainbow Bridge Company

This bridge is supporting 191lbs of weight. It didn’t break, that was all the weight we could fit in the trash can. I believe this is close to the maximum load it could hold because the platform the jig is resting on was significantly bowed. The rest of the structure was rock solid. The construction of the bridge is popsicle sticks.

These are some pictures of a bridge I built mainly out of boredom, but also since I have an interest in engineering. It’s a large cantilever bridge, meaning it has two arms that extend beyond their piers that hold up a center suspended span. It’s 10′ 2″ long and as of right now is hanging across my bedroom ceiling. One nice thing about cantilever bridges is how the longer top-chord is in tension, while the shorter, straight bottom chord is in compression.

The bridge was built using both sizes of popsicle/craft sticks held together with high-temperature hot melt glue. In the pictures (on my Photobucket, as WordPress wanted to crop them down too far) you can see the I-beams made out of sticks and made into the chords, diagonals and bracing. Two of the trusses were built by “building up” girders rather than making I-beams. The top-chord lateral bracing is different on the one cantilever arm, as it was built earlier in the year and I came up with the idea for the other bracing in the downtime.

For decoration I added blue LED lights to the bridge.

See more pictures at the Photobucket Album.

The design procedure was completed over a 2 week period allowing sufficient time for segment to dry before moving onto the next. Heavy duty pegs were used to fasten the joints that had already been sanded to allow for maximum strength in the joints.

As the design took shape, I made several smaller models and tested the joints to come up with a final design for the joints that would allow for more weight to be added before the joints failed. Failure in the joints would mean that it wasn’t the truss design that caused the failure, but a failure in the construction of the truss.

After allowing 24 hours for the 1 layer truss to dry the pegs were then removed. As the image illustrates the truss consists of one smaller heavily layered triangle and 2 larger reinforced triangles. The top side when under a pinpoint force will be under compression and the bottom tension. The central joint that will be directly under the metal element issuing the force was to be constructed so that the glue joins if they were to fail would not be able to move due to the surrounding sticks creating a ‘nest’ for the failed member.

The original design was to be compiled of 12 layers each consisting of 18-19 sticks. But after weighing the truss structure it was calculated that the truss would have a final weight of over 300grams which would lead to a decrease in efficiency. This was therefore reduced to an 8 layer truss that weighed 220 grams.

When designing the truss care was taken into using sticks that were not only straight but did not contain any knots or weak points, these sticks were discarded.

After the truss was completed all that was needed was to place angled bracing on the truss to stop any bending in the beams under compression. But throughout the whole design efficiency was always key and when bracing was placed on the structure it was placed in selected areas as once again placing sticks in places that was not needed would lead to a decrease in efficiency. Decking was also placed on the end supports to allow for even distribution of force throughout the layers.

The final design structure was 54cm long and 10.5cm wide it had an average depth of 5cm and weighed 220grams.

Nic’s design held a weight of 224kg and had a final efficiency of 1020. It did not shear or snap at any member, but bowed in the second third of member C-E. This member did have angled bracing to stop bowing and this brace did not snap it was only the glue joins in this brace that failed. The truss ballooned out at the point when 2.2kN of force was applied. It was at this point that the test was stopped.

If the experiment was to be completed again I would have braced this area more heavily but from what I had researched I thought the amount of reinforcement already applied would be sufficient as to negate any bowing in this area.

As you can see, I’ve modelled it on the Pratt Truss. Although, I didn’t do any “real” plans, just rough drawings (no force/load drawings).

Let me know what you think!

Cheers, 

Joe.