Working on a recent story for a community paper, I got to interview Austin Granger, holder of the Guinness world record for the largest KNEX ball machine structure in the world, which is located in the Brickmania offices in Northeast Minneapolis. During our chat, he started telling me about the most elaborate and difficult part of the machine, a 40-foot long bridge.
Here’s what Austin told me about the bridge, describing how he designed and built it.
[Rough transcript begins.]
Austin Granger: The bridge is about 40’ long, has 20,000 pieces, and can support 100 pounds. It took two months to design and build it, basically working non-stop.
Me: Oh my god. It’s amazing.
AG: It was very difficult to install. The top half is under constant compression forces, and it’s not meant to handle being lifted it up 11 feet in the air. It took the entire staff [of Brickmania] to hoist the bridge up into place. It was quite the undertaking, but it’s ended up standing just fine.
Me: So it has what bridge people call redundancy or something?
AG: It can hold more than 3 times the load that it needs to …
Me: How did you design that?
AG: It’s a lot like a bridge in Pittsburgh, the high bridge. It’s supported by these two towers with these giant anchors on the side. The bridge is entirely free standing, though, and is not connected to the walls.
Me: Does the bridge have a name?
AG: No. The closest thing I’ve been able to find is an “inverted suspension bridge”, but its slightly more than that because a standard inverted suspension bridge doesn’t have the arch component on the top. But the forces at play are similar.
Me: Did you ever think about going into civil engineering? I think you might have a knack for it. You should see the material lab down in the civil engineering building at the University of Minnesota. Apparently you can play around with steel and all kinds of things…
AG: I’ve been in the civil engineering building, but never been to the lab itself. That sounds like fun. I initially went to school for mechanical engineering, but skipped into computer science. I still have a love of all things physical and tangible, anything that I can put together with my own hands.
It’s not often people engineer and build their own bridges. (And for good reason, too!) So it’s pretty cool that one guy (and a lot of help) could make something so structurally impressive.
PS. On the Youtube video comment thread, Granger describes a few other technical aspects of the bridge. Enjoy!
Q: How did you make the arch so slight with KNEX?
A: The arch on the top was constructed as a straight truss which was then bent into the arch shape. The bottom edge of the truss was designed to be very strong in compression by using shorter rods and a denser construction. The top edge is designed for tension, with long parallel rods running the length of the structure. The tension to curve the truss is provided by the twin suspension cables on the bottom, while the shape of the curve is maintained by the pairs of diagonal tension rods running between the vertical beams.
Q: Is there much strain on the anchorages? How did you get the anchorages to keep upright? Are they tied to the walls of the building?
A: The bridge is a hybridization of an arch bridge and a suspension bridge, meaning the load is split 50/50 between a compressive span and a tensile span, this results in a net zero lateral force on the anchorages due to the arch and the suspension cables cancelling each other out, so the entirety of the weight is directed straight down into the columns, which allows them to remain freestanding, not fastened to the building in any way.
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