First, there was the fact that there was no penalty for having a heavy bridge. Many bridge designs for similar circumstances use a hundred or two hundred sticks. Our team made it a goal to use as many of the 1000 sticks as we could glue together in time. It was ugly. It was heavy. It was not well designed. But it was strong. Another team brought an iron due to the increased glue-melting capacity managed to use 998 of their sticks. It was formidable. But it had a fatal flaw common to many other bridges…
Then there was the scale. In order to weigh how much load was being placed on the bridges, the contestants stood on a bathroom scale placed on the bridge. And for whatever reason (maybe to help with stability) the bathroom scale was placed on a cutting board. Here’s our rag-tag bridge being tested in this manner:
You’ll notice that the weight is fairly evenly distributed along about 13" of the 15" gap. This significantly changes the design goals from a traditional truss bridge. Other bridge contests put a point load in the middle, which is not too dissimilar to a real bridge — it tends to try to buckle in the middle. But in this setup, the bridge is just being crushed vertically.
Most of the bridges were fairly strong against vertical crushing. In fact, the only bridges that failed in this capacity seemed to do so because they weren’t centered properly — one side had just an inch or two on the block and it snapped off. But the others all failed by sheering. Being imperfect humans, the weights were shifting forwards and backwards a fair bit — perpendicular to the axis of the bridge. Very few of the bridges had any diagonal bracing against this. The top, bottom and sides can all be perfectly strong, but if the corner joints fail to hold a 90 degree angle, it parallelograms into flatness. In my observation, this is how every properly centered bridge failed.
It’s an interesting social phenomenon seeing the same design mistake in every bridge. It’s understandable for many reasons. First off, real truss bridges that hold cars don’t have any bracing in this direction. They couldn’t. The braces would get in the way of the cars. Most model bridges don’t either. If your load isn’t active along the side-to-side axis, it’s not a huge deal.
Also, it’s rather difficult to get bracing in at those angles. For us it was something of an afterthought. We looked at it when it was somewhat assembled (23 minutes in) and said "we need diagonal braces!" But the popsicle sticks weren’t well suited to attaching at the odd angles necessary. Keeping with the design philosophy of the team, I heaped a bunch of glue on the end of a reinforced double-thick stick and slid it into the middle of the bridge. Then I dribbled glue onto the other end until it seemed like it might hold. I repeated this process a few times, and got some nice burns in the process. (I wish I had a picture down the interior of our bridge. Maybe I’ll add one.)
It’s hard to know how much this helped, but our bridge was quite strong. It held Jen and Eric at the same time!