Elevated Bridge B/C

[Greg Doe]

To anyone interested,
Finally I have found time to report on the Tennessee State SO "B" Division competition from Mar. 27. I don't have a lot of results, but you can fill in the missing data with educated gusses.
1. Cedar Springs Home School Group (no results)
2. Harpeth Hall Middle School 1103 (approximately)
3. Bearden Middle School (no results)
4. St. Rose Of Lima Catholic School 1014 (approximately)

The reason I know the second and fourth place efficiencies is because I coached those teams, so
we knew the bridge weights. Both bridges held all the weight. The lighter second place bridge
failed at 15+ kilograms, and the heavier fourth place bridge failed at 16+ kilograms.

A while back there were some negative comments about "Arch bridges being beautiful, but
usually finishing last". In 2009 my students finished first and second at the state level, and this
year they earned second and fourth with arch designs. Last year between the two schools we
built 8 bridges, and this year we built 5. The kids were very satisfied with the results considering
the limited amount of time and effort invested in bridge building. Last year I commented here that
their design could benefit from some improvements, but would probably never achieve the efficiencies that some of the top teams are getting.

We won't post pictures, but a friend summed up the design by describing it as a 'rainbow'?
We're done for this year as neither of my teams are going to the Nationals. Good luck to everyone
who is still competiting, and remember to 'read this forum before asking questions'. The answeres
you are looking for are probably already here.
Greg Doe
Smyrna, TN

[BeeVolturi]

My partner and I just finished our bridge. It's name is The Epic Fail.

[StampingKid]

What amazes me is how many different bridge designs like arches and suspension that you see In the real world that you don't see applied here on a more regular basis.

[nejanimb]

It's not really that amazing, since the task and parameters are so different. "real" bridges have to hold up a continuous roadway or path, whereas these bridges only need to support one spot in the center. Real bridges don't have to be "Efficient" by weight, necessarily, and are more concerned with cost efficiency and the ability to support the maximum load while staying up and, in some cases, looking cool.

In those particular cases though, the main difference is probably more about the parameters. Suspension bridges, for example, require the ability to brace laterally and anchor something into the ground, and they also have pillars in the middle of the bridge instead of on the ends. You can see here my attempt at modifying a suspension design such that it would make sense for a science olympiad structure, but I abandoned this pretty quickly. Arches, especially the ones used in ancient times, were useful because they could span a gap and leave an opening by using a material with essentially infinite compressive strength (with relation to the loads they were applying), rock. Balsa, especially when you're going for efficiency by weight, definitely does not have infinite compressive strength.

It shouldn't be surprising to see that the designs used are very different. Those who designed those bridges, and the competitors designing these ones, are engineers (in some form of the word), and engineers should have a solution to a problem that is specific to the task and parameters. Since those are so different, it's not surprising that the solutions would be, too.

[blue cobra]

I just finished building my next bridge that will hopefully be good enough to be my States design. It replaces 1/8 square bass with 3/32 sq bass and 1/8x1/32 bass paired strips with 1/8x0.020 bass paired strips. It also has a lean in of ~1cm in total. The mass amazed me. My Regionals bridge was over 14 grams. This bridge, with fresh glue, is 7.71 grams. With completely dried glue it may lose a few hundredths as well (EDIT: Actually, it's 7.73g now). That mass would give me a maximum efficiency of 1945, but I'm not that lucky. I'm going to be only slightly disappointed if I get around my last efficiency, happy if I break 1200, very happy if I break 1300, and ecstatic if I push 1500. Now, I need to stop saying break...

Nejanimb, I believe it was you who inspired the "upside-down T" lean-in jig. It worked great, and was much easier than what I was doing before! I will definitely be using that design again.

[lllazar]

I just finished building my next bridge that will hopefully be good enough to be my States design. It replaces 1/8 square bass with 3/32 sq bass and 1/8x1/32 bass paired strips with 1/8x0.020 bass paired strips. It also has a lean in of ~1cm in total. The mass amazed me. My Regionals bridge was over 14 grams. This bridge, with fresh glue, is 7.71 grams. With completely dried glue it may lose a few hundredths as well. That mass would give me a maximum efficiency of 1945, but I'm not that lucky. I'm going to be only slightly disappointed if I get around my last efficiency, happy if I break 1200, very happy if I break 1300, and ecstatic if I push 1500. Now, I need to stop saying break...

Nejanimb, I believe it was you who inspired the "upside-down T" lean-in jig. It worked great, and was much easier than what I was doing before! I will definitely be using that design again.

Break...break...break....BREAK!!!

But dude, that is NICE. 7.71 grams? Thats almost 50% of the original weight...good luck

[blue cobra]

Thanks.

Does anyone have dimensions of the test base on a standard hopper? I'm trying to figure out a way to use my safety tower with the hopper.

[AlphaTauri]

Thanks.

Does anyone have dimensions of the test base on a standard hopper? I'm trying to figure out a way to use my safety tower with the hopper.

I think the hopper is supposed to be off to the side (you are talking about the thing that pours sand into the bucket when you move the lever, yes?). Every one of those I've seen has an angled piece of PVC (?) pipe so it can funnel sand into the bucket from off to the side. I can't see why you would have any issues with a safety tower and a hopper...if it's what I think it is.

[blue cobra]

Yes, that's the hopper, but my safety tower's legs come down to form an approximately equilateral triangle with sides 2ft long. The test base (what you actually put the bridge on) is not that large, so I need to find something of the right size to clamp onto the test base to extend it so it can support the safety tower.

[seasponge]

I have a question that's been burning on my mind...if the bridge dips down into the Minimum Clearance area (the 30 x 12.5 area) while being tested (because it's sagging due to the weight of the load), is the bridge disqualified?