Elevated Bridge B/C

[nejanimb]

Sounds like you guys have done a lot of work this year! I think 1500+ will be in the top 10 for B division, but we'll see!

[Balsa Man]

Since this is a new event, does anyone know wut a placing efficiency is? My team has been working FOREVER on this. we are the b division team for nebraska and our nationals bridge will be #45. If some other nationals team wouldn't mind saying what they predict for nationals...

If you just go back in this thread, you will find lots of.....knowledgeable discussion of exactly what you're wondering.

I agree w/ nejanimb, though I'm not so sure there will be 10 B bridges >1500; top 3....?....I believe its going to take 1850 to 1900

[matthew8817]

u really think someone will be able to hold all 15kg with a 10g or less bridge? ',:-\

[Balsa Man]

u really think someone will be able to hold all 15kg with a 10g or less bridge? ',:-\

That's already been done. Check the scoreboard, and past posts. My son's C bridge did full load at 10.0 - and went to over 18kg in later testing, and as I've posted, a B version is significantly lighter.....

[matthew8817]

thats amazing! . i wish i had the design you do. but, of course, its a competition and that information isnt something you want to give out.

[Balsa Man]

thats amazing! . i wish i had the design you do. but, of course, its a competition and that information isnt something you want to give out.

That's already been done, too.

Check out the Image Gallery; Bridge Building folder. There is design sketch w/ forces; then go to the Balsa Man subfolder, and there are 13 photos - the bridge from various angles, the jigs used for construction, testing equipment. I posted these some weeks ago.

Check out post a few days ago on the philosophy of sharing vs not sharing. Yes, we're all in a competitive environment, but we're also in a.....scientific one. There are many of us that are trying to get basic, and often detailed information "out there", to expand the knowledge base we all all have to work from. Its all about taking what's known, adding your own new ideas or knowledge, and leaving the knowledge base bigger and better.... Am I worried about "sharing secrets" with "the competition"? - no, because we keep finding new "secrets."

With this thread at 108 pages, as I find myself saying.....every few weeks when someone asks about something that has already been discussed and answered, there is A LOT OF GOOD, IMPORTANT INFORMATION right here. I'm not just saying this to you - read, learn, enjoy. Before you know it, you'll have knowledge to contribute.

[matthew8817]

wow. i feel like i just read the end of a book. that looks almost exactly like the one i built but fit to c division specifications and a whole 5g lighter. also, did you weigh your wood before you used it?

[matthew8817]

is it better to pour the sand fast...or pour slow; to evenly distribute the sand around in the bucket attatched to the loading block?

I'm sure there are as many opinions out there as there are... but I believe that consistency is far more important than speed. If hand loaded, minimize the thumping of the sand hitting the bottom of the bucket, and keep the bucket from swaying. If machine loaded, find a speed that minimizes the sand being released in waves. Once these skills have been mastered, release it as quickly as possible within the above parameters. There is always a specific reaction time... the time between when the bridge breaks, and the judge releases the cut-off. It may only be a 1/10th of a second, but it's there... more sand will go into the bucket and be counted as load if running quickly, than if barely dribbling in.

As for slowing down at the end, I don't buy it... the longer a structure must resist a load, the more likely it will fail.

Much more important than the speed of the loading is the levelness of the bridge, the levelness of the testing platform, the angle of the hole through the loading block, and even the length of the eye bolt. If the loading block does not sit level on the top of the bridge, or the testing table is not level, or the hole is not drilled perfectly at a 90 degree angle to the block, the eye bolt will not be perfectly vertical. If the eye bolt is not perfectly vertical, when a load is applied it will be pulled vertical, causing an unbalanced load between the two sides of the bridge. This not a good a thing, and will cause the bridge to "torque." The longer the eye bolt is, and the tighter the hole is, the more this condition is exacerbated. I've see eyebolts that are as much as 12" long, and have hung as much as 1" out of plumb

So... take a pocket level or torpedo level with you and check the testing platform, and how the top of the bridge looks before placing the loading block... you probably will not be able to get the testing platform adjusted, even if it's out of level (not likely at Nats... theses guys are GOOD) but you may want to rotate the bridge 180 degrees to see if one way sits better than the other. Same goes with the loading block. Place it, but then look at it from the end... Does it look plumb?... I mean REALLY plumb?... if not, rotate the block and look again. If the bridge is not symmetrical, rotating the block won't help, but if the hole is slightly off of 90 degrees, here will be one rotation that will be better than the others.

Good Luck at Nats guys!

i know that loading slower does indeed make the structure have to support the weight longer and may weaken it as it goes, but think of it this way... would you rather have someone slowly lower a 80-lb weight on to your shoulders or have it dropped on you all at once and which do you think you would support better? i think as long as you keep the weight distributed evenly, you should stop some of the swaying, letting it hold more in the end, regardless of speed. i mean, obviously dont take 20 minutes for each scoop, but dont worry too much on if you are going fast enough.

[Balsa Man]

wow. i feel like i just read the end of a book. that looks almost exactly like the one i built but fit to c division specifications and a whole 5g lighter. also, did you weigh your wood before you used it?

That's cool (design similarity) - I'm not really surprised; the geometry is pretty much the minimum mumber of lines you need to make a structure around the dimensional limits in the rules.

Reading time, now.....

Wood weighing, and a lot of other info is discussed at length in previous posts. For example - From the design work, wood, for each piece, was bought at a specific density (for example, 1.1gr/36" sticks - different for each member). The stiffest sticks at the desired weight were picked out. Then those sticks were cut into pieces about 2cm longer than final length, and weighed (to a 10,000th of a gram). For members where there are 4 in the bridge, 12 to 16 were cut/weighed. Then the 4 to be used were picked - at the design density, and very closely matched in weight.......... that's what the colored markings in the photos are about- an easy way to know which piece is which.

If you go back and read through previous posts (use the search by author function), and study the photos, you will have a pretty darn good handle on where your extra weight is coming from, and how to get it off. Enjoy. And, as I said earlier today, the B Div version is significantly lighter than the 10g C version.......

[matthew8817]

thank you very much for this information. now if only i had discovered this site sooner. this should still help for next year though. i already built the nebraska nationals bridge and it is on its way to georgia by means of school van and teachers driving.