Elevated Bridge B/C

[jander14indoor]

What are you building your test bases with?

I spent the better part of an hour looking around Lowes for nice solid and flat pieces of wood, but wasn't terribly happy with what I ended up with :\
(judging a regional Saturday)

Flatest is probably some sort of particle board covered by laminate for smoothness. To keep cost low, look for an off cut from a counter top. Then cut a hole in it of appropriate size.

I've also seen two tables space apart appropriately used, trick is keeping distance from changing throughout the competition. If it were me doing that, I'd bring a few extra boards and clamps to connect the legs of the tables a fixed distance apart.

Jeff Anderson
Livonia, MI

[dholdgreve]

What are you building your test bases with?

I spent the better part of an hour looking around Lowes for nice solid and flat pieces of wood, but wasn't terribly happy with what I ended up with :\
(judging a regional Saturday)

i've seen a table with a whole cut in the middle, but the table was like one of those tables you would find in a science classroom(in my school anyways) the top of them are black, its pretty smooth, i'm not sure what it is...(or it its just painted black)

That's something like what they have at my competition.

A really great testing platform is a old laminated cabinet door like you would find in doctors or dentists office... Surely you must have some custom cabinet shops in you area... maybe check with them as see if the can help you... You may even find one that will cut the hole in for you! If not, check with Lowe's to see if they have any discontinued door samples of cabinets. I used a large plastic laminated cabinet door, cut out the center, then screwed a 2 x 4 edge ways near each end, and drilled a 5/16" hole near both ends of each 2x 4 for a total of 4 holes, one near each corner... A quick trip to my local ACE hardware, and I installed Tee nuts at each corner that allowed me to screw in threaded feet like you have under an appliance. With the help of a couple of torpedo levels, it's now a snap to make sure the testing table is PERFECTLY level, which is so critical this year! One word of caution... We would place the testing table between 2 student desks, so the 2 x 4s sat on the desk tops, facing each other... the coefficient of frriction between the metal feet and the desktop was virtually non-existent, allowing slipage, so when you are at ACE, pick up a pack of the self adhesive rubber disks and apply to the feet bottoms... problem solved!

[sr243]

there is only one way a truss can be counterproductive and only building the real bridge, not on graph paper. It can be built bad if you glue so it makes the structure bend out of shape. An example is like using a stick of balsa on the bridge and pushing the top and bottom out of their place. Otherwise there is no way i can think of trusses can be not beneficial.

I take this comment back, realizing how wrong I can be. There are ways to decrease your weight you hold if you add bad trusses. An example of this is adding excessive vertical trusses in the middle of the bridge. If you put down, you just increased the stress on the bottom middle which is the weakest point of the bridge. The whole point of trusses is to move the stress to the outside. So do not make bad trusses that the energy go to into the middle. Vertical trusses IMO is not any good because you are transferring the energy from the top to the bottom directly so all the weight goes right into the middle where there is no support. If you have just the outline of bridge, push down on the middle and you will find it is very easy to slide the legs and break it. That is why some really complex and overbuilt structures are not even better than simple ones.

[croman74]

I just got back from regionals and got a 9th place with an efficiency of 620. I was one spot away from a medal! I didn't get to see it, however, cause I was doing pentathlon.

[11madmic]

I just got back from regionals today and our bridge's weight was 8.2 grams(0.3 more than expected) and held 9.32kg. with an eff. of 1137. At least i saw where the bridge broke so i can improve it. My coach thinks we could of held the full 15 if i would of cross braced the ends. What do you think the minimum eff. to get a medal at a competitive state like Michigan?

[11madmic]

What do people think about getting specialized wood that weighs a certain amount? And what type of glue are people using? I use CA glue

[croman74]

I just got back from regionals today and our bridge's weight was 8.2 grams(0.3 more than expected) and held 9.32kg. with an eff. of 1137. At least i saw where the bridge broke so i can improve it. My coach thinks we could of held the full 15 if i would of cross braced the ends. What do you think the minimum eff. to get a medal at a competitive state like Michigan?

Are you in region 12? That's where I'm from. What school are you and did you medal?

[11madmic]

No i am in region 3. it is not competitive at all so we basically get first in all events( not all). I did get first in elevated bridge. we do good in build it events at state but not the book it events.

[kp9ssa]

what type of wood do u use, and what type of design do u use

[11madmic]

I use balsa different sizes depending if it is a tension or compression member and the bracing and design i will not leak until after states since we got an eff of around 1800 with it. ( Of course not at the competition!)