Elevated Bridge B/C

[lllazar]

couple Q's, state is in one week and I need to tweak my bridge

Should tension be as light as possible?

Does anyone use different glues for tension and compression?

And finally, is it bad to microwave a bridge as a faster and cheap alternative way than baking the bridge? , If time is not your friend...

I use a few drops (literally) of 5-15 second CA for lamination of the compression, if that's wat u mean...i don't understand wat you mean. Because there are a lot of points on the bridge where tension and compression connect...but im just an idiot so w/e...

[cypressfalls Robert]

I mean from tension on top of compression, paired strips, and compression to compression

[StampingKid]

Can CA go bad?

[lllazar]

Yes, im pretty sure it can...keeping it in cool conditions can however extend shelf life. But i build so many structures that theres not enough time for a bottle to go bad so ya...

[soccerisforlosers]

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?

probably not. they dont measure while your testing so and its not in the rules so you shouldnt have to worry about that

[dragonfly]

Can CA go bad?

If a bottle has been opened, with use the glue is constantly being exposed to more air, making it become slowly thicker and could eventually even completely harden if not taken care of properly. I keep my CA separate from the rest of our team's builders since they do not care for their glue in the same way that I do. While building I make sure to wipe off the cap and keep the nozzle unclogged so that the cap can fit on perfectly. If there is too much glue buildup and proper covering cannot be accomplished, your CA can dry out and not work as well as it did initially. Case in point -- TAKE CARE OF YOUR GLUE.

Should tension be as light as possible?

Technically, everything should be 'as light as possible', but what exactly that possibility is is the question I suppose you mean to ask. As previously discussed, I think it has been unanimously decided that when using balsa, tensions can use less dense wood than compression pieces. Again, as stated prior, mass of pieces are specific to their placement and usage however, but you can use this as a general rule.

And finally, is it bad to microwave a bridge as a faster and cheap alternative way than baking the bridge? , If time is not your friend...

I have never EVER baked a bridge (if you want to learn more about it, I do believe there were a few discussions on it last year) but if your purpose is to simply dry out your bridge, I would recommend keeping it in an area as dehydrated as possible in relatively normal conditions. I've never found a need to bake anything, and unless for possibly making arches (which we have also already found inefficient) do not see it's purpose.

[blue cobra]

I tested my new bridge today, using the hopper with a safety tower. It performed... satisfactorily I suppose, with a 1400. If I can recreate that, I may be able to squeeze out a medal, but I'm not confident at all.

Failure was in one of the outside leg members, in compression. It bowed inward a lot, and then buckled. However, none of the other 3 corresponding members had significant bending. I'm thinking it's possibly a bad piece of wood- I did notice a discoloration- since it bowed so much while the others did not. I'm not sure yet, but I might just rebuild exactly the same, or I may try to brace those members, as in something like 1/8x3/32 balsa between that and the inside leg member. Might that help?

[Balsa Man]

I tested my new bridge today, using the hopper with a safety tower. It performed... satisfactorily I suppose, with a 1400. If I can recreate that, I may be able to squeeze out a medal, but I'm not confident at all.

Failure was in one of the outside leg members, in compression. It bowed inward a lot, and then buckled. However, none of the other 3 corresponding members had significant bending. I'm thinking it's possibly a bad piece of wood- I did notice a discoloration- since it bowed so much while the others did not. I'm not sure yet, but I might just rebuild exactly the same, or I may try to brace those members, as in something like 1/8x3/32 balsa between that and the inside leg member. Might that help?

I'd say 1400 is .....quite satisfactory; nice work.
Isn't it nice to know what failed how/why?

From what you describe, two reasonable possibilities. First is a bad piece of wood, as in defect, or one leg significantly lighter density (hence lower strength). Carefully look at pieces for the new one. Weigh them; they should all be pretty darn close. Ideally, weigh legs from the one you tested. The new ones should be of comparable density (as in not lower). The other possibility is that assymetry was resulting in higher load on one leg. It doesn't take much. If the top is not really level, and its not sitting squarely and equally on all 4 legs (and/or if the two sides are not .....constructed almost exactly alike), more load will end up oon one or two of the legs. When you've put it together, check it really closely. If it has any.....3-leg teeter-tottering, carefully sand/shim leg ends to fix. Check your lean-in to make sure it is really even on both sides. Make sure the load block is sitting totally flat on what it rests on.

Good luck!

[cypressfalls Robert]

I know this was discussed before but anyway, how should I pack my brdige for the competition? I Have a cardboard box and two different sized plastic ones...and last year I heard something about rice???

When I test bridges at school I Leave the bridges in my locker and by the end of the day they have lost at least .10 grams

[Balsa Man]

I know this was discussed before but anyway, how should I pack my brdige for the competition? I Have a cardboard box and two different sized plastic ones...and last year I heard something about rice???

When I test bridges at school I Leave the bridges in my locker and by the end of the day they have lost at least .10 grams

Rice, if it starts out really dry, is a....somewhat efficient dessicant; like silica gel, it will absorb moisture out of the air. However, unless you have a truely sealed/air-tight container, it - or silica gel - won't have any effect. The difference in weight between really dry (like on those 10% relative humidity days we sometimes get in Colorado) and humid - like 90% rel humidity will run in the 0.2 to 0.4gr range. A "dry" structure will come to equilibrium with .....wetter air pretty quickly; light balsa taking 5-10 min, bass 10-15; most of the change in the first half of the time window. Bottom line, unless your event is being run in a way that you can go from dry, air-tight box to the official scale in less than 5 min, you gain nothing.

Pack it to protect it for the trip. Couple foam blocks glued into the box so it can't slop/rattle around work nicely. Goood luck.

State for us is Saturday