Elevated Bridge B/C

[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

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

Thanks, our state is on friday and saturday , good luck!

[AlphaTauri]

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

What we did last year was take a box slightly bigger than the bridge and stuff packing peanuts all around the bridge (you do want to be careful not to accidentally break any really thin pieces, though). It makes quite a mess when you take the bridge out, but at least it keeps it safe with almost no work needed.

[nejanimb]

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.

I think you meant this sort of rhetorically to say that it's not worth it, but this usually can be done. We bring out box all the way with us until right before they need to check in our bridge, and we usually ask that the first thing they do is put it on the scale. So, usually it's out of the box for less than a minute before it's on the balance. Even if it has absorbed more moisture by the time it gets onto the load table, the mass was recorded first, so then we get a few lower than we would have otherwise. This was definitely helpful in Augusta last year, with air that was basically wet.

Good luck at States! I'm excited to see pictures and hear reports when you return.

[cypressfalls Robert]

Ok so what I got out of this was , that I do need an airtight container, with a bit of rice?

Also, can someone explain to me, that why my bridge would lose .1 grams becasue of sitting in my locker for 6-8 hours during school?

EDIT: I have posted my 2008-2009 season bridges, this season will come in two weeks and depending on state results friday and saturday

[Littleboy]

Ok so what I got out of this was , that I do need an airtight container, with a bit of rice?

Also, can someone explain to me, that why my bridge would lose .1 grams becasue of sitting in my locker for 6-8 hours during school?

EDIT: I have posted my 2008-2009 season bridges, this season will come in two weeks and depending on state results friday and saturday

When the glue dries more it hardens and loses weight.

[Balsa Man]

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.

I think you meant this sort of rhetorically to say that it's not worth it, but this usually can be done. We bring out box all the way with us until right before they need to check in our bridge, and we usually ask that the first thing they do is put it on the scale. So, usually it's out of the box for less than a minute before it's on the balance. Even if it has absorbed more moisture by the time it gets onto the load table, the mass was recorded first, so then we get a few lower than we would have otherwise. This was definitely helpful in Augusta last year, with air that was basically wet.

Good luck at States! I'm excited to see pictures and hear reports when you return.

Oh, you're absolutely right- if you can get it out of a.....controled humidity atmosphere and onto the scale in a minute or two, it will make a difference. The reason I made the "...run in a way that you can...." comment is the way the rules actually read-
5c - place on scale....at the time of testing
I haven't dug up last year's rules, but I believe this was added..... As discussed for various events, the folk running various competitions are not always fully up to speed on every word and nuance of the rules. I could be wrong, but I'll bet at Nationals, weights will be measured "at the time of testing."

We always bring the bridge in an underwater camera case, and yup, its dry inside, and.....go with the flow on check-in. If the weighing is done early in the process, the weight is lighter than it would be if done later....

I will get pics, and discussion of a couple interesting things we've figured out this year up after this weekend. May be of some help to folk heading to Nationals....Both our team's bridges are done; we think we know how we're sitting, but only Saturday will tell.

Ok so what I got out of this was , that I do need an airtight container, with a bit of rice?

Also, can someone explain to me, that why my bridge would lose .1 grams becasue of sitting in my locker for 6-8 hours during school?

A couple of possible explanations. If the scale only reads out to 0.1gr, it could be a change of....a couple hundredths near a read-out breakpoint, like from 10.54 to 10.56. Or it could simply be the slight variation that's inherent in a scale that's only precise to 0.1gr. Also, relative humidity is higher in the morning- as the day warms up, unless a wetter air mass moves in, the relative humidity drops..... It can change a lot; like a foggy morning (100% Rel humidity) to a sunny afternoon (30%).

As to Littleboy's drying glue thought- if you've used a solvent-based glue, yes. If CA, no- it doesn't "dry", it reacts and hardens. It actually gains a VERY small (so small you can't measure it) amount of weight

[cypressfalls Robert]

Thanks, I have the following scae, it reads out to two decimals:


http://www.lotioncrafter.com/jennings-js-50x-scale.html

EDIT: Last question(hopefully) how long in advance does everyone build their bridges, from competition day?

[StampingKid]

I am stuck in a conundrum. I have broken more bridges in more ways than I care to count and still have not been able to improve on regional score (1050)! State is this weekend--I need to make on last push tonight to see if I can make any progress then I'll have three bridges to choose from. I am trying to decide what angle to set the upper compression joints at each end of the bridge. The option is but at +/- 45 or at a right angle. With the latter I would need a gusset which I have had success with the problem is the gusset takes the joint out of line when trying to connect the tension members but with the +/-45 I should be able to gusset across the joint with the tension member. is there a better way to do this?--my failures have been usually at this joint or running from this joint to the main tension member,

[quickestwinne]

Hey guys from Kansas,

This year Kapuan Mt. Carmel (the 3 time defending champ of the balsa event) got a score of 2500 at state. However, they will not be going to nationals. My dad said they could have probably gotten close to 2750 for nats. Collegiate (second from last year) got around 1700s? I think.

Also All Saints middle school will not be going but their bridge was around 2100-2200. Can't remember what my dad said.

Good luck to all who are going to nationals or still have their states coming up. May your bridges break at 15 kg!

[icyfire]

What kind of bridges are you guys trying? Are you building relatively heavy bridges (>10g) that can hold a lot of weight or a less heavy bridge that doesn't hold as much but just going for efficiency?