Elevated Bridge B/C

Locked
Balsa Man
Coach
Coach
Posts: 1318
Joined: November 13th, 2008, 3:01 am
Division: C
State: CO
Location: Fort Collins, CO
Has thanked: 0
Been thanked: 0

Re: Elevated Bridge B/C

Post by Balsa Man » January 25th, 2010, 11:04 am

croman74 wrote:I just want to make sure i'm understanding this right. Are you saying that if you were using say a 1/8 x 1/8 piece, it would be lighter and stronger to use a 3/16 x 1/16?
First, I'm only talking about tension pieces
Second, I'm saying that 1/64th paired strips are significantly lighter than any reasonable square cross-section stock; for instance, if you're using 1/8 x 1/8 for other (compression-loaded and coulmn bracing pieces, the comparison would be against 1/8 x 1/8 for tension pieces. Don't follow your 3/16 x 1/16 math
If you cut a piece of 1/8 square to 1/16th, you have 1/4 x 1/16th = 1/2 x 1/32 = 1/64th x 1"

Specifics from spreadsheet on densities and tensile strengths by cross-sections- pieces able to carry a 12kg tensile load;
1/8 x 1/8; about 5#/cubic foot = 0.009 gr/cm x 30 cm x 2 per sides x 2 sides = 1.08 gr
1/64th x 1/4- at about 14 #/cf = 0.0055 gr/cm = 0.66 gr
About 0.4 gr lighter; not a lot in total amount, but percent-wise>33%, and 0.4g is....0.4 gr

What makes the effect even greater is that lower density tehds to have more variability/imperfections, so the theiretical tensile for light 1/8 square is.....less likely to actually be there, so to be safe, you have to bump up to a higher density.
Len Joeris
Fort Collins, CO

evbassboy13
Member
Member
Posts: 49
Joined: October 15th, 2009, 4:43 pm
Division: Grad
State: MN
Has thanked: 0
Been thanked: 0

Re: Elevated Bridge B/C

Post by evbassboy13 » January 25th, 2010, 1:08 pm

Balsa Man wrote:
croman74 wrote:I just want to make sure i'm understanding this right. Are you saying that if you were using say a 1/8 x 1/8 piece, it would be lighter and stronger to use a 3/16 x 1/16?
First, I'm only talking about tension pieces
Second, I'm saying that 1/64th paired strips are significantly lighter than any reasonable square cross-section stock; for instance, if you're using 1/8 x 1/8 for other (compression-loaded and coulmn bracing pieces, the comparison would be against 1/8 x 1/8 for tension pieces. Don't follow your 3/16 x 1/16 math
If you cut a piece of 1/8 square to 1/16th, you have 1/4 x 1/16th = 1/2 x 1/32 = 1/64th x 1"

Specifics from spreadsheet on densities and tensile strengths by cross-sections- pieces able to carry a 12kg tensile load;
1/8 x 1/8; about 5#/cubic foot = 0.009 gr/cm x 30 cm x 2 per sides x 2 sides = 1.08 gr
1/64th x 1/4- at about 14 #/cf = 0.0055 gr/cm = 0.66 gr
About 0.4 gr lighter; not a lot in total amount, but percent-wise>33%, and 0.4g is....0.4 gr

What makes the effect even greater is that lower density tehds to have more variability/imperfections, so the theiretical tensile for light 1/8 square is.....less likely to actually be there, so to be safe, you have to bump up to a higher density.
Thanks for the clarification. Saved me a lot of experimenting. :lol: Another advantage to using 1/64 x 1/4 is that you have a lot more surface area for a solid lap joint without using gusset plates, which saves building time and (a trivial amount of) weight, and should make your joints sturdier. I think. Just thought of that while I was reading.

User avatar
lllazar
Member
Member
Posts: 839
Joined: November 19th, 2009, 7:20 pm
Division: C
State: IL
Location: Probably at my laptop, multitasking while on AJAX chat....
Has thanked: 0
Been thanked: 0

Re: Elevated Bridge B/C

Post by lllazar » January 25th, 2010, 6:22 pm

Hello

I just recently built a birdge....now bear with me as i say this, i used 1/8 x 1/8 in pieces for EVERY PART OF THE BRIDGE.

Yes, i am a bridge noob, and this is my first bridge....i was ok with the results, it got an efficiency of 272.

I was wondering, wat pieces should i make smaller, like wat parts of the bridge? And i know if i gave u the design it would give u guys a better idea but i lost it (i fail, a lot). But it does look an awful lot like this:

http://lh5.ggpht.com/_mKf1-yzBt_A/SRZJq ... G_6722.JPG

Plz note i didnt completely rip off the design, i just typed in elevated bridge on google and got this design....seemed easy and actually was so i just drew something similar and made it.

Thanks in advance.
2011 Season Events~

Fossils (Regionals ~1st) (State ~6th)
Towers (Regionals ~1st) (State ~3rd)
Helicopter (Regionals -3rd gahhh) (State ~5th)
Wind Power (Regionals ~1st) (State ~3rd TIERED!)

Hooray for getting everything i wanted?

dragonfly
Member
Member
Posts: 75
Joined: May 3rd, 2009, 6:18 pm
Division: Grad
State: PA
Has thanked: 0
Been thanked: 0

Re: Elevated Bridge B/C

Post by dragonfly » January 25th, 2010, 6:30 pm

I think the key for you first is to nail down a good design and then plan out densities and wood sizes second. Often wood size will simply change depending on design and then modifying can occur later. While size and shape of wood is very important, having a successful design with a lot of room to explore comes first!

One thing I can almost assure you though is that no matter what your crosspieces (to connect your bridge) will be lighter/smaller, since for most designs it works out that way. Other than that, not much else can be done if you're just starting out. Play with ideas for a bit longer, and it'll all make sense.
``````( ) ( ) /
------------
``````( ) ( ) \

PA 2009, 1st Bridges : 2010, 1st Bridges, 1st WM : 2011, 1st ED, 3rd Towers, 4th Heli
Nats Augusta 2009, 4th Bridges : Illinois 2010, 3rd Bridges, 9th ED : Wisconsin 2011, 3rd Heli, 5th Towers : Orlando 2012, 2nd ED, 5th Towers
Event Supervisor Balsa, ED

Bjsong
Member
Member
Posts: 30
Joined: January 25th, 2009, 6:27 pm
Division: B
Has thanked: 0
Been thanked: 0

Re: Elevated Bridge B/C

Post by Bjsong » January 25th, 2010, 7:17 pm

I'm a second year bridge builder and I wonder how you experienced bridge builders remove glue off of wood to re-glue the piece of wood.

andrewwski
Admin Emeritus
Admin Emeritus
Posts: 954
Joined: January 12th, 2007, 7:36 pm
Division: Grad
Has thanked: 0
Been thanked: 11 times

Re: Elevated Bridge B/C

Post by andrewwski » January 25th, 2010, 7:19 pm

Generally, you don't.

If there's a circumstance where you have to remove a member, you can carefully cut it off and try to re-glue, but I wouldn't do that for a competition bridge.

If it's just a matter of repairing a broken joint to re-test (if using a safety tower) then you usually can just add more glue or some splints, however if it's the glue failing you need better glue or better application techniques.

Bjsong
Member
Member
Posts: 30
Joined: January 25th, 2009, 6:27 pm
Division: B
Has thanked: 0
Been thanked: 0

Re: Elevated Bridge B/C

Post by Bjsong » January 25th, 2010, 7:44 pm

My problem is that I messed up gluing the wood in the first time and I tried to glue it the second time so is there anyway to quickly take out the glue before it permanently stays in the wood like washing it off?

andrewwski
Admin Emeritus
Admin Emeritus
Posts: 954
Joined: January 12th, 2007, 7:36 pm
Division: Grad
Has thanked: 0
Been thanked: 11 times

Re: Elevated Bridge B/C

Post by andrewwski » January 25th, 2010, 8:23 pm

What kind of glue are you using?

If it's CA, they sell CA debonder (acetone works too, but normal nail polish remover is too low of a concentration), but I'd be hesitant about using it on a bridge, especially if you are attempting to glue the piece again.

Bjsong
Member
Member
Posts: 30
Joined: January 25th, 2009, 6:27 pm
Division: B
Has thanked: 0
Been thanked: 0

Re: Elevated Bridge B/C

Post by Bjsong » January 25th, 2010, 8:54 pm

I'm using a glue called Weldbond
I think its PVA

andrewwski
Admin Emeritus
Admin Emeritus
Posts: 954
Joined: January 12th, 2007, 7:36 pm
Division: Grad
Has thanked: 0
Been thanked: 11 times

Re: Elevated Bridge B/C

Post by andrewwski » January 25th, 2010, 9:16 pm

Should be water soluble then.

Locked

Return to “2010 Build Events”

Who is online

Users browsing this forum: No registered users and 1 guest