Designs B/C

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iwonder
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Re: Designs B/C

Post by iwonder » October 20th, 2014, 8:07 pm

Yup. I would buy cheap wood from a local place, get some practice in with the designs, and then buy some specialized wood (I never buy it without the density classification, they rank it extra light to extra heavy). That way it's easier to play around with different weights and to find large pieces of moderately uniform wood. Also I only buy sheets when I switch to specialized and use a balsa stripper to cut my own strips. It gives more control over it and sheets are typically more consistent.
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Re: Designs B/C

Post by PatrickMcPherson » October 21st, 2014, 5:56 am

Specialized is very pricey, and it can take a while to get your wood. Until you build your competition model you can get about all the wood you need from a good local hobby store. You just need to know what you're looking for.

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Re: Designs B/C

Post by dholdgreve » October 21st, 2014, 6:01 am

I've bought a lot of bass and balsa from the Zimmer clan over the years, and from an experience and technical standpoint, there is none better... but if you know what you want, and budget is an issue, consider National Balsa. I will say this though... Do Not spend the extra money to specify densities with these guys, If you spec a certain density, they will find a block that averages the density requested, and slice it up for you,,, It may have individual pieces that have 4 pounds / CF... and there may be some 16 pound / CF material... the assortment is no better than if you didn't specify. just order enough unspecified to be able to sort out what need... and build towers with the rest :)
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Re: Designs B/C

Post by embokim » October 26th, 2014, 4:19 am

Has anyone tried a Baltimore prattbtruss design? I thought it might work and I aam waiting to test it, so any comments would be nice.
Thanks
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Re: Designs B/C

Post by someusername » October 27th, 2014, 2:34 pm

embokim wrote:Has anyone tried a Baltimore prattbtruss design? I thought it might work and I aam waiting to test it, so any comments would be nice.
Thanks
I have always used a normal pratt truss in the past and I have never had issues with that. If you want to try it, I would encourage you try them both to see if there are any differences between the two efficiency wise. If they both hold the same amount of weight then i would suggest going with the pratt simply because it's lighter.
simplicity is key...sometimes

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Re: Designs B/C

Post by retired1 » October 27th, 2014, 2:40 pm

The Baltimore bridge is a fair bit stronger than the standard Pratt, but it weighs more and has more joints. Try building it with smaller wood of the same #/cuft.

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Re: Designs B/C

Post by bernard » October 27th, 2014, 11:36 pm

Several FAQs regarding Bridge Building have been posted on the official Science Olympiad website:
Science Olympiad wrote:Must the outside width of the Bridge be at least 5.0 cm for the entire span? What does "at any height along its span" mean? (section: 3 / paragraph: e / line: 1)
Yes, the outside width of the Bridge must be at least 5.0 cm for the entire span (Div. B - 35 cm and Div. C - 45 cm). "At any height along its span" means that the 5.0 cm width for the entire span can be at any height on the bridge as long as it is not below the top surface of the Test Supports.

If the outside width of the bridge at the bottom chord is 5cm out to out, can the top chords be less than 5cm out to out in order to better support the loading block?
Yes, as long as the width of the Bridge is at least 5.0 cm for the entire span it doesn't matter at what height the 5.0 cm minimum width is located.

May the loading block be placed at any height of the Bridge as long as it is in the center of the span? (section: 3 / paragraph: f / line: 1)
Yes, the loading block may be placed at any height of the Bridge as long as it is in the center of the span and not below the top surface of the Test Supports prior to testing.
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Re: Designs B/C

Post by bearasmith » October 28th, 2014, 7:25 am

Wow. Thanks for that information. Now I can try a new and stronger design. :D
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Re: Designs B/C

Post by embokim » October 31st, 2014, 2:49 pm

I just tested both designs and my baltimore design held 15 kilograms and weighed 25 grams. My pratt bridge weighed 21 grams and held only 13 kilograms. I am going to stick with the the Baltimore design and try to make it lighter. :mrgreen:
Also does anyone know what a good score for the bridge is? Love to know.
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Re: Designs B/C

Post by retired1 » October 31st, 2014, 3:16 pm

Someone posted 6.15g, held 15Kg for a score of 2439.
Cutting down the size of the members and using less dense wood will give you a better score.
The tension members can be less than 1/4 the weight of the compression members, so do some homework on line and seriously reduce those. The real bridge was designed for rolling stock and winds, so everything was built heavier and then they threw in a safety factor that you do not need.

25g is massively too heavy.

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