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bridge building is fun

hey

2238! Best bridge yet
I'm having a bit of trouble with the bottom members in my bridge-it's always that member no matter what type of wood I use, width/length etc.
Any solutions out there?
Thanks:)

Entomology wrote:2238! Best bridge yet
I'm having a bit of trouble with the bottom members in my bridge-it's always that member no matter what type of wood I use, width/length etc.
Any solutions out there?
Thanks:)

What division, mass, and/or load?

Not directly liked to Olympiad, put I did a similar event this past Monday for Engineer's Week. I built two exhibition bridges to demonstrate for grade school kids and a few middle school who built their own bridges to test. The event was open to any bridges "constructed of wood and bonded by glue". However, most kids built theirs out of toothpicks and Popsicle sticks, because in years past that was the only thing which was allowed to be entered.

The span was 56 cm. Max height of 30 cm. Layout was same as Bridge Building last year for SO.

One of the ones I made was a 74g "beam" bridge. It had two parallel 4" tall balsa wood beams with laminated wood on top and bottom to provide extra support with truss-like bracing on the either sides of both plates. It held 220 lbs for a efficiency of about 1,300.

Second bridge was 23.1 g and held 150 lbs. for a score of about 2,950. Main compression members were 1/4" x 1/8", bracing was 1/16", and bottom tension cords were 1/16" x 1/8". Design was similar to many bridges i saw last year. Profile view was trapezoidal in shape with an "X" in it.

Learning point- Balsa wood is MUCH stronger than most people give it credit and the vast majority of bridges are over-built and/or fabricated insufficiently. For SO, we only load 33 lbs (15 kg) MAX for a bridge, a far cry from 150 lbs.

kinghong1970 wrote:i think many teams came in with the impression that their bridge NEEDS to hold 15kg.

my daughters' bridge tests at home started at 5.06g with total load of 3089gr and latest before contest came in at 3.52g which held 2754g

was hoping to break 2k... definitely possible... but alas, event is over for them.

This triggered an interesting thought experiment in my head: I wonder if it's possible to be competitive by making the lightest bridge possible that ONLY supports the loading block. Here's a rough analysis:

-Last year at Nationals, the 6th place team in Div B got a score of 3288, while the 6th place team in Div C got a 2466.
-The loading block is 5 cm x 5 cm x ~2cm = ~50 cm^3
-Assuming it's made out of Pine, which is common for lumber, the density is ~500 kg / m^3
-Thus the loading block will weigh ~25 g

To place at Nationals, bridges would need to weigh:
Div B: 25 / 3288 = 7.6 mg
Div C: 25 / 2466 = 10.1 mg

So is that even possible? Well:
-the lightest readily available balsa wood has a density of ~70 kg / m^3
-likewise, the smallest cross section you can get is 1/32" x 1/32" (0.8mm x 0.8mm) = 0.64 mm^2
-Thus, this super light and thin balsa will weigh 44.8 ug / mm = 0.448 mg / cm

Using the above weights, and assuming no glue weight, you'd be able to use:
Div B: 7.6 / 0.448 = 16.9 cm
Div C: 10.1 / 0.448 = 22.5 cm

Which in neither case is sufficient length for even a single stick to span the required clear span areas (35cm and 45cm). Just to have 2 bare sticks spanning the gap would require 4 times the length/mass, resulting in 1/4 the score, which in itself wouldn't likely place at virtually any regional competition.

Hence, supporting only the loading block with the lightest possible bridge is NOT a competitive technique by any stretch of the imagination.

chalker wrote:

kinghong1970 wrote:i think many teams came in with the impression that their bridge NEEDS to hold 15kg.

my daughters' bridge tests at home started at 5.06g with total load of 3089gr and latest before contest came in at 3.52g which held 2754g

was hoping to break 2k... definitely possible... but alas, event is over for them.

This triggered an interesting thought experiment in my head: I wonder if it's possible to be competitive by making the lightest bridge possible that ONLY supports the loading block. Here's a rough analysis:

-Last year at Nationals, the 6th place team in Div B got a score of 3288, while the 6th place team in Div C got a 2466.
-The loading block is 5 cm x 5 cm x ~2cm = ~50 cm^3
-Assuming it's made out of Pine, which is common for lumber, the density is ~500 kg / m^3
-Thus the loading block will weigh ~25 g

To place at Nationals, bridges would need to weigh:
Div B: 25 / 3288 = 7.6 mg
Div C: 25 / 2466 = 10.1 mg

So is that even possible? Well:
-the lightest readily available balsa wood has a density of ~70 kg / m^3
-likewise, the smallest cross section you can get is 1/32" x 1/32" (0.8mm x 0.8mm) = 0.64 mm^2
-Thus, this super light and thin balsa will weigh 44.8 ug / mm = 0.448 mg / cm

Using the above weights, and assuming no glue weight, you'd be able to use:
Div B: 7.6 / 0.448 = 16.9 cm
Div C: 10.1 / 0.448 = 22.5 cm

Which in neither case is sufficient length for even a single stick to span the required clear span areas (35cm and 45cm). Just to have 2 bare sticks spanning the gap would require 4 times the length/mass, resulting in 1/4 the score, which in itself wouldn't likely place at virtually any regional competition.

Hence, supporting only the loading block with the lightest possible bridge is NOT a competitive technique by any stretch of the imagination.

I heard stories about how one year some invitational was running bridges and used a scale which couldn't measure masses less than 1 gram, so it would read 0 grams at 1 gram, and one team brought 2 very light sicks of balsa which held the block and the bucket and then quickly collapsed, and ended up getting first with an efficiency of INFINITY (mass held/0 grams). Not really too helpful but just a funny story about this strategy.

At NJ States, 1st place has a 2400s efficiency and second place was 2250.

At NC Charlotte-Regional just completed today, Jay M. Robinson got first place with 3363 efficiency in Div. B.

xinu777 wrote:At NC Charlotte-Regional just completed today, Jay M. Robinson got first place with 3363 efficiency in Div. B.

Looks like we'll be seeing some more 4000+ scores at Nationals