Calling all Civil Engineers!

[Daniel]

Hey guys, I just need some advice, please? :)

Is this a good bridge design to use?
http://img706.imageshack.us/img706/2711/conceptbridge.png (http://imageshack.us/photo/my-images/706/conceptbridge.png/)

Basically our project goal is to hold the hold the heaviest load. This is accomplished by the teacher placing the handle of an empty 2kg bucket onto the centre of the structure, and then filling it with 2L water at a time. As you can see from the image above, the bridge has to span 450mm.

Our bridge is to be a truss bridge and made out of Hoop Pine wood and is 8mmx5mm and 600mm in length (we can use up to six pieces). Parallel pieces have to be at least 50mm apart and cannot be stuck right on top of each other, neither can we use too much hot glue.

So yeah, are there any improvements I should make on this design (even though it is a basic truss bridge) to make it hold a heavier load? :)

Regards,
Daniel. :)

EDIT: Quick query, should I try and divide it into more, smaller triangles?
EDIT2: I now realise that design cannot happen, due to me not taking into account the other side of the bridge (the hidden side) :( NOTE: It is NOT a 2D bridge. It is meant to be 3D. :/

[KingKong]

build it the other way, something like this:

And the cross sectional must be a triangle as well.(6-7 triangles on each side should be enough). And yes thats a strong bridge, i did something like that(i think the materials were the same size as urs? but im not sure) rougly the same size and it held almost 25 kgs.

E: use some rope if u can? its a truss bridge after all.

[Main]

Is this for highschool or for uni? There is a bunch of master civil eng people I could pass this question to. But if its only for highschool, I could probably throw in some stuff later today:P

[Daniel]

build it the other way, something like this:

And the cross sectional must be a triangle as well.(6-7 triangles on each side should be enough). And yes thats a strong bridge, i did something like that(i think the materials were the same size as urs? but im not sure) rougly the same size and it held almost 25 kgs
Thing is, similar projects in the past have managed to hold 75kg (that was the highest the teacher's ever seen).

Basically, to get full marks (100%), we have to have the strongest bridge. It's competition. So yeah, hence why I came to SRL for advice :)

Also, wouldn't an upside-down bridge like so cause more more tension in the upper part of the the bridge and thus will break quicker under a lighter load than if the members were in compression in that area?

Is this for highschool or for uni? There is a bunch of master civil eng people I could pass this question to. But if its only for highschool, I could probably throw in some stuff later today:P

It's for high school :) Grade 12.

[Main]

Check out this
http://vcity.ou.edu/demoModules/analysis/truss/truss.htm
http://emweb.unl.edu/negahban/em223/note12/note12.htm

Especially the zero force members.

[Daniel]

Check out this
http://vcity.ou.edu/demoModules/analysis/truss/truss.htm
http://emweb.unl.edu/negahban/em223/note12/note12.htm

Especially the zero force members.

Okay, thank you :)

I'll have to check them out tomorrow, as it is late and I have to go to bed. I quickly skimmed through it and they seemed decent and helpful :) Thank you :)

[KingKong]

Thing is, similar projects in the past have managed to hold 75kg (that was the highest the teacher's ever seen).

Basically, to get full marks (100%), we have to have the strongest bridge. It's competition. So yeah, hence why I came to SRL for advice :)

Also, wouldn't an upside-down bridge like so cause more more tension in the upper part of the the bridge and thus will break quicker under a lighter load than if the members were in compression in that area?


It's for high school :) Grade 12.

Im pretty sure no one will even top 20kgs(when i did it 2 years ago, that was the closest that anyone came to mine). From what ive seen in my class everyone else had bridge like yours and mine was the only upside down bridge and mine won :p

http://en.wikipedia.org/wiki/Truss_bridge#Roadbed_types

[Daniel]

I don't know KingKong, I just cannot see that bridge design working. Because you're pretty much putting all the weight and stress on only two members, the two which are resting on the two sides of the table. We cannot have any ground supports.

[Daniel]

Just wondering, what would be a more suitable top design?
1.
http://img256.imageshack.us/img256/1574/conceptbridgetopview.png (http://imageshack.us/photo/my-images/256/conceptbridgetopview.png/)

2.
http://img528.imageshack.us/img528/7466/conceptbridgetopview2.png (http://imageshack.us/photo/my-images/528/conceptbridgetopview2.png/)

Regards,
Daniel. :)

[I Am Legend]

in the truss design i would put the corner near the end of the table .

[KingKong]

number 1 for top design

[Bramble]

And another thing is, if the handle is going to be placed ontop of your model how would that even be relevant for topside supported bridges. if this is the case i would go with supports from underneath.
I dont know if everything is 100% relavent but i know quite a bit of it works, depending on what material your using it could change a little bit, i cannot remember my design but i do remember putting it together, i didnt sand mine or anything of that nature instead i took the rounded shape i want out of sheets, and i cut a bunch out of a pattern to get the rounded arch i applied a nice amount of glue but when i pressed the pieces together gently with a clamp most of the glue came out, i remember being at least 15 kg over the next highest bridge when i did it in school. also when the sheets werent long enough, i would modify each piece inbetween in the ones i cut out to overlap each other by about 3 cm. i found that creating joints were inevitably weak with what we were allowed to use(not meaning dont have any but as few as possible).

EDIT:: i didnt take into account you can take the handle off the bucket put it through and reattach. although it would be easier to just use a bolt and a couple pieces of twine.

http://www.balsabridge.com/
At the 31st annual contest, held March 15 2011, a ten year-old record was broken as Physics 12 student Gino Circo, the winner of the 2010 contest, blew past the previous mark to set a new record of 219.95 (rounded to 220.0) kg. The previous record had been deemed out of reach by students for the past decade. Gino's bridge, based on his successful structure design from his entry in his Physics 11 year, was copied by others but none of those structures were up to the standards of construction set by this graceful entry.
]
Use a small balsa wood saw (about $3) instead of an exacto knife to make cuts
Cut small notches to connect bridge components
Use a basic carpenter's glue
Yellow glues contain aliphatic resin, used in the majority of winning bridges
Fewer pieces mean fewer problems
Keep pedestals (feet) simple
Clamp glued pieces for about half an hour (use protective strips to avoid damaging the balsa). If you don't have any clamps you can use clothes pegs. C-type clamps are available quite inexpensively at Canadian Tire
Design for strength at the load application point
Construct roadway of thin, narrow strips of balsa
Don't glue down ends of road ways…they usually bend upwards under load
Use minimal support under roadway, except at load application points
Roadway must support a small Hot Wheels-type car
Most bridges bend inwards (as viewed from one end); consequently they require horizontal bracing
Write your team number prominently on a main horizontal beam of your bridge and on both sides of the bridge
Double check that a 40 cm-long board will fit between the pedestals (feet) of your bridge
Double check that a 5 cm cube will fit underneath your bridge and along your roadway. You can make a nearly perfect 5 cm cube from Lego bricks
Do not cover your bridge with any material. Glue should be used only to join components
Use light sandpaper (number 150 or higher) to gently clean your bridge and remove excess glue

[Main]

Top would be best, but how is it being hanged? is it by 1 spot?

[Daniel]

Top would be best, but how is it being hanged? is it by 1 spot?

Yes, one spot, on top (up to student, teacher said he recommends on top), in the middle (it has to be in the middle).

Main, is it possible to get your MSN? Also, which design do you think to be better? KingKong's upside-down one, or my initial upright one?

[cause]

Hey bit of a gravedig, but did you win? We did this same style competition at my highschool but with a limited number of popsicle sticks. My partner and I did alright but we used 2 different kinds of hot glue, with one being significantly weaker than the other.

[Daniel]

Hey bit of a gravedig, but did you win? We did this same style competition at my highschool but with a limited number of popsicle sticks. My partner and I did alright but we used 2 different kinds of hot glue, with one being significantly weaker than the other.

Nah, I managed about 24 litres of water. My thing couldn't stand up straight so I had to improvise and put it through a weaker, less supported, centre.

My report will hopefully own though :) So, that's hopefully where I should get the marks (I wrote like 15 pages, while the rest wrote ~3-5 :p)

How did you manage to go? :)

[cause]

Hey well that's pretty good! I can't even remember how we did exactly, except we weren't first and we weren't last.