Structural engineers, please help! School assignment

LeRoux Strydom · 08:46 AM

Actually, a school assignment for my 7th grade (13yr old) daughter. I'm feeling dumb just thinking about it.........

She has to build a structure not less than 1m tall (39 inches) with only 30 drinking straws each about 20 cm or 8 inches long, capable of holding a full 2-liter (2 quarts) plastic coke bottle on the top (weight should be 2 kg or 4.4 lbs). The connectors can be anything, she already thought of triangular folded/glued bits of carton that can hold 3 or 4 straw ends together.

I know I could eventually figure it out, but I'm lazy. Can anyone point me into the optimal solution or offer any suggestions from which we can work? I'll try to impart the lesson to my daughter, as I think the logical reasoning transferred to her is worth more than the credits for the assignment.

I'll google around, but is anyone up for some lateral thinking on structures? cegerer?

LeRoux

RallyJon · 03-29-2005, 08:36 AM

Does the structure have to support itself--balance with the bottle on top?

Can you cut the straws?

pwd72s · 03-29-2005, 08:36 AM

We have some architects on this board...I'm hoping one will weigh in....

LeRoux Strydom · 03-29-2005, 08:43 AM

Yes, the structure has to be self-supporting and balance with the waterbottle on top. It is allowed to cut the straws and use both pieces.

LeRoux

lendaddy · 03-29-2005, 08:44 AM

Well, it has to be a triangle (pyramid) design with a modified point to balance the bottle. Oh, one liter is NOT 4 quarts, one liter is almost exactly 1 quart.

LeRoux Strydom · 03-29-2005, 08:46 AM

Correction - the water bottle is 2 liters in volume (2 quarts) - thanks lendaddy. I'll edit the original post.

LeRoux

competentone · 03-29-2005, 08:53 AM

Quote:

Originally posted by LeRoux Strydom
...capable of holding a full liter (4 quarts?) plastic coke bottle on the top (weight should be 2 kg or 4.4 lbs).

A liter is slightly more than one quart; the weight of a liter of water is one kilogram (about 2.2 lbs).

Edit: I see it's already been corrected.

kach22i · kach22i's Garage

Quote:

Originally posted by lendaddy
Well, it has to be a triangle (pyramid) design with a modified point to balance the bottle.

Yes, that's the way to go. I built a truncated pyramid in community college for a similar extra credit/first week of class assignment over twenty years ago. It beat the pants off of anything else.

We were not allowed to use tape. I think I got by with pinching the straw ends into each other and some string - no glue. It was so long ago - heck my heart had not really been broken yet.

IROC · IROC's Garage

One goal would be to design the structure to place as many of the straws as possible in tension. Then your main goal will be coming up with a joint design that can react the loads. I would imagine that an 8 inch long straw is going to have a pretty low critical load (from a crippling) standpoint. Time for some Euler-Johnson calculations! :>)

The straws you do have in compression will need to be braced so that they don't easily deflect and buckle.

Mike

LeRoux Strydom · 03-29-2005, 09:03 AM

I also figured out the triangle issue, but need to find a way to build a tower of some sort with triangular sub-structures. Can anyone provide a drawing or sketch with some ideas? I'll post pics of the winning structure!

LeRoux

competentone · 03-29-2005, 09:04 AM

Quote:

Originally posted by LeRoux Strydom
... is anyone up for some lateral thinking on structures?

Why don't you post the exact wording on the instructions. I'm thinking that the most significant strength is the tensile strength of the straws. If the instructions allow anchoring to a solid foundation, some type of "suspension bridge" design might be your best solution.

kach22i · kach22i's Garage

I seem to recall bending for the joints- cut short straw bend in the middle and pinch so that the verticals and horizontals cover it up. The straws expand a bit and make a very tight friction fit - you have one shot at - don't pull it apart once together. Building four sides (four sided structure) and a bottom and doubling or running parallel straws will build in redunancy.

It held up 2-1/2 bricks, or at least double of anyone elses design.

LeRoux Strydom · 03-29-2005, 09:10 AM

My original post is as close to the actual instructions as possible, except to say that it is not allowed to use external anchors. So I'm not sure the suspension bridge idea would float. The structure has to be completely self-supporting and balance.

LeRoux

LeRoux Strydom · 03-29-2005, 09:16 AM

Apparently it is OK to use a folded & glued "corner" made from stiff carton for glueing the straw ends together, like in the picture:

LeRoux

stevepaa · 03-29-2005, 10:15 AM

You know this is like the assignment I gave 30 years ago in my classes. Then we used straws and pins only to build bridges spanning a two foot gap and the goal was to see how much weight each would support. The kids worked in teams of three. All of it was done in class so no parents could get involved.

I wonder why the teacher is making this an individual contest rather than a team effort.

I would suggest making minimal suggestions to her, this is a challenge for her not for you.

kach22i · kach22i's Garage

I'm having flash backs - why did you do this to me?

I recall driving around in my 1966 Mustang, and also riding on my 1978 Honda Hawk going to all the fast food places and finding straws that fit snugly into each other. They were not all the same sizes, the straws for shakes were larger and the smaller straws fit into them. Plus the differing suppliers had slighly different tolerences which allowed even more combinations.

The base is in tension, the corners want to kick outward - all I had was friction on my side.

I laid a bridge of straws side by side on the flat top of the structure- just like a log raft. This made a continuous load for the bricks to rest on, and not a destructive point load.

Avoid point loads, distribute the load as evenly as possible.

You may not need as much diagonal bracing as you think if you make the straws double walled as I did.

Above all follow the KISS Method (keep it simple stupid).

stevepaa · 03-29-2005, 12:35 PM

See, you did learn alot

LeRoux Strydom · 03-29-2005, 07:56 PM

I'm not intending to build it for her, just to point her into the right direction so she can figure out the rest herself. I'll get her to start working with the pyramid structures as basic building blocks and see how far she gets.

Thanks for ideas, guys. Anyone else?

LeRoux

M.D. Holloway · 03-29-2005, 08:34 PM

We did the same thing in 7th grade - we couldn't use glue or gussets. The fact that you can is going to make it easy. The strength you can garner from the gussets and glue will easily allow for structural intergity to be relized.

Go with the Tetrahedron structure, double wall the strews like kach22i et al suggested. If cutting the straws is allowed, split them in order to have 3 connevting at the gussets. I would use a stiff cyano-gel glue and thick cardstock (or use a glue gun!). Corregated may work as well, the strength to weight ratio makes sense in 2 of the 3 planes - if weight is not important do the cardstock. Stay away from the polystyrene foam board, the glue will melt it.

M.D. Holloway · 03-29-2005, 08:40 PM

if they don't have any restrictions on the amount of glue you can use, look to fill each straw with glue and let dry. The strews will be as stiff as heck! You could even make a mixture of sawdust and Woodglue, inject it into each straw and let dry. The freaking things could support a medium size dog!