balsa wood bridge project?

[samdale]

i've been given the task of building a bridge using just 1 sheet of balsa wood.
dimensions of the wood are 900x75x3mm and the finished bridge must span 400-500mm (yet to be confirmed)
no glue, tape etc is allowed
having trouble thinking about how to make joints strong as the bridge must hold as much weight as possible.
any engineers, bored students out there with any bright ideas???

[illmonkey]

Nail it to some 2x4.

HTH

[Justayellowbadge]

Cut into 4 strips, cut slots and fit together as a box section?

Think of a noughts and crosses board type affair.

[BarRefaeli]

What you want is something like g:



Edit: I have not got a clue if that's correct.

Edited by BarRefaeli on Thursday 4th February 14:25

[Shar2]

We do this project every year with our first year students, but we allow them to use glue.

Edited by Shar2 on Thursday 4th February 14:22

[WorAl]

BarRefaeli said:

What you want is something like g:

I would say more like Q myself, being only 3mm thick I doubt dove tailing would be possible, maybe a combination of the 2?

A 'V' Shaped slot in both pieces, one the thickness, one the length.

Edit, just spotted, in fact yes, exactly like Q.

Edited by WorAl on Thursday 4th February 14:33

[Matt Evans]

What about a "house of cards" style design?



Note technical Microsoft Paint diagram

Edited by Matt Evans on Thursday 4th February 14:36

[drivin_me_nuts]

Very simple actually. Create latice girders with rectangular slots in the lattice. Use the offcuts to create U shaped keys, that the girder csn slot into. These in effect hold the two sides of the lattice together. If you cut accurately and enough of them you will have a structure that slots together and the keys will hold the girders in place. Place the 'roadway' piece on top, all it has to do is rest on top of the girders.

You need a very sharp scalpel and patience and a template.

(Did this donkeys years ago as a civil engineering student) You will be amazed at how much the bridge can carry)

[pacman1]

Is it the sort of 'bridge' that will have a weight of some sort suspended from the middle to test it?
Then stress the top of the entire span from one end to the other, like this '|)', look at it on it side, with the curved bit on top, obviously. Cut slots either end of the straight piece so that either end of the top span can slot in to. If you build it with two top pieces, like '|))' it will be narrow with the wood you have, but super strong. But all the down force will be going into the slots (on a normal bridge this would be where the buttresses are), so make sure the slots are well inboard of the ends of the bottom straight piece to prevent splitting. Then work out where to put a few vertical proplike braces, perpendicular to the span in between the bottom piece and top two pieces along its length. use slots again to hold them in place. Hope this makes sense.
eta, if the top arc is minimal, it can be used as the roadway. It would look and be stressed like an archery bow.
PS, I'm no bridge engineer, by the way!
Here you go,
http://www.hruska-modelle.de/popup_image.php?pID=8...
But without the tank though!

Edited by pacman1 on Thursday 4th February 15:17