1. Balsa Bridge Construction
The bridges depicted in this presentation are not intended to be good designs, they are only intended to help you understand construction techniques. No one should build a bridge that designed like this

2. 1. Design your bridge. Must span 20 cm gap
Internal height at least 5 cm
Width 6.6-8cm
Gap for loading block
Sides and bottoms compatible

3. 2. Build your bridge from paper strips
Strips are about the average width of balsa
Cut them out
Glue them over your copy of the design

4. The strips represent how much balsa you could buy with your budget
Remember you will need to make two sides
Save some for the top
Long balsa strips are 92 cm and cost $200
Short balsa strips are is 61 cm long and cost $140
Scrap is available at bargain prices.

5. You have to decide how to make pieces join
Different joins have different strengths

6. 3. Put your working blueprints on cardboard
Cover it with wax paper
Use pins or tape to hold the wax paper, make it easy to remove

7. 4. Measure and cut each piece of balsa wood for one side.
Cut only on top of a cutting block
Cut pieces long and use sand paper to remove the weak jagged ends

8. Pin pieces to blueprint
Do not put pins where glue will go
Pin out the entire piece before applying any glue

9. Use Sandpaper to shape angles.
You can also use sandpaper to improve the fit

10. 5. Remove key pieces, add glue and pin them back in place
Let dry overnight

11. 6. Remove wax paper
Remove pins and peel the wax paper away from the wood
Add glue to the backside if necessary, let dry
Use sandpaper to remove excess glue

12. 7. Cover your design with a new piece of wax paper and make another side

13. 8. Assemble your bridge
Pin both sides down on top of the bottom design.
Keep the pins clear of where Joists (crossbeams) will go.

14. Add Joists (crossbeams)
More joists makes a stronger bridge.
Put in all the joists you can.
Never glue till beams are in place.

15. Put in long beams More beams = stronger bridge
Never pin where glue will go.
Don’t glue the road-bed unless you have time to add the top.

16. Add the top crossbeams
It does not take much, but you should have a simple truss
Start with two crossbeams
Paper strips can hold the sides together while the top dries.

17. Add at least one diagonal and your have a truss.
When everything is done, add one or more diagonals to make triangles (trusses) from the top.
Add any finishing colors, and your team and individual names.

18. Add final touches, sand away excess glue, etc
Add final touches, sand away excess glue, etc. and you bridge is ready to bust

19. Adding gussets is optional.
Glue or gussets may not extend more than 0.5 cm from the joint
Glue or gussets on joints past 0.5 cm may lead to disqualification

20. Ways to Improve Your Bridge
These joints should be lap joints, not butt joints
Concentrate on spreading forces away from the loading block, this is where most bridges break

21. Lap vs. Butt
A butt joint is when the end of one piece meets the end or side of another.
These are weaker than lap joints

22. Lap is short for overlap

23. Lamination is not allowed
Laminating (gluing together two or more pieces of wood along their length
This bridge is disqualified

24. Warning!
Bridges may not be painted nor strengthened with any applied substance.
Lamination (gluing two or more pieces together along their entire length is not allowed.