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

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

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

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.

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

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

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

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

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

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

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

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

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

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

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.

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.

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.

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

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

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

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

Lap is short for overlap

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

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.