Replying to Help a Brother Out with Building a Bridge

Hello NS, I have a project to do and I wanted to ask you guys if you had any input on what i should do, or if you had any tips for me. My project is that I have to build a balsa wood bridge, and I am going to test it with my teacher and partner for its efficiency.

these are basically the rules and what the bridge must have...

1. The bridge must be 30 cm long

2. There must be a road bed which must consist of two pieces of balsa wood which are 2.5 cm apart and has a 5 cm clearance for a hot wheels car to roll across and to hang weight from

3. No vertical supports

4. The wood cannot be treated to change its strength

5. Wood must be bonded together with glue given to us only at joints

6. No splitting or laminating the wood (basically you cannot split the wood or glue it all together to make one board of wood)

So if anyone has any ideas, suggestions or tips please share.
Thanks guys, you're awesome!