Building a Bridge with Balsa Wood

[subhareads]

I need to make a bridge using Balsa Wood, but I can't decide whether to use the Howe or Pratt Truss.

Requirements
maximum mass of 25 grams
maximum test load will be 15kg
must allow a 5.0 (∼2”) cm cube to pass along the roadway without touching
the structure
must be an opening at the center of the roadway of at least 1 cm x 1 cm. This will
allow the loading block bolt to pass through the roadway.

Any thoughts?

 

[anvesh111]

try to draw in a sheet ,or make a design in autocad for best construction