Here I have complied what I believe to be the best list of websites that you will find useful. All of them have relevant model structure, bridge and tower information, some sell balsa and basswood, and others contain useful tips. Some of these sites have a specific competition or goal in mind, but you can always take the principles and adapt them to your needs.
Bridge and Tower Design:
A gold mine of links and resources to model bridge building related websites.
Johns Hopkins Truss Simulator
This is a free program that allows you to design a virtual truss, add a load, and see where the forces end up. This may be a a bit difficult to understand, but very useful.
ABCD’s Bridge Design Tips for Kids
Pictures of balsa and spaghetti bridges, and a few tips.
Building a Popsicle-Stick Bridge (Web Archive)
How to build a popsicle bridge. Includes photos of several designs.
Balsa Wood Inc
Balsa Wood Inc sells a wide variety of shapes and sizes of Balsa wood. They have sticks, sheets, and blocks to fit your model making needs.
Balsa Sheets from Midwest Products
Midwest Products has been selling Balsa wood for a long time, and they know their business.
A recognized leader in the industry. Specializes in Balsa wood, but also sells Bass and Birch, and many other wood accessories. Recommended.
This is a online company that sells balsa and bass wood for your models.
Sells several different types of wood. Minimum order of $50.00.
Bridge Building Competitions
Website about an annual bridge competition. Contains many photos of bridges from past competitions. Also has many links to bridge sites.
International Bridge Building Contest
This competition is probably the biggest in the nation. Given rules and materials, you must design and build the most efficient bridge. Contains pictures of past bridges, as well as their efficiency scores.
Bridge Builder Games
In the mood for some fun? Check out this site.
Information about Balsa Wood:
Balsa Wood Bridge Info
Article explaining how to build a balsa bridge. Also has a chart comparing the strength of Balsa to other woods.
Interesting Facts About Balsa Wood
In-depth study on the properties of Balsa wood. Shows the three grain types, and explains their uses.
Kits and Software:
Home of ModelSmart, and model bridge design program. Contains several pages of good information, including a model bridge project.
Balsa Wood Bridges
A CD jammed packed with pictures that will help you design and build your bridge.
Trusses and Truss Design:
How to identify different trusses.
Truss Bridge Library
Explains how a truss works. Includes pictures of a Warren, Howe, and Pratt truss.
Craft Stick Bending
Great resource for bending popsicle sticks and any other kind of wood. Highly recommended that you check out this website for a simple method for bending wood for your arch bridges or other projects.
Engineering Education Service Center
The Engineering Education Service Center provides service and resources for mentors and pre-engineering students.
Has links to many aspects of real and model bridges. Includes a message board.
Hub Hobby Shop
Resource of links about all aspects of modeling. Includes a online store.