West Point Bridge Designer 2012 (for Windows)
The West Point Bridge Designer 2012 will introduce you to engineering through an authentic, hands-on design experience. This software provides you with the tools to model, test, and optimize a steel highway bridge, based on realistic specifications, constraints, and performance criteria.
The version for Mac is available on their site, but I can't test it to see if it has viruses (it is a government site so it probably doesn't) . You can never be too careful.