These are just some of the many sites that address key concepts in structures. Searching compression and tension in structures, bridges, and forces and motion are some key search words that will open many doors. Note that there are many sites that offer lesson plans. Be careful...be sure the lesson plans model inquiry based instruction...think of the Learning Cycle Model as you evalaute the plans.

http://education.sdsc.edu/SE/physicsbridgestud.html

http://education.sdsc.edu/SE/physicsbridgeteach.html

http://travel.howstuffworks.com/bridge1.htm

http://www.pbs.org/wgbh/buildingbig/lab/forces.html

http://www.bearwoodphysics.com/3schemproject3.3.4.htm

http://www.idahoptv.org/buildingbig/bridges.html

http://eduspace.free.fr/bridging_europe/br_lesson2.htm

http://www.roehampton.ac.uk/education/science/secs/BAQTS_3_year_degree/Year%202%20autumn/2aweek3/compression&tension.pdf