I second both Tristan and Karen’s suggestions. The Building Big videos/DVDs from PBS are great. (Check the library.) I used to teach a huge bridge building unit in my teacher life. We used the book that Tristan suggested and built all the bridges in the K’Nex kit she mentioned.
Something else we did, which might be fun, was design, build and test toothpick bridges. The kids had a budget (I think $1,000,000) for everything, and they had to write checks at the “store” to buy supplies (toothpicks, string, glue, design paper), get permits, etc. I did it with 4th graders, and they rose to the challenge of working within a budget and keeping an accurate check ledger.