Some schools have an MS non-thesis program where you just take classes and sometimes take an exam at the end. Then there's M.Eng. where there's a project built in instead of a thesis. Not many schools offer an M.Eng degree in civil/structural yet. Cornell, Lehigh, and Berkeley are ones that I applied to, but I'm quite sure Michigan and UIUC don't. You'd probably be able to get into an M.Eng. program with a GPA of 3.15. It's still above a 3.0, which is the cut-off for a lot of places. I think the best think you need to do is start researching programs. They'll often list a preferred or required GPA once you start digging around their websites. Every places is different. I think you should calculate your major GPA, and if that's higher you can always use that as evidence of your academic strength in civil/structural engineering when you write your SOP.