Hi,
it's a bit hard to tell where or why you're going wrong without a question in front of me to look at, but if its a real sticking point for you then you'd be best to buy a basic mechanics of structures/solids book or look up questions of that ilk on the web. Otherwise, if you can describe the particular situation/question you're thinking about then i'll try and help you out.
Also, some of us wrote a bit about the later selection stages in a thread called 'Britannia final stage' that you might be interested in.
C