Help for my thesis

[ri89]

I Know it's almost impossible to find the construction detail but for my thesis I have to analyze the development of skyscraper's envelope in Chicago and NY in the years.
I've identified three generations of envelopes in order with the definition of Banham.
For each generation I have to pick and analyze the first ten skyscrapers according to the height, I do this in order to have a scientific base and not a casual selection (wich is not objective). Having said this how do you suggest to continue my study? How to choose the right skyscrapers to analyze?

Sorry again for my english
Thanks