Sandman nailed it! The quickest and easiest calculation you can make is determining whether the steel will support the new sign. In any case, in most municipalities you will need a stamped engineers drawing, but you can easily calculate this yourself with fairly good accuracy using the formulas in Peter Horsley's "Sign Structures and Foundations", which has been around for a long time. I find this is good enough for estimating, and when I get the job I'll pay the engineer to review my drawings. The engineer's job is simplified if I have already spec'd out the structure (he/she has to run fewer calculations if I have already done the design), and in most cases they can save you some material costs (value engineering). If you want to learn more about engineering sign structures, I would recommend Benjamin Jones' "Engineering Sign Structures" (not a light read, but if you have a basic understanding of trig and can work through some algebra it is not too difficult to understand).
A more recent phenomenon is obtaining FAA clearances for structures over 20' that are within 20,000' (approx. 3.79 miles) of an airport runway, which will likely trigger a study in most cities (
https://oeaaa.faa.gov/oeaaa/external/portal.jsp ). You will need clearances for both your structure and to deploy your equipment. An even newer development is clearing radio frequencies used to communicate with remote signs. You will definitely want to clear those hurdles before you commit to a project. These studies don't cost anything (other than your time), but you may likely need them to get your permit (yes, I agree... does that mean I need an FAA study to put a ladder up to clean my gutters? Well, you don't need a permit for that, but I digress... ).
And don't forget the crane operators certification (
https://www.signs.org/cranes ).
Today's more savvy city planners and inspectors are requiring much more to get a permit than they used to. The old-school sign guys are getting run out of business. Some great opportunities here!