A tower, a ballast mount, or an attachment? They are all fairly the same.
1) Design the component for the vertical and lateral load at height. Treat the component as independent of the building first.
2) Take the resultant forces and apply them to the building. Determine if the building can handle those loads. In order to proceed with Step 2, you need building drawings and a field review of the conditions. In my experience, the building's construction is usually the most difficult part of the task, as most building owners/management companies don't have or don't want to give up the building drawings for inspection and you are limited in budget, time, effort, etc. in surveying the construction.
3) I treat the tower+equipment as dead load. Check the structure supporting the tower for vertical, lateral, and overturning. Design the attachments to the structure. Attachments have some difficulties because the unknowns of the structure start to emerge. Most building owners don't want to see through-bolted plate systems. Most contractors just want to drop in expansion anchors or adhesives. You need to use your engineering judgement to decide what strategy to use.
4) In my opinion, you need to consider deflections of the tower so that your antennas remain in line. I rarely considered the deflection at the top of the structure as additive in a mathematical sense; that is, I would rarely if ever consider the building's top deflection and the tower's deflection all at once. You end up having to beef up the telecom structure which defeats the purpose of having low-weight construction thirty floors up. And, deflection for antenna service is something that the RF designers should be specifying but rarely do. Look at the rotation more than the absolute deflection.
5) Consider if you are in a seismic zone as well. In my area, the jurisdiction passed a new-ish building code that required the positive seismic restraint of all non-structural components. It really changed how telecom structures were treated around here because they couldn't just put a light weight mounts loaded with antennas on the roof. These components now need to tie into the structure to have seismic restraint. It starts to blow up with building envelope and diaphragm problems, so beware. Keep an eye on your contractor and construction.
In Canada these structures fit under CSA S37. I believe in the States TIA 222 governs and provides really good guidance on how to compute the wind loads. But there is A LOT of ambiguity on how these structures pertain to the building codes. My opinion is that, they generally do and you need to follow those guidelines. Some designers take the ambiguity as a signal of "do what you want" which I don't agree with.
