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Steel that will be exposed to the elements is prepainted grey (towers and external concourses), whereas steel that is going to be internal after completion is uncoated. It will eventually be sprayed with a fire deterrent foam that adheres better to the untreated surfaces. The slight layer of rust formed over these couple of weeks will have no effect on the integrity of the steel.
If you study the pics you can see examples of all three.
In general trems, it's not external vs internal. It's simply fireproofing vs no fireproofing. Internal steel doesn't always have fireproofing. You see internal painted steel all the time.
Of course I have no idea what the specs are on this project specifically. I imagine there will be quite a bit of exposed structure, so my question is, with there really be large amounts of exposed fireproofing on this project, even in the "nicer" areas? They do make a fireproofing coating that looks more like paint, so you don't have to see the ugly sprayed fireproofing, but that may not be an option.
Whether or not a structure requires fireproofing is dependent upon a lot of factors spelled out in the building codes. Building type, construction type, allowable square footage, and occupancy type all factor in determining the necessary fire resistance of a structure.
There are also provisions that state if the primary structural elements are at a certain height above the occupied floor, fire resistance is not needed; you typically see this for indoor sports arenas.
It is possible to gain fire resistance of structure with many methods: spray-applied, intumescent paint, and simple gypsum wall board. Intumescent is generally the most expensive and spray-applied is the most widely used.