Quote:
Originally Posted by biophilic
I've never heard of this before, and I'm not sure that it's all that accurate.
Per code, the entire floor system needs to be fire rated. Using a certain thickness of concrete on top of an approved metal deck system gets you the fire-rating for the slab. You then need to fireproof the beams so that the whole floor assembly is fire resistive.
I believe beams and girders need a one or two hour rating, while columns need a 4 hour rating.
Anybody know for sure?
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You got the closest...
To build a building of this height (and pretty much any building more than a few stories tall) the building code will require you use Type 1-A construction which is the most restrictive type in terms of passive fire protection (active protection is sprinklers, required anyways) The CBC will set forth hour ratings for each building element, bearing walls, partitions, columns, ...
Once you have your req'd ratings you go to the UL book (underwriters lab, there are a few other companies that do this) for approved designs for different building assemblies. These are the people that tell you that a metal stud wall with 5/8 drywall on each side will give you 1 hour rating. Once you have your rating requirement and your design that you would like to use (if your intended design doesn't appear in the UL book you have to calculate it yourself <- sucks to do) you then need to design details that will maintain the rating for the entire occurrence of the assembly. It is okay if you switch designs, as long as you maintain the rating on all dependent elements. So, for example, you can't have a 2 hr. column enclosure holding up a 3 hr. slab. What's the point if the column fails, who cares if your slab is a billion hour rated.
So i am guessing that the metal deck and slab gives you 3 hrs. without any further treatment and the spray insulation gives you 3 hrs. Thus you have a continuous 3 hr. enclosure. They likely overspray to be safe. I couldn't imagine the liability if you skimp on the fireproofing and it dooms the building.
If you want to learn more about assembly ratings check out USG's listing for rated wall enclosures. The UL book sucks to go through and should only be consulted as part of a rookie architect hazing ritual.
Just to round out the explanation, you rarely find requirements more than 3 hrs. because, lets face it, if there is a fire going for 3 hrs and the fire department hasn't come yet, you're dead from inhaling smoke. At least that is the case with the International Building Code. The screwy and often sucky chicago code actually has a circumstance whereby which you would have to build an 8 hour wall. Go figure... I can't wait for them to finally switch in a few years.
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And like always, this is a fantastic photo, harryc! 
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