• Jon,

I would argue that your understanding is correct.  Yes, you need to be able to a) determine what the occupancies are from Chapter 3 and then b) determine from that Table 508.4 what the separation requirement is.  If you can do those two tasks, I personally feel that you will be covered for such questions on the exam.

Separate question: is a "séance room" one where everyone is under a trance, and if so, are they now incapable of self-preservation?  Deep thoughts.  Please don't answer this!

• thanks a /ton/ david...

• David,

I'm wondering if I could ask you to please confirm a few things for me, it would be such a great help to know I have this right:

1. Primary versus ancillary. When faced with multiple occupancies, is it just simply whichever area is greater (unless otherwise clarified in Chapter 3)? (Example: a restaurant with a shop inside, versus a shop with a restaurant inside).
2. Ancillary versus mixed. If the ancillary occupancy is greater than 10% of the (fixed) building area, is this basically the criteria for determining  a mixed occupancy condition?
3. Nonseparated versus separated. Table 508.4 is what we use to decide whether a mixed condition is a nonseparated (no fire rating req.) or separated occupancy (fire rating req.), is that correct?

Thank you so much,

• (Edited )

I’ll address each one:

1. Essentially you’re correct. You look at the building’s main function that you’re doing.  As you say, a restaurant with a small shop in side would be a restaurant (Assembly Use).  A large store with a small café inside would be a Store (Mercantile Use).   That’s how I’d first decide what the main Use Group is.

2. As to whether or not the secondary use of the space needs to be quantified with an actual Use Group (in the example above, an A-2/M building), yes you use the 10% rule.  If the secondary function is < 10% of the building area, you can consider than at an accessory occupancy and do not need to apply separated mixed-use criteria at that point.  Check out some of the exceptions in Chapter 3 under Assembly Use as well, there’s a few other instances where large spaces don’t need to be classified separately.

3. Table 508.4 isn’t what you use to first determine if you need to be separated mixed use vs. non-separated mixed use.  What determines that requirement is Table 503.  When you have two separate Use Groups in a building, but you would like to avoid having to provide any fire rated separation between them, you need to take the most restrictive of the two Use Groups and first see if the ENTIRE building could be built under the single, more restrictive classification.  Using your Construction Type and that Use Group, run the calculations (taking into account sprinkler and frontage increases too).  If it passes the test, then you can proceed with non-separated mixed use.  Now, on the drawings, it is STILL a mixed-use building, and you still have both those Use Groups in there, but you can qualify it as non-separated mixed use and the two occupancies can be open to one another.

4. It’s only when this calculation effort using the most restrictive Use Group does NOT pan out that you have to pursue a separated mixed-use design.  You still have to run both Table 503 calculations for both Use Groups first, but once you do that and you confirm that the building works, THEN  you refer to Table 508.4, and that Table will tell you what the fire rating needs to be between the two.

I hope this helps!

• David,

Really, thank you so very much for responding and taking the time to explain.

I think I've got most of it - but unfortunately Step 3 has me confused. Those calculations (Section 506, right?) aren't covered in my version of Building Codes Illustrated (2015). Do you think we'll need to know it for the exam?

Just so I can understand the following statement, at least qualitatively: "...you need to take the most restrictive of the two Use Groups and first see if the ENTIRE building could be built under the single, more restrictive classification."

What does this mean? I thought the secondary use could always, in theory, be built to the more restrictive construction type. I'm trying to think of an example using Table 503 of where that wouldn't be the case...

A

• (Edited )

I think you may have misunderstood what I meant there.  Your initial question was about determining non-separated and separated mixed-use classifications for a building.  When you have two Use Groups, let’s use the A-2 and M example, you first see if you build the building as non-separated mixed-use because that allows everything to be open and more flexible for us as designers.  The Code says “OK, take the most restrictive of those two Use Groups.  If you can prove to us that you could build this building as big as you’re proposing in area, height, and in number of stories and in the construction type you’re proposing with JUST that one Use Group, then we’re good to go.  We don’t care how many other less restrictive Use Groups are in this building because those are safer than the one you just analyzed.”  To assign a “non-separated mixed use” classification, we must prove that you could build this building ENTIRELY of this more restrictive Use Group, and in doing so, of course Code is going to allow you to build the building with a less restrictive Use Group inside it as well. Does that make sense?

Now when it comes to separated mixed-uses, meaning you tried the non-separated route above and it didn’t pan out, Code now makes you analyze each of the Use Groups with Table 503 individually.  It’s actually very easy, but the verbiage in the Code is terrible.

First look at Table 503's height and number of stories requirements and check to make sure, for each Use Group, that you do not exceed the maximum building height and allowable number of stories.  There's a kicker to this: if you have a multistory building, you can take into consideration on what floors your Use Groups are located.  Here's an example using A-2 and M and assuming Type VA Construction:

A-2 - 2 stories max, 50' in height

M - 3 stories max, 50' in height.

Let's assume as well that this is sprinkled, so we take the sprinkler increase of an added story and 20' in height for both:

A-2 - 3 stories max, 70' in height

M- 4 stories max, 70' in height

Let's assume you are building a 4-story building with A-2 on floors 1-3 and M on all four floors.  Are we ok?  The answer is yes because you do not have A-2 on the 4th floor, thus in your building the A-2 uses are within a 3-story building footprint.  If you had A-2 on the 4th floor, you'd be in trouble.

Now for area use Table 503 and take Use Group A-2 and figure out how big in area the building can be per floor, including sprinkler and frontage increases, and set that number aside as the Allowable A-2 number.  Now do the same exact thing for M and come up with the Allowable M number.  Now Code makes you do a little math.  You now have to go into your floor plans, floor by floor, and figure out how many SF of A-2 you actually have (Actual A-2) and how many SF of M you actually have (Actual M).  Then for each floor you do this:

(Actual A-2/Allowable A-2)    +   (Actual M/Allowable M)   =  X

X is the sum of the ratios of Actual Area to Allowable Area.  You’d do this calculation for every floor of your building, and for every floor, Code says that X must be less than 1.  If it is, you are good to go with calling this building a separated mixed-use building.  Go to Table 508.4 and find out what rating is required between A-2 and M.  Design your building accordingly.

I personally did not have to do this type of equation on the test, but I do think it’s worth knowing how to do.  Hope this helps.

• David,

Finally...I've got it! I had to spend quite some time going through it, but it makes so much sense now. The only way I could ever understand was by sketching it :)

Thank you so much for your clarification, this has been very valuable for me!!!