Good Afternoon Sandy,
The building code information you provide is a bit confusing. That may be a large part of the miscommunication with the plan reviewer. In order to answer your question(s), I will have to make a few assumptions. From what you have described it is not clear if you are designing to meet the IRC or the IBC design requirements. This is your choice and it must be stated on the permit application in order for the plan reviewer to know what you want. This is particularly true if you want the residential building to be reviewed under the IBC requirements (which is what it sounds like you desire to do). I will use the 2018 IBC for all references.
From the words you have used in your question, I will assume that when you describe the building as a duplex we are talking about a single property (lot) with one building on it and that building will contain two dwelling units which must be properly separated from each other. The Occupancy Type of the building will be R-3 and the Construction Type will be VB (fully fire sprinklered).
With the aforementioned parameters, Section 420.2 and 420.3 will require the two dwelling units in the building to be separated by "Fire Partitions" (walls) designed to meet the requirements of Section 708 and, in your case, a Horizontal floor/ceiling assembly designed to meet Section 711. For both the wall and floor assemblies, the fire rating must be a minimum of 1/2 hour. The fire rated partition(s) must meet the requirements of Section 708.4 (using both of the two conditions stated). The rated wall(s) can be offset as long as they begin and terminate at rated horizontal assemblies as required in 708.4.
The vertical fire partition can bear on a slab on grade, the top of the foundation wall, or it can bear on the top of a fire rated floor assembly. The separation just has to be continuous (the wall can be offset). I would suggest you keep the rated assemblies simple and select approved assemblies from Tables 721.1(2) and 721.1(3). These are just as acceptable as using a UL or other tested assemblies.
The detailing of both the wall(s) and the floor assemblies must each maintain continuous rated assemblies. The rated floor assemblies must be rated for the entire span. The supporting walls will most likely not be required to be rated assemblies.
What you want to do is allowed. You just need to guide the plan reviewer to the exact thing you want to do.
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Bruce Fischer AIA
Associate Professor
University of Nebraska, Lincoln
Lincoln NE
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Original Message:
Sent: 08-24-2020 18:20
From: Sandy Liu
Subject: Type V: When a fire-rated wall meets fire-rated floor, and vice versa
How do I find a set of details (preferably UL listed or ESR rated, if possible) that solve the intersection between a 1-hour rated wall and a 1-hour rated floor assembly, in Type V construction?
I'm working on a couple of residential duplexes. In each duplex, I have two units that start out side by side on the ground level, then on the second floor, one of the units goes over part of the other unit. The plan checker is insisting that a rated wall cannot end at a rated floor (as in a T-intersection, if viewed in section), that a rated floor cannot end at a rated wall (as a T-intersection if viewed in section), and that a rated wall and rated floor cannot form an L-intersection (if viewed in section). The plan checker says the 1-hour separation wall must extend from foundation to roof. Previous similar projects were permitted, so now I am confused. Have I learned this incorrectly?
Thanks in advance for your advice and guidance!
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Sandy Liu AIA
Pasadena CA
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