Exploring the Building Code - IIIA and III B Construction Quirkiness

One of the most common mistakes when working with the building code is not reading the table footnotes. This month we will be pulling from an actual code issue that occurred in the office as well as one of my favorite ways to gain insight into the code, an ICC Code Opinion.

You may already be familiar with the footnotes to Table 601 – Fire Resistance Rating Requirements for Building Elements. If not, this table provides the fire rating requirements based on construction type of structural building elements. Footnote F, which is referenced by both Bearing Walls and Primary Structural Frames seems at first glance innocuous. Even after reading section 704.10 you may not be concerned, after all it only applies to load bearing structural members located within exterior walls or on the outside of the building and the rating requirements seem obvious.  1: As required for the type of construction and the building element per table 601. 2: As required for the type of construction and exterior bearing walls and 3: As required by table 602.

If you missed it, don’t worry items 1 and 3 are no brainers, item 2 is where it gets interesting. If you look back at table 601 you will realize that in all but one construction type the rating for primary structural frame and exterior bearing walls are the same. In type III construction it’s not, in both IIIA and IIIB the exterior bearing wall is to be 2hr rated but the primary structural frame is 1hour for IIIA and 0 hours for IIIB. What this means for type III construction is that if the primary structural frame is within the exterior wall or on the outside of a building (Perhaps supporting a covered portico that is an extension of the building roof) then these structural elements need to be 2 hour rated. In IIIA construction this is probably not a severe inconvenience after all the structural elements were already being rated, but in IIIB going from no rating to a two hour rating could require a significant redesign.

Now in my introduction I promised an ICC Code Opinion and how it comes into play on an M+A project. We sought an opinion of the text “within the exterior wall”. We interpreted it to mean literally within or at least engaged with the exterior wall framing. On one of our projects we had received a correction letter because we had a column adjacent to the framing and that column was considered engaged within the exterior wall because it would interrupt the interior gyp board being shown on the inside of the exterior wall. Since there are buildings without interior finishes on the exterior wall (Warehouses and Garages) to name a few and the code does not include interior finish or skin on the inside face in any definition for an exterior wall, we had confidence in our interpretation of the code. However the code opinion we received relied upon prior code language:

“While Section 704.10 addresses loadbearing structural members “within” exterior walls, this section has also historically included loadbearing structural members located “along the outer lines” of a building or structure. While the primary structural frame may not be required to be rated due to Table 601, if the structural members are located along an exterior wall that is required to be rated either based on the type of construction or the fire separation distance, then those structural members are also required to be rated.”

We think this is a good example of a code section that may need further interpretation from the ICC, after all why was the code language changed from “along the outer lines” to “within”? And since there is no separation distance to define “along the outer lines” how far from the exterior wall does the primary structural frame need to be? So stay tuned we may be updating this in the future, as for our M+A project we were able to address the problem by going to a different construction type. In our case going from IIIB to VB but depending on your projects height and area that may not be a solution.