IEBC 2018 Substantial Structural Alteration

[L_Bey]

I am working on a renovation to an existing building, and one section of the IEBC is causing a bit of disagreement within the office. The section in question reads as follows:

[BS] SUBSTANTIAL STRUCTURAL ALTERATION.
An alteration in which the gravity load-carrying structural
elements altered within a 5-year period support more than 30
percent of the total floor and roof area of the building or
structure. The areas to be counted toward the 30 percent shall
include mezzanines, penthouses, and in-filled courts and
shafts tributary to the altered structural elements.

We have a small unreinforced masonry structure, roughly rectangular, with joists running parallel to the long face with two interior supports (three roughly equal spans). The first support is a beam framing to a column at the center. The second is a CMU wall for half the distance, and then a beam for the rest. The architect would like to replace the CMU wall with a beam. What we are arguing about is whether the area for the 30% threshold should be the full length of the supported element, or just the tributary area to the supported element. Since the building is pretty small, the difference between the full length of the supported joists/beams vs just the area of the roof that is tributary to the altered element is the difference between needing to show that the lateral system of the altered building satisfies IBC for wind and seismic loads.

What say you? Tributary area would be the standard way to look at altered elements, but it's not super clear. It seems like it could be interpreted to mean that the full length of the supported framing. See the attached sketch showing the two options for interpreting the affected area.

 

[kissymoose]

"tributary to the altered structural elements"
That last line, though referring to specific spaces, indicates the method of looking at tributary areas.
Also, in this example, you have 4 supporting lines for 3 joists spans. The floor area supported by each adds up to 100% of the space. If one beam line "supported" 2/3 of the space because the joists touching it made up the 2/3 of the floor area, then the total area "supported" by each line would be more than 100%.
I'd personally move forward with the tributary definition and argue as such against a plans reviewer or in court if needed.

 

[L_Bey]

That makes sense - having the supported area add up to more than 100% doesn't really track. It also doesn't make sense that removing 1/3 of one of the bearing lines (when there are 4) would trigger a 30% threshold. The engineer originally looking at the project read the current code and assumed that it would be the full length of any supported joist, but your arguments are pretty compelling so I will bring it to them as well. Thanks!