Concrete Load Bearing Wall Live Load Reduction

[ChiEngr]

Hello,

I am working on a project in which there is a proposed penthouse (CFSF framed) being built on top of an existing roof structure of a two-story building. The penthouse walls actually coincide with 24 ft long existing concrete load bearing walls below, constituting a square core in the middle of the building. I am checking the existing footings for the added load due to the new penthouse construction. My question is with regards to live load reduction for the concrete bearing walls. Below are my assumptions:

1. The tributary width of the walls is 12 ft, so my tributary area for the walls are 24 ft x 12 ft = 288 ft^2.
2. Since the existing roof level is now housing a mechanical room and covered with the penthouse roof, I am considering this a "floor" for the purpose of total tributary area on the existing strip footing: 288 ft^2 x 2 floors = 576 ft^2.
3. I am using a LL element factor, KLL = 2, considering the wall as an interior beam.

The assumptions above seem reasonable to me, but I am mainly unsure of item #2. Maybe it is inappropriate to consider the roof level as a "floor", even if a new penthouse structure is built on top of it. Also, I am unsure if I should use the full length of the wall in calculating my tributary area. I know some people use 1.5x(floor to floor height) as a maximum length of wall per previous threads on this site.

Any help or suggestions would be greatly appreciated.

 

[KootK]

Maybe it is inappropriate to consider the roof level as a "floor", even if a new penthouse structure is built on top of it.

I think you're good there. It should be based on occupancy rather than location in space etc.

Also, I am unsure if I should use the full length of the wall in calculating my tributary area.

I would use the full length of the wall. Maybe even more of the wall of the core than just that one too. If you're checking a foundation that is a raft pad or pile cap, then all of the load coming to that element from various places should be fair game for LLr. It might be a different story were you just checking the wall immediately below the PH.