Concerning the 5% increase for gravity load, the IBC sections in question are 3403.3 and 3404.3. They state that if the addition or alteration causes an increase in design gravity load of more then 5%, the member shall be strengthened, supplemented, replaced, or otherwise altered as needed. Note that it refers to load, not stress, or moment, or interaction ratio. So if your beam has 1 k/ft originally, you can go up to 1.05 k/ft without having to modify the beam. If your beam is supporting a concentrated load, I would apply this 5% increase to the new concentrated load.
For elements resisting lateral load, the code refers to the demand-capacity ratio, and says that this ratio can be increased by 10%. Here, you would multiply your demand-capacity ratio of the existing member by 1.1 to find the new limiting demand-capacity ratio. If it's at 10%, you can up it to 11%. If it's at 90%, you can up it to 99%.
What this tells me is that for gravity members, you don't even need to worry yourself with the interaction ratio. I guess it's assumed to be OK by virtue of being "existing." As long as you increase the applied load by no more than 5%, you're OK.
For lateral members, though, you do have to check the interaction ratio. It's not enough to simply limit the applied load in a member to 110% of the original.