I spent quite a bit of time trying to figure this out today so I'm going to share and you can ignore me...I *think* you have to use the fully mobilized skin friction to calculate the downdrag force on a drilled shaft, even for the service limit state condition where we do not necessarily consider the skin friction to fully mobilize. Thanks for listening. My brain hurts. If I'm wrong, you can tell me. I can take it. #morecoffee