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Re: diaphragm drift limitation

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The delta-M diaphragm drift you're talking about is not out-of-line with many of the designs I've seen and done myself so I know where you're coming from. In regards to roof diaphragm drift, I think you're applying limits to something that isn't intended. I have always felt that what the code is limiting is the drift of the vertical elements of the lateral system, i.e. the end shearwalls (in-plane drift with concrete tilt-up shearwalls is typically not a problem). There is a difference between drift and diaphragm deflection. The code doesn't really limit the diaphragm deflection, it just says that anything attached to the diaphragm has to be capable of supporting itself when the diaphragm deflects and everything attached to it moves out-of-plumb. This brings us to two important elements that are hanging out at the center of the diaphragm, the columns and the out-of-plane panels. I tend to agree with your friend that if a column is modeled and designed as pinned top and bottom, moving the top 14" out-of-plumb doesn't add flexure, the eccentric load is simply resisted by lateral forces at the top and bottom that create a resisting couple... draw the FBD. In reality, the base isn't really pinned, there's some fixity and hence the load being pushed out-of-plumb at the top does create some flexure in the column. If you go this route however, you should also be able to take advantage of the fact that k for your column isn't really 1. Basically standard practice is to model it pinned top and bottom and I've found this to be adequate. Our office deals with some of these unknowns by adding an accidental eccentricity to the vertical load (in addition to any eccentricities that might be present). Walls are similar to the columns. Where it get's fuzzy to me is the case where you have dock-high panels that are connected at the slab and 5' or so below at the footing (to account for a 4' dock). In this case, the panel is pretty well fixed at the base and the question becomes "does the panel really have the capacity to be pushed out-of-plumb 14" at the top?" At a certain point, the question is "what's more rigid" and when does the panel start to push back on the diaphragm?

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