Understanding Drift Results

Question

Hello, I am trying to understand drift results for my structure using RAM SS. We have semi-rigid diaphragms modeled and are trying to use the drift at control points to check the drift ratios. We have chosen select frame nodes coordinates as our control points, closest to the perimeter of the building. A few of the results are puzzling: 
 
 We have a frame column that cuts through diaphragms, without any frame beams connecting, and drift results only display for a few of the lower floors and not the upper floors. 
 Some frame columns do not display any drift results through their entire height. 
 Some of the columns display expected drift results then at the upper most floor, their total displacement is noted as 0. The drift results indicate a reverse displacement occurring as a story drift at the upper most floor, when compared to the floor below. This is resulting in very large drift ratios and does not match the deflected shape of those load combinations when comparing to the drift results. 
 
 Note: We are using load combinations to check drift due to a mechanical screen wall at the roof. The attached drift results are just considering Wind in the X and Y directions to simplify/shorten the output. PDF

Seth Guthrie · Answer

If you want the displacement at the nodes it's more precise to use nodal displacements. 
 Assuming the diaphragms are rigid, the drift report is mostly useful for the final checks that compare the control point displacements vs average displacements to see if there is a torsional irregularity. It will only give values for a level if there is, in fact, a diaphragm at the control point location. Where levels are partial and do not cover the control point 0 values are expected. This also causes some confusion in the story drift ratio because the program is then going to look at the non-zero deflection vs the two-story height. 
 If the diaphragm is semi-rigid you can still get results, but these can deviate from the frame displacements when the diaphragm is soft. A review of the deflected shape is useful for confirming that condition.