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structural PERFORMANCE 3-D Snow Drifts

B the 2016 version of the American Society of Civil Engineer's ASCE 7 Load Standard, Minimum Design Loads for Buildings and Other Structures, all snowdrifts were two dimensional. The height and width (horizontal extension) of the leeward roof step drifts were taken to be constant all along the roof step. The same holds for windward roof step drifts, parapet wall drifts, and over-the-ridge gable roof drifts. As such, the wind direction of interest was nominally perpendicular to the geometric irregularity, i.e., perpendicular in plan to the roof step, the parapet wall, or the gable roof ridgeline. There are some roof geometries where two, 2-D drifts overlap or occupy the same roof area. One such roof geometry is the Northwestern corner of a roof with parapet walls along both the North and West edges. Wind out of the South would result in a two-dimensional drift along the North wall, while wind out of the East would result in a two-dimensional drift along the west wall. Presumably, each of the 2-D drifts would control design for most of the bays along the North and West sides, respectively. The 2-D drift footprints overlap at the corner. Prior to ASCE 7-16, the design snowdrift for the NW corner bay was open to question. Some structural engineers may well have designed the bay for each of the 2-D drifts separately. Other structural engineers may have designed the corner bay for the sum of the two 2-D drifts. ASCE 7-16 clarifies the situation by specifying that the snow depth at any point in the overlap area is taken as the larger of the two 2-D drift depths at that point, as shown in Figure 1. That is, a 3-D drift is not a new drift; instead, it is the drift at locations where well established 2-D drifts overlap. Note that this approach is consistent with the ASCE 7 approach for roof step drifts. In that case, both the leeward and windward drifts are determined, and the larger (not the sum) is used for design. Other roof geometries can lead to 3-D snowdrifts, i.e., the overlapping of two 2-D drifts. A simple gable roof, with a N-S ridgeline and a pediment or parapet at the North end wall, is one such example. For wind out of the South, there would be a regular 2-D parapet wall drift, while for wind out of the West, there would be a regular 2-D gable roof drift on the East side of the gable. These two, 2-D drifts would overlap along a portion of the parapet wall East of the ridgeline, as sketched in Figure 2. Extension of the current 3-D drift provision for parapet wall corners and re-entrant corners to other roof geometries is currently under consideration by the ASCE 7-22 committee. One issue is whether such new guidance, which by necessity would increase the length and complexity of the code provisions, is needed. As noted above, there is a single wind direction of interest for the 2-D drifts (e.g., nominally perpendicular to the ridgeline for gable roof drifts). However, there are two wind directions of interest for 3-D drifts (e.g., wind out of South and wind out of the East for the Northwest parapet wall corner case discussed above). This raises the question of the likelihood of multiple wind directions in wintertime. For a site with a single, predominant wind direction (e.g., the winter wind is almost always out of the North), the potential for significant 3-D drift formation would seem limited. On the other hand, 3-D drift formation would seem more likely if winter winds from multiple directions were expected. Boston’s 2014-15 winter season was an example of the latter.