mansard roof and mwfrs
 

mansard roof and mwfrs
how do others handle windloads for a large flat roof structures with a steep slope mansard roof around the perimeter?
using asce7 low-rise building, the steep slope will significantly increase the mwfrs wall pressures compared to a flat roof. it doesn't make sense that adding a mansard roof above a 3rd or 4th floor should increase the wall pressures at the 1st floor.
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well you don't say which version of asce-7 you are using so i'm going to answer based on 7-98 since i have that edition in front of me. i would assume a similar answer for more recent editions.
7-98 has 2 figures to address mwfrs pressures, 6-3 and 6-4. since you state 'low rise buildings' i'm assuming you are using something similar to figure 6-4 which is for h<60. from what i've heard, this figure was developed based on requests from the metal building association in order to reduce their loads, notice the table title states walls & gable roof (and thats what the pictures show) since that is what metal buildings typically look like. if you have a steep mansard around the perimeter of a multi-story building, i would think fig 6-4 may not be the best figure to use.
if you use figure 6-3, notice the wall pressure table does not take into account roof pitch or corner zones. this is a more general table (albeit more conservative for some structures) that can be used for most buildings.