Mansard roof and MWFRS

[CTSeng]

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.

 

[CFSEng]

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.