I have a building with a low roof (office space) and a high roof (maintenance space) without an expansion joist. I modeled the high roof as a 2nd story. The low roof is to the west of the high roof.
When the wind is applied north-south directions, both diaphragms appear to have the correct load (i.e. tributary wall area * pressure). When the wind is applied in the east-west direction, the high roof wind load is not correct.
It appears that the load on the high roof is based on 1/2 of the change in roof elevation (half of the exposed wall height of the partition wall) * bldg depth * wind pressure. The low roof appears to have the wind pressure from half of the first level (wall height) and half of the second level (wall height).
When the wind is from the west, the low roof walls will shield the high roof. When the wind is form the east, the upper diaphragm should have all of the windward pressure. What are my options?
User defined story forces are probably best. See: RAM Frame - Wind Loads FAQ