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This projected area method is usually sufficiently accurate, but if there are steep hip ends, there may be a significant amount of building mass unaccounted for. It may also be more convenient to enter the self-weight of the actual roof material than it's projected weight. However, the sloped approach leads to more complicated array of masses.
Depending on the direction of the seismic force relative to the roof panel, they may be split into two or three segments to create triangular, trapezoidal, or partial uniform line loads. It is necessary to do this independently for the N-S and E-W directions. The self-weight of the roof panel will be multiplied by the depth of the roof panel segment, along the slope and in the in the direction of the loading, at each point along the extent of the roof panel segment.
For building masses generated for forces that are parallel to the front edge of the panel; that is parallel to ridge for side panels, the intensity is the length of the side panel at a particular point multiplied by the self-weight times the secant (1/cos) of the slope of the panel. This is to account for the greater expanse of material along the slope than along the projected distance of the building mass object.
The line loads are located in such a way that they combine with building masses for walls and floors, to minimize the clutter on the screen. For panels that are inclined in the same direction as the seismic force, the location of the line load is the location of the wall on which the roof rests. For panels that are inclined perpendicular to the force, the location is that of the wall line at each end of the roof panel.