Seismic Design Question

[strguy11]

I have a 2 story building with a flexible roof diaphragm. (Metal deck untopped with concrete shearwalls). In one area between the the shearwalls there is a gym area, where basically there is no floor. The demising shearwall between the 2 areas will protude thru the roof, breaking the roof into 2 sections.

When applying the EQ loads vertically to each of the floors, I noticed that for the area where the gym is, the weight for that portion of the roof is greater due to the added wall trib height, than the other roof areas.

Should I treat these areas as 2 separate roof diaphragms and apply the forces individually, OR, do I just combine the weights into one mass for the roof, and design the diaphragm for the resulting load. I cant find any examples that address this, however, the one example I did find talks about an idealized mass theory which seems to suggest the latter option is correct, however, If I use the other method I get a slightly higher diaphragm design load.

I am going to use the worse case, but was wondering what others are doing.

 

[JAE]

The cases where you apply the mass to the diaphragm at one point is an idealized condition where the diaphragm is rigid and the lumping of the mass at the center of mass doesn't affect the design results.

For an irregular configuration, like yours, and where the diaphragm is flexible, you want to apply the seismic demand on the mass exactly where the mass really is. Don't try to lump it to some idealized center of mass. It would be distributed relative to the position of the various mass elements and applied to the diaphragm more like a series of uniform loads vs. point loads.

So I think you would first figure out the vertical distribution of the base shear to each level based on total mass at each level per the code.

Then distribute the resulting calculated force at each level based upon the horizontal layout of the various masses at that level.