I have a (2) part issue. I have a large rectangular building with 1/4" slope in 2 directions away from a center ridge. The building is structurally separated by an expansion joint down the center of hte building and about 1/3 of the right half is separated as well. This is due to special freezers that need structural separation. I have created separate frame list, break in the concrete bearing walls, and in the deck (separate slab lines that do not cross). Gravity runs fine with no errors. The seems to run fine but I am getting some strange results. I am using a Pseudo Flexible Diaphragm due to the ridge in the deck (semi rigid needs to be flat or a continuous plane or a nodal error occurs).
I would expect the story shear forces to be applied in the center of each diaphram since rigidity is not considered in the load distribution (percentages have been provided for the Pseudo Flexible Diaphragm). But when I view the Building Story Shears, they do not appear to be applied in the correct locations. For Wind_X load case, the left box portion appears to be applied at an offcenter location closer to the concrete walls (more rigid elements) like I would expect from a rigid diaphram, the upper right rectangle appears to be skewed toward the concrete walls, and the lower right rectangle has half of the story shear skewed toward the the concrete walls and the other half at what appears to be the COR for the entire building. Then, for the Wind_Y load case, the upper right story shear is applied in the same location as the Wind_X load case, the lower right is applied at the same location as half of the Wind_X load case where it was skewed toward the concrete walls, but this time half of the left box portion is applied at the same location as Wind_X load case and the other half is applied at the COR of the entire building. Based on the percentages applied to the frames in the Pseudo Flexible Diaphragm Properties suggest that the Building Story Shears would be applied in the center of each diaphragm.
Another issue is when the braced frames are viewed with Axial Member Forces shown (or in a report), the roof beam of the braced frame does not appear have the proper axial load applied to it. Where the force is 46.36 kips applied to the brace, the axial load in the roof beam is only 9.27 kips.
I will send the RAM file for clarification shortly.
When the diaphragm is pseudo-flexible, the location of the graphic for the diaphragm shear is largely irrelevant. This wiki indicates that it's somewhere near the geometric center.
If you look at the building story shear report, you'll see 4 lines in the table for diaphragms 1,2,3 and "None". There is a "None" diaphragm because some of the chevron braces are not connected to any diaphragm, so the force in those braces shows up in the None row (and also on the graphic, near the very center). That's explained on the same wiki. You can change the criteria - diaphragm - disconnect option to prevent that. Since you don't have a rigid diaphragm it make no difference in other results.
I think the beam axial forces are OK. Keep in mind, you have nodal loads applied at all three joints of the chevron frames typically.
Finally, it's worth noting that the program generated wind loads on this structure won't be correct. We will apply a full amount of windward and leeward pressure to each diaphragm like we would to three towers. I would recommend using defined loads. More on wind loads here:
Answer Verified By: Jeffrey Champlin