I am having problems getting more than 90% modal mass participation in my response spectrum analysis.
The structure consists of 2 framed levels below grade, a large 3 story podium above grade, and then three towers extending up from the podium for a total of 21 stories (including basement). The shared podium and basement make the structure very bottom heavy.
When I set the base shear to come out at grade (Level 1), I can get >90% using the first 24 modes. However, we are using concrete shear walls that are continuous down to the basement (B3). The software utilized laterally restrains the base shear level and all levels below it (in this case L1, B1, and B2). This causes an unrealistically high portion of the overturning moment to be resolved as a horizontal couple at L1 and B1 instead of a vertical couple at the foundation. In order to consider some deformation at and below grade, I need to set the base shear to come out at the basement instead of at grade as well as consider the stiffness of the diaphragm (non-rigid). When I do this, I need to run many, many more modes. In fact, running 100 modes only gets me to 88% modal mass participation. Has anyone else had these issues? Any suggestions other than going to the equivalent lateral force procedure?
One workaround is to disconnect the shear wall nodes from the diaphragms at grade and below, and then reconnecting the walls to the diaphragm using beams calibrated to the stiffness of the diaphragm. Getting the beam properties "right" will be very tricky, especially with respect to torsional stiffness (we have cores, not blade walls). I'm not enthusiastic about this option.
Is there any way to justify using less than 90%? I suspect the bulk of the missing mass is below grade, which should not have any meaningful impact to the structural behavior.
The structure consists of 2 framed levels below grade, a large 3 story podium above grade, and then three towers extending up from the podium for a total of 21 stories (including basement). The shared podium and basement make the structure very bottom heavy.
When I set the base shear to come out at grade (Level 1), I can get >90% using the first 24 modes. However, we are using concrete shear walls that are continuous down to the basement (B3). The software utilized laterally restrains the base shear level and all levels below it (in this case L1, B1, and B2). This causes an unrealistically high portion of the overturning moment to be resolved as a horizontal couple at L1 and B1 instead of a vertical couple at the foundation. In order to consider some deformation at and below grade, I need to set the base shear to come out at the basement instead of at grade as well as consider the stiffness of the diaphragm (non-rigid). When I do this, I need to run many, many more modes. In fact, running 100 modes only gets me to 88% modal mass participation. Has anyone else had these issues? Any suggestions other than going to the equivalent lateral force procedure?
One workaround is to disconnect the shear wall nodes from the diaphragms at grade and below, and then reconnecting the walls to the diaphragm using beams calibrated to the stiffness of the diaphragm. Getting the beam properties "right" will be very tricky, especially with respect to torsional stiffness (we have cores, not blade walls). I'm not enthusiastic about this option.
Is there any way to justify using less than 90%? I suspect the bulk of the missing mass is below grade, which should not have any meaningful impact to the structural behavior.