ASCE 7 Response Spectrum with less modal mass less than 90%
ASCE 7 Response Spectrum with less modal mass less than 90%
(OP)
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.






RE: ASCE 7 Response Spectrum with less modal mass less than 90%
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: ASCE 7 Response Spectrum with less modal mass less than 90%
RE: ASCE 7 Response Spectrum with less modal mass less than 90%
Two additional things to consider:
1) Regardless of how you model your structure for design purposes, I would have no problem with using a separate, above grade only model to justify that you satisfy the 90% business.
2) If you've got a big podium with three towers and no expansion joints, you probably satisfy the requirements for ASCE's two stage podium analysis procedure. And that's essentially saying that you can consider the superstructure separately.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: ASCE 7 Response Spectrum with less modal mass less than 90%
I agree regarding your point about diaphragm flexibility. As to the others:
1. Separate models would work, but I would rather not maintain 2 models. Transferring the reactions from walls above to walls below will also be cumbersome. Still, it's a valid approach.
2. My structure doesn't qualify for ASCE's two stage procedure, and even if it did, I don't have any need for it. The towers on the podium are working just fine - same building materials and same lateral system type. There might be some advantage to reducing the seismic shear in the towers, but those are primarily governed by wind anyway. Seismic is only playing a role where I have to use amplified load combinations.
RE: ASCE 7 Response Spectrum with less modal mass less than 90%
RE: ASCE 7 Response Spectrum with less modal mass less than 90%
No consideration of soil-structure interaction. The perimeter foundation wall is idealized (perhaps poorly) as a shear wall.
RE: ASCE 7 Response Spectrum with less modal mass less than 90%
Some years back I had a building where I was using the response spectrum history analysis too. I just modeled the bases is "fixed" (for all 6 dof). And I just wasn't getting there. But I made some reasonable guesses at the base stiffness and got the mass well over 90% participating.
RE: ASCE 7 Response Spectrum with less modal mass less than 90%
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: ASCE 7 Response Spectrum with less modal mass less than 90%
RE: ASCE 7 Response Spectrum with less modal mass less than 90%
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: ASCE 7 Response Spectrum with less modal mass less than 90%
RE: ASCE 7 Response Spectrum with less modal mass less than 90%
RE: ASCE 7 Response Spectrum with less modal mass less than 90%
I blanked the mass at grade and below (well, actually set it to 1 k-s^2/ft) and assigned those three diaphragms (Level 1, B1, and B2) as rigid. Then I disconnected the core nodes from the diaphragm and reconnected them using custom beams that provide restraint similar to considering below grade flexibility.
With these two changes (dropping below grade mass and core attachment) I get achieve >90% modal mass participation with 15 modes.
Thanks for the help!
RE: ASCE 7 Response Spectrum with less modal mass less than 90%
It's great that things are working out. I'd love to hear more about how the custom beams are set up to simulate below grade flexibility. Can you elaborate?
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: ASCE 7 Response Spectrum with less modal mass less than 90%
Back in the full model, I could add beams at the corners of the core and calibrate the area and modulus to match the stiffness. In the spring_beams image, ignore spring144. I was just too lazy to give those beams a different property. Their only purpose is to support the meaningful springs. spring247,576,511,and 562 are my "spring beams". spring247 is a beam that is 247 in^2 with E = 10,000 ksi. I proportioned the springs so EA/L matched on both faces of the core, hence the larger area on the longer beam.
RE: ASCE 7 Response Spectrum with less modal mass less than 90%
First thanks for following up, so rarely people do.
Couple quick questions as I've been following along...
- So the spring beams are representing the flexibility of the diaphragm at grade?
- What program do you use? Looks like RAM elements/ Ram SS?
ThanksEIT
www.HowToEngineer.com
RE: ASCE 7 Response Spectrum with less modal mass less than 90%
RE: ASCE 7 Response Spectrum with less modal mass less than 90%
I don't know much about ETABs modeling, so I can't give you any suggestions on how to deal with the rigid boundary conditions at L1, B1, et cetera.
RE: ASCE 7 Response Spectrum with less modal mass less than 90%
RE: ASCE 7 Response Spectrum with less modal mass less than 90%
Regardless, RAM probably also has a residual mass option. Doesn't it? Sure, you could try to model the basement levels as having no mass and that would get you above 90% participation. The overturning moment should be essentially the same, so it's not a bad option. However, I would think those other floors should still get a ZPA acceleration.... making the residual mass vectors a better option (at least in my opinion).
Caveat: I work for RISA, which has a program that can be considered a direct competitor to RAM or ETABs. Even though I try to keep my posts mostly neutral, I wouldn't want to misrepresent myself as being completely unbiased.
RE: ASCE 7 Response Spectrum with less modal mass less than 90%
EIT
www.HowToEngineer.com