Period and Mass participation of the building in RAM SS

Question

Hi, 
 I've currently modelled a 22 story building and period of the structure doesn't look right (I'm getting over 3 s for the first natural period, while I would expect around 1.5 s). 
 Also, I can't find any mode with mass participation over 90% (all of them are less than 50%). 
 
 (file uploaded) 
 
 Please advise.

Seth Guthrie · Answer

If the diaphragms are semi-rigid then the masses are spread over the full diaphragm and it can be harder to get enough mass participation. If the diaphragm is highly flexible, or worse sloped, then long modes representing diaphragm waves can happen. You can usually see this using Process - Results - Mode Shapes. Check out eignevalue and semi-rigid diaphragm posts below for more. communities.bentley.com/.../ramss-eigenvalue-error.aspx communities.bentley.com/.../ram-ss-semirigid-diaphragms.aspx

Seth Guthrie · Answer

I ran the model (Oracle_VB5.rss) in v15.0 using the same options you had. As noted, the mass participation is low using 10 mode shapes. The general shape of the modes and period values seem reasonable to me, but since so much of the mass is at the bottom levels it may just require more mode shapes to get to 90% (bear in mind that since the diaphragms are semi-rigid there are literally thousands of mass points involved). I'm trying 30 modes now.

Seth Guthrie · Answer

30 modes (using the improved Lanczos solver which is new in version 15) yielded great results, well over 90% in both directions.

Seth Guthrie · Answer

That solver option is under Criteria - General (the Ritz option is still pretty good too). The first period was right at 5 seconds and the 2nd just slightly shorter, as I recall.
There are two factors about using rigid vs semi-rigid diaphragms. The first is that the diaphragm mass is lumped to a single point when the diaphragm is rigid. The second, more important, nuance is that when the diaphragm is rigid it's out of plane stiffness is neglected in the lateral load analysis. i think this also applies to the eigensolution. So I like to use semi-rigid diaphragms to all concrete two way decks. See our post on diaphragms for more: communities.bentley.com/.../ram-ss-semirigid-diaphragms

Seth Guthrie · Answer

Here are a couple of contributing factors. First you have set the cracked section factor on the walls fairly low (0.4 in plane), reducing the stiffness. Second you have P-delta analysis on, which in Ram Frame, results in a slightly longer period. Finally, it looks like there is a significant rotational component to the mode shapes that results in a longer period than you might expect. This seems logical since the cores are centrally located and the other 2D walls are only in the Y direction.